Abstract: Canopy stomatal conductance (G_s) is an important parameter quantifying the stomatal performance at canopy level, being able to characterize the exchange dynamics of vapor and energy of forest canopy. In this paper, the sap flow in a Schima superba stand in southern China was continuously monitored with Granier’s thermal dissipation probes, and the canopy transpiration of the stand was obtained through data upscaling methodology. In combining with the microclimatic observations, the G_s was estimated by using PenmanMonteith equation, and the responses of the G_s to vapor pressure deficit under different soil water conditions (wet season vs. dry season) were compared. The estimated G_s based on the sap flow measurement was in the same order of magnitude as that of the leaf stomatal conductance (g_s) determined by gaseous exchange method, and similar patterns of the diurnal variations of both G_s and g_s were observed. The responses of the G_s to vapor pressure deficit in wet and dry seasons had obvious difference. In wet season (soil water content θ> 33%), the G_s was more sensitive to vapor pressure deficit (partial coefficient -0.316); in dry season (θ< 23%), the G_s was more sensitive to photosynthetically active radiation (PAR) (partial coefficient 0.885). The decoupling coefficient (Ω) of stand canopyatmosphere was close to 1 (maximum value) in wet season but substantially smaller in dry season, indicating that vapor pressure deficit had little effects on G_s in wet season probably due to the thicker boundary layers on leaves, while PAR was the main environmental factor controlling G_s.

Key words: temporal and spatial distribution, environmental factors, Portunus trituberculatus, biological characteristics, Laizhou Bay.