WOODY PLANT-GROWTH RELATED TO PLANTING TIME AND CLIPPING OF A C-4 GRASS

The density of woody plants has increased in grasslands throughout the world, but the cause has been elusive, although changes in herbivory seem central. In this study, the importance of relative time of planting and clipping of Bouteloua curtipendula, a C-4 grass, on Prosopis glandulosa, a woody seedling, was evaluated in a 2-yr field study in central Texas, USA. Secondarily, effects of planting date, presence of the woody seedling, and clipping of the grass on the growth of the grass were evaluated. Relative growth rates (RGRs) of Prosopis glandulosa (honey mesquite) seedlings grown alone were seasonal, generally high in spring and decreasing as summer progressed. When P. glandulosa was planted earlier than Bouteloua curtipendula (side-oats grama), the grass had little or no effect on the woody seedling RGRs. However, the dry mass of P. glandulosa increased 2.3 times when the grass was clipped but was reduced 54% when the grass was unclipped. When the two species were started at the same time and the grass was not clipped, P. glandulosa RGRs decreased and remained zero over the 2 yr, dry mass was reduced 99.8%, and mortality was 80%. If both species were started at the same time and the grass was clipped, 1st- and 2nd-yr growth of P. glandulosa was comparable to that of seedlings grown alone; however dry mass was reduced 79%. When planted later than the clipped grass, dry mass of P. glandulosa was reduced 98% compared to dry mass when planted alone, while in unclipped plots there was 100% mortality. When B. curtipendula was planted early in the growing season, clipping increased aboveground grass dry mass 1.63 times; there was no increase when the grass was planted late. Evidence suggests that woody seedling growth in undisturbed C-4 grasslands would be very limited, and establishment requires gap formation.