Runoff Supply Is a Key Resource for Vegetation Performance: Evidence from a Runoff Exclusion Experiment

Water is a limiting resource for dryland vegetation, and vegetation biomass, composition, and phenology are directly dependent on highly variable water availability and close linkages between water availability and vegetation dynamics that characterized arid and semiarid ecosystems. Dryland perennial vegetation commonly forms isolated patches interspersed with bare soil areas that usually exhibit high runoff rates, while vegetated areas act as runoff sinks, re-infiltrating most of this run-on. Thus, dryland ecosystems usually act as complex ecohydrologically coupled systems in which water and sediment redistribution from bare areas controls ecosystem functioning and increases vegetation productivity. In the present study, we analyze the response of Macrochloa tenacissima to run-on exclusion during two years in a semiarid ecosystem located in Almeria (SE Spain). To do this, we established eight experimental plots, including one plant of M. tenacissima per plot. For four of them, run-on from upstream areas was excluded, whereas the rest of plots were maintained as control open plots (receiving runoff from upstream interplant spaces). All the plants were scanned by using a terrestrial laser scanner, and we measured their spectral response, at the beginning and the end of the experiment. From the plant point clouds, we estimated total above ground biomass per plant and total photosynthetically active biomass. Also, we periodically monitored plant phenology and productivity, measuring leaf photosynthesis on the different plants and their spectral response. In addition, we continuously monitored soil water content at two depths and soil CO2 molar fraction. The results showed that plants receiving run-on showed more photosynthetically active biomass and net C uptake rates than plants under run-on exclusion treatment. The observed differences between treatments increase with time and were especially accentuated after rainfalls, when the differences in water availability were more pronounced. Our findings highlight the important role that the supply of water from run-on plays for dryland vegetation. In this sense, any alteration on unvegetated spaces may also have an important effect on vegetation productivity, like those pushed by the alteration in precipitation pattern, reducing vegetation biomass and their capacity to respond to water pulses.