Responses of plant water uptake to groundwater depth in limestone outcrops

Abstract

The study of plant water uptake from groundwater is important to garner an improved understanding of ecohydrological processes and groundwater management. However, little is known about how fluctuations in groundwater depth influence plant water absorption in carbonate outcrop regions. This study quantified the plant water uptake patterns and plant responses to different groundwater depths at four distinct sites. The aquifers of the boreholes CF10 (4 to 10 m groundwater level) and CF12 (2 to 9 m groundwater level) basically consisted of porous and fissure media. The aquifer of borehole CF13 (historically sustained 0 to 1 m groundwater level) was dominated by fissure and conduit media, while that of borehole CF18 (2 to 3 m groundwater level) was comprised on rocks with considerable small-sized dissolution pores and seams. Borehole water, referred to as rock water in this study, is defined as a potential water source that plants can absorb. The IsoSource model, based on the dual stable isotopes of δ D and δ18O was employed to estimate soil and rock water contributions to the plant xylem water. The results revealed that the groundwater depth of the CF10, CF12, and CF13 boreholes had significant negative correlations with the total precipitation of the previous ten days. At the CF10 site, Toona sinensis and Caesalpinia decapetala absorbed soil water during the rainy season, and rock water during the dry season. At the CF12 site, Toona sinensis primarily absorbed rock water during rainy season and soil resident moisture during the dry season, while the water utilization proportion of Caesalpinia decapetala fluctuated significantly in different seasons. At the CF13 site, rock water was the dominant water source for both plant species during the rainy and dry seasons, while soil water was the main water source for both plants in the CF18 site. The proportion of rock water absorption by plants did not directly statistically correlate with groundwater depth; however, it did with the total precipitation of the previous two and fifteen days, respectively. Nevertheless, groundwater depth is a comprehensive embodiment of precipitation and the structure of karst water-bearing media. Therefore, the plant absorption of rock water is not only contingent on groundwater depth but also on the karst water-bearing media. For the first time, the isotopic values of borehole water were employed to estimate the contributions of plant water sources to improve calculations. The above results indicated that field surveys of hydrogeological structures and high precision monitoring were equally indispensable toward facilitating the restoration of vegetation and groundwater management in fragile rocky desertified ecosystems.