Effects of Invasive Grasses on Ecosystem Processes Along an Invasion Gradient in South Texas, U.S

摘要

Invasive grasses pose a threat to biodiversity, ecosystem stability, and wildlife habitat. Increasing evidence suggests invasive grasses alter ecosystem processes, such as energy and nutrient cycling and soil microbial dynamics. Southern Texas, in particular, has experienced increases of several invasive grasses including Lehmann lovegrass (Eragrostis Lehmanniana Nees) and tanglehead (hereafter, H. contortus; Heteropogon contortus [L.] P. Beauv. ex Roem & Schult.), which has spread rapidly across the south Texas Coastal Sandsheet. H. contortus is a native, warm season, perennial bunchgrass common in much of southwestern U.S. I conducted a study to assess the effects of H. contortus and E. Lehmanniana in an invasion gradient on (1) microbial community composition and size, (2) seed bank dynamics, and (3) seasonal production and growth patterns of native plants in its surrounding plant community. Research was conducted in a controlled greenhouse setting on Texas A&M University-Kingsville campus as well as on a local ranch in Jim Hogg County, Texas. Microbial community size differed between H. contortus dominated plant communities and native plant communities. Conditioning soil with H. contortus seedlings and subsequent plant establishment did not result in changes to the microbial community during an 18-month period. Seed bank dynamics were characterized from the emerged seed bank for H. contortus and E. Lehmanniana, another invasive grass found in this area. Species diversity indices of the emerged seed bank did not differ in either invasive treatment; however, a greater number of seedlings emerged for each of these species compared to strictly native sites. Plant communities including H. contortus had greater foliar canopy cover and greater standing crop of grasses, but less of native grasses and forbs. Native plant communities without H. contortus had higher percent bare ground. My findings suggest H. contortus alters ecological processes including soil microbial dynamics, native plant characteristics, and seed banks.