Ecophysiological Effects of Groundwater Drawdown on Phreatophytes: Research Trends during the Last Three Decades

A systematic synthesis of phreatophytes’ responses to groundwater drawdown would provide a more complete picture of groundwater-related research aimed at the sustainable management of groundwater-dependent ecosystems amid climate change. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, the ecophysiological effects of groundwater drawdown on phreatophytes and methodological approaches were synthesized from peer-reviewed articles published from 1988 to 2022. The highest relative count of studies was found in arid and semi-arid high-income countries, such as Australia and North America (18–24%), while the lowest relative count to no data was found in hyper-arid countries, such as north African countries (0–3.65%). The groundwater depth effects on phreatophyte ecophysiology had the highest relative count (53.65%), followed by large-scale tree plantation effects on the groundwater characteristics (44.37%) and groundwater depth and biological invasion relationship (1.99%). The results revealed that as the groundwater depth increased, the phreatophytic vegetation growth, productivity, and community structure decreased across the ecosystem types. A groundwater withdrawal also had a significant impact on the physiology of the phreatophytes, specifically on the transpiration rate, xylem water potential, hydraulic conductance, and photosynthetic rate. Many of the reviewed studies concluded that large-scale tree plantations can deplete groundwater resources due to an increased evapotranspiration rate. Further, species’ diversity, evenness, dominance, composition, and distribution, as well as the Normalized Difference Vegetation Index (NDVI), are commonly measured parameters in the reviewed studies through vegetation and groundwater monitoring. Amid applied and contemporary problems, this synthesis may provide researchers with cues to conduct studies relevant to the integrated and sustainable conservation and management of groundwater-dependent ecosystems, particularly in data-poor, hyper-arid countries