| The stems not only account for the largest storages of the biomass in forest ecosystems and important economic output for forest managers,but also represent a long-term and huge global carbon store for ecologists.However,Respiration from all the living cells of the stem tissue takes place continuously throughout the whole year,and results in substantial amounts of photosynthetically fixed CO2 being released back to the atmosphere.Stem CO2 efflux(Es),the diffusion of CO2 from tree stem surface into the atmosphere,therefore plays a key role in controlling the productivity and annual carbon balance of forest ecosystems and regulating the atmospheric CO2 concentration.The temperate forest in northeastern China has a significant effect on maintainence of carbon balance of terrestrial forest ecosystems because of its high latitude,big area,great carbon storage,and sensitive responses to climate change.However,there are still many uncertainties in forest carbon budget estimate in this area,one of which is lacking of systemic quantification of woody tissue respiration.Up to date,the temporal(diurnal,seasonal,and interannual)variations in ES and related drivers and stand-level ES have not been well quantified for the Chinese temperate forests.The diurnal dynamics in ES was measured in situ for four major coniferous tree species coexisting in the temperate forests in northeastern China from October to November 2011 and April to May 2011,respectively.The seasonal dynamics of ES was also investigated on the same tree species from May 2012 to July 2014.The tree species included three evergreen(Korean pine[Pinus koraiensis],Korean spruce[Picea koraiensis],and Mongolian pine[Pinus sylvesris var.mongolica])and one deciduous(Dahurian larch[Larix gmelinii])tree species.Ten to twelve replicate trees with various diameters at breast height(DBH)were randomly sampled for each tree species.The objectives of this study were to:(1)examine the temporal variations in Es and their relationships to related biological and environmental factors,and(2)quantify stand-level ES with various methods in attempt to establish a more accurate experimental protocol estimating stand-level ES.These results will provide solid data for developing and validating the carbon cycling model for the temperate forests,and be important for mechanistically understanding the forest carbon cycling.The major results were shown below:(1)The diurnal variation in ES largely followed the change in stem temperature(Ts)for all the tree species in both autumn 2011 and spring 2012 with a "unimodal" or "bimodal" pattern,but the daily maximum value,occurring time and amplitude of ES varied with tree species and seasons.TS explained more than 50%variations in ES for all the species except for Mongolian pine in spring,which suggested that TS be a key environmental factor regulating the ES.However,ES lagged behind TS by 1.5 h(3 h for Mongolian pine in spring).Normalized ES at 10?(ES10)was significantly lower in autumn than that in spring for each species,ranging from 0.54 ?mol CO2 m-2 s-1 for Dahurian larch to 0.78 ?mol CO2 m-2 s-1 for Korean spruce in autumn and from 0.87 ?mol CO2 m-2 s-1 for Korean pine to 1.10 ?mol CO2 m-2 s-1 for Mongolian pine in spring,respectively.The mean ES10 was about 40%higher in spring than in autumn.There was no significant difference in temperature sensitivity(Q10)of ES between autumn and spring for each species(P>0.05),ranging between 1.52 for Mongolian pine to 1.82 for Korean spruce.The Es10 was significantly positively correlated with DBH for all the tree species in both seasons,while the Q10 was negatively with DBH(all P<0.05).This suggested that DBH can be used as a proxy for predicting the Es for these temperate coniferous tree species.(2)The seasonal variation in ES for the four tree species overall followed the change in Ts from July 2013 to July 2014,with the maxima occurring in the summer months(late May to early July)characterized by higher temperature and rapid stem growth and the minima in spring(late March to April)or autumn(October)having lower temperature.TS accounted for 42%-91%and 56%-89%of variations in ES during the growing(May to September)and non-growing(other months)seasons,respectively.Furthermore,apart from Ts,we also found significant regression relationships between Es and daily stem circumference increment,relative air humidity,and sapwood nitrogen concentration during the growing season,but their forms and correlation degree were species-dependent.These results indicated that Ts was the dominant environmental factor affecting seasonal variations in ES,but the magnitude of the effect varied with tree species and growth