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 C4 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. gladulosa was planted earlier than Bouteloua curtipendula (side—oats grama), the grass had little or no effect on the woody seedlings 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 C4 grasslands would be very limited, and establishment requires gap formation.
Recent increases in woody plant density have occurred in grasslands worldwide, but both the cause and mechanisms involved in these changes have been elusive. Changes in grass biomass mediated by high levels of constant herbivory seem to be the pivotal reason. In this field study in central Texas, USA, effects of aboveground and belowground interference on the growth of seedlings of a deep‐rooted, woody heliophyte, Prosopis glandulosa, were tested. The effects of two positions (gap or Bouteloua curtipendula grassland), two levels of aboveground interference (high or low light), and three levels of grass root interference (2, 20, and 40 cm deep root excluders) on P. glandulosa aboveground, belowground, and total dry mass were measured. The exclusion of belowground interference significantly increased aboveground, belowground, and total P. glandulosa dry mass, with the reduction of belowground interference to a depth of 20 cm maximizing P. glandulosa dry mass. Aboveground, belowground, and total dry mass of P. glandulosa were not significantly different when grown in gaps compared to grasslands, nor were there any significant differences when aboveground interference (shade) was imposed. However, the trend was for greater dry mass in gaps and high light. Data indicate that belowground interference from grass roots significantly reduces the dry mass of P. glandulosa, while aboveground interference has a lesser effect.
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