Steppe to Tundra
Steppe (a grassland) to tundra (mosses and shrubs growing in waterlogged soils) is a regime shift that can occur in cold terrestrial ecosystems. Tundra and steppe regime shift is typically found where permafrost occurs. Steppe and tundra are primarily found in the Arctic, north of the tree line, where mean temperature below 10-12oC for the warmest month (Jonasson et al. 2000). Climate change and changes in the density of large herbivores are the main drivers of regime shifts between steppe and tundra. Climate changes that reduce soil moisture can favor steppe over tundra, and vice versa. Tundra is favored by moss growth, which is more limited by water than by nutrients. Steppe is favored by grass growth, which is improved by drier soils with available nutrients. Large herbivores can shape ecosystems through their impact on vegetation species composition, soil structure, and ecological dynamics. Large herbivore trampling and grazing can slow moss growth and convert tundra to steppe vegetation. At the end of the last ice age (12,000 yr BP), human hunting greatly reduced populations of large herbivores which may have contributed to a shift from grass-dominated steppe to moss-dominated tundra. In the 21st century, climate change together with the presence of horses, bison, and musk oxen could lead to shifts between steppe and tundra vegetation.
Key direct drivers
- Harvest and resource consumption
- Species introduction or removal
- Global climate change
- Extensive livestock production (rangelands)
Key Ecosystem Processes
- Primary production
- Nutrient cycling
- Wild animal and plant products
- Climate regulation
- Aesthetic values
- Knowledge and educational values
- Spiritual and religious
- Food and nutrition
- Health (eg toxins, disease)
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
- Social conflict
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms
Links to other regime shifts
The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).
The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.
Drivers and causes of the regime shift
Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.
Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.
How the regime shift works
Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).
In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.
Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).
Shift from Tundra to Steppe
Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.
Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.
Impacts on ecosystem services and human well-being
Shift from Steppe to tundra
Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).
The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.
Tundra to steppe
Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).
Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).
To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change
Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.
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