Stem Respiration And Leaf Photosynthetic Capacity Of Larix Gmelinii And Their Responses To Climate Warming

Since the industrial revolution,atmospheric concentrations of CO₂ and other greenhouse gases have been rising,and global warming is occurring at an unprecedented rate,with a potential annual average temperature increase of 1.1-6.4°C by this century.Compared to plant leaf respiration,stem respiration is difficult to quantify precisely,and relatively little is known about it.There are still great uncertainties in the effects of climate warming on stem respiration,leaf photosynthesis and other carbon metabolic processes.An in-depth understanding of the effects of climate change on stem respiration and leaf photosynthetic capacity and their regulatory mechanisms is important for exploring the effects of climate warming on the carbon balance of trees and for constructing models of tree growth and forest ecosystem carbon budget.In this study,we simulated climate warming by transplanting four different latitudes of Larix gmelinii to a common garden at lower latitude,and analyzed the main regulatory factors affecting the changes of stem CO₂ efflux to explore the effects of climate warming on stem CO₂ efflux and temperature sensitivity of Larix gmelinii.The intra-species variability of stem CO₂ efflux and its temperature sensitivity,stem phloem non-structural carbohydrate（NSC）content and its components（soluble sugars and starch）and leaf photosynthetic capacity and their response to climate warming were analyzed to explore the response mechanism of stem CO₂ efflux and its temperature sensitivity to climate change,and the link between leaf photosynthetic capacity and stem CO₂ efflux response to climate warming,so as to provide data and theoretical bases for carbon balance models of forest ecosystems.The main results are as follows:（1）Climate warming significantly increased A_max（23.42%-35.15%）,while V_cmax and J_maxincreased significantly,indicating that warming-induced carboxylation and photosynthetic electron transfer rate are responsible for the up-regulation of A_max.The climate warming effect CWE)was correlated with the magnitude of warming and increased with temperature,increasing with temperature,photosynthetic capacity increased,and leaf carbon assimilation accumulated more photosynthetic products.Significant intraspecific differences in photosynthetic capacity were found in both common garden and original sites,demonstrating the local adaptation of plants to the original climate.There was a significant linear positive correlation between stem CO₂ efflux and A_max,and the slope of the regression equation between stem CO₂ efflux and A_max was greater than 1,indicating that stem CO₂ efflux was more sensitive to climate warming under the same magnitude of climate warming.Increased temperature increased stem respiration,accelerated the consumption of photosynthetic products during transport,and reduced the content of photosynthetic products in the phloem.（2）The daily variation of stem CO₂ efflux of larch at the four latitude sites was similarly to the daily variation of stem temperature with single-peaked curves,but there was a time lag of 0-1 h（original site）and 0-4 h（common garden）between the peak of stem CO₂ efflux and stem temperature.The daily variation of stem CO₂ efflux of larch decreased with increasing latitude,and climate warming also decreased the daily variation of stem CO₂ efflux.The significant decrease of Q₁₀ with the stem temperature increased indicated the thermal acclimation of stem respiration to the increase of temperature after long-term climate warming,because thermal acclimation of respiration leads to a decrease of temperature sensitivity of respiration.There was a significant positive correlation between stem CO₂ efflux and DBH,and a significant negative correlation between Q₁₀and DBH.This indicates that DBH can be one of the parameters for estimating the stem CO₂ efflux and temperature sensitivity of larch.There were significant intraspecific differences between E_S and DBH in common garden and original sites,showing local adaptation of plants to the original climate.（3）Stem temperature was one of the dominant environmental factors driving stem CO₂efflux variation at four latitudinal sites of larch during the growing and non-growing seasons,but air humidity also influenced stem CO₂ efflux dynamics to varying degrees.The stem CO₂efflux and its Q₁₀ values of larch differed significantly among the four latitudes during the growing and non-growing seasons,so the use of a single Q₁₀ value as a model parameter in the model may lead to an underestimation or overestimation of stem CO₂ efflux,and future studies,need to explore more deeply the accurate response of stem CO₂ efflux to the dynamic changes of each biological and environmental factor in different seasons.（4）Soluble sugars content of stem phloem varied significantly（P<0.05）among original sites,increasing with the latitude of the site,and starch content decreased（P<0.05）with the latitude of the site.After transplanted to common garden,climate warming decreased soluble sugar content,significantly increased starch content,and significantly increased total non-structural carbohydrates（NSC）.There was a relationship between temperature sensitivity(Q₁₀)and NSC content of stem CO₂ efflux in larch,indicating that substrate concentration is also an important factor in regulating stem CO₂ efflux.In summary,temperature,air humidity,non-structural carbohydrate content and stem diameter are all regulators of stem CO₂ efflux,so a reasonable upward extrapolation scheme should be used in estimating stem CO₂ efflux and their scale upward extrapolation,and corresponding prediction models should be established according to different tree species,so as to improve the estimation accuracy of the models.There were significant intra-specific differences in stem CO₂ efflux and photosynthetic capacity of Larix gmelinii,and such differences did not disappear when transplanted to the same environment,demonstrating the local adaptation of plants to the original climate.Compared with photosynthetic rate,stem CO₂efflux is more sensitive to climate warming under the same warming amplitude.These findings provide some physiological mechanisms for the response and acclimation of tree photosynthesis and stem respiration to temperature changes,which are important for the simulation and prediction of tree growth and forest carbon cycle under climate warming.