1. The effects of moss on northern ecosystem function
Several reviews have assessed the role of mosses in food web dynamics, plant competition, and carbon and nutrient cycling (cf. Van Breemen, 1995; Turetsky, 2003; Nilsson & Wardle, 2005; Cornelissen et al., 2007; Lindo & Gonzalez, 2010; Turetsky et al., 2010). Mosses can dominate primary productivity in northern environments, on average contributing to 20 and 50% of above-ground NPP in boreal forests and wetlands, respectively (Turetsky et al., 2010). Mosses are important for nitrogen cycling and availability for vascular plant uptake because of their low nitrogen-use efficiency, high cation exchange (leading to nitrogen interception and retention), and slow decomposition rates (cf. Hobbie et al., 2000; Malmer et al., 2003; Turetsky et al., 2010). By regulating soil temperature and moisture, mosses also control concentrations of plant-available nitrogen (cf. Gornall et al., 2007). Additionally, some species, particularly feather mosses and Sphagnum, can serve as an important source of ecosystem nitrogen by facilitating biological nitrogen fixation (Basilier, 1979; DeLuca et al., 2007; Markham, 2009; Kip et al., 2011). Mosses are also important to phosphorus cycling in that they absorb phosphate and reduce availability for vascular uptake (Chapin et al., 1987). On the other hand, Crowley & Bedford (2011) found that moss created more oxidized conditions in shallow fen soils, and overall enabled greater phosphorus acquisition by forb species. This highlights the fact that mosses can have positive or negative interactions with vascular plants, depending on species and abiotic conditions. Gornall et al. (2011) present a conceptual model of the impacts of the moss layer on vascular performance in which negative effects begin to outweigh any positive effects with increasing depth of the moss layer.
Because of their influence on soil climate, NPP, nutrient content, and decomposition rates, mosses facilitate surface peat accumulation and thus long-term soil carbon accumulation. As a result of slow rates of soil carbon accumulation over millennia, large stocks of carbon are currently held in frozen mineral soils and in frozen and unfrozen peatland soils, all of which may be vulnerable to soil warming and changing disturbance regimes (Grosse et al., 2011). Based on c. 620 peat cores collected across western Canada (Zoltai et al., 2000), bryophytes (mostly Sphagnum remains) were estimated to represent c. 50% of the total peat volume (Turetsky, 2003). In model simulations of > 8000 yr of peat accumulation, vascular plants accounted for 65% of total NPP but only 35% of the remaining peat mass. Vascular plants had < 5% of their total NPP remaining in the peat, whereas moss groups had > 10% remaining (Frolking et al., 2010).
By regulating soil climate and peat accumulation, mosses strongly influence vascular plant recruitment and regeneration. Because organic soils tend to be poor seedbeds, in particular for broadleaf species that have small seeds, mosses indirectly control vascular regeneration (cf. Johnstone & Chapin, 2006; Astrup et al., 2008; Camill et al., 2010; Soudzilovskaia et al., 2011). Mosses also appear to directly inhibit vascular plant germination through allelopathy (cf. Steijlen et al., 1995), although Soudzilovskaia et al. (2011) found strong negative relationships between bryophyte phenolics and seedling germination in the laboratory but not in field settings. Decreases in moss abundance and resulting decreases in soil moisture can reduce habitat quality for wildlife in some cases (Hodson et al., 2010). In general, mosses play an integral role in northern ecosystem food webs, because they are eaten by some animals (Prins, 1982), regulate key habitat conditions such as soil climate, and serve as important habitat for soil organisms that interact to form the detrital food web (Lindo & Gonzalez, 2010).
Mosses and peat play an important role in permafrost stability by buffering surface soils and permafrost from fluctuating air temperatures. In the growing season, surface soil temperatures are negatively related to the thickness of organic soil layers (Harden et al., 2006; Romanovsky et al., 2008). Both observational and manipulative studies have quantified the importance of mosses to ground heat flux (van der Wal & Brooker, 2004; Gornall et al., 2007; Blok et al., 2011). Also, modeling studies incorporating thick organic soils have shown its importance in permafrost dynamics (e.g. Yi et al., 2007; Lawrence et al., 2008; Wisser et al., 2011). While some moss species have high moisture retention, in general the porosities of organic soil layers are higher than mineral soils (Yi et al., 2009). Thus, drainage occurs with seasonal ice thaw and the ensuing drier conditions further protect surface permafrost.
