Understanding and predicting ecosystem responses to projected increases in climate extremes in water limited ecosystems requires a thorough assessment of the role plant root systems. Land-use change and shrub encroachment may drastically change root distribution in the soil profile affecting water and nutrient uptake. However, the relationship among plant rooting distribution patterns, climate change and ecosystem functioning has been understudied. Our objective was to study the effect of rooting distribution patterns ranging from very shallow to very deep on plant transpiration across sites along precipitation gradient. We used SOILWAT, a multi-layer, daily time-step model that simulates soil water content by depth in the soil profile. Processes simulated in SOILWAT include water interception and subsequent evaporation from the plant canopy and litter, water infiltration into the soil, vertical water flow among soil layers, evaporation and transpiration from each soil layer, and soil water content by layer. We hypothesized that (1) plant-root distribution has a significant effect on plan transpiration. (2) The effect of rooting depth pattern changes from arid to mesic ecosystems. In order to test these hypotheses, we modeled water losses and soil-water availability at 100 dryland locations with virtual root distributions that simulate shallow and deep root systems. We evaluated the effect of contrasting rooting distribution on maximum plant transpiration, a proxy for plant growth and primary productivity.
Results/Conclusions
Our results showed that plant rooting depth has a significant effect on plant transpiration, soil evaporation and deep percolation. The rooting depth profile that maximized plant transpiration changed along a precipitation gradient. Arid ecosystems maximized transpiration with significantly shallower roots than more mesic ecosystems. Differences along the precipitation gradient are related to the relative contribution of water fluxes to the overall water balance. In arid ecosystems, soil water losses occur predominantly through soil evaporation while in mesic ecosystems the main water loss path is deep percolation. Therefore, shallow roots minimized water losses in arid sites (Evap = -1.12 + 8.94 * rooting depth, R2 = 0.3, P = 0.006) and deep roots minimized water losses in mesic sites (Percol = 60.4 – 18 * rooting depth, R2 = 0.4, P = 0.008). The effect of biotic exchange and land-use change resulting in novel plant communities with different root traits will affect the water-use efficiency of water limited ecosystems. Changes in rooting distribution patterns will impact primary productivity because plant transpiration is tightly related to photosynthesis.