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Keywords:

  • Carbon sequestration;
  • Interdisciplinary;
  • Land use;
  • Lowland river;
  • Mountains;
  • Nature management;
  • Tidal wetlands

Abstract

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References

Question

How does the interaction between silicon (Si) and vegetation affect local and global ecological processes, higher levels of ecological organization, and terrestrial- and watershed-scale Si fluxes?

Location

We selected several ecosystems throughout the world, from river headwaters to estuaries, being examples of (i) terrestrial vegetation, (ii) aquatic and floodplain vegetation, and (iii) tidal wetland vegetation.

Methods

We provide examples of the importance of linking Si use by terrestrial and aquatic vegetation, to larger-scale Si flux consequences towards and through rivers. Cross-disciplinary studies achieve the best understanding of vegetation effects on the global Si cycle, and the role of Si as a plant functional trait.

Conclusion

Si use by plants has not always received the research attention of other elements. Yet, today the importance of Si for plant functioning is slowly becoming better understood. Silicon is a crucial element for many plant species, being important for decomposition processes, plant competitiveness and stress tolerance. The inclusion by vegetation scientists of Si uptake as a plant functional trait is important to assess links between plant physiology, plant distribution and plant tolerance to environmental changes, but also to understand the role of vegetation on Si fluxes through the watershed. However, lack of knowledge regarding the biological control of the Si cycle hinders accurate quantification. Only a concerted effort bringing scientists together from a broad array of disciplines will provide this new direction for research on vegetation–Si cycling.


Nomenclature
Temperate species: Flora Europea (http://rbg-web2.rbge.org.uk/FE/fe.html);

(Sub)tropical species: Tropicos (http://www.tropicos.org)

Background

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References

Silicon (Si) is usually considered a non-essential element for plants, as almost all higher plants (except Equisetum spp.) can grow and reproduce in Si-deprived media. This has caused a small number of researchers to address Si functioning in plants, vegetation and ecosystems, and compare it to other major elements. However, all plant species contain Si and it can be present in high amounts, e.g. 19% in giant cane (Arundinaria gigantea (Walter) Muhl.), 8% in rice (Oryza sativa L.) and 6% in reed (Phragmites australis (Cav.) Trin. ex Steud.) dry biomass (Struyf & Conley 2009). Raven (1983) and Epstein (1994) were among the first to address this discrepancy between Si studies and those on the more ‘classical’ nutrients such as nitrogen and phosphorus. They emphasized the importance for of Si in plant resistance to a number of biotic and abiotic stresses. Si physiology has been best studied in rice, and several competitive advantages have been highlighted, including increased plant performance under shading and increased resistance to salinity, herbivory and Al, Fe and Mn toxicity (Agarie et al. 1992; Savant et al. 1996; Ma & Yamaji 2006). This is also the reason for its incorporation in a wide range of fertilizers on agricultural land (Currie & Perry 2007). Present-day biologists are now learning that plant ecology is actually more siliceous than previously thought, as recently summarized by Cooke & Leishman (2011a). Several plant species, the accumulators of Si, have specialized in using Si to increase their competitiveness, especially grasses and sedges (Epstein 2009). Most research on Si in plants has focused on agricultural species belonging to the grass family, which profit from Si availability to relieve multiple stressors (Datnoff et al. 2001; Cooke & Leishman 2011a).

Research in the last decade has clearly shown that Si uptake by plants in natural ecosystems exerts an important control on Si cycling (Conley 2002; Derry et al. 2005; Struyf & Conley 2009), with the majority of the weathered Si first passing through vegetation before it eventually reaches rivers and ultimately coastal zones (Derry et al. 2005). Neotropical grasses, for instance, growing on soils with more easily weatherable Si, are more silicified than their paleotropical counterparts (Sarmiento 1992). The turnover and storage efficiency of the vegetation within a watershed are thus prime drivers for eventual fluxes of Si towards rivers. Hence, vegetation use of Si determines the conditions under which Si is released. In plants, Si is present in the form of hydrated amorphous Si (SiO2.nH2O), usually referred to as biogenic silica (BSi) and mostly deposited as phytoliths (siliceous plant bodies; Broadley et al. 2012). Diatoms (algae), testate amoebae (single-celled protists) and sponges (animals) also contribute significantly to the BSi pool of terrestrial and aquatic ecosystems; these organisms are coated with an amorphous siliceous exoskeleton (Aoki et al. 2007; Smol & Stoermer 2010), or have an endoskeleton stiffened by spicules (sponges; Maldonado et al. 2010). The structure of BSi is generally characterized by its relatively low thermodynamic stability, which makes its dissolution kinetics an order of magnitude faster than that of silicate minerals (e.g. quartz) in water (Cornelis et al. 2011). Due to the efficient uptake of weathered Si into terrestrial biota, large amounts of reactive BSi are currently stored in biomass and consequently in soils, which is crucial in determining eventual riverine Si fluxes (Struyf & Conley 2012).

To understand the impact of vegetation on the biogeochemical Si cycle, and the role of Si in plant and vegetation dynamics, it is essential to establish a link between the uptake, processing and functioning of Si within vegetation and fluxes of Si at the ecosystem level. Hence, we must link the plant Si accumulation rates, plant Si physiology and the abundance of Si accumulating plant species in a watershed, to eventual terrestrial–aquatic fluxes (Bartoli 1983; Fulweiler & Nixon 2005; Conley et al. 2008). Cornelis et al. (2010) showed that differential Si uptake by tree species directly affects BSi content of the soil. BSi in soils is an important factor for pore water dissolved silica (DSi) concentrations (Farmer & Miller 2005), and indirectly for other pedological processes, such as DSi adsorption onto soil particles and neo-formation of secondary silicates (Cornelis et al. 2011). These processes greatly affect Si transport through the watershed, but also soil formation, and hence local vegetation dynamics.

The understanding of vegetation Si processing is still at an embryonic stage (Struyf & Conley 2012). Plant Si processing, as well as plant Si concentration in different organs, might well be important functional plant traits, determining both the response of vegetation to environmental disturbance and the effect of vegetation on ecosystem functions, such as biogeochemical cycling. Plant Si uptake has, as previously indicated, a strong potential effect on plant competitive ability, but is also correlated with factors such as cellulose, lignin and phenols, affecting decomposition efficiency (Schoelynck et al. 2010; Schaller et al. 2012a) and uptake of other elements (C, N, P; Schaller et al. 2012b). It thus impacts the remineralization of crucial resources and soil nutrient turnover rates, and the trade-off between resource efficiency and plant stabilization and defence (Schaller et al. 2012b). Hence, Si could play an important role in vegetation competition, associated functional traits and ecosystem functioning, but has received little attention from vegetation scientists in this context (Diaz & Cabido 1997; Lavorel & Garnier 2002).

