| Canopy conductance（gc）can reflect stomatal activity at the ecosystem scale,and as an important biological factor,it directly affects the process of carbon and water exchange between plants and the atmosphere.Evapotranspiration（ET）is an important component of the water cycle.In the context of global change,variations in forest ET and its underlying mechanisms have received extensive attention.The gc is one of the most important biotic factors affecting evapotranspiration.Understanding the responses of gc to the environmental factors and conducting further studies on the effects of environmental and biotic factors on ET would help ecologists obtain further insight into carbon and water exchange processes and forecast future changes in ecosystems under global climate change.Therefore,we investigated the seasonal and interannual variations in the gc and ET based on 12 years of flux data that were observed by an eddy covariance system in a subtropical coniferous forest.Furthermore,the corresponding environmental factors and leaf area index（LAI）data and the correlation analysis,stepwise regression analysis and structural equation model（SEM）methods were used to explore the underlying mechanism of variations in gc and ET.Moreover,the effects of biological and environmental factors on gc and ET in different climatic years were analyzed.The main results are as follows:（1）Variations in gc and its specific responses to environmental factors.By teasing out the dominant factors step by step,the specific gc responses to environmental factors were clarified,and the divergent responses in contrasting climatic years were determined.The multiyear mean gc was 3.46±0.36 mm·s–1,and the gc peaked in April and declined to a minimum in October in most years.The vapor pressure deficit（VPD）was the most important factor that inhibited the gc,and the gc decreased sharply when the VPDs were greater than1.8 k Pa.Without high VPD stress,the gc increased linearly with air temperature（Ta）when the Ta was below 27°C,after which it decreased.When the high VPD and Ta stresses were excluded,the gc increased with the net radiation（Rn）following a logistic growth model.The gc increased logarithmically with the soil water content at 5 cm depth（SWC5）.The effects of the soil water content at 50 cm depth（SWC50）emerged under water stress or temperature stress conditions,which indicated that the forest utilized deep soil water to defend against environmental stress.（2）Divergent responses of the gc to environmental factors in different climatic years were found. In the dry year,gc decreased linearly with VPD,and VPD could explain 97%of the variations in gc.In cold years,gc decreased linearly with SWC50,and SWC50 could explain 98%of the variation in gc.The shallow soil water content SWC5 showed the least driving effect on the ecosystem gc during cold years.（3）The variations in ET and the effects of environmental and biological factors on ET.The multiyear average daily ET value was 2.02±1.23 mm.It peaked in July,and the minimum value appeared in winter.The net radiation（Rn）was the most important factor affecting ET.When Rn was greater than 20 MJ m-2,ET decreased significantly.The response of ET to VPD and Ta also showed a trend of increasing and then decreasing,with thresholds of 1.9 k Pa and 30°C,respectively.ET changed with canopy wind speed（wind speed,WS） following a quadratic function.When WS was less than 3 m s-1,ET gradually increased with increasing WS.When WS was greater than 3 m s-1,ET decreased as WS increased.ET decreased linearly with precipitation（PPT）.Rn,Ta and VPD drove ET significantly on the daily scale,but the effects of the three factors on ET varied on different time scales.Rn showed the strongest effects on ET at the daily and annual scales（r2=0.76;r2=0.31）;VPD had the strongest effects on ET at the monthly scale（r2=0.17）.The responses of ET to LAI were different in different climatic years.When temperature or water stress occurred,the effects of LAI on ET weakened.（4）The effects of environmental and biotic factors on ET analyzed by SEM in different climatic years.According to the analysis of the SEM,in cold years,the total influence coefficients of environmental factors on ET were in the order of Rn>VPD>Ta>SWC20.In dry years,the total influence factors of environmental factors on ET were in the order of Rn>gc>VPD （negative impact）,SWC50>Ta,LAI>WS.On average,the order of influence of environmental factors on ET was Rn>VPD>SWC20>Ta>LAI>gc,which indicated that Rn was the main driver of ET.However,in different climatic years,Rn would be coordinated with different environmental factors or biological factors to regulate ET.On average,gc had a weak positive drive on ET,a small impact on ecosystem ET in cold years,and a certain positive effect on ecosystem ET in dry years. |
