Specific responses of canopy conductance to environmental factors in a coniferous plantation in subtropical China

Understanding the responses of canopy conductance (gc) to environmental factors would help ecologists to obtain further insight into the 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 based on 12 years of flux data that were observed by an eddy covariance system in a subtropical coniferous forest. Furthermore, 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 than 1.8 kPa. 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. Additionally, divergent responses of the gc to environmental factors in different climatic years were found. The results of this study provide detailed information on the gc variations and their specific responses to the environmental factors, which would improve the understanding of gc and help accurately estimate gc.