| 英文摘要: | Rapid climate warming in the tundra biome has been linked to increasing shrub dominance1, 2, 3, 4. Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost2, 5, 6, 7, 8, yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an underused resource to explore climate–growth relationships. Here, we analyse circumpolar data from 37 Arctic and alpine sites in 9 countries, including 25 species, and ~42,000 annual growth records from 1,821 individuals. Our analyses demonstrate that the sensitivity of shrub growth to climate was: (1) heterogeneous, with European sites showing greater summer temperature sensitivity than North American sites, and (2) higher at sites with greater soil moisture and for taller shrubs (for example, alders and willows) growing at their northern or upper elevational range edges. Across latitude, climate sensitivity of growth was greatest at the boundary between the Low and High Arctic, where permafrost is thawing4 and most of the global permafrost soil carbon pool is stored9. The observed variation in climate–shrub growth relationships should be incorporated into Earth system models to improve future projections of climate change impacts across the tundra biome. The Arctic is warming more rapidly than lower latitudes owing to climate amplification involving temperature, water vapour, albedo and sea ice feedbacks5, 7. Tundra ecosystems are thus predicted to respond more rapidly to climate change than other terrestrial ecosystems4. The tundra biome spans Arctic and alpine regions that have similar plant species pools and mean climates, yet vary in topography, seasonality, land cover and glaciation history. Concurrent with the recent high-latitude warming trend7, repeat photography and vegetation surveys have shown widespread expansion of shrubs1, 2, 3, characterized by increased canopy cover, height and abundance. However, climate warming7 and shrub increase2, 10 have not occurred at all sites. Models predict that warming of 2–10 °C (ref. 11) could convert as much as half of current tundra to ‘shrubland’ by the end of the twenty-first century8, but the uniformity of the frequently cited relationship between climate change and tundra shrub expansion5, 12, 13, 14, 15 has yet to be quantified across the tundra biome as a whole. Shrubs are woody perennial species that can live from decades to centuries. In seasonal climates, they form annual growth rings, allowing analysis of radial growth over time. Many shrub species are widely distributed across the tundra biome and are often dominant, owing to their canopy height, longevity and ability to outcompete low-growing plants. With wide geographic distributions and annual growth records, shrubs are ideally suited for quantifying tundra vegetation responses to climate warming. Assembled annual growth records from sites across the tundra biome provide a unique opportunity to test competing hypotheses of shrub responses to climate change over the past half-century. Previous ecological monitoring and dendroecological studies have identified temperature, growing season length, summer precipitation and snow cover as important variables explaining spatial and interspecific variation in shrub growth1, 10, 13, 14, 16, 17, 18. However, there is a lack of consensus regarding which climate variables best explain growth across all tundra ecosystems. We therefore do not know whether climate–growth relationships are consistent in direction, strength and magnitude among species and among sites where plant composition, climate trends and environmental parameters differ. At present, most large-scale vegetation models assume high climate sensitivity and a uniform growth response to warming among shrub species and populations8, 19. These models predict pronounced positive climate feedbacks as a result of tundra vegetation change5, 8. Yet, if shrub growth responses to climate are constrained, then changes in shrub dominance should vary regionally, and feedbacks across the tundra biome as a whole could be weaker than predicted at present. We quantified the climate sensitivity of shrub growth—that is, the strength of relationship between annual growth and climate variables (including temperature and precipitation, specific calculations described below)—to test four hypotheses: (1) The greatest climate sensitivity of growth should occur at northern or high-elevation range edges if plant performance is more climate limited in the harsher growing conditions at range edges than in the centre of species distributions20, 21, 22. (2) Climate sensitivity of growth should be greatest in the centre of species distributions if populations growing under more stressful conditions at range edges have evolved conservative life history strategies limiting their ability to respond when conditions improve23. (3) Climate sensitivity of growth should vary along spatial gradients if the response of species to warming is limited by other factors, such as soil nutrients, soil moisture or biotic interactions21. Alternatively, (4) climate sensitivity of growth could be uniform across the tundra biome. We synthesized published and unpublished time series of shrub growth across the tundra biome. Our data set extends beyond previous analyses by including sites across the circumpolar Arctic, comprising dwarf, low and tall canopy species, and encompassing 60 years of annual-resolution shrub growth. We used crossdated, radial and axial growth measurements spanning 1950–2010, collected at 37 sites, and for 25 shrub species in 8 genera. We analysed climate–growth relationships for 46 genus-by-site combinations using linear mixed models to estimate climate sensitivity, with 33 candidate climate models as predictors of shrub growth increments. All data were normalized before analysis and model terms included seasonal temperatures and precipitation as fixed effects and year as a random effect (see Supplementary Information). We calculated four complementary indices of climate sensitivity from the mixed model analysis for each genus-by-site combination: (1) the difference in Akaike information criterion (AIC) between the best climate model and a null model (ΔAIC), (2) the R2 for the best climate model, (3) the absolute value of the slope of the relationship between growth and summer temperature and (4) the proportion of individuals that had significant linear relationships between growth and summer temperature (the best predictor from the overall analysis). We assessed these indices of climate sensitivity across abiotic (wet day frequency, soil moisture, growing season length) and biotic gradients (distance to range edge and species-level maximum canopy height, see Supplementary Information). In Fig. 1, we report both ΔAIC and model slopes to illustrate spatial variation in climate sensitivity (all indices reported in Supplementary Fig. 12). In Fig. 2 we report the percentage of models indicating climate (temperature or precipitation) sensitivity in the model comparison analysis; Fig. 3 shows relationships between all four climate-sensitivity indices across different gradients.
- Elmendorf, S. C. et al. Plot-scale evidence of tundra vegetation change and links to recent summer warming. Nature Clim. Change 2, 453–457 (2012).
- Myers-Smith, I. H. et al. Shrub expansion in tundra ecosystems: Dynamics, impacts and research priorities. Environ. Res. Lett. 6, 045509 (2011).
- Tape, K. D., Sturm, M. & Racine, C. H. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Glob. Change Biol. 12, 686–702 (2006).
- IPCC Climate Change 2014: Impacts, Adaptation, and Vulnerability (eds Field, C. B. et al.) (Cambridge Univ. Press, 2014).
- Chapin, F. S. et al. Role of land-surface changes in Arctic summer warming. Science 310, 657–660 (2005).
- Blok, D. et al. Shrub expansion may reduce summer permafrost thaw in Siberian tundra. Glob. Change Biol. 16, 1296–1305 (2010).
- Hinzman, L. D. et al. Trajectory of the Arctic as an integrated system. Ecol. Appl. 23, 1837–1868 (2013).
- Pearson, R. G. et al. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Clim. Change 3, 673–677 (20
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