Water use strategies of a young Eucalyptus urophylla forest in response to seasonal change of climatic factors in South China

Abstract. To depict the wet (April with a soil water content, SWC, of 37 %) and dry (October with a SWC of 24.8 %) seasonal changes in the water use and physiological response of a Eucalyptus urophylla plantation in subtropical South China characterized by monsoon climate, the whole-year (June 2012 to May 2013) transpiration of E. urophylla was monitored using the TDP method. Daily transpiration (E_T) in October averaged 5.7 ± 2.9 kg d⁻¹ and was 58.0 % higher than that in April (3.6 ± 2.3 kg d⁻¹). The difference is consistent with that of the radiation and evaporative demand of the two months, while the nocturnal transpiration (E_T-NOC) in the wet season (0.18 ± 0.021 kg d⁻¹) was almost twice that in the dry season (0.11 ± 0.01 kg d⁻¹). Trees displayed a higher stomatal conductance (G_S) (53.4–144.5 mmol m⁻² s⁻¹) in the wet season and a lower G_S (45.7–89.5 mmol m⁻² s⁻¹) in the dry season. The leaf-soil water potentials (Ψ_L) of the two months (April and October) were −0.62 ± 0.66 and −1.22 ± 0.10 MPa, respectively. A boundary line analysis demonstrated that the slight improvement in the G_S by SWC in wet season was offset by a significant decrease in D, and the slope of G_S sensitivity to D (dG_S/dlnD) in response to G_Sref (references G_S at D = 1 kPa) was affected by the variance of radiation instead of SWC. Specific hydraulic conductivity (k_s) of trees of different sizes decreased by 45.3–65.6 % from the wet to the dry season. Combining the decreased maximum reference G_S at D = 1 kPa (G_Sref-max) by 22.4 % with the constant max G_S (G_Smax) when Ψ_L < −1.2 MPa, we shed some light on the mechanism underlying the high water-use efficiency (WUE) of this Eucalyptus specie. With a slight change in G_Sref-max and high sensitivity of k_s to decreasing Ψ_L, large trees used water more efficiently than small ones did. In addition, the −m in the dry season (0.53 ± 0.007) was lower than that in the wet season (0.58 ± 0.01) due to the difference in the ratio of G_S to the boundary layer conductance (g_b) in the two months. The negative relationship between −m (except when light is limited) and Q proved to be a plastic response to environmental changes for E. urophylla but did not change with decreased k_s as expected.