Print PageLand-use change (LUC) in semiarid grasslands of Northern Mexico has driven a decline of plant cover and alterations in species composition. Additionally, differential land-uses over small areas have provoked highly heterogeneous landscapes in vegetation structure and function. Implications of these ecosystem changes in ecosystem processes like evapotranspiration (ET), however, are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition, and how a number of biotic and abiotic drivers modulate water vapor fluxes over fragmented areas of short-grass steppe in Central Mexico. Five grassland communities were included following a gradient of disturbance: 1) a 28 years exclosure, 2) a site dominated by commonly subordinated grasses, 3) an overgrazed site, 4) an overgrazed site with shrub encroachment and exotic species invasion and 5) a site converted into rainfed oat crop. ET was recorded on 10 dates from October 2008 to September 2009 using the geodesic dome method. Six plots per site were established and gas exchange was measured four times throughout the day (08:00, 12:00, 16:00 and 20:00 h). A repeated measures ANOVA (α=0.05) was performed to compare ET among sites. Linear and non-linear regression analyses between water fluxes and drivers were performed.
Results/Conclusions
Photosynthetic photon flux density was the main environmental driver which controlled ET in a daily time scale; while soil water content and leaf area index controlled fluxes throughout the year. Daytime ET was two fold larger for the growth season for sites with optimal plant cover (Exclosure and Moderate grazing) compared to sites with poor plant cover (Overgrazed and Shrub encroachment) (types of plant cover included; optimal >30% and poor <10%). In dry months (January-May), water fluxes were similar among all sites (<0.1 mm d-1). Whenever soil water content was high, differences in ET among sites were small, probably due to an enhancement of evaporation fraction in communities with large proportion of bare soil. Differences in species composition also defined the sensibility of these communities to environmental controls. Daytime ET in communities dominated by C3 plant was attenuated by the closure of stomata at high temperature and vapor pressure deficits observed at midday in summer, which produced negative slopes in PPFD response curves.
The ET was larger for sites with optimal plant cover, but was counterbalanced by higher evaporation rates in sites with poor plant cover; whereas differences that depended on species composition were the result of functional groups dominance.
