Seasonal variation in stem sap flow and its driving mechanisms across different leaf phenophases in Hevea brasiliensis

doi:10.17521/cjpe.2025.0120

Abstract

Abstract:

Aims To clarify how water use in rubber (Hevea brasiliensis) plantations varies across leaf phenophases and to identify the key environmental drivers of stem sap flow dynamics, thereby improving the assessment of impacts on regional water cycling and informing water-resource management.

Methods We continuously monitored stem sap-flux density (F_d) with the thermal dissipation probe (TDP) method across four phenophases—defoliation, early leaf-expansion, mid leaf-expansion, and late leaf-expansion. Environmental variables (soil and atmospheric temperature and humidity, etc.) were recorded synchronously. We then applied XGBoost coupled with the SHAP interpretability framework to quantify the relative contributions of candidate drivers.

Important findings (1) Phenology strongly modulates water use: F_d and tree-level water use differed significantly among phenophases, in the order of late leaf-expansion > mid leaf-expansion > defoliation > early leaf-expansion; the daily mean F_d in late leaf-expansion was 4.4-6.1 times that of the leafless periods (defoliation and early leaf-expansion), indicating a significant increase in water consumption. (2) The dominant environmental drivers showed dynamic shifts with phenology. During defoliation, air temperature (T_a; 32.39%) and volumetric soil water content (VWC₅; 24.38%) at 5 cm depth were the primary drivers. In the early leaf-expansion, soil temperature (T_s; 36.61%) and photosynthetically active radiation (PAR; 19.55%) prevailed. During mid leafexpansion, vapor pressure deficit (VPD; 52.16%) become dominant. In late leaf-expansion, T_a re-appeared as the key driver (62.75%). Mechanistically, leaf phenology modifies canopy leaf area (and thus canopy conductance), reshaping the soil—plant—atmosphere water-potential gradient, so that limitation of water use shifts from soil water availability and temperature toward atmospheric evaporative demand; this shift amplifies phenophase-dependent differences in sap flux and water consumption. These findings delineate the phenology-dependent dynamics and driver transitions of sap flow in H. brasiliensis, and provide an evidence base for water resource management and strategy development in tropical plantations.

Key words: Hevea brasiliensis, phenophase, machine learning model, environmental factor, water use

GUO Xin-Wei, WANG Guan-Ze, SUN Rui, XU Wen-Xian, WU Zhi-Xiang. Seasonal variation in stem sap flow and its driving mechanisms across different leaf phenophases in Hevea brasiliensis[J]. Chin J Plant Ecol, 2026, 50(6): 1436-1447.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2025.0120

https://www.plant-ecology.com/EN/Y2026/V50/I6/1436

Figures/Tables 9

Table 1 Major parameters of Hevea brasiliensis in Danzhou experimental site, Hainan

编号 No. of sample trees	树高 Tree height (m)	胸径 Diameter at breast height (cm)	边材面积 Sapwood area (cm²)
R1	14.15	22.15	284.94
R2	14.00	24.05	302.78
R3	13.50	20.99	254.90
R4	15.31	27.39	382.77

Table 2 Onset and cessation timing of different leaf phenophases of Hevea brasiliensis in Danzhou experimental site, Hainan

叶片物候期 Leaf phenological stages	开始时间 Onset time	结束时间 Cessation time	持续天数 Duration days
落叶期 Defoliation	2018-02-05	2018-03-01	25
展叶前期 Early leaf-expansion	2018-03-02	2018-03-13	12
展叶中期 Mid leaf-expansion	2018-03-14	2018-03-31	17
展叶后期 Late leaf-expansion	2018-04-01	2018-04-14	14

Fig. 1 Daily variation of environmental factors in the sample plot of the rubber plantation at Danzhou, Hainan. PAR, photosynthetically active radiation.

Fig. 2 Daily variation of rubber tree sap flux density at different leaf phenophases (mean ± SD). A, Defoliation. B, Early leaf-expansion. C, Mid leaf-expansion. D, Late leaf-expansion.

Fig. 3 Daily variation (mean ± SD) (A) and comparative analysis of sap flux density (B) in rubber tree stems during different leaf phenophases. Different lowercase letters indicated significant differences in stem sap flux density between leaf phenophases (p < 0.05).

Table 3 Daily water use (Qtree) of individual rubber trees in different leaf phenophases (mean ± SD)

叶片物候期 Leaf phenological stages	单日水分利用量 Daily water use amount (kg)
叶片物候期 Leaf phenological stages	R1	R2	R3	R4
落叶期 Defoliation	4.72 ± 1.20^C	4.17 ± 1.22^C	3.95 ± 1.33^C	-
展叶前期 Early leaf-expansion	3.86 ± 0.65^C	3.91 ± 1.13^C	1.84 ± 0.39^D	5.01 ± 1.21^C
展叶中期 Mid leaf-expansion	7.10 ± 0.93^B	20.24 ± 3.28^B	12.09 ± 1.64^B	14.01 ± 2.76^B
展叶后期 Late leaf-expansion	11.58 ± 4.09^A	31.33 ± 4.73^A	17.33 ± 2.85^A	22.00 ± 3.16^A

Fig. 4 Fitting curves of predicted and observed sap flux density values of rubber trees across different leaf phenophases based on the XGBoost model. A, Defoliation. B, Early leaf-expansion. C, Mid leaf-expansion. D, Late leaf-expansion. MSE, mean squared Error; RMSE, root mean squared error; MAE, mean absolute error.

Fig. 5 SHAP summary figures of rubber plantations during different leaf phenophases. A, Defoliation. B, Early leaf-expansion. C, Mid leaf-expansion. D, Late leaf-expansion. In this figure, SHAP values > 0 indicate that the corresponding feature increases the predicted value of instantaneous liquid flow density (Fd) output by the model. Green and purple dots represent high and low feature values, respectively. Features are ordered vertically by their overall contribution to the model output. PAR, photosynthetically active radiation (μmol·m-2·s-1); RH, relative air humidity (%); Ta, air temperature (℃); Ts, soil temperature (℃); VPD, vapor pressure deficit (kPa); VWC5 and VWC50, volumetric water content (%) at soil depths of 5 cm and 50 cm; WS, wind speed (m·s-1).

Fig. 6 SHAP feature importance bar plots for stem sap flow in rubber trees across leaf phenophases. A, Defoliation. B, Early leaf-expansion. C, Mid leaf-expansion. D, Late leaf-expansion. PAR, photosynthetically active radiation (μmol·m-2·s-1); RH, relative air humidity (%); Ta, air temperature (℃); Ts, soil temperature (℃); VPD, vapor pressure deficit (kPa); VWC5 and VWC50, volumetric water content (%) at soil depths of 5 cm and 50 cm; WS, wind speed (m·s-1).

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