Effects of fire and vegetation cover on ecohydrological processes in semiarid grasslands of central Mexico
Drylands are characterized by highly variable rainfall. Boutelua gracilis (Bg) and Muhlenbergia rigida (Mr) are abundant grass species in semiarid grasslands in Mexico. They differ in growth, structure and tussock form, which may have contrasting effects on hydrological processes. Hence, to understand water dynamics in open semiarid grasslands, it is important to consider the vegetation-soil interface at the species level and to examine how infiltration and runoff may be ecohydrologically linked. However, little is known as to how natural fire influences the growth dynamics and cover of different grass species and their effects on ecohydrological processes. Our objective was to compare the dynamics of infiltration and runoff for canopies of different grass species considering the effect of fire in semiarid grasslands of Ojuelos, Jalisco. We used a rainfall simulator with a sprinkler to control low (L=15mm/h) and high (H=47mm/h) rainfall intensity at two soil humidity conditions (dry and wet) to determinate infiltration (Ki, %) and runoff (Ke, %) coefficients in treatments with previous fire (wf) and without fire (wof). Four treatment combinations (Bg/wf, Bg/wof, Mr/wf, Mr/wof) were applied to five 1x1m plots. Additionally, we used Beerkan´s model to determine the rate of water infiltration for each treatment combination.
Preliminary data showed that total infiltration was significantly affected by fire and species (P<0.05). Total infiltration was higher for Mr (wf:101.66+/-10.48mm and wof:159.26+/-10.48mm) than for Bg (wf:66.30+/-10.48mm and wof:106.10+/-10.48mm) in both fire treatments. Fire treatment and species also significantly changed the rate of infiltration (P<0.05). Mr had higher infiltration rates (wf:2.25+/-0.086mm/min and wof:3.52+/-0.18mm/min) than Bg (wf:1.44+/-0.051mm/min and wof:2.29+/-0.15mm/min) in both fire treatments. With the rainfall simulator, fire and rainfall intensity significantly affected Ki (P<0.05). Also, there was a significant species by rainfall intensity interaction (P<0.05). In both species, Ki was higher under low (Bg/L:95.58+/-1.18%, Mr/L:99.87+/-1.18%) than under high (Bg/H:78.62+/-1.44%, Mr/H:94.79+/-1.44%) intensities. For a given rainfall intensity, Ki was significantly lower in sites with Bg than in sites with Mr. Independently of rainfall intensity, fire differently affected Ki with respect to species (P<0.05). Ki was higher in sites with Mr than with Bg and fire enhanced this species effect. It is suggested that higher tiller density and higher litter cover (in case of wof) of Mr in comparison to Br favored infiltration responses. Hence, tussock functional attributes of grasses may markedly influence ecohydrological processes at two levels, 1) tiller density of individual plants, 2) abundance and spatial distribution of species.