Response And Adaptation Of Leaf Ecophysiological Characteristics Of Larix Gmelinii To Changing Environments

The effect of environmental changes on tree survival, growth and development is mainly determined by magnitude of the changes and adaptation capability of tree ecophysiology. The natural distribution of forest trees reflects the combination of phenotypic acclimation and genotypic adaptation to the external environment. Partitioning these two components is critical to understanding the mechanisms underlying tree adaptation to environemtal changes, and provides theoretical framework for predicting vegetation distribution under the global change scenarios. Comducting such partitioning in situ and underlying drivers is still challenging. Dahurian Larch (Larix gmelinii) is one of the dominant tree species in Eurasian boreal forests with a broad bio-geographic range under divergent habitats, and thus an ideal tree species for exploring tree adaptation to environmental changes. Leaf is a key functional organ closely associated with tree carbon metabolism, water relations, growth and development, and also sensive to environmental changes. In this study, I measured the leaf phenology, photosynthesis, carbon use efficiency (CUEF), water use efficiency (WUEF), elemental stoichiometry of 30-year-old Dahurian larch trees from six provenences in a common garden at the Maoershan Forest Ecosystem Research Station (45°24’N,127°40’E) for three years (2009-2011). The six provenances were located across the natural distribution range of the larch, spanning～4° in latitude (48-52°N) and～5 °C with a gradient of mean annual temperature from -2.3 °C to 2.6 °C and mean annual precipitation from 425 mm to 622 mm. The differences in temperature (AT) and aridity index (AAI) between current sites and original sites of the seed-sources of the provenances were used as indeces of environmental change gradients (i.e., six provanences were represented as six levels of environmental changes). My goal was to explore impacts of environmental changes on leaf phenology and ecophysiological characteristics, and stoichiometry of the larch and their driving factors. The main results were summarized as follows:1. The leaf phenology did not differ significantly among the six levels of environmental changes. There were no significant differences in bud bursting date, leaf unfolding date, leaf shedding date, and growing season length among the six levels. However, the leaf phenology differed significantly across the three consecutive measuring years for each level (P< 0.05). The starting dates of bud bursting and leaf unfolding were both negatively correlated with the mean spring temperature and the ccumulative temperature above 0 °C before bud bursting or leaf unfolding in spring. The starting date of leaf shedding was positively correlated with the mean annual or autumn temperature. These indicate that the leaf phenology displays a phenotypic thermal acclimation to the environmental changes.2. The maximum net photosynthetic rate (Pmax), dark respiratory rate (Rd), apparent quantum yield (AQY), maximum rate of carboxylation (Vmax), maximum electron transport rate (Jmax), triose phosphate utilization rate (TPU), and mesophyll conductance (gm) all increased significantly (P< 0.05) with increasing AAI, while the light compensation point (LCP) showed an opposite trend. This suggests that the leaf photosynthetic characteristics genetically adapted to the original site conditions of the tree seed-sources, among which the AI is the major driving factor. Pmax was positively correlated with AQY, Vmax, Jmax, TPU, gm, leaf nitrogen concentration (NL), leaf phosphorus concentration (PL), leaf mass per area (LMA) and leaf Chlorophyll (a+b) (Chl(a+b)), but negatively with LCP (P<0.05). However, the trees with higher ΔAI had higher sensitivity of Pmax associated with other characteristics.3. The photosynthetic capacity had different seasonally among the six levels of environmental changes. The photosynthetic capacity with higher ΔAI was lower in early growing season, and increased more rapidly in the middle growing season, and maintained higher in late growing season. Across the whole growing season, Pmax, AQY, Vmax, Jmax, TPU, and gm were positively correlated with the mean seasonal temperature, while LCP and Rd were negatively correlated (P< 0.05). However, trees with higher AAI were more sensitive to temperature changes. These results illustrate that leaf photosynthetic capacity is jointly affected by phenotypic acclimation and genotypic adaptation to the environmental changes.4. The CUEF decreased significantly (P< 0.05) with AAI increasing, while the WUEF increased significantly. CUEf was positively correlated with NL, PL, LMA, and Chl (a+b), but the trees with higher AAI had higher increase rates of CUEF.WUEF increased exponentially with stomatal conductance (Gs):increased rapidly when the Gs was less than 0.2, and was relatively stable when the Gs was greater than 0.2. WUEf was positively correlated with NL and LMA. The trees with higher AAI had greater increase rates of WUEF with increasing NL and LMA.5. Leaf carbon concentration (CL),NL, PL, carbon to nitrogen ratio (C/N), nitrogen to phosphorus ratio (N/P), and carbon to phosphorus ratio (C/P) all differed significantly among the six levels of environmental changes (P< 0.05). CL, NL, C/P, and N/P all increased with AAI decreasing, while PL and C/N showed a decreasing trend. CL was negatively correlated with PL and NL. However, CL with higher ΔAI was more sensitive to PL and NL increasing.The results above suggest that the responses of leaf eco-physiological characteristics to changing environment were jointly controlled by both phenotypic acclimation to current site conditions and genotypic adaptation to the original environment of the seed-source. Phenotypic acclimation was the dominant component of the responese of leaf phenology to environmental changes, while combinaton of genotypic adaptation to the climate of original site and phenotypic acclimation to the current site conditions contributed to the variability in leaf photosynthesis, carbon and water use efficiencies, and elemental stoichiometry. These findings provide insights on understanding the survival, reproduction and distribution of Dahurian larch to variable environments.