Impact of environmental factors on the decoupling coefficient and the estimation of canopy stomatal conductance for ever-green broad-leaved tree species

Abstract

Aims Accurate simulation of canopy stomatal conductance (G S ) 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 G S with two different methods of Köstner (G S1 ) and in-versed Penman-Monteith equation (G S2 ). The effect of environmental factors on canopy decoupling coefficient (Ω) was evaluated before they were adopted to assess the reasonability of G S simulated by the two methods. Important findings Results showed that the G S 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. Multivariate correlation analysis also found weak relations between those environmental factors and Ω. Boundary line analysis revealed that the increase 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 decreases exponentially with the increase of u. Compared with S. superba, E. urophylla has higher G S. The annual averages G S2 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 G S2 /G S1 ratio of E. urophylla and S. superba was 0.92 (R 2 ≈ 0.70) and 0.98 (R 2 ≈ 0.76), respectively, implying the overestimated canopy stomatal conductance for G S1 (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 (G Siref ) for G S1 and G S2 is closely related to Ω. Based on the estimation, G S1 was relatively reliable when Ω = 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.