Grazing, fire and grass-species composition shape ecohydrological processes in a semiarid grassland landscape in Central Mexico

Background/Question/Methods

Semiarid grasslands evolved under highly variable climate, nomadic grazing, and periodic fire events. In Central Mexico, two-hundred years of human appropriation of the grassland biome has conferred fundamental shifts in the original grazing and fire regimes resulting in changes in grass species composition, vegetation cover and soil physicochemical characteristics. These modifications in grassland ecosystems are expected to influence water infiltration, run-off and ultimately the provision of ecosystem services. The question arises: how has historic grassland use under different fire and grazing systems shaped grassland structure and function and in particular ecohydrological processes? In June 2011, we set up 76 (2x3m) run-off plots in 13 different grassland sites in Vaquerias, Jalisco. Between June 2011 and May 2012, we recorded each rainfall event in a network of 7 pluviometers situated maximum 8 km apart. After each rainfall event we measured volumetric water run-off, and monitored weekly soil water content at 15 and 30 cm. We also determined soil organic matter, soil compaction, vegetation cover, number, area and organization of vegetation patches. We compared these variables in grasslands under different grazing (continuous, rotational, seasonal, 30 and 10-yrs grazing exclosures) conditions, the influence of an introduced grass species and the effect of prescribed fire.

Results/Conclusions

We found a highly spatiotemporally variable rainfall distribution, with more the 50% of the rainfall events being smaller than 5.0 mm and September being the month with the highest rainfall in all sites. Only 50% of the rainfalls occurred simultaneously in all sites. Considering different grazing treatments, both 30 and 10 year exclosures exhibited half the runoff volume compared to sites with continuous and rotational grazing and on average maintained 10% greater soil moisture content. When comparing exotic and native grass species no differences in runoff volume and soil moisture were found. Finally, when considering grazing and fire, runoff volume was significantly greater on sites where prescribed fire was applied three years ago. Overall, vegetation cover and patch distribution and number as well as vertical and horizontal soil organic matter distribution explained partially these differential run-off patterns. Semiarid grasslands under continuous grazing pressure cause substantial water loss directly affecting the long-term provisioning of ecosystem services. Reintroducing fire as an inherent disturbance agent in grassland ecosystem may stimulate primary productivity, however with the trade-off of enhancing water loss.