Long-term proliferation of large annual thistles in dry Mediterranean rangelands

Spiny plants comprise a large, ecologically and economically important functional group in Mediterranean rangelands. The increasing abundance of large annual thistles in grazed grasslands entails major consequences for ecosystem services. This study aims to quantify the contribution of large annual spiny plants to rangeland vegetation cover and productivity and to identify management- and climate-related drivers of spiny plant distribution and performance. Using a 15-year dataset from an extensive herbaceous pasture experiment in northeastern Galilee, Israel, we examined the effects of livestock management and climate on the cover and biomass of two dominant annual thistle species, Scolymus maculatus and Carthamus glaucus. These thistles constitute a significant proportion of pasture productivity, contributing more than 50% and 25% under high and moderate stocking density, respectively. Despite the similarity in life-form and life-history characteristics, each demonstrates a distinct response to both management intensity and annual climatic parameters. High stocking density had a positive effect on both cover and biomass of S. maculatus. Conversely, C. glaucus biomass estimates were higher under moderate stocking density. Distance from feeding site had significant contrasting effects on each of the focal thistles. Relative humidity and temperature were the most sensitive climatic parameters in explaining the variation in cover and biomass of the two thistles. The directionality of the two species' responses suggests that S. maculatus attains higher biomass than C. glaucus under dry and warm conditions. 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%.