- Herbivore density – moss coverage mechanism (local, well-established) (R3):Mosses are susceptible to trampling, a disturbance, by large herbivores. Moss coverage increases in the presence of low herbivore density (van der Wal 2001). Herbivore density is maintained below carrying capacity since moss are low in nutrients and palatability. Moss dominated sites are avoided by herbivores (Wolff 1980).
- Moss coverage – soil moisture mechanism (local, well-established) (R4): The lack of root system prevents water up take from soils by transpiration and dense moss layer decreases evaporation therefore increasing soil moisture. The combination of low evapotranspiration rates and low soil temperature allows the development of water-logged soils (Zimov 1995), which maintains conditions for moss-dominated vegetation. Water-logged soils combined with low soil temperatures reduce decomposition rates of dead matter, so nutrient availability is low within the system.
- Moss coverage – land availability mechanism (regional, well-established) (B2): Moss create a local environmental conditions that exclude grasses. Moss coverage is spatially limited within Arctic ecosystem, so land availability within the Arctic serves as a natural boundary. Competition for available land space will naturally limit moss coverage. The Arctic tundra landscape is a microrelief characterized by mounds and depressions influenced by specific combination of microclimate patterns such as moisture, temperature, light, wind exposure and snow coverage
- Moss-Permafrost (local, well-established) (B4): Moist conditions promote moss coverage. Moss coverage insulates the soil and therefore reduces permafrost thawing (Kelley et al. 2004); in addition to that, moss prevents evaporation (Zimov et al. 1995), both of which promote soil moisture. In spite of moss cover, air temperature rising – due to climate change – can increase the depth of permafrost’s active layer. When it happens, thawing provides water, which maintains moist conditions for mosses that then prevents accelerated evaporation and progressive active layer deepening (Tenhunen et al. 1992).
- Water-Permafrost (local, well-established) (R4):Permafrost thawing can cause accumulation of water. These impounded places are more susceptible to increasing of air temperatures and have a bigger impact on permafrost degradation (Jorgenson et al. 2006).
- Grass coverage mechanism – nutrient availability (local, well-established) (R2): Grasses are highly productive with high evapotranspiration rates, which decrease soil moisture, creating drier soils (Zimov 1995). Dry soil conditions (thawing of the active layer) increase nutrient cycling including nitrogen availability, promoting fertile landscape for grasses (Nadelhoffer et al.) Furthermore, grasses can shade out or physically smothers mosses, which leads to decline moss abundance (Chapin et al. 1995).
- Grass coverage – land availability (regional, well-established) (B3): Grasses create a local environmental conditions that exclude mosses. The Arctic environment serves as a natural boundary for grass coverage. Competition for available land space will naturally limit grass coverage and the carrying capacity of herbivores.
- Herbivore density - grass coverage (local, well-established) (R1): Herbivores accelerate nutrient cycling in grassland, and can also import nutrients, through their feces and death (decomposition). Grass growth is stimulated by nutrient inputs and grazing (Zimov, 1995). Grasses dominated areas attract herbivores, because they are better to eat than moss.
- Herbivore density – grass coverage mechanism (local, well-established) (B1): Nutrient rich grasses are highly palatable by herbivores.Grazing pressure by herbivores decreases the grass density. Grasses compensate by increasing primary production, through overcompensation grasses may maintain dominance under grazing at fertile sites (Bråthen et al. 2007).
- Climate-Permafrost (regional, well-established) (R5): Increasing of air temperature has been associated with permafrost warming (Jorgensen et al. 2006). Thawed permafrost increases soil moisture through melting ground ice (Natali et al. 2015). Soil moisture increase permafrost degradation as accumulation of surface water affects heat flux in to soils and promotes increase thawing (Jorgensen et al. 2010). Exposing previously frozen carbon to microbal processes, CO2 and CH4, shifting the permafrost function from a carbon sink to a carbon source (Hollesen et al., 2011). The thawing of permafrost contributes to greenhouse gas emission. Over time but not abruptly, the emission released is one of the factors that lead to global warming (Schuur et al. 2015).
Steppe to Tundra regime
- Climate change (global, uncertain): Climate change from arid to humid during the shift between the Pleistocene and Holocene epoch. Megafuana survived the climate shift into Holocene. Bison, horses and muskoxen continued to graze on the steppe, maintaining the grassland.
Internal direct driver
- Decline in Herbivory (local, well-established): Scientists explained disappeared of animals in northern grasslands coincides with human hunting during (Zimov 1995). Herbivore feces inputs, grazing, and trampling maintained the steppe coverage. In the absence of herbivory, grass litter accumulated on the soil surface insulating the soil. In turn, soil fertility declined along with increasing soil moisture created suitable habitat for moss expansion.