This review provides examples of how local vegetation impacts terrestrial, watershed-scale Si fluxes, and how Si availability affects vegetation dynamics. We have selected several ecosystems, from the river headwater to estuaries, illustrating the role of (i) terrestrial vegetation, (ii) aquatic and floodplain vegetation, and (iii) tidal wetlands (Fig. 1). Our examples demonstrate the importance of linking local vegetation-scale processes in the watershed to larger-scale flux consequences towards the rivers and ultimately towards coastal zones. Finally, we provide a range of examples where interaction of different research disciplines may result in a leap forward in our understanding of plant–Si cycle interactions.

image

Figure 1. Si weathering, uptake and cycling through vegetation, and fluxes in waterways vary across the river continuum, indicating that position in the landscape and vegetation type affect Si fluxes at a site and also further downstream. Top panel: Meltwater on a glacial moraine in front of the lower Arolla glacier in the Alps near Mount Callon (Switzerland). Vegetation: alpine grasslands dominated by Nardus stricta L. and sub-alpine Picea forest. Middle panel: Lowland river (Biebrza) at Biebrza National Park near Lipsk (Poland). Vegetation in the river: Nuphar lutea Sm. and Potamogeton crispus L.; at the floodplain: Phragmites australis (Cav.) Trin. ex Steud., Carex acuta L. and several other sedges. Lower panel: Ungrazed salt marsh at the Wadden Sea coast at Sönke-Nissen-Koog (Germany). Vegetation at the creek: Spartina anglica C.E. Hubb; back and front: Elymus athericus (Link) Kerguélen

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The river continuum: from headwaters to estuary

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References

Terrestrial vegetation

In many headwaters, high hydrologic flushing rates and minimal soil development limit chemical weathering (Clow & Sueker 2000). Here, run-off is negatively correlated with vegetation cover in the watershed, as high rates of evapotranspiration (e.g. in water-saturated meadows) and physical deceleration of water currents reduce flushing rates (Clow & Sueker 2000). Alpine meadows, for instance, cover only small areas of a catchment, but have a surprisingly large influence on stream water in general (Clow & Sueker 2000). Vegetation thus prolongs water residence and contact times, increasing the potential processing of Si (Hornberger et al. 2001; Scanlon et al. 2001). Average BSi production in sub-alpine grassland communities can be one order of magnitude higher than that of alpine grassland communities, adjacent heaths and nearby conifer forests (Carnelli et al. 2001). In these meadows, more intense biological activity leads to rapid mineralization and humification of organic matter and to more rapid dissolution of phytoliths, resulting in higher turnover rates of BSi (Carnelli et al. 2001). This impacts availability of Si to the plant communities, but also ecosystem control on Si fluxes. Here, a potential time lag between dissolution and uptake will be a crucial factor for the Si retention capacity of the ecosystem. The dissolution of BSi from dead organic matter does not necessarily coincide with the period when most DSi is taken up, e.g. due to seasonal growth of the vegetation, or preferential uptake of Si by vegetation during young or adult biomass production.

Silicon availability and accumulation capacity of occurring plant species are important factors in determining longevity of leaves (Cooke & Leishman 2011b). Ge et al. (2011) showed that leaves (needles) and branches of conifers in the Changbai Mountains (China) were more silicified than those of broad-leaved species, where phytoliths were scarce and fragile. The proposed explanation for the low Si content in broad-leaved species is that broad-leaved trees have <5 mo to grow leaves, while needles of coniferous species live longer, allowing them to silicify their tissues more strongly (Ge et al. 2011). This could potentially contribute to reduced herbivory on conifers, as Si increases food abrasiveness (Sangster & Hodson 2001). In this way, Si content of conifer needles potentially provides an important positive feedback loop to sustain needles over multiple years and could provide conifers with a competitive advantage. It should be emphasized that other studies have actually found higher Si content in broad-leaved species compared to gymnosperms; it is thus not a general observation that Si content is highest in conifers (e.g. Cornelis et al. 2010).

To understand both the effect of Si availability on vegetation structure and longevity, and the eventual influence of plant Si processing on Si fluxes, soil Si fractions (both dissolved and solid) and riverine Si fluxes should be linked to processing of Si by terrestrial vegetation, and to vegetation functioning and characteristics (competition between conifers and broad-leaved species, biomass production, vegetation seasonality). Carey & Fulweiler (2013) have recently demonstrated that uptake of DSi by terrestrial vegetation can temporarily decrease in-stream DSi concentrations (by 2.7 μmol·L−1·d−1). Decomposition is an important process in regulating both Si availability and fluxes. Research in the eastern Pyrenees pointed to temperature and rainfall as crucial factors regulating decomposition rates in high-altitude Pinus sylvestris L. (Pausas 1997), but also indicated that other currently unknown factors are important too. Recent research on Si and decomposition emphasized that Si uptake by vegetation is likely one of these unknown factors (Schaller et al. 2012b).

Fauna can also impact the transport of Si through rivers through its interaction with vegetation. In a study in the Shenandoah National Park (USA), sudden declines in the DSi concentration in streams, that could not be explained by short-term variations such as differences in hydrological flow paths determining the mineral contact time, coincided with a gypsy moth (Lymantria dispar Linnaeus, 1758) defoliation event (Grady et al. 2007). This species is invasive in North America and can cause intense defoliation and tree dieback. The defoliation event causes a pulse in stream water nitrate; in addition, more light is able to penetrate the vegetation canopy. The authors showed this stimulated diatom populations to bloom year after year, explaining the observed DSi decrease in the streams. This example demonstrates that vegetation can also influence Si dynamics through indirect effects, such as shading and impact on other nutrients.

Herbivory can also alter uptake of Si by vegetation, or could cause the vegetation to become dominated by more Si accumulating species (e.g. grasses). Population density of voles was linked to a fluctuating Si content in grazed grass species (Massey et al. 2008). During periods with high vole densities, grasses took up more Si, eventually rendering grazing less efficient and reducing the vole population. Si was therefore possibly contributing to the cyclic abundance of voles (Massey et al. 2008). In a similar way, high Si levels in perennial savanna grasses of the Serengeti have been explained as long co-evolution between grasses and large grazers (McNaughton et al. 1985), thereby maintaining biotic diversity of both grasses and grazers in the savanna ecosystem. This co-evolutional relation affects storage and recycling of Si in soils and thus potentially Si fluxes through rivers. Vegetation–environment relationships are stronger in moderate to heavily grazed situations compared to (near) ungrazed situations (Cingolani et al. 2003), and consumer control (grazing/fire) is one of the most important controllers (along with environmental factors, e.g. climate, hydrology, etc.) on the distribution of vegetation (Bond 2005). Plant Si metabolism is an important physiological trait in response mechanisms of vegetation to grazing disturbance.

Previous examples clearly show that terrestrial vegetation, and external impacts on vegetation, can directly and indirectly change processing of Si. Hence, small or large alterations in vegetation structure may affect Si fluxes in the watershed. Here, a new field is opening for vegetation scientists and biogeochemists to understand how observed differences in Si processing in different ecosystems relate to vegetation Si metabolism. This could be crucial in e.g. the context of studying response of ecosystems to climate change. Major shifts in vegetation are expected due to climate change, e.g. in alpine regions (Walther et al. 2005; Weber et al. 2007). Plant functional types provide a powerful tool in climate change research, as they bridge the gap between plant physiology, community and ecosystem processes (Diaz & Cabido 1997). Here, we hypothesize that Si plays an important functional role in vegetation shifts, due to its important role in coping with environmental stressors, especially in grasses. Coordinated studies are required to further elucidate how interactions among climate, disturbance and vegetation distribution influence carbon dynamics and water and energy exchange (McGuire et al. 2002), and a good understanding of the role of functional elements (including Si) in these interactions is essential.

