Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var.glandulosa) on sites differing in herbaceous biomass and grazing history

Summary Emergence and survival of honey mesquite (Prosopis glandulosa var.glandulosa Torr.) seedlings was quantified on sites with contrasting grazing histories: long-term continuous grazing (LTG) and long-term protection (LTP) from grazing by cattle. On each site, different levels of heroaceous defoliation were imposed at monthly intervals (no defoliation=ND, moderate=MD and heavy=HD). The two weeks following seed dissemination appeared to be the most critical toProsopis establishment on LTP-ND plots. Openings in the herbaceous layer created by moderate defoliation of grasses on the LTP site increased germination and/or survival 7-to 8-fold during this period. However, increasing the degree of defoliation from moderate to heavy did not stimulate additional emergence on either the LTP or LTG site. Emergence from scarified seed placed in cattle dung (17 to 30%) was lower than that of bare seed placements in various microhabitats (43–60%). However, deposition of scarifiedProsopis seed in dung in conjunction with graminoid defoliation may be the most likely combination of events when livestock are present. Emergence from seeds transported into grasslands by other fauna likely would be low, unless seeds were deposited in areas where grasses had been defoliated.Prosopis survival was comparably high in dung and bare seed placements after one growing season. survival of seedlings present two weeks after seed dissemination ranged from 74 to 97% at the end of the second growing season. Seedling survival and shoot development (biomass, leaf area and height) were similar on LTP and LTG sites, regardless of the level of herbaceous defoliation or seed placement. In addition, the magnitude and patterns of net photosynthesis, stomatal conductance and xylem water potential were comparable among one-year-old seedtings on ND, MD and HD plots, even though differences in herbaceous species composition and above- and below-ground biomass between these treatments were substantial. Such data suggest competition for soil resources between grasses andProsopis may be minimal early in the life cycle ofProsopis. High rates ofProsopis emergence and establishment on LTP-MD plots are counter to the widespread assumption that long-term and/or heavy grazing is requisite forProsopis encroachment into grasslands. Results are discussed with regard to factors contributing to the recent, widespread invasion of this woody legume into grasslands of southwestern North America.Abbreviations LTG long-term grazed - LTP long-term protected from grazing - ND non-defoliated - MD moderate defoliation - HD heavy defoliation     

Acacia karroo invasion of grassland: environmental and biotic effects influencing seedling emergence and establishment

Acacia karroo Hayne is the most important woody invader of grassland in South Africa, and can greatly reduce the productivity of grassland. A field experiment was conducted to test the hypotheses that emergence, growth and the 1st year's survival of Acacia karroo would be enhanced by (1) defoliation of the grass sward, (2) increased irradiance, (3) increased moisture availability and (4) its germination within cattle dung pats. The study was conducted on one site above and one below the natural altitudinal treeline of this species in grassland of the eastern Cape, South Africa. Not one seedling emerged from dung pats. Neither location nor the other treatments affected the density of emerging seedlings, although only 40.4 seedlings m^–2 emerged of the 200 seeds m^–2 planted. Shading dramatically increased the density of surviving seedlings. In the open, only 3 and 1.5 seedlings m^–2 remained respectively at the end of the growing season or the beginning of the next, compared to 23.3 and 19.5 seedlings m^–2 under shading for these respective times. This was attributed to the effect of shade on moisture availability in a season which received only 54% of average rainfall. Seedling survival until the end of the growing season was enhanced (30%) by shade at both sites, but also by supplemental water (24%) and defoliation of the sward (7%) at the site above the treeline. Across sites and treatments, seedling survival was related to moisture availability, with no or poor survival for < 500 mm rainfall, indicating this species can only establish in certain rainfall years. Seedling survival over winter was not influenced by treatment, but was greater for larger seedlings. Treatments affected seedling size, in particular seedlings growing under shade and within a dense grass canopy were etiolated. A. karroo seedlings are capable of establishing and surviving within a dense grass sward for at least a year, tolerant of low irradiance and of interference, which, because most seeds do not persist for much longer than a year, suggests this species forms predominantly a seedling bank. This has implications for the invasion of grassland by woody species.