Local ecotypic and species range-related adaptation influence photosynthetic temperature optima in deciduous broadleaved trees

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 (Topt) 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 Topt 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 Topt. 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 Topt could be due to either fixed genotypic differences (e.g., adaptive Topt), acclimation of Topt, or both. In contrast, the Populus species with the colder home range, P. tremuloides, showed significantly (P < 0.05) lower Topt 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.