| 英文摘要: | Populations near the warm edge of species ranges may be particularly sensitive to climate change1, 2, 3, 4, but lack of empirical data on responses to warming represents a key gap in understanding future range dynamics. Herein we document the impacts of experimental warming on the performance of 11 boreal and temperate forest species that co-occur at the ecotone between these biomes in North America5. We measured in situ net photosynthetic carbon gain and growth of >4,100 juvenile trees from local seed sources exposed to a chamberless warming experiment that used infrared heat lamps and soil heating cables to elevate temperatures by +3.4 °C above- and belowground6 for three growing seasons across 48 plots at two sites. In these ecologically realistic field settings, species growing nearest their warm range limit exhibited reductions in net photosynthesis and growth, whereas species near their cold range limit responded positively to warming. Differences among species in their three-year growth responses to warming parallel their photosynthetic responses to warming, suggesting that leaf-level responses may scale to whole-plant performance. These responses are consistent with the hypothesis, from observational data and models4, 7, 8, 9, 10, that warming will reduce the competitive ability of currently dominant southern boreal species compared with locally rarer co-occurring species that dominate warmer neighbouring regions. Co-occurring species at boreal–temperate ecotones may respond differently to climate warming, triggering changes in their competitive hierarchies, and thus in species composition1, 2. One metric that could indicate such differences is the location of local populations relative to key features of species geographic distributions, such as range limits3, 7, 8, 9, 10. Even locally adapted populations of co-occurring species might differ in terms of how well suited they are to the local thermal environment. Boreal species with generally colder distributions and greater cold tolerance11, 12 may have lower capacity to improve their performance with projected higher temperatures than less cold-tolerant temperate species, because of trade-offs between cold tolerance and growth capacity13, 14, 15. Boreal species may also be more sensitive to heat waves and associated droughts than temperate species. Moreover, as widely distributed species commonly display ecotypic variation across the environmental gradients spanned by their ranges16, 17, 18, intraspecific genotypic variation within co-occurring species could further shape differences in their responses to climate warming3, 4, 19. For example, populations near the cold edge of a species’ distribution may receive genes from populations from warmer climates, and thus contain genetic material that enables successful growth in a warming climate. In contrast, populations near the warm edge of their range cannot receive genes from populations in warmer climates, because such populations do not exist. Any, or all, of these differences could result in boreal species near the warm edge of their range having limited capacity to respond positively to further warming compared to temperate species near their cold range limit. These kinds of species differences in responsiveness to climate warming could lead to major compositional shifts at broad ecotones, including the boundary between the vast boreal and temperate forest biomes7, 9, 10, 20. Many tree species co-occur at the boreal–temperate ecotone, but otherwise have markedly distinct geographic distributions. For example, in northern Minnesota, USA, roughly half of the abundant species5 are boreal (extending to northern Canada but not much further south in the US) and half are temperate (extending further south in the US, but with northern range limits not much beyond the US/Canada border)21, 22. The co-occurrence in the southern boreal ecotone of species with markedly distinct ranges provides an opportunity to address the hypothesis that boreal tree species near their warmer range limits will exhibit negative or neutral responses to future warming, whereas coexisting temperate species near their cold range limits will have neutral or positive responses, facilitating forest compositional change. This hypothesis is implicit in ‘climate-envelope’ models, even those with modifications for plant sensitivities to resources and environments10, 23, 24, 25, 26. However, it is also possible that, despite large differences in overall geographic distribution, strong local adaptation of near range-edge populations could result in co-occurring species having a similar capacity to respond physiologically to future warming. Herein we present results of a three-year chamberless field experiment6 that tested these hypotheses by exposing juveniles of 11 tree species to ambient and elevated (+3.4 °C) growing season temperatures and measuring their physiological and growth responses. Juveniles (~3 years old in 2009) of ten native and one naturalized species from northern Minnesota seed sources were planted in 2008 into existing vegetation in both open (cleared) and closed canopy (understory) forest habitats at two sites (~150 km apart) in northeastern Minnesota, USA (Supplementary Table 1). Plants grew in ecologically realistic densities of neighbouring herb, shrub and tree species, and thus the observed performance of each species represents their response to warming in a setting that included interactions, such as competition, with other plants. Although the study species are often lumped into boreal and temperate groups, their distributions represent continua (Supplementary Fig. 1), and we evaluated whether two complementary indices of distributions were related to species’ responses to climate warming. One index, based on a mapped continent-wide distribution21, 22, is the centre of the latitudinal range in central North America. A second, and more regional, index quantifies for each species the percentage of their regional relative abundance that occurs in the northern half of six ecotonal counties in northeastern Minnesota. See Supplementary Information for details. The two measures of geographic distribution are significantly (r = 0.90, P < 0.001) linearly correlated (Supplementary Fig. 2). As hypothesized, over three growing seasons, net photosynthetic carbon gain and juvenile tree growth were adversely impacted by experimental warming for boreal species growing furthest south of the centre of their range, near their warm range limit, but were stimulated for co-occurring temperate species growing north of the centre of their range, near their cold range limit (Table 1, Figs 1 and 2, and Supplementary Fig. 3). The analyses of variance of experimental treatments across all species, sites and canopy conditions showed significant interaction between species and warming treatment for both growth and net photosynthesis (P < 0.0001, Table 1); species differed in the direction (positive, neutral, negative) and magnitude of response to warming. Overall responses of net photosynthesis and growth to warming (on average across species) did not differ by site (that is, no site ∗ warming interactions, P > 0.05).
- Walther, G-R. Community and ecosystem responses to recent climate change. Proc. R. Soc. Lond. B 365, 2019–2024 (2010).
- Pucko, C., Beckage, B., Perkins, T. & Keeton, W. S. Species shifts in response to climate change: Individual or shared responses? J. Torrey Bot. Soc. 138, 156–176 (2011).
- Davis, M. B. & Shaw, R. G. Range shifts and adaptive responses to Quaternary climate change. Science 292, 673–679 (2001).
- Reich, P. B. & Oleksyn, J. Climate warming will reduce growth and survival of Scots pine except in the far north. Ecol. Lett. 11, 588–597 (2008).
- Friedman, S. K. & Reich, P. B. Regional legacies of logging: Departure from presettlement forest conditions in northern Minnesota. Ecol. Appl. 15, 726–744 (2005).
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