Grazing-induced morphological and growth rate changes in Anarthrophyllum rigidum, a Patagonian leguminous shrub

Abstract

Grazing influences the morphology and growth rate of shrubs, and consequently, their population dynamics. It has been shown that grazing directly affects the growth of shrubs. On the other hand, the reduction of grass biomass by herbivores reduces soil-water competition between grasses and shrubs, and indirectly, could enhance the growth of shrubs. However, the lack of long and homogeneous records of plant dynamics and primary production in the arid Patagonia hampers the assessment of the long-term effects of grazing on the growth of shrubs. We combined allometric and growth-ring analyses to assess the direct effect of grazing on individuals of Anarthrophyllum rigidum, a leguminous shrub widely distributed across the Patagonian steppe. The evergreen, protein-rich leaves of A. rigidum constitute an important complement for the sheep diet, particularly in winter when the abundance of grasses is reduced. Our observations indicated that individuals nearby the water source are smaller in size (35.5 cm vs. 67.39 cm), and present a larger number of basal branches (23 vs. 12), and show slower rates of growth (8.2 mm yr-1 vs. 13.3mm yr-1) than those located far from the water source. Our first quantification of the long-term effects of grazing on A. rigidum individuals suggests that beneficial effects of grazing through the reduction of grasses that compete for soil water should be more obvious for non-preferred than preferred shrubs in the dry Patagonian steppe.