Aquatic and floodplain vegetation

Rivers and their floodplains are highly dynamic ecosystems, providing a wide range of ecosystem goods and services, such as nutrient control, water supply and flood protection (Hoeinghaus et al. 2009). Vegetation in these ecosystems is a potentially crucial controller in the global Si cycle, with Si playing a role in the potential of vegetation to withstand abiotic stress. Common aquatic and wetland plant species contain significant amounts of Si (Hodson et al. 2005; Struyf & Conley 2009; Schoelynck et al. 2010), which influences the physical strength. Hydrodynamic stress in the Biebrza River (Poland) was identified as a trigger for BSi incorporation in Yellow water lily (Nuphar lutea Sm.), which was also demonstrated experimentally for two submerged macrophytes: Egeria densa Planch. and Limnophila heterophylla (Roxb.) Benth (Schoelynck et al. 2012a). Individuals growing in a hydrodynamic environment (rivers) incorporated more Si than individuals from stagnant water (oxbow lakes), regardless of DSi concentration in the water (Schoelynck et al. 2012a). The Si content of plant tissue can be linked to cellulose and lignin content, two molecules known to provide structural rigidity to plants (Schoelynck et al. 2010; Schaller et al. 2012a). Another example of the role of Si in relieving abiotic stress is found on the floodplains. Tussock formation in wet meadow plants is a fairly common occurrence and is observed in several (Si accumulating) graminoids (Costello 1936; Taylor et al. 2001). It appears to be a stress avoidance strategy for plants to escape waterlogging and associated low oxygen levels and soil toxicity by elevating their rooting substrate (van de Koppel & Crain 2006). Inside these tussocks, apparent aerobic conditions may enhance nutrient cycling (Olde Venterink et al. 2002), including Si cycling. In a study comparing the BSi stock in the above-ground biomass of Carex appropinquata A. Schumach. of unmanaged (tussock) parcels and managed parcels in the valley of the Biebrza River in Poland, six times more BSi was found in the unmanaged parcels (Opdekamp et al. 2012). This could partly be ascribed to the repetitive removal of biomass (and hence BSi) from the mown fields, causing a degradation of the stock. Another explanation might be the tussocks’ capacity to trap organic material, thereby stimulating intense internal recycling of nutrients from which almost no Si leaks out. This process could provide the tussock-forming grass species with a competitive advantage; it creates strong availability of Si for the tussock grasses, which in turn helps them to sustain their rigidity in a dynamic environment.

Several studies have argued that water velocity is the main factor in regulating aquatic macrophyte distribution, composition, biomass and metabolism in rivers (Marshall & Westlake 1990; Franklin et al. 2008). Macrophytes (partly due to incorporation of Si) withstand the current and physically hamper the water flow (De Doncker et al. 2009). This results in the development of self-sustaining vegetation patchiness (linked to occurrence of low- and high-flow zones in rivers), with vegetation controlling litter accumulation and element cycling (Schoelynck et al. 2012b). There is also increasing evidence that Si concentration in aquatic species litter is a major factor driving decomposition rates, due to its impact on phenol, cellulose and lignin content and litter stoichiometry (Schaller et al. 2012a,b). This can have large effects on ecosystem productivity (Hilton et al. 2006) and nutrient availability, which in turn directly affect vascular plant distribution (Bragazza & Gerdol 2002).

In many parts of the world, current land use and increased intensification of agricultural practices force lowland rivers to drain larger water quantities during ever-shorter time periods. Drainage, however, is hampered by human artefacts such as dams and bridges as well as by naturally occurring aquatic vegetation. High biomass can lead to river obstructions, rising water levels and increased flooding frequency (Bal & Meire 2009). To avoid flooding and water-related problems, river managers opt for removing aquatic vegetation in certain circumstances. Yearly weed cutting in Danish lowland rivers may favour the growth of species like Sparganium emersum Rehmann (commelinoid monocot; Riis et al. 2000) because shoots can originate from their basal meristem (Sand-Jensen et al. 1989) giving them a competitive advantage. Analysis showed that S. emersum in the Zwarte Nete (Belgium) contains about twice the amount of BSi as the eudicot species Callitriche platycarpa Kütz. that was presence before weed cutting activities (J. Schoelynck, unpubl. data). This corresponds to Hodson et al. (2005), who described the phylogenetic variation in shoot Si concentrations within angiosperms, and observed that shoot Si concentrations decreased in the order commelinoid monocots > non-commelinoid monocots > eudicots. This shows that mowing might have the same effect on vegetation as herbivory: an increased dominance of Si accumulating species.

The above examples demonstrate how plants’ ability to functionally use Si can help them adapt to a dynamically changing environment in floodplains and rivers on a relative short time scale (orders of years); this may also change vegetation composition. There is an intricate range of processes and functions linked to Si uptake by aquatic and wetland macrophytes, and again only multidisciplinary research of both scientists trying to unravel ecosystem influence on Si processing and scientists aiming to unravel the role of Si in plant functionality will allow us to achieve a better understanding of these processes and functions.

Tidal wetlands

Tidal marshes contain large stocks of BSi, due to retention of plant and diatom Si in the sediments, which is deposited on the marsh surface during tidal inundations and after litter fall (Norris & Hackney 1999; Struyf et al. 2005b; Jacobs et al. 2013). With a continuous flow of soil seepage water from these – often water-saturated – marsh sediments, DSi can be exported to the estuarine channel, the key transition zone between rivers and oceans, supplying an additional source of DSi to the estuary and the coastal zone (Struyf et al. 2006; Jacobs et al. 2008; Vieillard et al. 2011). Compared to ocean surface water, where DSi concentrations are usually <2 μmol·L−1 (Tréguer et al. 1995), the global average for rivers is relatively high at 160 μmol·L−1 (Dürr et al. 2011). However, in spring and summer, DSi can be completely depleted in estuaries (Boderie et al. 1993). In these seasons, discharge from tidal marshes with DSi concentrations of up to 320 μmol·L−1 (Struyf et al. 2006) can become an important DSi source for diatoms. As an important food source for primary consumers, diatoms fill a key position in the estuarine food web (Sullivan & Moncreiff 1990). By exporting DSi to the estuary, tidal marshes consequently contribute to productivity of the coastal zone (Hackney et al. 2000).

Besides processing Si in estuarine ecosystems, BSi accumulation in tidal marshes contributes to the functioning and composition of the vegetation. Si is able to increase the ecological resilience of a marsh by improving the resistance of plants to various stressors (Currie & Perry 2007; Liang et al. 2007; Cooke & Leishman 2011a). Against the background of a rising sea level and further expected constructional changes in estuaries associated with economic development in many regions of the world (Tate et al. 2004; Castro & Freitas 2011), Si-induced attributes may become even more valuable for plants. First, Si-induced rigidity might increase the plants’ resistance to waves and currents caused by enhanced ship traffic and stronger flow surges (Rafi & Epstein 1999). Second, the impacts of salt water intrusion in the estuary due to sea level rise and dredging activities might be ameliorated by Si-induced salt tolerance (as already observed for e.g. wheat; Ali et al. 2012). Finally, it has also been shown that Si can increase resistance of some plants to chemical contaminants (Kidd et al. 2001; Nwugo & Huerta 2008), which are still considered to be one of the major threats to estuaries worldwide (Kennish 2002). On the plant level, Querné et al. (2012) did not find increases in BSi concentration in Spartina alterniflora Loisel. with increasing abiotic stresses (waves and salinity). At community level, however, it could be expected that plants that are able to utilize Si to their benefit would be favoured under changing environmental conditions in estuaries.

Silicon uptake might also contribute to the competitiveness of some invasive species. For instance, a non-native genotype of the Si-accumulating P. australis is currently invading North American coastal wetlands, threatening local species composition (Meyerson et al. 2000; Hirtreiter & Potts 2012). The native Si-rich grass Elymus athericus (Link) Kerguélen invades European salt marshes due to change in land use (de Bakker et al. 1999; Pétillon et al. 2005; Bockelmann et al. 2011), and Arundo donax L., a Si-rich tall-growing grass (Chauhan et al. 2011) known to be one of the world's most invasive alien species, is invading subtropical wetlands on all continents (Herrera & Dudley 2003). All three plant species have very high biomass production in common. According to Ehrenfeld (2010), this trait is typical of the majority of invasive species and can lead to substantial changes in soil biogeochemistry and nutrient cycling. In terms of Si, it is conceivable that invaders with high Si uptake rates will stimulate Si transport from deep soil layers to the surface, and therefore promote Si export to estuaries (Struyf et al. 2007). However, in the case of low decomposition rates accompanied by frequent flooding, a considerable fraction of Si could be buried in sediments, as reported for carbon in E. athericus-dominated salt marshes (Valéry et al. 2004). We hypothesize that any change in Si pathways in tidal marshes, due to changes in dominant plant species with different Si metabolism, could strongly affect the role of tidal wetlands as importers of BSi and exporters of DSi.

