Critical advances in understanding ecohydrological processes of terrestrial vegetation: From leaf to watershed scale

The study of ecohydrological processes within vegetated terrestrial ecosystems is one of the new frontiers in ecology, hydrology and global change studies, and one of the most theoretical areas in ecohydrology. In recent years, a great deal of research has been conducted on the interactions between vegetative ecological processes and hydrologic processes, ranging from plant cells scales to whole continent scale. These studies have made great progress in changing the way we think and approach both ecology and hydrology. However, from the perspective of Ecohydrology, a discipline that integrates Ecology and Hydrology, there is an urgent need to systematically summarize the progress of multi-scale theories and methods and to present new ideas on the theoretical frontiers in this emergent field. This review synthesizes and evaluates recent findings and future challenges in the science and modeling of ecohydrological progresses in terrestrial vegetative systems including: (1) Plant water use and regulation; (2) coupled carbon-nitrogen-water cycle processes and modeling; (3) ecological effects of key processes in the hydrologic cycle; (4) influence of vegetation on the generation and alteration of runoff and overland flow; (5) feedback effects from changes in terrestrial vegetation ecological water cycle on localized and regional precipitation pattern. The main progress in these five aspects of ecohydrology at multi-scales and across-scales is systematically summarized, with special attention given to the development of process-based quantitative models. Based on our synthesis of progress in the ecohydrology of terrestrial vegetation systems, we identify the four primary avenues for future research. First, it is necessary to clarify the effectiveness of the "triangular trade-off relationship" among water transmission efficiency and safety, and those mechanisms underlying plant water use strategies given that plant water use strategies and water regulation mechanisms under changing environmental conditions are the foundation of ecohydrological theory. Accordingly, an exploration on the internal associations among this "triangular trade-off relationship" and root hydraulics redistribution, ecohydrological separation, and isohydric characteristics will be needed. Special attention is needed for a better understanding of the multi-scale mechanisms for plant water use and regulation at the individual-community scale. Second, in the light of marginal water consumption during vegetative carbon assimilation, we explore the multi-coupling relationships among marginal water consumption and environmental factors such as light intensity and water availability and variations in the relationship among various vegetation types and growth stages. The ecosystem carbon-water coupling parameters that have clear ecological significance across scales need to be determined and highlighted. It is also important to develop a cross-scale carbon and water flux modeling method based on the canopy optimal conductivity theory. Third, how and to what extent the water availability and its dynamic changes restrict ecosystem carbon and nitrogen process is one of the frontier bottlenecks that need to be resolved to advance global change ecology. It is necessary to systematically clarify the complex interactions and threshold conditions between the water availability, nitrogen supply, and carbon benefits of the ecosystem. Development of mechanism models for the full coupling of ecosystem carbon and nitrogen cycling and hydrologic cycle processes are also crucial. Fourth, the vegetation factors and mechanisms in the terrestrial water cycle remain key issues for continued attention in the future. The core issue is the identification and quantitative description of ecological factors in the formation and dynamic changes in runoff. On a larger spatial scale, it is also vital to accurately analyze the atmospheric feedback and contribution range of changes in precipitation from the vegetation cover changes in the relevant regions based on the understanding of the impact of land surface vegetation structure changes on the land-atmosphere coupling relationship.