![]() ![]() Here we develop a dynamic shrub patch-scale suitability model capable of predicting the present and projected ceiling (i.e. While our understanding of the factors that influence shrub dynamics is increasing 2, 6, 10, 14, 27, our ability to predict the spatially heterogeneous patterns of shrubification across the Arctic remains limited. However, shrubs cannot expand homogeneously, even within similar gradients of warming 3, 4, 15. Minimizing uncertainties in key biogeophysical feedback processes in response to projected climate and environmental change, however, relies on understanding patch-scale shrubification dynamics.Īs climate and environmental conditions change, shrubs will likely continue to thrive in warmer and longer growing seasons 2, propagating across the landscape 14, 27. Due to the complex ecological dynamics that govern the rate and direction of shrub cover change at local to regional scales, our ability to anticipate patch-scale patterns of vegetation change across space and time remains limited. In contrast, thermokarst (surface subsidence from permafrost thaw) may facilitate the transition from well-drained to poorly drained terrain, drowning shrubs with increased soil inundation 15, 20. seasonally thawed soil layer), also promoting shrub recruitment 10. Disturbances including frost heave, permafrost thaw, and wildfire may expose mineral soils and/or deepen the active layer (i.e. normalized difference vegetation index and aboveground biomass) 14. Warmer summer temperatures have also been linked with increased productivity (i.e. Forced and passive warming experiments have demonstrated positively enhanced shrub height and width 8, similar to that observed in long-term monitoring plots in response to climate warming 26. Soil moisture and water availability have been shown to regulate the kinetics of reactions involved with shrub growth 2, leading to increased shrub expansion in moist soils but limited expansion in saturated soils due to anoxic conditions 23, 24, 25. Observations of shrub expansion (often termed shrubification) are prevalent across Alaska 15, 16, Canada 17, 18, Russia 19, 20, and northern Europe 21, 22, where the rate and direction of shrub cover change vary with and modify a suite of environmental variables. The widespread success and increased dominance of deciduous shrubs across many tundra regions contribute to accelerating regional warming by altering surface energy balance (i.e., decreased albedo) 2, 6, 10 and positive trends in ecosystem productivity and greenness across the pan-Arctic 10, 12, 13, 14 Alder shrubs have the additional advantage of a symbiotic relationship with nitrogen-fixing bacteria, increasing the local availability of nitrogen in soils 11. These shrubs are strong competitors for resources, able to outcompete prostrate and low-statured plant types for light via tall canopy architecture 8, increase soil insulation and moisture via snow accumulation 9, and efficiently acquire nutrients via mycorrhizal associations 2, which together contribute to rapid expansion rates 2, 10. One of the earliest observed and most ubiquitous of these shifts is the expansion of tall deciduous shrub canopies 5, 6, 7, particularly in alder, birch, and willow shrubs ( Alnus spp., Betula spp., and Salix spp., respectively). High-latitude regions are warming nearly four times faster than the global average 1, leading to widespread shifts in tundra vegetation 2, 3, 4. These findings will be useful for constraining and projecting vegetation-climate feedbacks in the Arctic. ![]() These observations suggest that increased nitrogen inputs from nitrogen-fixing alders contributed to a positive feedback that advanced overall tall-shrub expansion. Models show the heterogeneous patterns of tall-shrub expansion are not only predictable but have an upper limit defined by permafrost, climate, and edaphic gradients, two-thirds of which have yet to be colonized. Using fine-scale remote sensing, we modeled the drivers of patch-scale tall-shrub expansion over 68 years across the central Seward Peninsula of Alaska. To constrain the effects of widespread shrub expansion in terrestrial and Earth System Models, improved knowledge of local-to-regional scale patterns, rates, and controls on decadal shrub expansion is required. As Arctic regions warm, shrubs expand heterogeneously across their ranges, including within unburned terrain experiencing isometric gradients of warming. ![]() Tall deciduous shrubs are critically important to carbon and nutrient cycling in high-latitude ecosystems. ![]()
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