rhythm.Mean Es for each of the four tree species was significantly higher in the growing season than in the non-growing season,whereas within the season there were also significant differences in mean ES among the tree species(all P<0.05).The Q10 of Es did not differ significantly among the tree species during the growing season,ranging from 1.64 for Dahurian larch to 2.09 for Mongolian pine,but did differ during the non-growing season which varied from 1.80 for Korean pine to 3.14 for Dahurian larch.Moreover,Korean spruce,Mongolian pine and Dahurian larch had significantly greater Q10 values in the non-growing season than in the growing season(P<0.05).These findings suggested that the differences of the response of ES to temperature change for different tree species were mainly from the non-growing season.Because the seasonality and inter-specific variability in Es for these temperate coniferous tree species were primarily collectively controlled by multiple factors such as temperature,we conclude that using a single annual temperature response curve to estimate the annual ES may lead to more potential uncertainty.(3)The estimated annual total ES(ET-chamber)at chamber level was 196.8,365.9,234.6,and 376.6 g C m-2 stem surface area yr-1 for Korean pine,Korean pine,Mongolian pine,and Dahurian larch,respectively,from July 2013 to June 2014.The annual maintenance respiration(EM-chamber)calculated for the three evergreen coniferous tree species averaged 79%of the total respiration during the whole year.However,for the deciduous tree species Dahurian larch,the year round contribution of EM-chamber to ET-chamber was approximately equal to that of the annual growth respiration to ET-chamber.This indicated that the coniferous tree species with contrasting leaf phenology may have divergent growth strategy and carbon allocation patterns to support growth and respiration.The ET-chamber and its components for all the studied tree species were almost positively correlated with annual stem circumference increment,sapwood volume(excluding Mongolian pine),and DBH.Moreover,the comprehensive models containing annual stem circumference increment and DBH significantly improved the accuracy of estimation in ET-chanmber and its components for Dahurian larch.This result suggested that annual stem circumference increment,sapwood volume,and DBH were not only the major determinants of intra-and inter-species variations in ET-chamber and its components,but also could be used as powerful proxies for modeling and predicting ET-chamber and its components.However,the functional forms and their coefficients of regression of ET-chamber and its components and the three predictive variables varied with tree species,suggesting that the models of species-specific ES estimation should be developed to reduce the uncertainty and improve model predictions of forest carbon cycling models in the local area and the region.(4)Applying the calculation schemas that take the seasonal respiratory characteristics(i.e.ES10 and Q10 value)and appropriate up-scaling unit would effectively improve accuracy of stand-level Es(ET-stand)estimation without considering the inter-individual variations and with the uniform calculation method in respiratory characteristics.The ET-stand estimated with seasonal(growing and non-growing season)respiratory features averaged 10%higher than that estimated using annual respiratory characteristics.The ET-stand was generally lower when calculated on a stem volume basis compared to a stem surface area or sapwood volume basis,indicating that using the stem volume as an up-scaling unit would bring about underestimation in ET-stand.According to the above calculation schemas,the estimated ET-stand based on stem surface area and sapwood volume averaged 165 and 166 g C m-2 ground area yr-1 for Korean pine,respectively,and 183 and 283 g C m-2 ground area yr-1 for Dahurian larch;the former were 10%and 41%lower than the latter,respectively.The differences in ET-stand among the three years were obvious,but the magnitude of the difference depended on up-scaling unit used to estimate ET-stand.The other tree species contributed 45%to ET-stand for the Korean pine stand,suggesting that in some cases it is necessary to carefully consider the way of dealing with other tree species in the stand when estimating the annual stand-level flux.Some potential sources of error in ET-stand estimation were also discussed.This study emphasizes the importance of taking the temporal dynamic in ES and the reasonable up-scaling protocol into account in improving the accuracy of estimation of stand-level Es. |
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