Effect of the 2003 heatwave on Eryngium alpinum demography under different management regimes

In the summer of 2003, Europe was impacted by an extreme heatwave that altered ecosystem productivity and increased plant mortality in natural areas. We analyse the effect of this extreme climatic event on the demography of the protected alpine plant Eryngium alpinum in relation to local ecological conditions and management regimes (mowing, grazing and unmanaged). Spatiotemporal variation in the vital rates of different plant life-stages (seedlings, juveniles, vegetative and reproductive adults) was estimated in seven sites of E. alpinum in the French Alps between 2001 and 2010. The effects on population dynamics (deterministic and stochastic population growth rates, λ and a) were studied using matrix population models and life table response experiments. Reductions in survival rates were observed following the extreme 2003 summer. λ was smaller during the heatwave and a decreased in simulations where the occurrence probability of a 2003-like event was increased. Adult survival rates and fecundity were negatively affected by heavy spring grazing, leading to lower λ and a. There were few differences in population dynamics between mowed and unmanaged sites. While greater rates of heatwave occurrence did increase extinction probability, only heavily grazed sites showed considerable extinction risk. As a consequence, heavy spring grazing must be discouraged in sites where the conservation of E. alpinum is a priority, while it is important to quantify acceptable levels of grazing in semi-natural areas where plant conservation has to be achieved in accordance with human development needs.