Influence of precipitation pulses on long-term Prosopis ferox dynamics in the Argentinean intermontane subtropics.

Biological processes in arid communities are associated with episodic precipitation pulses. We postulate that annual to decadal-scale precipitation pulses modulate the dynamics of the intermontane Prepuna woodlands. To study this hypothesis, we have assessed the influence of precipitation pulses on the rates of growth and survival of Prosopis ferox in the Prepuna woodlands during the past century. Tree ages from several P. ferox stands were used to reconstruct the establishment patterns at each sampling site. Ring-width chronologies provided the basis to assess the influence of annual versus multiannual precipitation pulses on radial growth and establishment over time. Both the radial growth and the stand dynamics of P. ferox at the regional scale were found to be largely modulated by climate, with precipitation the dominant factor influencing interannual variations in P. ferox ring-widths. Our analysis of dendrochronological dating data on 885 individuals of P. ferox revealed a period of abundant establishment from the mid-1970s to beginning of 1990 s, which is coincident with an interval of remarkable above-average precipitation. However, tree-growth and establishment patterns at the local scale in the Prepuna also reflected land-use changes, particularly long-term variations in livestock intensity. The P. ferox dynamics documented here substantiates the hierarchical concept of "resource-pulse" in dry ecosystems, with precipitation pulses of different lengths modulating distinct dynamic processes in the P. ferox woodlands. Interannual variations in precipitation influence year-to-year patterns of P. ferox radial growth, whereas multiannual oscillations in rainfall influence episodic events of tree establishment. The long-term interval considered in this study enabled us to disentangle the roles of natural versus human controls on P. ferox dynamics in the region.