Implications of local adaptation in tree physiology and phenology in a common garden

Background/Question/Methods Tree species distributions are influenced by environmental conditions and competitive interactions.  Predicting population responses to climate change requires understanding the extent to which populations are physiologically adapted to their local environment, the degree to which they can adjust to environmental changes, and the consequences of those patterns for carbon gain. To better understand the extent of local adaptation in tree physiology, we examined seasonal patterns of photosynthesis and leafing phenology in a common garden located in St. Paul, Minnesota during 2005. The garden, planted in 1999, is comprised of Quercus rubra, Quercus alba and Acer rubrum populations ranging in geographic origin from Arkansas to Minnesota. We hypothesized that despite later leafout, Quercus species would have higher seasonal carbon gain than Acer due to higher photosynthetic rates. Further, we hypothesized that despite longer leaf lifespans, southern populations would have lower seasonal carbon gain due to increased susceptibility to early and late season frost damage.

Results/Conclusions Among species, Quercus rubra showed the highest seasonal photosynthetic rates, followed by Quercus alba and Acer rubrum respectively. Within species, northern populations had higher seasonal photosynthetic rates than southern populations throughout the season, despite the fact that northern populations deployed lower leaf areas than southern populations.  Furthermore, we found that northern populations leafed out earlier and held their leaves longer than southern populations.  This suggests that tree physiology within species is variable along latitudinal gradients, and that local physiological adaptation has important consequences for forest growth and carbon gain.  Within species, southern populations appear unable to advance their leaf deployment and maximize carbon gain earlier in the season.  Our results imply that southern tree populations, whose distributions may shift northward in response to climate change, may be at a competitive disadvantage if unable to adapt to the altered temperature and light regimes.