SOIL SEED BANK OF A SEMIARID TEXAS GRASSLAND UNDER 3 LONG-TERM (36-YEARS) GRAZING REGIMES

Species Composition and density of the readily germinable soil seed bank of three long-term (36-yr) grazing treatments (heavy continuous, moderate deferred rotation and ungrazed exclosure) were determined for a summer and spring period for a semiarid grassland of the western Edwards Plateau, Texas. Total seed densities (2252 to 4409 seeds/m2) did not differ among grazing treatments, but did vary seasonally. Species composition varied among treatments. Heavy continuous grazing had a high proportion of early-seral, annual-dicot taxa, whereas the ungrazed treatment had a high proportion of late-seral, perennial-monocot taxa. Seeds of late-successional midgrass species (e.g., Bouteloua curtipendula, Eriochloa sericea) were not stored in the soil of any treatment and appeared to be transient. Many other species appeared to maintain persistent seed banks, including seeds of the current mid-successional dominant shortgrass, Hilaria belangeri. Similarity of composition between germinable seeds in the soil and existing plant communities was low. Soil seed bank is a primary control of secondary succession in these grasslands, and absence of late-successional species in the seed bank impairs the rate of succession.