Forest structure and function strongly depend on and concurrently influence environmental conditions. Tree performance is generally governed by its genetics and environment; thus, recent hotspots in this field include tree genotype × environment, phenotype × environment, and functional trait × environment interactions. The editorial, review, and 22 original research articles in this Special Issue, “Tree ecophysiology in the context of climate change”, highlight ecophysiological phenomena (e.g., climate hormesis, seed germination, tree mortality), processes (e.g., tree metabolism, photosynthate allocation, nutrient uptake and transport), indicators (e.g., carbon sequestration, pollutants), measurements (e.g., thermal time methods, soil quality indices, vegetation spectral index, and near-infrared leaf reflectance), and modeling (e.g., climate correlations with tree growth, photosynthetic phenology, hydraulic strategies, OliveCan model) in the context of global climate change. Understanding forest–environment interactions from an ecophysiological perspective as climate changes provides insights into species fitness in suboptimal environments, species competition for limited resources, and phylogenetic divergence or convergence of species, and predicting species distributions.

Woody plants contribute to the stability and productivity of terrestrial ecosystems and are significantly affected by climate change. According to the concept of environmental hormesis, any environmental stressors can cause hormesis, that is, stimulation in low doses and inhibition in high doses. Numerous studies have demonstrated plant hormesis under low doses of various abiotic stressors. However, the hormetic responses of woody plants to abiotic stressors from climate change are insufficiently studied. This review analyses data on the stimulating effects of low doses of climate stressors in experiments and in real ecosystems. Numerous laboratory and field experiments show that single and combined exposure to various climate stressors (temperature, humidity, and elevated carbon dioxide concentrations) can cause hormesis in various species and functional types of woody plants, which can be accompanied by hormetic trade-offs and preconditioning. In addition, there is evidence of climate hormesis in woody plants in ecosystem conditions. Field experiments in various ecosystems show that elevated temperatures and/or precipitation or elevated carbon dioxide concentrations causing hormesis in dominant tree species can stimulate ecosystem productivity. Moreover, climate hormesis of the growth and reproduction of dominant forest tree species contributes to the spread of forests, that is, climate-driven ecological succession. The main commonalities of climate hormesis in woody species include: (1) Low-dose climate stressors cause hormesis in woody plants when strong (limiting) stressors do not affect plants or these limiting stressors are mitigated by climate change. (2) Hormesis can occur with the direct impact of climatic stressors on trees and with the indirect impact of these stressors on plants through other parts of the ecosystem. (3) Climate stressor interactions (e.g., synergism, antagonism) can affect hormesis. (4) Hormesis may disappear due to tree acclimatization with consequent changes in the range of tolerances to climate factors. This review highlights the need for targeted studies of climate hormesis in woody species and its role in the adaptation of forest ecosystems to climate change.

Climate change is causing more frequent and severe climatic events, such as extreme heat and co-occurring drought, potentially accelerating tree mortality. Which tree species will cope better with those extreme events is still being researched. This study focuses on heat as a physiological stress factor and interspecific variation of thermal tolerance and sensitivity traits in 15 temperate coniferous and broad-leaved tree species. We investigate (1) whether thermal tolerance and sensitivity traits correlate with a drought-related physiological trait, particularly the leaf turgor loss point (π_tlp, wilting point), and (2) how thermal tolerance and sensitivity traits co-vary within different tree-functional types classified by morphological and physiological traits of the leaf, i.e., leaf mass per area (LMA) and percentage loss of area (PLA). The study was carried out in the Traunstein Forest Dynamics Plot of the ForestGEO network in Germany. The temperature response of the maximum quantum yield of photosystem II (F _v/F _m) on leaf discs was determined, from which various physiological leaf traits were estimated, one of which is the breaking point temperature (T ₅), the temperature at which F _v/F _m declines by 5%. Additionally, the temperature of 50% (T ₅₀) and 95% (T ₉₅) decline in F _v/F _m was evaluated. The decline width between T ₅₀ and T ₅ (DW_T50−T5) was taken as an indicator of the species’ thermal sensitivity. The breaking point temperature ranged from 35.4 ± 3.0 to 47.9 ± 3.9 °C among the investigated tree species and T ₅₀ ranged between 46.1 ± 0.4 and 53.6 ± 0.7 °C. A large interspecific variation of thermal tolerance and sensitivity was found. European ash (Fraxinus excelsior L.) was the most heat-sensitive species, while Wild cherry (Prunus avium L.) was the least heat-sensitive species. Species with a more negative π_tlp tended to have a higher breaking point temperature than species with a less negative π_tlp. A lower thermal sensitivity characterized species with a higher LMA, and high PLA was found in species with low thermal sensitivity. Accordingly, species with thicker and tougher leaves have lower thermal sensitivity which coincides with a lower wilting point. We conclude that species that develop drought-adapted foliage can cope better with heat stress. Further, they might be able to maintain transpirational cooling during combined heat and drought stress, which could lessen their mortality risk during climatic extremes.

Seventeen morphological and anatomical characteristics of the leaves were selected from five natural populations to explore the variation in leaf traits of Litsea coreana var. sinensis and the effects of geographical environment on these variations. Nested analysis of variance, multiple comparisons, principal component analysis (PCA), and correlation analysis were conducted to explore the variations within and between populations and their correlation with geographical and climatic factors. Significant differences in the 17 leaf traits were observed within and among populations. On average, the relative contribution of within population variation to total variation was 24.8%, which was lower than among population variation (54.6%). The average differentiation coefficient of the traits was 65.8%, and the average coefficient of variation 11.8%, ranging from 6.7% for main vein thickness to 21.4% for petiole length. The PCA results showed that morphological characteristics were divided into two categories, and the level of variation was greater than that of leaf anatomy. Most of the leaf traits were significantly correlated with geography and climate and showed a gradual variation with longitude, latitude, and altitude. In areas with high temperatures, less rainfall, and strong seasonal rainfall, the leaves are larger, longer and thicker. This study shows that variations in leaf traits of L. coreana var. sinensis mainly come from variations among populations. The level of trait differentiation among populations is high and the level of variation within populations low. These findings help further understand leaf morphological characteristics of this species and can provide a valuable reference for the protection and sustainable utilization of this natural resource.

Understanding what environmental factors are genetically linked to a phenological event is critical for predicting responses to climate change. Photosynthetic phenology often varies among a species of evergreen conifers due to local adaptation. However, few empirical studies have revealed relevant relationships between climatic factors in provenance environments and photosynthetic phenology. This study evaluated the effects of environmental conditions of the growing site and seed source provenance on the seasonal changes in maximal photochemical quantum yield of photosystem II (F _v/F _m) in a common garden experiment with 2-year-old seedlings of Sakhalin fir (Abies sachalinensis), a representative species with local adaptation, from four seed source provenances. A logistic model was constructed to explain the seasonal variation of F _v/F _m from July to October and the relationships between the estimated model parameters and representative factors featuring provenance environments were evaluated. The landscape gradient of the detected model parameters responsible for the provenance environments was visualized in a map of the distribution area. The lowest temperature was the most plausible factor in the growing environment to explain the seasonal changes of F _v/F _m. Among the representative meteorological factors of provenance environments, the lowest temperatures in July showed significant relationships with two model parameters, explaining the lower limit of F _v/F _m and the higher sensitivity of autumn F _v/F _m decline. The estimated spatial maps of model parameters consistently showed that the higher the lowest temperature in July in the provenance environment, the lower the F _v/F _m in October and the greater the decrease in the autumn F _v/F _m decline. Therefore, the lowest summer temperature could be associated with the local adaptation of autumn photosynthetic phenology in A. sachalinensis.