Tree-grass interactions and the establishment phase of the woody plant life cycle: Contrasts in above- and belowground competition and edaphic characteristics in grazed ecosystems

Woody seedling (Prosopis glandulosa) establishment was evaluated with respect to site grazing history/landscape position, herbaceous defoliation (surrogate for current years' grazing pressure), and soil depth on the Edwards Plateau in central Texas. In a central Texas savanna site, spatial heterogeneity in canopy gaps and belowground biomass was quantified in a Schizachayriuim-Paspalum matrix and experimentally evaluated with respect to the establishment of Prosopis. Additional experiments examined (1) above- and belowground growth of species representing contrasting grass growthforms (shortgrass vs. mid-grass) with respect to rooting barriers with fissures; survival and growth of Prosopis with respect to (2) clay-pan root barrier; and (3) subsurface bedrock fissures.; Prosopis establishment was influenced by grazing history/landscape position. Herbaceous defoliation tended to increase establishment on long-term grazed upland sites but had little influence on long-term protected sites. Soil temperatures in alluvial sites tended to be comparable to, or higher than, those in uplands and, along with a possible increase in evaporation, may have contributed to the higher seedling mortality in alluvial lowlands. Establishment was not positively correlated with soil depth or herbaceous neighbor identity. Distance to nearest neighbor did affect establishment. Ungrazed or lightly grazed alluvial sites dominated by late seral grasses would be quite resistant to Prosopis invasion in years of normal to slightly below-normal seasonal or annual rainfall. Emergence and short-term survival data both suggest recruitment of Prosopis can occur on upland sites.; Within the four grass stands (central Texas), 62% were unoccupied by perennial grasses with herbaceous root biomass being comparable among the stands. Prosopis seedling establishment trials confirmed that gaps ≥10 cm diameter are sufficient for successful establishment after one and two growing seasons, if they occur on microsites where herbaceous root biomass is low. Available volume in the vertical dimension was more important to establishment than that in the horizontal dimension.; A root barrier effect was not evident based on the growth of the two herbaceous species. The growth and survival of Prosopis seedlings along the fissure gradient was greatest away from the fissure. The growth of Prosopis was unaffected by the partial disruption of the potentially restrictive argillic rooting barrier.