Mosses also are important to the formation of microtopography – elevated mounds (hummocks) separated by low hollows – in both forests and peatlands. Hummocks tend to vary in height from a few cm to 1 m (Ivanov, 1981). Paleoecological and modeling evidence suggests that these microtopography features are long-lived (e.g. Nungesser, 2003). The formation of hummock-hollow microtopography creates a gradient of distance to water table, increasing the diversity of microhabitats and niches for moss species. Hummock species must be able to retain and use water efficiently and avoid desiccation, more so than species living in hollows (Titus et al., 1983; Rydin, 1993). For example, species in the Sphagnum section Acutifolia form dense populations of small individuals, leading to greater water retention and transport than hollow communities. Sphagnum communities in hollows are often dominated by species in the section Cuspidata, which tend to have larger capitula diameters and lower population densities. Because of such traits, hollow species are highly productive but prone to desiccation, and thus are competitively excluded from hummocks (Rydin, 1993). Several studies have documented substantial variation in decomposition rates among dominant Sphagnum spp., generally with slower decomposition rates in hummock than in hollow species (Johnson & Damman, 1991; Turetsky et al., 2008; Lang et al., 2009b; but see Hogg, 1993; Turetsky et al., 2010). The biochemical mechanism leading to differential decomposition rates is not well understood but appears to be related to nitrogen (cf. Lang et al., 2009b) and/or structural carbohydrates such as sphagnan (cf. Turetsky et al., 2008; Hájek et al., 2011). Regardless of the mechanism, differences in decomposition rates between hummock and hollow mosses are thought to affect peat properties and hydraulic transmissivity over time, reinforcing moisture differences among microhabitats (cf. Nungesser, 2003; Belyea & Baird, 2006).
By influencing the diversity of habitats for moss species, microtopography increases the diversity of functional traits related, in particular, to water balance. This in turn increases resistance at an ecosystem scale to drought as well as wildfire (Benscoter & Wieder, 2003; Belyea & Baird, 2006). Despite being situated further from the water table, hummocks burn less frequently and severely than hollows in continental forests and peatlands. This is because high water retention by hummock mosses inhibits both the initiation and downward propagation of combustion (Benscoter et al., 2011). Across c. 80 black spruce forests in Alaska, the area of unburned Sphagnum hummocks was a strong predictor of how much organic soil escaped burning at a stand scale (Shetler et al., 2008). In situ survival within these unburned ‘islands’ could also be important to recolonization and rates of succession post-fire. Hylander & Johnson (2010) documented that small-scale refugia can be important for forest bryophytes after fire, but concluded that their role in recolonization is not well understood.
2. Using models to explore the role of moss in ecological resilience
Because of the already-mentioned difficulty of using mosses in plant-level experiments, and the cost and difficulty of performing field experiments in remote northern areas, ecological models are an important tool for understanding the function and resilience of mosses. The strong role of moss in both fast (cf. soil heat fluxes) and slow (cf. permafrost aggradation, peat accumulation) ecosystem dynamics means that models are sometimes the only available tool, given their ability to examine vegetation and ecosystem responses over long time-frames. On the other hand, model results are fundamentally circumscribed by what pools and processes are actually built into the model and how they are represented, and model results can never be perfectly confirmed (Oreskes et al., 1994). Despite these limitations, model simulations are critical to identify knowledge gaps, formulate hypotheses, and examine long-term dynamics. In this way, empirical studies will continue to inform modelling work, and vice versa.
Dynamic vegetation models (DVMs) represent a synthesis of research in four main groups of processes: plant geography, plant physiology and biogeochemistry, vegetation dynamics, and biophysics. The most important unique feature of DVMs is their ability to simulate vegetation dynamics, that is, transient changes in vegetation structure in response to variations in the external environment. DVMs generally simulate the effects of changes in climate on natural vegetation in a spatially explicit manner. Within a grid cell, vegetation may be modeled by fractions or strata occupied by multiple plant functional types (PFTs). Competition among PFTs is considered for basic resources, including light, nitrogen, and water. Vegetation growth in DVMs is represented by NPP, which is usually modeled explicitly as the balance of carbon uptake by photosynthesis and release by autotrophic respiration. Plant succession processes, such as establishment, competition, and mortality, are often included.
