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 planti ng and clipping of Bouteloua curtipendula, a C-4 grass, on Prosopis gl andulosa, a woody seedling, was evaluated in a 2-yr field study in cen tral Texas, USA. Secondarily, effects of planting date, presence of th e 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 pl anted 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 b ut 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. glandu losa RGRs decreased and remained zero over the 2 yr, dry mass was redu ced 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. glan dulosa was comparable to that of seedlings grown alone; however dry ma ss was reduced 79%. When planted later than the clipped grass, dry mas s of P. glandulosa was reduced 98% compared to dry mass when planted a lone, while in unclipped plots there was 100% mortality. When B. curti pendula was planted early in the growing season, clipping increased ab oveground grass dry mass 1.63 times; there was no increase when the gr ass was planted late. Evidence suggests that woody seedling growth in undisturbed C-4 grasslands would be very limited, and establishment re quires gap formation.