In addition to aquatic ecosystems, Arctic warming is reshaping the structure and functioning of terrestrial environments across vast spatial and temporal scales. Increased productivity is evident through satellite imagery, whereas photographic records have revealed increased shrub cover and northward boreal forest expansion. Such shifts in terrestrial ecosystems are likely to trigger numerous feedback loops in the global climate system (Normand et al., 2013).
Before delving into terrestrial ecosystem responses, it is important to understand the distinct biomes that characterise Arctic environments. Arctic tundra covers around 20% of the Earth's surface and typically has low biotic diversity. It is characterised by plants with shallow root systems including low-growing shrubs, mosses, lichens and grasses that can survive in the shallow active soil layer of thawed permafrost during summer months. The Arctic treeline, marking the boundary between taiga and tundra, represents the northern limit of tree growth and is a sensitive indicator of climate variability.
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| The northward advancement of the boreal forest into Arctic tundra |
Before delving into terrestrial ecosystem responses, it is important to understand the distinct biomes that characterise Arctic environments. Arctic tundra covers around 20% of the Earth's surface and typically has low biotic diversity. It is characterised by plants with shallow root systems including low-growing shrubs, mosses, lichens and grasses that can survive in the shallow active soil layer of thawed permafrost during summer months. The Arctic treeline, marking the boundary between taiga and tundra, represents the northern limit of tree growth and is a sensitive indicator of climate variability.
Tundra responses
A warming Arctic climate is inducing earlier thawing, greening and longer growing seasons of tundra plant communities and is likely to promote the advancement of tundra into the polar deserts. Shifts in plant assemblages can influence productivity, nutrient cycling, active layer depth, decomposition, snow distribution and surface albedo. Elmendorf et al. (2012) quantified the climatic effects on tundra ecosystems by investigating responses to experimental warming over time periods of up to 20 years at 27 different locations. The results illustrated increased canopy height, amount of dead material and abundance of shrubs and graminoids (herbaceous plants with grass-like features). On the other hand, species diversity and the abundance of lichens and mosses declined in correspondence with warming.
What's also apparent in this study is the spatial and temporal variation in ecosystem responses. Temporally, the effects of rising temperature influence a range of ecosystem processes including photosynthetic rates, soil organic matter, and biogeochemical cycling and thus the direction and extent of climate change responses vary over time. Spatially, the tundra biome covers a range in average summer temperature of more than 10°C and an array of moisture contents from wetlands to polar desert, with variations in nutrients, organic matter content, pH, and herbivore communities. For example, caribou and muskoxen herbivory counteracts the positive effect on shrub expansion that warming has, favouring the growth of graminoids (Post and Pedersen, 2008). There are also regional variations in the structure and composition of species, for example, vascular species are more abundant at warmer sites and the shrub canopy is generally taller and denser.
Despite the comprehensiveness of the study and agenda to investigate spatial variation in ecosystem responses, sites in the Siberian tundra are lacking. This has led to a bias towards Canada, Greenland and northwest Europe, neglecting a significant tundra environment. In addition, a substantial amount of unexplained variation exists regarding effect size, particularly for long-term impacts.
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| Impacts of experimental warming on (a) community attributes and abundance (b) vegetation height (Elmendorf et al., 2012) |
What's also apparent in this study is the spatial and temporal variation in ecosystem responses. Temporally, the effects of rising temperature influence a range of ecosystem processes including photosynthetic rates, soil organic matter, and biogeochemical cycling and thus the direction and extent of climate change responses vary over time. Spatially, the tundra biome covers a range in average summer temperature of more than 10°C and an array of moisture contents from wetlands to polar desert, with variations in nutrients, organic matter content, pH, and herbivore communities. For example, caribou and muskoxen herbivory counteracts the positive effect on shrub expansion that warming has, favouring the growth of graminoids (Post and Pedersen, 2008). There are also regional variations in the structure and composition of species, for example, vascular species are more abundant at warmer sites and the shrub canopy is generally taller and denser.
Despite the comprehensiveness of the study and agenda to investigate spatial variation in ecosystem responses, sites in the Siberian tundra are lacking. This has led to a bias towards Canada, Greenland and northwest Europe, neglecting a significant tundra environment. In addition, a substantial amount of unexplained variation exists regarding effect size, particularly for long-term impacts.
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| Study sites in the Arctic tundra (Elmendorf et al., 2012) |
Arctic treeline advancement
The position of the Arctic Front (the southern boundary of Arctic air) during the summer determines the position of the northern treeline. Therefore, ocean-atmospheric phenomena and teleconnections are important determinants of the northern treeline (Sulphur et al., 2016). The increase in productivity that has been recorded in correlation with Arctic warming is also linked to the range expansion of birch, willow and alder. This shift to taller, darker canopies, with reduced albedo, and elevated evapotranspiration rates, will act as a positive feedback to warming. It is estimated that boreal forest coverage can reduce surface albedo by 25-50% compared to Arctic tundra (Macdonald et al., 2008).
In addition, the areal coverage of northern terrestrial ecosystems is a fundamental component of the global carbon cycle, influencing the Earth’s radiative balance by acting as a sink. Although expansion of woody species that are more biologically productive, creating more litter, significantly increases the carbon content of above-ground biomass, this may be modest when compared to the potential loss of carbon stored in tundra soils. An increased abundance of fungi associated with increased forest cover, potentially enhanced carbon cycling with litter from new plant species, and increased winter soil temperature may enhance the decomposition of older carbon stores (Parker et al., 2015).
The projected contraction of tundra and expansion of taiga will evidently have major impacts on Arctic environments and produce wider global feedbacks, the extent to which will vary on a spatial and temporal scale.
In addition, the areal coverage of northern terrestrial ecosystems is a fundamental component of the global carbon cycle, influencing the Earth’s radiative balance by acting as a sink. Although expansion of woody species that are more biologically productive, creating more litter, significantly increases the carbon content of above-ground biomass, this may be modest when compared to the potential loss of carbon stored in tundra soils. An increased abundance of fungi associated with increased forest cover, potentially enhanced carbon cycling with litter from new plant species, and increased winter soil temperature may enhance the decomposition of older carbon stores (Parker et al., 2015).
The projected contraction of tundra and expansion of taiga will evidently have major impacts on Arctic environments and produce wider global feedbacks, the extent to which will vary on a spatial and temporal scale.
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