B4WarmED forest warming experiment: Effects on photosynthetic temperature optima of temperate and boreal tree species

Background/Question/Methods

With the predicted increase in air temperature caused by anthropogenic climate change, plant responses to increasing temperature have become a major area of concern. Boreal Forest Warming at an Ecotone in Danger (B4WarmED) is a manipulative open-air experiment at two sites in northern Minnesota, addressing the potential for climate warming to alter tree function and species composition at the boreal-temperate forest ecotone through effects on juvenile physiology, growth, and survival.  The goal of this study was to compare the optimal temperature of photosynthesis (T_opt) and maximum photosynthetic rates (A_max) of 7 tree species grown under two temperature regimes (ambient and warmed 3.4 °C belowground and aboveground) to determine whether shifts in T_opt and A_max occur, and whether the magnitude of those shifts varies for temperate and boreal species.  The in situ experiment uses infrared heat lamps and soil heating cables to heat 7.1m² plots continuously during the growing season.  Temperature response curves were measured on detached foliage using the Li-Cor 6400 from a subsample of 11,000 seedlings planted in 2008.  Leaf samples were cut each morning and kept hydrated throughout the day in a growth chamber, which was used to aid in achieving measurement temperatures of 12, 17, 22, 27, 32, and 37 °C. 

Results/Conclusions

A total of 1219 photosynthetic temperature-response curves were made in 2009, 2010, and 2011. Results show a significant shift in T_opt (of ≈1.3 °C on average for plants in the warmed 3.4 °C treatment) at both sites and in both habitats. The shifts occur similarly among all species (as well as across sites and canopy conditions), suggesting that all temperate and boreal species likely have some capacity to shift their temperature response functions to better match changes in temperatures.  A_max was positively affected by the warming treatment in 2009, but there was no significant effect of warming in 2010 or 2011.  There were, however, significant species differences in A_max in all years that generally corresponded with shade tolerance rankings.  Better understanding of patterns of T_opt and A_max, which reflect species’ abilities to adapt to a changing global temperature, could be used to improve bioclimatic models.