Plant functional types are plant species grouped according to their effects on community or ecosystem function (Chapin et al., 1998; Walker et al., 1999), how they respond to disturbance (Friedel et al., 1988; Smith et al., 1997), and/or similarities in resource use (Grime, 1979). PFT classification is central to DVMs, yet the sets of PFTs adopted by existing models can be arbitrary and the appropriate parameter values for particular PFTs are sometimes not well established (but see Kattge et al., 2011). This is particularly true for mosses – given that bryophyte species are often lumped into a single PFT in models or not included at all. A number of DVMs and ecosystem models have included moss as a single or as multiple PFTs (e.g. Pastor et al., 2002; Nungesser, 2003; Zhuang et al., 2006; Bond-Lamberty et al., 2007; Yurova et al., 2007; Heijmans et al., 2008; Euskirchen et al., 2009; Wania et al., 2009; Frolking et al., 2010). These models vary in time step as well as how moss is parameterized (Table 1). In empirical studies, mosses are often analyzed as groups according to substrate preference (e.g. rock vs log; Frisvoll & Presto, 1997; Astrom et al., 2007), overstory canopy preference (Astrom et al., 2007; Baldwin & Bradfield, 2007), and microtopographic position, all of which reflect the response of mosses to moisture, light, and chemical gradients (cf. Robinson et al., 1989; Glaser et al., 1990; Bridgham et al., 1996). Quantifying variation in traits related to drought tolerance and avoidance (such as stem density, leaf/branch morphology, water retention and capillary wicking, metabolic change following re-wetting), light acquisition and tolerance (such as leaf area, chlorophyll content, pigmentation), and nitrogen use and economy (such as nitrogen-use efficiency, affiliation with N2 fixers, cation exchange capacity) would allow for more robust classification of response traits (Lavorel & Garnier, 2002; Waite & Sack, 2010).
Table 1. Comparison of characteristics (related to moss PFTs) of the models as they were used in this paper
|Model||Moss PFTs||Controls on moss NPP||Moss PFT parameters||Competition with vascular PFTs||Time Step/Duration||Reference|
|Biome-BGC||One – feather moss or Sphagnum depending on dominant moss type||Temperature, water content, [CO2], nutrients, light||Water: carbon at full turgor Fraction external water Leaf water potential at full and zero turgor Reduction in conductance as a result of cuticular water||Water, light, nutrients||Daily time step for seasons to decades|| Bond-Lamberty et al. (2007) |
|TEM-DVM||Two –Sphagnum and feather moss, which were parameterized depending on the ecosystem type in which they are located||Temperature, water content, [CO2], nutrients, light||Parameterized and calibrated with PFT-specific: GPP NPP Maximum potential NPP Nitrogen uptake Nitrogen availability Net nitrogen mineralization Vegetation C and N Soil C and N||Light, nutrients||Monthly time step for seasons to decades|| Euskirchen et al. (2009) |
|HPM||Five – brown moss, three Sphagnum (lawn, hummock, hollow), feather moss||Water table depth, peat depth (as proxy for nutrients)||Optimum water table and peat depth NPP response to suboptimal conditions Maximum potential NPP Initial litter decomposition rate||Not directly; inferred from PFT realized niches in water table and peat depth||Annual time step for decades to millennia|| Frolking et al. (2010) |
There is surprisingly little information directly linking moss traits to specific ecosystem functions (effect traits, Suding et al., 2008; but see Cornelissen et al., 2007; Waite & Sack, 2011). Studies have commonly grouped mosses by life history traits such as spore size, frequency of sporophyte production, growth form, and presence of vegetative reproduction (cf. During, 1992; Baldwin & Bradfield, 2007; Darell & Cronberg, 2011). Some studies have related moss life history traits to water retention and economy (e.g. Hedderson & Longton, 1996). Chapin et al. (1996)’s classification of arctic plant species first partitioned cryptogam species into moss and lichen, and then further divided mosses into Sphagnum vs nonSphagnum spp. based on properties related to peat accumulation. Segregating Polytrichum spp. from other moss species also has been suggested based on life history and water economy traits (cf. Gordon et al., 2001). Elumeeva et al. (2011) quantified water retention across 22 bryophyte species at both the shoot and colony levels, and their analyses provided support for six PFTs based on water economy and habitat preference. Studies that screen moss species for a variety of traits will be useful for PFT classification, but linking traits to ecosystem processes is necessary (Suding et al., 2008).
Frolking et al. (2010) used relationships between relative NPP, water table depth and peat height to represent five moss PFTs in the Holocene Peat Model (HPM), a one-dimensional model that simulates peat accumulation at an annual time step as the net balance between above- and below-ground productivity and litter or peat decomposition. In a new simulation using the HPM, we explored the role of moss in the response of peat accumulation to a drying perturbation. We imposed climatic drying as a linear increase in evapotranspiration (ET) starting 3000 yr after the beginning of the simulation and ending with a 30% increase by the end of the 5000 yr simulation. We then conducted the same drying simulation, this time completely removing the moss cover at the onset of drying and increasing vascular NPP so that total NPP was essentially unchanged. The baseline 5000 yr simulation represents a typical raised bog, where the transition between the fen and bog phases occurs after c. 1500 yr, and where the vegetation cover of the last 3500 yr comprises primarily Sphagnum mosses and shrubs. Gradual drying reduced peat accumulation, though the effect was small for the first 1500 yr of drying, by which time ET had declined by 23%. After this, once a threshold was reached where the loss of water through ET was too high to be compensated by precipitation, the system responded with a relatively abrupt drop in water table and a period of net peat loss. In the ‘drying + no moss’ scenario, C accumulation slowed earlier (by c. 500 yr) in the period of declining ET, leading to wetter conditions as the peat could not outgrow the water table. At the same time the deeper peat is relatively unaffected, as it resists drying because of its low hydraulic transmissivity. When the simulated water table is near the surface, it responds to dry periods (low precipitation generated stochastically) with much greater variability (Fig. 1). Across the no disturbance, drying, and drying + no moss scenarios, the total accumulated peat mass was 260, 210, and 186 kg C m−2, respectively (of which 136, 103 and 64 kg C m−2 were derived from moss litter). In these HPM simulations, moss presence maintained two ecosystem characteristics – carbon accumulation rate, and water table depth along with its associated vegetation composition – for much longer in the face of a slowly but persistently increasing ‘press’ disturbance (drying).
