Seasonal transpiration dynamics of evergreen Ligustrum lucidum linked with water source and water-use strategy in a limestone karst area, southwest China

Transpiration dynamics of karst ecosystems are not conditioned only by the shallow-soil water status, mainly because of the presence of deep-water source pools formed within the underlying bedrock if trees can develop their deep roots to the pools. However, the strength of these relationships and how as well as why they can vary from an area to another and from season to season are poorly understood, due primarily to high heterogeneity of karst ecosystems. Therefore, the present study was conducted during 2018–2019 rainy and dry seasons in a subtropical (limestone) karst area on southwest China to explore the functional responses to reduced water availability of evergreen Ligustrum lucidum (Chinese glossy privet) growing on the shallow soils. Seasonal variations in the transpiration, water-use patterns, and water sources were investigated for Ligustrum lucidum through high-resolution monitoring of the micrometeorology, sap flow, and soil moisture data, in combination with the carbon stable isotope composition of the tree leaves, as well as oxygen and hydrogen stable isotope composition of the tree stem, soil, and deep water. Our findings showed that high transpiration rates of Ligustrum lucidum were mainly associated with the soil moisture during rainy seasons (87% in total) but the low rates were associated primarily with the deep-water sources during dry seasons (13% in total). During the rainy season, when plenty of water was available to the plant, Ligustrum lucidum tended to obtain water inexpensively through the soil layers (on average, 59%) and consumed it profligately through reduction of its water use efficiency (WUEi) (mean WUEi rainy-season = 82.6 μmol/mol). In contrast, during the dry season, when limited water was available to the plant resulting from the significant reduction in rainfall, Ligustrum lucidum had to obtain the water expensively (albeit at a small amount) through deep-water source pools (on average, 62%), and consumed it conservatively through increasing its WUEi (mean WUEi rainy-season = 108.6 μmol/mol). Our findings can provide the insights into temporal transpiration dynamics as well as the strategies and mechanisms adopted by the plants to counteract the seasonal drought stress associated with the shallow soils in karst ecosystems.