Given prior evidence for local ecotypic and species-specific adaptation in trees, we hypothesized that: (1) Acer rubrum and Quercus rubra provenances with different climate origins should differ in photosynthetic temperature optimum (T opt) even after long-term growth in a common environment; (2) congeneric species Populus tremuloides and Populus deltoides with differing but overlapping ranges should not differ in T opt when co-occurring, due to the likelihood of both ecotypic thermal adaptation and phenotypic thermal acclimation. To address these questions, we investigated the temperature responses of pairs of A. rubrum and Q. rubra provenances planted in a common garden and the temperature responses of P. tremuloides and P. deltoides at four sites where the species ranges overlap in Minnesota, USA. Both studies showed significant signals of temperature adaptation. The provenances of both A. rubrum and Q. rubra that originated from northern sites with lower ambient temperature had lower T opt. This supported the hypothesis about the dominance of local ecotypic adaptation of photosynthesis to temperature despite opportunity for both long-term (12-year) acclimation to the common-garden temperature regime and short-term temperature acclimation. However, acclimation capacity to the temperatures experienced in the days and weeks before the gas exchange measurements differed among the contrasting provenances suggesting that the observed differences in T opt could be due to either fixed genotypic differences (e. g., adaptive T opt), acclimation of T opt, or both. In contrast, the Populus species with the colder home range, P. tremuloides, showed significantly (P < 0.05) lower T opt on average than co-occurring P. deltoides. Thus, despite the opportunity for both ecotypic adaptation and local acclimation, phylogenetic inertia still constrained the species with the colder overall range to a different temperature optimum than the one with a warmer overall range. Our results also imply that rapid but modest climate change may create mismatches between photosynthetic physiology and local climate because of lags in population or species-level adaptation.
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Acknowledgments We thank Prof. Jacek Oleksyn for thoughtful comments on the manuscript, Ms Cindy Buschena for excellent technical assistance, and the Wilderness Research Foundation for funding support. Piotr Robakowski was supported by the research scholarship from the Kosciuszko Foundation and Yan Li by the Chinese Government PhD Scholarship.
- Adaptation to temperature
- Broadleaved tree species
- Photosynthetic temperature optimum
- Photosynthetic temperature response curve