Importance and intensity of competition along a fertility gradient and across species

Questions: 1. Can the importance and the intensity of competition vary independently along a nutrient gradient? 2. Are these variations species dependent? Location: Sub-alpine pastures of the northern French Alps. Methods: Competition intensity measures how much competition decreases the performances of an organism. Competition importance measures how much competition contributes to affect performance, among other processes (such as environmental stress or disturbance). Competition intensity and importance were measured on three co-occurring species: Festuca rubra, a perennial grass, and two forbs of contrasting basal area, Chaerophyllum hirsutum and Alchemilla xanthochlora. A neighbour removal experiment was performed on Festuca rubra in three sub-alpine grassland communities differing in fertility and on Chaerophyllum hirsutum and Alchemilla xanthochlora in the two more fertile of these communities. The importance of competition was quantified using an index proposed by Brooker et al. (2005). Results: Competition intensity and importance showed different patterns of variation along the fertility gradient for Festuca rubra: competition importance decreased with decreasing fertility whereas competition intensity did not change. The largest forb was the least affected by competition. Our results suggest that the importance of competition for all three species depended on their individual tolerance to low nutrient availability. Conclusions: 1. The distinction between the importance and the intensity of competition is helpful to explain conflicting results obtained on the variations of competition indices along productivity gradients. 2. The choice of a phytometer can affect the conclusions drawn from empirical studies.