Mesquite effects on microhabitat alter community structure but not productivity in short grass prairie

Background/Question/Methods

The Prosopis glandulosa, or honey mesquite tree, is an invasive tree species to the American Southwest. It is particularly problematic in short-grass prairies dominated by C₄ grasses and other short-statured C₃ herbaceous species. Mesquite is unlike the vegetation in these systems in that it is woody, tall statured, legume and thus can have a strong effect on the microhabitat of the area where it establishes. Nonetheless, it is unclear how these microhabitat effects may impact the short-grass prairie community. The main questions we addressed were (1) Does mesquite-driven changes in microhabitat influence short-grass prairie communities? (2) What are the mechanisms driving this change? And (3) What do these effects mean for the grassland ecosystems moving forward? To answer each question, we set up paired plots under and 1 meter outside the canopy of 40 mesquite trees in a short-grass prairie in western Texas. We took soil moisture measurements once a week over the course of 9 weeks. We also measured community composition and plant biomass in all 80 plots during the peak of the growing season. We then assessed the microhabitat and plant community differences between plots under the trees and plots outside the canopy of the trees.

Results/Conclusions

We found the plots under the trees regularly had lower soil moisture than those away from the trees. This, combined with the light interception provided by the trees (mean leaf area index = 0.94 ± 0.07 m² m^-2), reduced resource availability to the understory plants. However, we found that the biomass outside the canopy was only marginally greater than that under the tree despite the lower light and soil moisture availability. This effect, combined with the additional biomass of the tree itself, suggests an overall increase in productivity as a result of mesquite invasion. Our community measurements indicated that plant diversity was unaffected by the mesquite presence, but that the non-mesquite communities were not made up of similar species. We found there was an 18% greater grass to forb biomass ratio in the plots outside the canopy than those under the canopy. These results suggest that with the invasion of the of the honey mesquite tree, the grassland community structure may become more productive overall without a loss in total diversity, but the species comprising the plant community will not resemble the pre-invasion community.