Long term dynamics of 2 populations of Prosopis caldenia Burkart.

Abstract

Invasion of pristine grasslands by calden (Prosopis caldenia Burkart), and increased densities of this species in savannas are well-known vegetation changes in the semiarid region of central Argentina although little is known about its rates and patterns. In this paper we studied the relationship between dynamics of 2 representative P. caldenia populations and factors that could control the invasion process such as range management, fire events and precipitation regimes. Rates of implantation and spatial patterns are quantified using the present age distribution and dendroecological techniques. The pristine landscapes of the 2 study sites were grassland plains with (Site 2) and without trees (Site 1). The present density of the calden is 586 and 1,259 shrubs/ha in Site 1 and 2, respectively. No evidence of clustering was found at the spatial scale of the study (p = 0.52, Site 1 and p = 0.08, Site 2 for n = 112). The ages of sampled individuals ranged from 3 to 65 years in site 1 and 8 to 55 years in Site 2 (only trees with diameter lower than 30 cm were sampled in Site 2). The importance of cattle as an effective disperser of calden seeds was confirmed, as changes in measured establishment rates coincided fairly well with changes in cattle management. Establishment rates during the period of sheep grazing were 0.99 plant/ha/yr (16 years) in Site 1 and 10 plant/ha/yr (15 years) in Site 2. However, 10 years after the introduction of cattle these values reach 12.7 plant/ha/yr and 48.5 plant/ha/yr, respectively. One fire event occurred at each site (1980 in Site 1 and 1964 at Site 2). This factor did not change the density trend at Site 1, and at Site 2 it coincided with cattle introduction and caused an impressive increase in tree establishment. Fire was not an effective means of controlling P. caldenia populations. No relationship was found between population dynamics and available precipitation data.