Canopy transpiration of two black locust (Robinia pseudoacacia) plantations with different ages in semi-arid Loess Plateau, China

Abstract:

Black locust (Robinia pseudoacacia) was widely planted to control soil erosion and restore degraded ecosystem in Loess Plateau. The water use of the plantations was concerned due to its potential effects on hydrological cycle and regional water resource. Although some studies estimated canopy transpiration (E_c) of the mature black locust plantation, variation in E_c in plantations with different ages was not clear. In this study, we selected two plantations with different ages (12 years and 27 years, denoted as young stand and mature stand, respectively) in similar topographical conditions in Yangjuangou catchment in the central of Loess Plateau. Sap flux density (F_d) and tree biometrics were measured in each stand during the growing season in 2014. Soil water content (SWC) in each plot and meteorological variables in the catchment were simultaneously monitored. Tree transpiration (E_t) was derived from F_d and tree sapwood area (A_s). Canopy transpiration (E_c) was estimated by a product of mean stand sap flux density (J_s) and stand total sapwood area (A_ST). The mean F_d of mature trees was 2-fold larger than that of young trees.However, tree-to-tree variation in F_d among sampled trees within mature stand was evident compared to that within young stand. Mean E_t in mature stand was higher than that in young stand. E_c in mature stand was significant higher than that in young stand,with cumulative value of 54 mm and 27 mm respectively. This is attributed to higher J_s in mature stand although A_ST in young is slightly higher than that in mature stand. The patterns of daily E_c during the growing season were similar in both stands during the study period. A exponential saturation model can explain the responses of E_c to vapor deficit pressure (VPD) and solar radiation (R_s) in both stands.The relationship between E_c and SWC was not detected. Our finding suggested that stand age should be taken into consideration when estimated vegetation water use in this region. Further researches on variations in E_c of black locust plantations along a chronosequence and responses of E_c to drought should be conducted to provide useful information on water resource and forest managements in this region.