Finally, Si reaches the coastal zone, where it is incorporated into diatom frustules or marine sponges (Maldonado et al. 2010). Another potentially important sink, yet highly under-studied ecosystem in this context, is (sub)tropical coastal mangroves. Studies in the Bhitarkanika mangrove system on the east coast of India (Chauhan & Ramanathan 2008) and in the mixed Rhizophora forests lining Coral Creek on Hinchinbrook Island in Australia (Alongi 1996) show these ecosystems are very efficient Si accumulators. Much of this Si will likely be consumed by diatoms present is this ecosystem (Smol & Stoermer 2010). However, Si import to this ecosystem may also be significantly driven by the consistently high rates of plant growth and productivity within the forests (Alongi 1996), where Si may serve as a reliever of abiotic stress. Deposition of Si in the roots reduces apoplastic bypass flow and provides binding sites for metals, resulting in decreased uptake and translocation of toxic metals and salts from the roots to the shoots (Ma & Yamaji 2006).

Outlook

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References

The human factor

Humans have been modifying natural landscapes and native vegetation for thousands of years. Yet today's growing population and rising food demands are causing land-use changes at the expense of natural vegetation systems at unprecedented levels. The implications of land-use changes for Si pools and fluxes, as well as vegetation Si dynamics, are just beginning to be understood (Conley et al. 2008; Struyf et al. 2010; Carey & Fulweiler 2012; Clymans et al. 2011). Croplands and pastures are often dominated by gramineous vegetation; hence, management is potentially strongly linked to plant Si dynamics. Harvesting of crops containing high levels of BSi (for agricultural or industrial purposes) reduces the amount of BSi that is released into the soil (Vandevenne et al. 2012; Schoelynck et al. 2013). Persistent cropping of winter wheat straw in the UK decreases available Si in the top soil, whereas a re-building of soil BSi pools is seen in reforested fields (Guntzer et al. 2011). Applying artificial Si fertilizers to enrich Si-depleted cultivated soils is one solution to abate the loss of soil BSi (Currie & Perry 2007). A similar effect can be achieved using economical crop species like bamboos, which have the ability to actively uplift Si from deeper layers to the surface (Parr et al. 2010). In Indonesia, local farmers have been practicing the ‘bamboo–kebun’ crop rotation system, consisting of annual food or cash crops (kebun) alternating with clumps of bamboo species. Bamboo plays a key role in both the cycling and retention of organic matter (Christanty et al. 1997) and nutrients (Mailly et al. 1997), restoring fertility during the fallow period of shifting cultivation systems. The traditional belief of Indonesian farmers that ‘without bamboo the land dies’ hints at the tremendous importance of bamboo vegetation for soil Si pools and pathways. First estimates for temperate regions point to a 10% reduction in BSi storage in soils since historical land-use changes occurred (Clymans et al. 2011). These land-use changes are responsible for up to 20% of the annual global riverine Si flux from land to ocean (Clymans et al. 2011).

Another interesting part of land-use change is nature management. Although pastures and meadows are man-made ecosystems, traditional management methods often result in large plant diversity and mosaic landscapes with high species richness (Weibull et al. 2003). Cessation of traditional management in such mosaic landscapes can result in species shifts towards generalist species, at the expense of specialists, due to landscape homogenization (Eriksson et al. 2002; Dullinger et al. 2003). In the Biebrza National Park in Eastern Poland, unreclaimed low productivity meadows have traditionally been mown for hay, but abandonment began in the 1970s. Around 1981–1983, ca. 30 000 ha were still mown annually (Banaszuk 1994), while today <5000 ha of meadows within the park boundaries are mown or grazed. On abandoned parcels, the original succession restarts, leading to a growing proportion of Si-rich monocots, especially grasses (Poaceae) and sedges (Cyperaceae), many of which form tussocks (Opdekamp et al. 2012).

Moreover, waterways transporting Si from upstream to downstream ecosystems are heavily impacted by human actions. Obstructions increase water residence times in rivers and floodplains, potentially enhancing the trapping of BSi-rich material, as previously observed in lakes and artificial reservoirs. Humborg et al. (2000), among others, showed that these artificial water bodies have the potential to significantly impact river biogeochemistry due to their BSi trapping efficiency as well as their reduction of DSi concentrations through diatom blooms.

Future research

The functional role of Si in plants and the resulting impact of vegetation on Si export fluxes all warrant a coordinated research effort between scientists working at community and ecosystem level, focusing on both vegetation dynamics and biogeochemistry. Multidisciplinary studies are required to better understand the broader effects of local Si–vegetation interactions. We conclude this review with a list of diverse scientific scenarios that such studies could address (Table 1). The questions are grouped according to three themes: natural ecosystems, agricultural ecosystems and nature management. All examples have worldwide significance because of ever-growing human environmental pressures. Other disciplines than those mentioned in the Table 1 maybe indispensable (e.g. hydrology, geology and microbiology). To facilitate interpretation of the Table 1, two of the scenarios are briefly elaborated here as examples.

Table 1. Examples of scenarios that could affect Si transformation and export from an ecosystem, together with an indication of scientific specializations required to tackle these new research horizons (indicated with +) and an indication of whether the Si flux is expected to be either high or low. The requirement for a scientist is based on their field of interest covering the various key factors that primarily determine ecosystem Si dynamics (based on Struyf & Conley 2012)
 Vegetation scientistBiogeochemistPhysiologist 
Key factorsPlant sociologyAbiotic conditionsWithin plant responseSi flux rate
Natural ecosystem
Invasive plant species
An invasive Si accumulator may impact abiotic conditions and Si dissolution rates because of increased Si uptake by the vegetation++ Low
Invasive animal species
Native vegetation is affected by an invasive herbivore that may also trample or burrow in the soil. BSi processing may increase due to herbivory, but the vegetation increase Si uptake as a herbivore defence+++High
Abiotic stress
Increased UV radiation, salinity, drought or heavy metal pollution may cause plant stress that can be relieved by Si. Plant reaction may involve Si uptake, ultimately reducing output of Si to rivers+++Low
Eutrophication
Increased nutrient availability will increase vegetation production and will induce a shift towards fast-growing species, with potentially higher uptake of Si and faster turnover (e.g. highly productive grasslands or nettles, which are relatively Si rich; Struyf et al. 2005a)++ Low
Agricultural ecosystem
Genetically modified organisms
Biofuels are less efficient, having more BSi in biomass material. Industry prefers highly productive crops with low BSi content. Growing these crops might alter vegetation control on Si fluxes  +Low
Grazing
Pastures are grazed by insects and large herbivores. The grass will show several growth cohorts, influencing the system's productivity, and the system will shift towards species with higher Si content (Reynolds et al. 2009; Garbuzov et al. 2011)+++Low
Reduced soil stability (Erosion)
In high-productivity arable farming the potential for soil erosion is high, leading to high exports of soil BSi in suspended material, resulting in soils depleted in plant available Si++ High
Water management
To increase productivity, hydrology is managed to keep ground water at an optimal level. Still, abiotic conditions can shift abruptly. Pulsed fluxes of Si are associated with flooding/drainage events++ High
Nature management
Succession fixation
Natural succession is prevented. Soil biogeochemistry is kept stable artificially++ Low
Fire
Fire rejuvenates succession. Abiotics suddenly shift, causing abrupt Si mobilization (ash). New dominant vegetation appears (e.g. Molinia caerulea (L.) Moench (Poaceae) encroachment in heathlands; Jacquemyn et al. 2005)++ High
Mowing
Dominant vegetation is mown to increase rare species diversity (e.g. Metsoja et al. 2012). Si is exported in harvests. Plants may invest in Si uptake (see herbivory)+ +High
Topsoil removal
Topsoil is removed, suddenly changing abiotic conditions. Rapid Si export flux occurs (as BSi in removed soil). Dominant vegetation completely shifts++ High
Rewetting
To restore wetlands, hydrology is managed. Hydrological connections are re-established, allowing increased exchange fluxes. With abrupt shifts in abiotic conditions, vegetation shifts entirely++ High

