Populations near the warm edge of species ranges may be particularly sensitive to climate change, 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 America. 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 belowground 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 models, 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.
Bibliographical noteFunding Information:
This research was supported by the Office of Science (BER), US Department of Energy, 385 Grant No. DE-FG02-07ER64456, by the Minnesota Department of Natural Resources, and by the College of Food, Agricultural, and Natural Resources Sciences and Wilderness Research Foundation, University of Minnesota. Assistance with experimental operation and data collection was provided by C. Buschena, C. Zhao, K. Gill, H. Jihua, X. Wei and numerous summer interns. We also thank N. Danz, J. Almendinger and K. Zhu for assistance in identifying the range limits and local ecotonal distributions of species, and A. Pierce for operational support.
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