GROWTH OF PROSOPIS-GLANDULOSA IN RESPONSE TO CHANGES IN ABOVEGROUND AND BELOW-GROUND INTERFERENCE

Recent increases in woody plant density have occurred in grasslands wo rldwide, but both the cause and mechanisms involved in these changes h ave 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 interfer ence on the growth of seedlings of a deep-rooted, woody heliophyte, Pr osopis glandulosa, were tested. The effects of two positions (gap or B outeloua curtipendula grassland), two levels of aboveground interferen ce (high or low light), and three levels of grass root interference (2 , 20, and 40 cm deep root excluders) on P. glandulosa aboveground, bel owground, and total dry mass were measured. The exclusion of belowgrou nd interference significantly increased aboveground, belowground, and total P. glandulosa dry mass, with the reduction of belowground interf erence to a depth of 20 cm maximizing P. glandulosa dry mass. Abovegro und, belowground, and total dry mass of P. glandulosa were not signifi cantly different when grown in gaps compared to grasslands, nor were t here 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 roo ts significantly reduces the dry mass of P. glandulosa, while abovegro und interference has a lesser effect.