How does disturbance affect the intensity and importance of plant competition along resource gradients?

Abstract

Ecologists have long argued about the modification of plant competition along abiotic gradients, and particularly along resource gradients. Through simulations, we evaluated the impact of bramble defoliation by roe deer on the response of oak seedlings to bramble presence along two resource gradients. We set up a controlled experiment crossing: (i) two light availabilities (10% and 30% incident radiation), (ii) two water regimes (normal rainfall and a reduced water regime) and (iii) three bramble defoliation modes (non-defoliated brambles, brambles defoliated in June and brambles defoliated in late July). Control plots contained no brambles. We found that, the intensity of the competition remained constant along the gradients, while the importance of the competition significantly increased with increasing resource availability. Our results further show that bramble defoliation causes a significant decrease in both competition intensity and importance and that the effect depends on both the demographic parameter and the defoliation period. Furthermore, defoliation did not change the patterns of competition intensity and importance along the resource gradients. We emphasize the importance of considering both disturbances, such as deer defoliation, and various demographic parameters related to plant phenology and plant response dynamics in the characterization of plant-plant interactions.