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Rising temperatures caused by climate change could negatively alter plant ecosystems if temperatures exceed optimal

temperatures for carbon gain. Such changes may threaten temperature-sensitive species, causing local extinctions and

range migrations. This study examined the optimal temperature of net photosynthesis (Topt) of two boreal and four

temperate deciduous tree species grown in the field in northern Minnesota, United States under two contrasting temperature

regimes. We hypothesized that Topt would be higher in temperate than co-occurring boreal species, with

temperate species exhibiting greater plasticity in Topt, resulting in better acclimation to elevated temperatures. The

chamberless experiment, located at two sites in both open and understory conditions, continuously warmed plants

and soils during three growing seasons. Results show a modest, but significant shift in Topt of 1.1 0.21 °C on average

for plants subjected to a mean 2.9 0.01 °C warming during midday hours in summer, and shifts with warming

were unrelated to species native ranges. The 1.1 °C shift in Topt with 2.9 °C warming might be interpreted as suggesting

limited capacity to shift temperature response functions to better match changes in temperature. However, Topt of

warmed plants was as well-matched with prior midday temperatures as Topt of plants in the ambient treatment, and

Topt in both treatments was at a level where realized photosynthesis was within 90–95% of maximum. These results

suggest that seedlings of all species were close to optimizing photosynthetic temperature responses, and equally so in

both temperature treatments. Our study suggests that temperate and boreal species have considerable capacity to

match their photosynthetic temperature response functions to prevailing growing season temperatures that occur

today and to those that will likely occur in the coming decades under climate change.

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