Background and Aims Canopy stomatal conductance is considered as a sensitive parameter of forest ecosystem in responding to environmental changes and can be used to monitor the impacts of water stress, air pollution, trace gas and other environmental factors on forest function. Sap flow of an Acacia mangium forest was measured to scale up stand transpiration and canopy stomatal conductance. It aimed at understanding water use of A. mangium forest in different environmental water conditions and its responses to soil moisture change at an ecosystem level.
Methods The Granier's thermal dissipation probes were applied to monitor sap flow of 14 sample trees in an Acacia mangium forest in hilly land of South China. The sap flow data were used to calculated whole-tree and stand transpiration, and to estimate mean canopy stomatal conductance (G_c) by combing with synchronous measurement of environmental factors.
Key Results Sap flux density (J_s) and whole-tree transpiration (E_t) were obviously affected by tree morphological features. Even though there were less individual trees with large diameter at breast height (DBH) in the sample plot, they comprised relatively larger proportions of total sapwood area and transpiration of the forest. Daily variations of J_s and E_t were mainly controlled by photosynthetically active radiation (Q_o) and vapor pressure deficit (D). Soil moisture (θ) had greater effects on J_s and E_t of trees with larger DBH than on those trees with smaller DBH. The differences of J_s and E_t across individuals reduced with decreasing θ. The highest stand transpiration (E) occurred in July when the radiation and hydrothermal condition were favorable, whereas the reduced water supply in soil during the time period of September-November resulted in lower E value and its sensitivity to D. G_c responded to major environmental factors in a similar way with E. Long-term lower θ would significantly decreased G_c, which would cause reduction in E.
Conclusions Mature A. mangium forest can maintain a vigorous transpiration under sufficient radiation and hydrothermal conditions, but could not stand well under long-term stress of soil moisture. Granier's probes have the potential to accurately estimate stand transpiration, canopy conductance and their responses to environmental factor, extending the temporal and spatial scales in studying the ecological process of forest ecosystem.