COS 116-5 - Proliferation of large annual thistles increases productivity but decreases the carrying capacity of dry Mediterranean rangelands

Abstract: In Mediterranean regions, increased abundance of spiny plants under grazing may decrease rangeland carrying capacity, and paradoxically, increase rangeland productivity, through altered plant size, density, and resource allocation. Increasing abundance of large annual thistles in grazed grasslands entails major consequences for ecosystem services. Our study aims to quantify the contribution of large annual thistles to rangeland vegetation cover and productivity and to identify management- and climate-related drivers of spiny plant distribution and performance.

Using long term data (15 years) from an extensive herbaceous pasture experiment in Karei Deshe Experimental Range Station, northeastern Galilee, Israel, we examined the effects of cattle stocking density, feeding sites and climate on the cover and biomass of two dominant annual thistle species, Scolymus maculatus and Carthamus glaucus.

The two thistle species constitute a significant proportion of rangelands productivity, contributing more than 50% and 25% under high and moderate stocking density respectively. Despite the similarity in species life-form and life-history, each demonstrates a distinct response to both management intensity and annual climatic parameters. Moderate stocking density leads to higher biomass of C. glaucus while high stocking density leads to higher cover and biomass of S. maculatus and a decrease in the amount of palatable biomass. Distance from feeding site had significant contrasting effects on each of the focal thistles. C. glaucus cover increased while S. maculatus cover decreased as the distance from the feeding site grew. Estimates of the susceptibility of C. glaucus and S. maculatus to grazing demonstrated that C. glaucus is grazed in its juvenile stages, more so than S. maculatus. Temperature and relative humidity were the most sensitive climatic parameters in explaining the variation in cover and biomass of the two thistles. The trends of the two species’ responses suggests that, under dry and warm conditions, S. maculatus attains higher biomass than C. glaucus.
The proliferation of both large thistles under variable stocking densities and climatic conditions increases productivity but reduces the palatable vegetation of Mediterranean rangelands by 27-50%.