In a first example, the effect of grazing by insects and large herbivores on the Si cycle is proposed. The expertise of vegetation scientists, plant physiologists and biogeochemists is needed to understand plant response to herbivory (e.g. induced Si accumulation), and the resulting changes in plant productivity and altered Si mobilization potential from faecal matter versus litter fall. The Si flux rate is expected to be relatively slow as it depends on many factors (hydrology, in-field recycling).

In a second example, the effect of fire as a management technique is discussed. An accidental fire at the heathland Kalmthoutse Heide (Belgium) in 1996 (and again in 2011) destroyed many hectares of Erica species and resulted in the encroachment of Molinia caerulea (L.) Moench (a Si accumulating species; Jacquemyn et al. 2005). The subsequent Si flux rate is initially expected to be high, as a large amount of stored BSi becomes suddenly available from ash (in parallel to other elements, N, P, Ca, Mg, K; Kucerova et al. 2008). This requires a combined scientific effort from vegetation scientists (vegetation shift), physiologists (Si accumulating species emerge, but also fire-resistant species) and biogeochemists (shifted abiotic conditions and altered Si mobilization potential from the ash) to quantify the impact of this fire on the Si cycle, and to elaborate how Si could potentially play a role in vegetation resistance to fire stress and invasion rates after the fire.

Table 1 is a testimony to the lack of knowledge that currently exists on interactions between plant Si metabolism and whole ecosystem Si cycling. We emphasize that multidisciplinary studies are potentially of high scientific value: they could change our understanding of ecosystem Si cycling, and hence of linked element cycles. Si delivery from the continents plays an essential role in the occurrence of eutrophication problems in the coastal zone: the ratio at which Si, N and P are delivered is a determining factor for the occurrence of nuisance algal blooms (which usually occur when increased N and P input coincide with Si limitation; Cloern 2001). The direct impact of Si dynamics on three of the major global carbon sinks is even more compelling (Ragueneau et al. 2006; Street-Perrott & Barker 2008). First, the weathering of mineral silicates is an important sink for atmospheric CO2 on geological time scales, as CO2 is absorbed in the weathering process to form carbonate and release silicate (Berner 1992). Second, the import of Si into coastal zones from the terrestrial environment is essential to sustain diatom growth. Dead frustules and associated carbon are ultimately reincorporated into sediments and play a key role in the oceanic carbon sink (Rabosky & Sorhannus 2009). Third, organic carbon occluded within phytoliths is highly resistant to decomposition compared to other soil organic carbon components. Its accumulation within the soil is an important process in the terrestrial sequestration of carbon (Parr & Sullivan 2005; Song et al. 2012). The importance of Si for carbon sequestration cannot be under-estimated. However, lack of knowledge regarding the biological control of the Si cycle hinders accurate quantification of these carbon sinks. Only a concerted effort bringing scientists together from a broad array of disciplines, as suggested above, will provide this new direction to research on vegetation Si cycling.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References

F.V. thanks Special Research Funding from the University of Antwerp (BOF-UA) for PhD fellowship funding, and E.S. thanks FWO (Research Foundation Flanders) for post-doctoral research funding. We also acknowledge FWO for funding the project ‘Tracking the biological control on Si mobilisation in upland ecosystems’ (project number G014609N) and BELSPO for funding project SOGLO. F.M. would like to thank the Bauer-Hollman Foundation and ESTRADE (Estuary and Wetland Research Graduate School Hamburg) as member of LExI (State Excellence Initiative), funded by the Hamburg Science and Research Foundation, for financial support. The authors thank Prof. Stijn Temmerman and Toon De Groote for the photos and Prof. Kai Jensen, Prof. Jos Verhoeven and Patrick Frings for proofreading this paper.

