Impact of environmental factors on the decoupling coefficient and the estimation of canopy stomatal conductance for ever-green broad-leaved tree species
(2.South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China 510650)
【Abstract】 Aims Accurate estimation of canopy stomatal conductance (GS) is quite important for the assessment of regional evapotranspiration. Methods In this study, two planted broad-leaved tree species, Eucalyptus urophylla (exotic species) and Schima superba (native species), were chosen to estimate their GS with two different methods of Köstner (GS1) and inversed Penman-Monteith equation (GS2). The effect of environmental factors on canopy decoupling coefficient (Ω) was evaluated before they were adopted to assess the reasonability of GS estimated by the two methods. Important findings Results showed that the GS of the two tree species was well coupled with meteorological conditions (Ω = 0.10 ± 0.03 for E. urophylla and 0.17 ± 0.03 for S. superba). Principal component analysis showed that photosynthetically active radiation (PAR) and vapor pressure deficit (D) significantly dominated the variations of Ω, while the effect of wind speed (u) was very weak. Correlation analysis showed weak correlation between those environmental factors and Ω. Boundary line analysis revealed that the increases of D and PAR would eventually force Ω approaching a constant value as determined by tree species (S. superba ≈ 0.20, E. urophylla ≈ 0.05), while Ω decreased exponentially with the increase of u. Compared with S. superba, E. urophylla had higher GS. The annual average values of GS2 of E. urophylla and S. superba were (33.42 ± 9.37) mmol·m−2·s−1 and (23.40 ± 2.03) mmol·m−2·s−1, respectively. Linear fitting showed that the GS2/GS1 ratios of E. urophylla and S. superba were 0.92 (R2 ≈ 0.70) and 0.98 (R2 ≈ 0.76), respectively, implying the overestimated canopy stomata conductance for GS1 (p < 0.01). In addition, the ratio of the sensitivity of canopy stomatal conductance to vapor pressure deficit to stomatal conductance at D = 1 kPa (GSiref) for GS1 and GS2 was closely related to Ω. Based on the estimation, the GS1 was relatively reliable when the Ω was 0.05–0.15 (83.1% of all the data) and 0.10–0.20 (47.8% of all the data) for E. urophylla and S. superba, respectively.
【Keywords】 plant transpiration; canopy stomatal conductance; decoupling coefficient; environmental factor;
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