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Photosynthetic biochemical limitation parameters (i.e., V-cmax, J(max) and J(max):V-cmax ratio) are sensitive to temperature and water availability, but whether these parameters in cold climate species at biome ecotones are positively or negatively influenced by projected changes in global temperature and water availability remains uncertain. Prior exploration of this question has largely involved greenhouse based short-term manipulative studies with mixed results in terms of direction and magnitude of responses. To address this question in a more realistic context, we examined the effects of increased temperature and rainfall reduction on the biochemical limitations of photosynthesis using a long-term chamber-less manipulative experiment located in northern Minnesota, USA. Nine tree species from the boreal-temperate ecotone were grown in natural neighborhoods under ambient and elevated (+3.4 degrees C) growing season temperatures and ambient or reduced (approximate to 40% of rainfall removed) summer rainfall. Apparent rubisco carboxylation and RuBP regeneration standardized to 25 degrees C (V-cmax25 degrees C and J(max25 degrees C), respectively) were estimated based on AC(i) curves measured in situ over three growing seasons. Our primary objective was to test whether species would downregulate V-cmax25 degrees C and J(max25 degrees C) in response to warming and reduced rainfall, with such responses expected to be greatest in species with the coldest and most humid native ranges, respectively. These hypotheses were not supported, as there were no overall main treatment effects on V-cmax25 degrees C or J(max25 degrees C) (p > .14). However, J(max):V-cmax ratio decreased significantly with warming (p = .0178), whereas interactions between warming and rainfall reduction on the J(max25 degrees C) to V-cmax25 degrees C ratio were not significant. The insensitivity of photosynthetic parameters to warming contrasts with many prior studies done under larger temperature differentials and often fixed daytime temperatures. In sum, plants growing in relatively realistic conditions under naturally varying temperatures and soil moisture levels were remarkably insensitive in terms of their J(max25 degrees C) and V-cmax25 degrees C when grown at elevated temperatures, reduced rainfall, or both combined.