References

  1. Top of page
  2. Abstract
  3. Background
  4. The river continuum: from headwaters to estuary
  5. Outlook
  6. Acknowledgements
  7. References
  • Agarie, S., Agata, W., Kubota, F. & Kaufman, P.B. 1992. Physiological roles of silicon in photosynthesis and dry-matter production in rice plants. 1. Effects of silicon and shading treatments. Japanese Journal of Crop Science 61: 200206.
  • Ali, A., Basra, S., Iqbal, J., Hussain, S., Subhani, M.N., Sarwar, M. & Ahmed, M. 2012. Augmenting the salt tolerance in wheat (Triticum aestivum) through exogenously applied silicon. African Journal of Biotechnology 11: 642649.
  • Alongi, D.M. 1996. The dynamics of benthic nutrient pools and fluxes in tropical mangrove forests. Journal of Marine Research 54: 123148.
  • Aoki, Y., Hoshino, M. & Matsubara, T. 2007. Silica and testate amoebae in a soil under pine–oak forest. Geoderma 142: 2935.
  • Bal, K. & Meire, P. 2009. The influence of macrophyte cutting on the hydraulic resistance of lowland rivers. Journal of Aquatic Plant Management 47: 6568.
  • Banaszuk, A. 1994. Grasslands and meadow management in the Biebrza valley. In: Wassen, M.J. & Okruszko, H. (eds.) Towards protection and sustainable use of the Biebrza Wetlands: exchange and integration of research results for the benefit of a Polish – Dutch Joint Research Plan, pp. 287313. Institute for Land Management and Grassland Farming, Warsaw, PL.
  • Bartoli, F. 1983. The biogeochemical cycle of silicon in 2 temperate forest ecosystems. Ecological Bulletin 35: 469476.
  • Berner, R.A. 1992. Weathering, plants, and the long-term carbon-cycle. Geochimica, Cosmochimica Acta 56: 32253231.
  • Bockelmann, A.-C., Wels, T. & Bakker, J.P. 2011. Seed origin determines the range expansion of the clonal grass Elymus athericus. Basic and Applied Ecology 12: 496504.
  • Boderie, P., Zwolsman, J., van Eck, G. & van der Weijden, C. 1993. Nutrient biogeochemistry in the water column (N, P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands). Aquatic Ecology 27: 309318.
  • Bond, W.J. 2005. Large parts of the world are brown or black: a different view on the ‘Green World’ hypothesis. Journal of Vegetation Science 16: 261266.
  • Bragazza, L. & Gerdol, R. 2002. Are nutrient availability and acidity–alkalinity gradients related in Sphagnum-dominated peatlands? Journal of Vegetation Science 13: 473482.
  • Broadley, M.R., Brown, P., Cakmak, I., Ma, J.F., Rengel, Z. & Zhao, F. 2012. Beneficial elements. In: Marschner, P. (ed.) Marschner's mineral nutrition of higher plants, pp. 249269. Elsevier, Amsterdam, NL.
  • Carey, J.C. & Fulweiler, R.W. 2012. Human activities directly alter watershed dissolved silica fluxes. Biogeochemistry 111: 125138.
  • Carey, J.C. & Fulweiler, R.W. 2013. Watershed land use alters riverine silica cycling. Biogeochemistry 113: 525544
  • Carnelli, A.L., Madella, M. & Theurillat, J.P. 2001. Biogenic silica production in selected alpine plant species and plant communities. Annals of Botany 87: 425434.
  • Castro, P. & Freitas, H. 2011. Linking anthropogenic activities and eutrophication in estuaries: the need of reliable indicators. In: Ansari, A., Singh Gill, S., Lanza, G., Rast, W., Khan, F., Gill, S. & Varshney, J. (eds.) Eutrophication: causes, consequences and control, pp. 265284. Springer, Dordrecht, NL.
  • Chauhan, R. & Ramanathan, A. 2008. Evaluation of water quality of Bhitarkanika mangrove system, Orissa, east coast of India. Indian Journal of Marine Sciences 37: 153158.
  • Chauhan, D.K., Tripathi, D.K., Kumar, D. & Kumar, Y. 2011. Diversity, distribution and frequency based attributes of phytoliths in Arundo donax L. International Journal of Innovations in Biological and Chemical Sciences 1: 2227.
  • Christanty, L., Kimmins, J.P. & Mailly, D. 1997. ‘Without bamboo, the land dies’: a conceptual model of the biogeochemical role of bamboo in an Indonesian agroforestry system. Forest Ecology and Management 91: 8391.
  • Cingolani, A.M., Cabido, M.R., Renison, D. & Solis, V.N. 2003. Combined effects of environment and grazing on vegetation structure in Argentine granite grasslands. Journal of Vegetation Science 14: 223232.
  • Cloern, J.E. 2001. Our evolving conceptual model of the coastal eutrophication problem. Marine Ecology -Progress Series 210: 223253.
  • Clow, D.W. & Sueker, J.K. 2000. Relations between basin characteristics and stream water chemistry in alpine/subalpine basins in Rocky Mountain National Park, Colorado. Water Resource Research 36: 4961.
  • Clymans, W., Struyf, E., Govers, G., Vandevenne, F. & Conley, D.J. 2011. Anthropogenic impact on amorphous silica pools in temperate soils. Biogeosciences 8: 22812293.
  • Conley, D.J. 2002. Terrestrial ecosystems and the global biogeochemical silica cycle. Global Biogeochemical Cycles 16: 68-168-8.
  • Conley, D.J., Likens, G.E., Buso, D.C., Saccone, L., Bailey, S.W. & Johnson, C.E. 2008. Deforestation causes increased dissolved silicate losses in the Hubbard Brook Experimental Forest. Global Change Biology 14: 25482554.
  • Cooke, J. & Leishman, M.R. 2011a. Is plant ecology more siliceous than we realise? Trends in Plant Science 16: 6168.
  • Cooke, J. & Leishman, M.R. 2011b. Silicon concentration and leaf longevity: is silicon a player in the leaf dry mass spectrum? Functional Ecology 25: 11811188.
  • Cornelis, J.T., Ranger, J., Iserentant, A. & Delvaux, B. 2010. Tree species impact the terrestrial cycle of silicon through various uptakes. Biogeochemistry 97: 231245.
  • Cornelis, J.-T., Delvaux, B., Georg, R.B., Lucas, Y., Ranger, J. & Opfergelt, S. 2011. Tracing the origin of dissolved silicon transferred from various soil-plant systems towards rivers: a review. Biogeosciences 8: 89112.
  • Costello, D.F. 1936. Tussock meadows in southeastern Wisconsin. Botanical Gazette 97: 610648.
  • Currie, H.A. & Perry, C.C. 2007. Silica in plants: biological, biochemical and chemical studies. Annals of Botany 100: 13831389.
  • Datnoff, L.E., Snyder, G.H. & Korndörfer, G.H. 2001. Silica in agriculture. Elsevier, Amsterdam, NL.
  • de Bakker, N.V.J., Hemminga, M.A. & van Soelen, J. 1999. The relationship between silicon availability, and growth and silicon concentration of the salt marsh halophyte Spartina anglica. Plant and Soil 215: 1927.
  • De Doncker, L., Troch, P., Verhoeven, R., Bal, K., Desmet, N. & Meire, P. 2009. Relation between resistance characteristics due to aquatic weed growth and the hydraulic capacity of the river Aa. River Research and Applications 25: 12871303.
  • Derry, L.A., Kurtz, A.C., Ziegler, K. & Chadwick, O.A. 2005. Biological control of terrestrial silica cycling and export fluxes to watersheds. Nature 433: 728731.
  • Diaz, S. & Cabido, M. 1997. Plant functional types and ecosystem function in relation to global change. Journal of Vegetation Science 8: 463474.
  • Dullinger, S., Dirnbock, T., Greimler, J. & Grabherr, G. 2003. A resampling approach for evaluating effects of pasture abandonment on subalpine plant species diversity. Journal of Vegetation Science 14: 243252.
  • Dürr, H.H., Meybeck, M., Hartmann, J., Laruelle, G.G. & Roubeix, V. 2011. Global spatial distribution of natural riverine silica inputs to the coastal zone. Biogeosciences 8: 597620.
  • Ehrenfeld, J. 2010. Ecosystem consequences of biological invasions. Annual Review of Ecology, Evolution and Systematics 41: 5980.
  • Epstein, E. 1994. The anomaly of silicon in plant biology. Proceedings of the National Academy of Sciences of the United States of America 91: 1117.
  • Epstein, E. 2009. Silicon: its manifold roles in plants. Annals of Applied Biology 155: 155160.
