Understanding and predicting ecosystem functioning in water limited ecosystems requires a thorough assessment of the role plant root systems. Widespread ecological phenomena such as shrub encroachment may drastically change root distribution in the soil profile affecting the uptake of water and nutrients. However, the relationship between plant rooting distribution patterns and ecosystem functioning has been largely overlooked. Our objective was to assess the role of a range of rooting distribution patterns 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 would have a significant effect on plan transpiration. (2) The effect of rooting pattern changes along a precipitation gradient. In order to test these hypotheses, we modeled water losses and soil-water availability at 55 grassland locations with virtual root distributions that simulate shallow and deep root systems. We evaluated the effect of contrasting rooting distribution on plant transpiration, a variable tightly correlated with plant growth and primary productivity.
Our results show that plan rooting distribution patterns have a significant effect on plant transpiration. Shallow root distribution showed significantly lower transpiration than deep root distributions, a mean difference of 8.2 cm/yr. Such effect changed along a precipitation gradient showing smaller effects of root distribution in arid sites than in mesic sites. Locally, plant transpiration increased with rooting depth up to a point where transpiration reached a plateau. The shape of such relationship and the maximum transpiration rate changed among sites. Arid ecosystems showed shallower transpiration rate increases as root depth increased and smaller maximum transpiration rates than mesic ecosystems. The effect of species invasions or changes in the relative abundance of plan types with different root traits will affect ecosystem functioning. Given the tight relationship that exists between transpiration and photosynthesis, we expect that changes in rooting distribution patterns will impact primary productivity with greater effects in mesic grasslands.