Plant hydraulics-modulated forest physiological and compositional responses to climate changes: an evolutionally optimal modeling analysis

Climate changes have significantly affected forest ecosystems across the world, causing large-scale tree mortality and triggering forest structural and compositional shifts. These shifts in turn alter the boundary conditions of the land surface on the atmosphere and affect the climate systems locally and globally. However, our understanding of forest responses to climate change is highly uncertain because of the diverse physiological and ecological processes involved in forest ecosystem responses. The hydraulic traits of a plant represent the trade-offs between biomechanical support and hydraulic safety and efficiency. They have different competitive advantages when climate changes, and thus drive changes in vegetation composition. We used a game-theoretic vegetation demographic model (BiomeE) to simulate forest responses to climate changes with the trade-offs of the underlying plant hydraulics-mortality mechanisms that determine the physiological and demographic properties of trees. We conducted experimental runs at four sites spanning tropical to boreal regions and explore the major drivers for different forest types. According to our simulation experiments, net carbon gain (net primary production, NPP) is a determining internal factor controlling the diversity of strategies. In low NPP regions, plants dont have many choices. In high NPP regions, trees can fully explore all the possibilities of plant hydraulic trait combinations in competition with other strategies. In warming climates, tropical forests will have more deciduous trees because of high precipitation variability and more frequent and intense drought events. Boreal forests will have more temperate deciduous species with warming and intensified disturbance and mortality. Tropical evergreen trees are advantageous for light competition in dry season, but with high mortality risk if the dry season is exceptionally longer than normal; deciduous trees can avoid the risks of mortality in dry seasons but lose the opportunity of photosynthesis at high sunshine of dry seasons. In boreal and temperate forests, the major global change factors, such as warming and drought, change vegetation recovery successional pathways through altering soil organic matter decomposition and competitive plant hydraulic properties.