  • Eriksson, O., Cousins, S.A.O. & Bruun, H.H. 2002. Land-use history and fragmentation of traditionally managed grasslands in Scandinavia. Journal of Vegetation Science 13: 743748.
  • Farmer, V.C.D. & Miller, J.D. 2005. The role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams. Geoderma 127: 7179.
  • Franklin, P., Dunbar, M. & Whitehead, P. 2008. Flow controls on lowland river macrophytes: a review. Science of the Total Environment 400: 369378.
  • Fulweiler, R.W. & Nixon, S.W. 2005. Terrestrial vegetation and the seasonal cycle of dissolved silica in a southern New England coastal river. Biogeochemistry 74: 115130.
  • Garbuzov, M., Reidinger, S. & Hartley, S. 2011. Interactive effects of plant-available soil silicon and herbivory on competition between two grass species. Annals of Botany 108: 13551363.
  • Ge, Y., Jie, D.M., Sun, Y.L. & Liu, H.M. 2011. Phytoliths in woody plants from the northern slope of the Changbai Mountain (Northeast China), and their implications. Plant Systematics and Evolution 292: 5562.
  • Grady, A.E., Scanlon, T.M. & Galloway, J.N. 2007. Declines in dissolved silica concentrations in western Virginia streams (1988–2003): gypsy moth defoliation stimulates diatoms? Journal of Geophysical Research -Biogeosciences 112: G1.
  • Guntzer, F., Keller, C., Poulton, P., McGrath, S. & Meunier, J.-D. 2011. Long-term removal of wheat straw decreases soil amorphous silica at Broadbalk, Rothamsted. Plant and Soil 352: 173184.
  • Hackney, C.T., Cahoon, L.B., Preziosi, C. & Norris, A. 2000. Silicon is the link between tidal marshes and estuarine fisheries. A new paradigm. In: Weinstein, M.P. & Kreeger, D.A. (eds.) Concepts and controversies in tidal marsh ecology, pp. 543554. Kluwer Academic, Dordrecht, NL.
  • Herrera, A.M. & Dudley, T.L. 2003. Reduction of riparian arthropod abundance and diversity as a consequence of giant reed (Arundo donax) invasion. Biological Invasions 5: 167177.
  • Hilton, J., O'Hare, M., Bowes, M.J. & Jones, J.I. 2006. How green is my river? A new paradigm of eutrophication in rivers. Science of the Total Environment. 365: 6683.
  • Hirtreiter, J.N. & Potts, D.L. 2012. Canopy structure, photosynthetic capacity and nitrogen distribution in adjacent mixed and monospecific stands of Phragmites australis and Typha latifolia. Plant Ecology 213: 821829.
  • Hodson, M.J., White, P.J., Mead, A. & Broadley, M.R. 2005. Phylogenetic variation in the silicon composition of plants. Annals of Botany 96: 10271046.
  • Hoeinghaus, D.J., Agostinho, A.A., Gomes, L.C., Pelicice, F.M., Okada, E.K., Latini, J.D., Kashiwaqui, E.A.L. & Winemiller, K.O. 2009. Effects of river impoundment on ecosystem services of large tropical rivers: embodied energy and market value of artisanal fisheries. Conservation Biology 23: 12221231.
  • Hornberger, G.M., Scanlon, T.M. & Raffensperger, J.P. 2001. Modelling transport of dissolved silica in a forested headwater catchment: the effect of hydrological and chemical time scales on hysteresis in the concentration-discharge relationship. Hydrological Processes 15: 20292038.
  • Humborg, C., Conley, D.J., Rahm, L., Wulff, F., Cociasu, A. & Ittekkot, V. 2000. Silicon retention in river basins: far-reaching effects on biogeochemistry and aquatic food webs in coastal marine environments. Ambio 29: 4550.
  • Jacobs, S., Struyf, E., Maris, T. & Meire, P. 2008. Spatiotemporal aspects of silica buffering in restored tidal marshes. Estuarine and Coastal Shelf Science 80: 4252.
  • Jacobs, S., Müller, F., Teuchies, J., Oosterlee, L., Struyf, E. & Meire, P. 2013. The vegetation silica pool in a developing tidal freshwater marsh. Silicon 5: 91100.
  • Jacquemyn, H., Brys, R. & Neubert, M.G. 2005. Fire increases invasive spread of Molinia caerulea mainly through changes in demographic parameters. Ecological Applications 15: 20972108.
  • Kennish, M. 2002. Environmental threats and environmental future of estuaries. Environmental Conservation 29: 78107.
  • Kidd, P., Llugany, M., Poschenrieder, C., Gunsé, B. & Barceló, J. 2001. The role of root exudates in aluminium resistance and silicon-induced amelioration of aluminium toxicity in three varieties of maize (Zea mays L.). Journal of Experimental Botany 52: 13391352.
  • Kucerova, A., Rektoris, L., Stechova, T. & Bastl, M. 2008. Disturbances on a wooded raised bog - How windthrow, bark beetle and fire affect vegetation and soil water quality? Folia Geobotanica 43: 4967.
  • Lavorel, S. & Garnier, E. 2002. Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail. Functional Ecology 16: 545556.
  • Liang, Y., Sun, W., Zhu, Y.-G. & Christie, P. 2007. Mechanisms of silicon-mediated alleviation of abiotic stresses in higher plants: a review. Environmental Pollution 147: 422428.
  • Ma, J.F. & Yamaji, N. 2006. Silicon uptake and accumulation in higher plants. Trends in Plant Science 11: 392397.
  • Mailly, D., Christanty, L. & Kimmins, J.P. 1997. ‘Without bamboo, the land dies’: nutrient cycling and biogeochemistry of a Javanese bamboo talun-kebun system. Forest Ecology and Management 91: 155173.
  • Maldonado, M., Riesgo, A., Bucci, A. & Rutzler, K. 2010. Revisiting silicon budgets at a tropical continental shelf: silica standing stocks in sponges surpass those in diatoms. Limnology and Oceanography 55: 20012010.
  • Marshall, E.J.P. & Westlake, D.F. 1990. Water velocities around water plants in chalk streams. Folia Geobotanica et Phytotaxonomica 25: 279289.
  • Massey, F.P., Smith, M.J., Lambin, X. & Hartley, S.E. 2008. Are silica defences in grasses driving vole population cycles? Biology Letters 4: 419422.
  • McGuire, A.D., Wirth, C., Apps, M., Beringer, J., Clein, J., Epstein, H., Kicklighter, D.W., Bhatti, J., Chapin, F.S., de Groot, B., Efremov, D., Eugster, W., Fukuda, M., Gower, T., Hinzman, L., Huntley, B., Jia, G.J., Kasischke, E., Melillo, J., Romanovsky, V., Shvidenko, A., Vaganov, E. & Walker, D. 2002. Environmental variation, vegetation distribution, carbon dynamics and water/energy exchange at high latitudes. Journal of Vegetation Science 13: 301314.
  • McNaughton, S.J., Tarrants, J.L., McNaughton, M.M. & Davis, R.H. 1985. Silica as a defense against herbivory and a growth promotor in African grasses. Ecology 66: 528535.
  • Metsoja, J.A., Neuenkamp, L., Pihu, S., Vellak, K., Kalwij, J.M. & Zobel, M. 2012. Restoration of flooded meadows in Estonia – vegetation changes and management indicators. Applied Vegetation Science 15: 231244.
  • Meyerson, L.A., Saltonstall, K., Windham, L., Kiviat, E. & Findlay, S. 2000. A comparison of Phragmites australis in freshwater and brackish marsh environments in North America. Wetlands Ecology and Management 8: 89103.
  • Norris, A.R. & Hackney, C.T. 1999. Silica content of a mesohaline tidal marsh in North Carolina. Estuarine and Coastal Shelf Science 49: 597605.
  • Nwugo, C.C. & Huerta, A.J. 2008. Silicon-induced cadmium resistance in rice (Oryza sativa). Zeitschrift für Pflanzenernährung und Bodenkunde 171: 841848.
  • Olde Venterink, H., Davidsson, T.E., Kielh, K. & Leonardson, L. 2002. Impact of drying and re-wetting on N, P and K dynamics in a wetland soil. Plant and Soil 243: 119130.
  • Opdekamp, W., Teuchies, J., Vrebos, D., Chormanski, J., Schoelynck, J., Van Diggelen, R., P., M. & E., S. 2012. Tussocks: biogenic silica hot-spot in a riparian wetland. Wetlands 32: 11151124.
  • Parr, J.F. & Sullivan, L.A. 2005. Soil carbon sequestration in phytoliths. Soil Biology and Biochemistry 37: 117124.
  • Parr, J., Sullivan, L., Chen, B., Ye, G. & Zheng, W. 2010. Carbon bio-sequestration within the phytoliths of economic bamboo species. Global Change Biology 16: 26612667.
