The nature of spatial transitions in the Arctic

Abstract

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change.Location Arctic tundra ecosystems of the North Slope of Alaska and the tundraforest region of the Seward Peninsula, AlaskaMethods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High-Arctic and Low-Arctic systems, (ii) the transition between moist non-acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change.Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non-acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development.Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming.

Experimental approaches to predicting the future of tundra plant communities

“…heating cables or lamps -either singly or in factorial combination) are usually restricted geographically to a few sites with suitable infrastructure (Harte and Shaw 1995;Johnson et al 2002). This carries with it the problem, however, that results might be difficult to extrapolate to regional or even local scales (Epstein et al 2004), depending on whether or not 'zonal', or other more specialised plant communities, were selected for investigation. A counter-argument in an arctic-alpine context, however, is that micro-or meso-topographic variations have a disproportionate effect on thermal environment and water-balance, and for this reason substantial community variability at the local scale (Walker 2000) can be exploited to make inferences about how ecosystems much further apart would respond to the same drivers of change.…”

“…Clearly, in interpreting the results of environmental manipulation experiments it is important that their spatial and temporal context is considered explicitly (Epstein et al 2004). How applicable are conclusions across an array of contrasting community and ecosystem types, and how useful are the results for making predictions for the future?…”

Measurements and modelling of snowmelt and turbulent heat fluxes over shrub tundra

“…Tall shrub canopies occupy less areal extent but are prevalent in both upland and sheltered riverine areas, consisting mainly of willow and alder (Alnus) species such as Alnus crispa (Jorgenson and Heiner, 2003). Epstein et al (2004) suggest that shrubs are sensitive indicators of climate warming and appear to be the most responsive of a wide range of tundra and treeline biomes considered; this pattern is reflected in repeat photography and satellite imagery suggesting recent expansions in Arctic shrub cover (Tape et al, 2006). Tundra may be snow-covered for 6 to 8 months per year in sub-arctic regions and up to 9 months in the Arctic ).…”

Calibration and Validation of Landsat Tree Cover in the Taiga−Tundra Ecotone

“…The geographic distribution of these factors may point to how the vulnerability of TTE structure (i.e., the likelihood of changes in spatial patterns and mosaics of trees) to changing climate is cast across the landscape, and the time lags associated with changes in structure. Monitoring the dynamics of this transition zone at multiple spatial and temporal scales is critical for understanding the likelihood of TTE structural change, the causes and consequences of these changes, and the elasticity of those changes [17,21,26,27]. At each scale of observation, the uncertainty of a mean TTE structure measurement provides the statistical basis upon which the smallest amount of change can be reliably identified.…”

“…At the finest scales, the climate-limited portions of the TTE respond to changing climate through the site-specific growth and dieback of trees [7]. These responses are controlled by a multi-scale suite of factors that, in addition to climate, include topography, edaphic conditions, disturbance history, and current tree species position and extent [20][21][22][23][24][25]. The geographic distribution of these factors may point to how the vulnerability of TTE structure (i.e., the likelihood of changes in spatial patterns and mosaics of trees) to changing climate is cast across the landscape, and the time lags associated with changes in structure.…”