| There is a general consensus that, for trees and other plants, northern range limits are determined by temperature minima. However, the determinants of a southern range limit are far more ambiguous. To assess whether differential metabolic acclimation and trends in carbon-use efficiency at least partially determine southern range limits, we designed an experiment to examine the response of boreal and temperate tree species along a latitudinal transect. Common gardens were established at three locations along an approximately 900 km latitudinal transect from northern Wisconsin to southern Illinois. Boreal species used were trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) while temperate species included eastern cottonwood (Populus deltoides var. deltoides) and sweetgum (Liquidambar styraciflua). Climate differed substantially across the three sites along our latitudinal transect during the growth interval. Namely, air temperature averaged 19.3, 22.3 and 28.5°C in northern Wisconsin, southern Wisconsin and Illinois, respectively. We observed no photosynthetic thermal acclimation in the boreal species or cottonwood, whereas sweetgum exhibited adjustments that led to an increase in the optimum temperature of photosynthesis with decreasing latitude (and rising growth temperature). On the other hand, none of the four species exhibited respiratory thermal acclimation along the transect. Overall, carbon-use efficiency, the efficiency with which assimilated carbon is converted into plant biomass, declined with decreasing latitude. This trend was associated with an increase in foliar dark respiration, the magnitude of which was exacerbated by the absence of respiratory acclimation to warmer climates. Results from this study indicate that anticipated climate warming may compromise the carbon balance of boreal and temperate tree species. Moreover, the limited ability of boreal and temperate species to acclimate metabolically to varying growing season temperatures could contribute to climate-driven shifts in species' southern range limits. |