  • Pausas, J.G. 1997. Litter fall and litter decomposition in Pinus sylvestris forests of the eastern Pyrenees. Journal of Vegetation Science 8: 643650.
  • Pétillon, J., Ysnel, F., Canard, A. & Lefeuvre, J.-C. 2005. Impact of an invasive plant (Elymus athericus) on the conservation value of tidal salt marshes in western France and implications for management: responses of spider populations. Biological Conservation 126: 103117.
  • Querné, J., Ragueneau, O. & Poupart, N. 2012. In situ biogenic silica variations in the invasive salt marsh plant, Spartina alterniflora: a possible link with environmental stress. Plant and Soil 352: 157171.
  • Rabosky, D.L. & Sorhannus, U. 2009. Diversity dynamics of marine planktonic diatoms across the Cenozoic. Nature 457: 183186.
  • Rafi, M.M. & Epstein, E. 1999. Silicon absorption by wheat (Triticum aestivum L.). Plant and Soil 211: 223230.
  • Ragueneau, O., Schultes, S., Bidle, K., Claquin, P. & Moriceau, B. 2006. Si and C interactions in the world ocean: importance of ecological processes and implications for the role of diatoms in the biological pump. Global Biogeochemical Cycles 20: GB4S02.
  • Raven, J.A. 1983. The transport and function of silicon in plants. Biology Reviews, Cambridge Philosophical Society 58: 179207.
  • Reynolds, O.L., Keeping, M.G. & Meyer, J.H. 2009. Silicon-augmented resistance of plants to herbivorous insects: a review. Annals of Applied Biology 155: 171186.
  • Riis, T., Sand-Jensen, K. & Vestergaard, O. 2000. Plant communities in lowland Danish streams: species composition and environmental factors. Aquatic Botany 66: 255272.
  • Sand-Jensen, K., Jeppesen, E., Nielsen, K., Vanderbijl, L., Hjermind, L., Nielsen, L.W. & Iversen, T.M. 1989. Growth of macrophytes and ecosystem consequences in a lowland Danish stream. Freshwater Biology 22: 1532.
  • Sangster, A.G. & Hodson, M.J. 2001. Silicon and aluminium codeposition in the cell wall phytoliths of gymnosperm leaves. In: Meunier, J.D. & Colin, F. (eds.) Phytoliths: applications in earth sciences and human history, pp. 343355. Taylor & Francis, London, UK.
  • Sarmiento, G. 1992. Adaptive strategies of perennial grasses in South American savannas. Journal of Vegetation Science 3: 325336.
  • Savant, N.K., Snyder, G.H. & Datnoff, L.E. 1996. Silicon Management and Sustainable Rice Production. In: Donald, L.S. (ed.) Advances in agronomy, pp. 151199. Academic Press, London, UK.
  • Scanlon, T.M., Raffensperger, J.P. & Hornberger, G.M. 2001. Modeling transport of dissolved silica in a forested headwater catchment: implications for defining the hydrochemical response of observed flow pathways. Water Resources Research 37: 10711082.
  • Schaller, J., Brackhage, C. & Dudel, E.G. 2012a. Silicon availability changes structural carbon ratio and phenol content of grasses. Environmental and Experimental Botany 77: 283287.
  • Schaller, J., Brackhage, C., Gessner, M.O., Baüker, E. & Dudel, E.G. 2012b. Silicon supply modifies C:N:P stoichiometry and growth of Phragmites australis. Plant Biology 14: 392396.
  • Schoelynck, J., Bal, K., Backx, H., Okruszko, T., Meire, P. & Struyf, S. 2010. Silica uptake in aquatic and wetland macrophytes: a strategic choice between silica, lignin and cellulose? New Phytologist 186: 385391.
  • Schoelynck, J., Bal, K., Puijalon, S., Meire, P. & Struyf, E. 2012a. Hydro-dynamically mediated macrophyte Si dynamics. Plant Biology 14: 9971005.
  • Schoelynck, J., De Groote, T., Bal, K., Vandenbruwaene, W., Meire, P. & Temmerman, S. 2012b. Self-organised patchiness and scale-dependent bio-geomorphic feedbacks in aquatic river vegetation. Ecography 35: 760768.
  • Schoelynck, J., Beauchard, O., Jacobs, S., Bal, K., Barao, L., Smis, A., Van Bergen, J., Vandevenne, F., Meire, P., Van der Spiet, T., Cools, A., Van Pelt, D., Hodson, M.J. & Struyf, E. 2013. Dissolved silicon and its origin in Belgian beers – a multivariate analysis. Silicon 5: 312.
  • Smol, J.P. & Stoermer, E.F. 2010. The diatoms: applications for the environmental and earth sciences. Cambridge University Press, Cambridge, UK.
  • Song, Z., Liu, H.M., Si, Y. & Yin, Y. 2012. The production of phytoliths in China's grasslands: implications to the biogeochemical sequestration of atmospheric CO2. Global Change Biology 18: 36473653.
  • Street-Perrott, F.A. & Barker, P.A. 2008. Biogenic silica: a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon. Earth Surface Processes and Landforms 33: 14361457.
  • Struyf, E. & Conley, D.J. 2009. Silica: an essential nutrient in wetland biogeochemistry. Frontiers in Ecology and the Environment 7: 8894.
  • Struyf, E. & Conley, D.J. 2012. Emerging understanding of the ecosystem silica filter. Biogeochemistry 107: 918.
  • Struyf, E., Van Damme, S., Gribsholt, B. & Meire, P. 2005a. Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium). Hydrobiologia 540: 6977.
  • Struyf, E., Van Damme, S., Gribsholt, B., Middelburg, J.J. & Meire, P. 2005b. Biogenic silica in tidal freshwater marsh sediments and vegetation (Schelde estuary, Belgium). Marine Ecology – Progress Series 303: 5160.
  • Struyf, E., Dausse, A., Van Damme, S., Bal, K., Gribsholt, B., Boschker, H.T.S., Middelburg, J.J. & Meire, P. 2006. Tidal marshes and biogenic silica recycling at the land-sea interface. Limnology and Oceanography 51: 838846.
  • Struyf, E., Van Damme, S., Gribsholt, B., Bal, K., Beauchard, O., Middelburg, J.J. & Meire, P. 2007. Phragmites australis and silica cycling in tidal wetlands. Aquatic Botany 87: 134140.
  • Struyf, E., Morth, C.M., Humborg, C. & Conley, D.J. 2010. An enormous amorphous silica stock in boreal wetlands. Journal of Geophysical Research-Biogeosciences 115: G04008.
  • Sullivan, M.J. & Moncreiff, C. 1990. Edaphic algae are an important component of salt marsh food-webs: evidence from multiple stable isotope analyses. Marine Ecology – Progress Series 62: 149159.
  • Tate, K.W., Dudley, D.M., McDougald, N.K. & George, M.R. 2004. Effect of canopy and grazing on soil bulk density. Rangeland Ecology & Management 57: 411417.
  • Taylor, K., Rowland, A.P. & H.E., J. 2001. Molinia caerulea (L.) Moench. Journal of Ecology 89: 126144.
  • Tréguer, P., Nelson, D.M., Vanbennekom, A.J., Demaster, D.J., Leynaert, A. & Queguiner, B. 1995. The silica balance in the world ocean – a reestimate. Science 268: 375379.
  • Valéry, L., Bouchard, V. & Lefeuvre, J. 2004. Impact of the invasive native species Elymus athericus on carbon pools in a salt marsh. Wetlands 24: 268276.
  • van de Koppel, J. & Crain, C.M. 2006. Scale-dependent inhibition drives regular tussock spacing in a freshwater marsh. American Naturalist 168: E136E147.
  • Vandevenne, F.I., Struyf, E., Clymans, W. & Meire, P. 2012. Agricultural silica harvest: have humans created a new and important loop in the global silica cycle? Frontiers in Ecology and the Environment 10: 243248.
  • Vieillard, A.M., Fulweiler, R.W., Hughes, Z.J. & J.C., C. 2011. The ebb and flood of Silica: quantifying dissolved and biogenic silica fluxes from a temperate salt marsh. Estuarine, Coastal and Shelf Science 95: 415423.
  • Walther, G.R., Beissner, S. & Burga, C.A. 2005. Trends in the upward shift of alpine plants. Journal of Vegetation Science 16: 541548.
  • Weber, P., Bugmann, H. & Rigling, A. 2007. Radial growth responses to drought of Pinus sylvestris and Quercus pubescens in an inner-Alpine dry valley. Journal of Vegetation Science 18: 777792.
  • Weibull, A.C., Ostman, O. & Granqvist, A. 2003. Species richness in agroecosystems: the effect of landscape, habitat and farm management. Biodiversity and Conservation 12: 13351355.