Figure 1. Holocene Peat Model (HPM) simulations of: (a) accumulating peat carbon over the 5000 yr simulation for the baseline scenario (solid black line), a gradual drying starting in simulation year 3000 (dashed green line), and removal of moss and drying starting in year 3000 (solid orange line); (b) 50 yr simple moving window average of simulated water table depth for the three scenarios; and (c) moss fraction of cumulative total net primary productivity (NPP). The parameterization of the model is similar to the one presented in Frolking et al. (2010) for the Mer Bleue peatland (45.40°N, 75.50°W), a 28 km2 ombrotrophic bog. The prescribed annual precipitation input is based on a regional precipitation reconstruction (Muller et al., 2003) corresponding to the first 5000 yr of peat accumulation of the Mer Bleue peatland, including random variability based on the confidence intervals of the reconstruction (variability during 0–3000 yr is caused only by stochastic precipitation).
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Simulations with another process-based model, the soil thermal model (STM) version of the Terrestrial Ecosystem Model (STM-TEM), have also explored the importance of moss in the resilience of carbon and nutrient cycling in boreal conifer forests following wildfire. Zhuang et al. (2002) compared recovery of soil moisture, temperature, and aspects of nutrient cycling in scenarios with and without moss recovery post-fire. For several decades after fire, soils in the burned stand are warmer and drier than in the unburned stand because more radiation reaches the soil surface, causing higher evaporation, and because of greater drainage associated with a thicker active layer after fire (Fig. 2a,b). During ecosystem recovery post-fire, the simulation with moss growth resulted in both lower NPP (Fig. 2d) and heterotrophic respiration (Fig. 2e) in comparison to the simulation without moss growth. Although the presence of mosses had little effect on the pattern of NEP after fire (Fig. 2f), it did affect the patterns of accumulation of carbon in the ecosystem, as vegetation carbon accumulation is greater in the simulation without moss growth (Fig. 2g) because of greater nitrogen uptake in warmer soils, and soil carbon accumulation is slightly greater in the simulation with moss growth (Fig. 2h), because of slower decomposition in colder soils. Thus, these simulations indicate that mosses play a role in the resilience of soil carbon recovery after fire in the ecosystem.
Figure 2. Soil thermal model version of the Terrestrial Ecosystem Model (STM-TEM) simulations of the dynamics of soil temperature, soil moisture, nitrogen cycling, carbon fluxes, and carbon stocks in the sensitivity analysis for moss growth. Scenarios include a stand with moss cover that was not burned (black line), a stand that was burned and moss allowed to grow during stand development (also known as the ‘standard’ scenario, light gray line), and a stand that was burned and moss not allowed to grow during stand development (dark gray line). (a) Soil temperature integrated over 20 cm of soil relative to the soil surface; (b) mean volumetric soil moisture of the humic organic layer from May to September; (c) annual net nitrogen mineralization; (d) annual net primary productivity (NPP); (e) annual heterotrophic respiration/decomposition (RH); (f) annual net ecosystem production (NEP); (g) vegetation carbon; (h) soil carbon. Reprinted, with permission, from Zhuang et al. (2002).
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The HPM and STM-TEM simulations show that loss of the ground moss layer can have important consequences for how ecosystem processes either resist or respond to perturbations associated with drying and wildfire. Both simulations considered the effects of large reductions in moss abundance and did not consider how changing moss species composition might influence ecosystem processes following disturbances. While some severe disturbances, such as prolonged drought or severe burning, may reduce total moss abundance, in many cases, changing environmental conditions are more likely to cause shifts in moss species composition. To our knowledge, no empirical or modeling study has systematically varied the relative abundance of individual moss PFTs within a community to ask questions about the effects of moss abundance vs composition on ecosystem processes. In Section III, we explore how northern moss abundance and composition likely will respond to climate change and disturbances, and how this could influence the recovery of ecosystem processes.