There is an uncertainty if climate change alone leads to a shift from productive steppe to less productive tundra. Similar climate shift occurred in previous interglacial periods, yet these did not cause catastrophic landscape reconstructions. It is difficult to disentangle the combine effects of decline herbivory and climate change on the steppe landscape, since they both overlap.
Tundra to Steppe regime
External direct driver
- Climate change (global, well-established): This driver affects soil moisture. Moist conditions promote moss coverage. Moss coverage insulates the soil and therefore reduces permafrost thawing; in addition to that, moss prevents evaporation. If air temp increases and permafrost thaws, it will provide moist conditions for grasses, hence mosses create a buffer effect against thawing.
Internal direct driver
- Increase in Herbivory (local, well-established): Herbivores trample the mosses, increasing evaporation rates which can cause soil moisture to decline. Drying soil and herbivore dropping can increase nutrients available for the establishment of grasses.
Slow internal system changes
- Nutrient availability (local, well-established): Assimilation/Mineralization of nutrients (nitrogen and phosphorus) is a slow process that is regulated by soil temperatures and excrement supply from herbivores. When condition are optimal (elevated soil temperatures and low moisture during the short summer) nutrients become available for grasses. Another temporal delay occurs in herbivore reproduction cycle, time between generations.
Summary of Drivers
|#||Driver (Name)||Type (Direct, Indirect, Internal, Shock)||Scale (local, regional, global)||Uncertainty (speculative, proposed, well-established)|
Shift from Tundra to Steppe
- Soil Moisture: There is a tipping point between moss and grasses that is controlled by soil moisture.
- Herbivore Density: Can impact soil moisture. Herbivores trample the mosses, and if trampling is dense and frequent enough evaporation rates increase and soil moisture declines causing soil to dry. As a result, soil nutrients are available for grass growth. As herbivore density increases nutrient level in the soil increase by feces deposition. Moss growth is limited by trampling opening spaces for grass colonization.
Shift from Steppe to Tundra
- Soil Moisture: The threshold is when conditions shift from dry soils to waterlogged. In the absence of herbivores, plant litter accumulates on the soil surface insulating the soil increasing soil moisture. As a result, soil fertility declines creating suitable moss habitat. Mosses are more limited by moisture than nutrients
- Herbivore Density: At low herbivore densities moss can outcompete grasses. The nutrient poor mosses are unpalatable for herbivores. An area dominated by mosses is not attractive for herbivores who will graze elsewhere.
Soil moisture (regional, well-established) is a crucial leverage point in maintaining mosses or grasses. Vegetation growth rates depend on soil moisture; mosses require waterlogged soils whereas grasses need dry conditions. Global warming, as a driver, directly affects soil moisture. A change in soil moisture induces a succession between the two vegetation types. Air temperature increases causing evaporation and promoting dry soils. Moss system shifts from mosses to grasses; however, in grass systems, grass growth is reinforced by drier conditions. Soil moisture is a defining variable in grass versus moss coverage reinforcing loop.
The mineralization (local, well-established) is an important process in the nutrient limited Arctic system. The delay occurs in nutrient assimilation from organic to inorganic, creating the stock of nutrient available in the soil. The process is critical for grass growth which maintains large herbivores populations (mosses are unpalatable to them). Large herbivores carrying capacity is sustained by grass coverage, which depends on nutrient availability. Mineralization is delayed by long winters, which is a temporal delay. Another temporal delay occurs in herbivore reproduction cycle, this is time between generations.
Summary of Ecosystem Service impacts on different User Groups
||References (if available)|
|Freshwater||+/-||Wrona et al. 2016|
|Feed, Fuel and Fibre Crops|
|Wild Food & Products||+/-||Huntington, 2013|
|Air Quality Regulation|
|Climate Regulation||-||Schuur et al. 2015 Ivanova, 2003|
|Soil Erosion Regulation|
|Pest & Disease Regulation|
|Protection against Natural Hazards||-||Schaefer, 2012|
|Aesthetic Values||-||Huntington, 2013|
|Cognitive & Educational||-||Huntington, 2013|
|Spiritual & Inspirational||-||Huntington, 2013|
Uncertainties and unresolved issues
- Vegetation assemblages at local and regional scale in the arctic depends on various variables such a relief, thickness of the permafrost layer, soil type, etc. In addition to that, vegetation stabilizes feedbacks with air and subsoil temperatures as with animals. How this complex system will look in the face of climate change is a question that still requires large research efforts.
- Vegetation dynamics interact with other processes such as climate, water dynamics (draining, lake formation), relief changes and anthropogenic impacts (mining, oiling and herding). There is a knowledge gap regarding these interactions, which is necessary to answer questions such as “will mosses could be replaced by other species if temperature raises?” or “what is the effect of toxic substances releasing in moss and grass communities?”