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 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).