Seasonal changes in Cyclobalanopsis glauca transpiration and canopy stomatal conductance and their dependence on subterranean water and climatic factors in rocky karst terrain

摘要

The presence of forest on south China karst is presumed to increase perennial epikarst spring flow, partly because there is adequate storage in bedrock fractures underlying the shallow soil in the forest. If true, transpiration of the ecosystem would not be strongly reduced by temperate drought if trees develop deep roots to reach the perched epikarst water. Therefore, in karst ecosystem the epikarst-soil-plant-atmosphere continuum (ESPAC) would be different from the SPAC in non-karst system. We measured transpiration and canopy conductance from a Cyclobalanopsis glauca (syn. Quercus glauca) stand on a rocky hill slope in South China during 2006-2007 by using the Granier's sap-flow method. Annual stand transpiration (836mmy-1) accounted for 48.7% of the rainfall during the experimental year. Per month, daily stand transpiration (E_c) maximums varied between 2.1mmd~(-1) in January (cool season) to 5.1mmd~(-1) in July (hot season). In the driest months, September and October, E_c of C. glauca was still high with maximum E_c 3.82mmd~(-1) and 2.96mmd~(-1) respectively. Solar radiation (PAR), vapor pressure deficiency (VPD), and air temperature were simple influences on transpiration of C. glauca, which contributed to a quadratic power model, while soil water content (SWC) moisture influence on transpiration was complicated, which SWC influenced E_c greatly under higher VPD, but did not influence Ec under low VPD. High stomatal openness occurred in C. glauca in the early morning and declined throughout the day. The relation coefficient between canopy stomatal conductance (G_c) and E_c was high when VPD was more than 1.0kPa, moderate when 0.5kPa