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

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 (836 mm y−1) accounted for 48.7% of the rainfall during the experimental year. Per month, daily stand transpiration (Ec) maximums varied between 2.1 mm d−1 in January (cool season) to 5.1 mm d−1 in July (hot season). In the driest months, September and October, Ec of C. glauca was still high with maximum Ec 3.82 mm d−1 and 2.96 mm d−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 Ec 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 (Gc) and Ec was high when VPD was more than 1.0 kPa, moderate when 0.5 kPa < VPD < 1.0 kPa, and low with VPD of less than 0.5 kPa. Under high VPD, stomatal control of transpiration is high. The pattern of seasonal change of transpiration and canopy stomatal conductance of the plant in karst regions is different from that in non-karst regions, with the stand transpiration and canopy stomatal conductance being high even during the dry season in the karst region because karst plants obtain water partially from the epikarst. With high stand transpiration and canopy stomatal conductance, karst vegetation is presumed to be the most important “pathway” for ESPAC.