PARENT SESSION
Contributed Oral Session 124: Grassland Ecology: Water Relations; Growth
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 518 C, Level 5, Palais des congrès de Montréal
Stress tolerance and competitive response ability determine the outcome of biotic interactions.
Liancourt, Pierre*,1, Callaway, Ragan Morrison2, Lavorel, Sandra1, Michalet, Richard3, 1 LECA, UMR 5553 CNRS - Université joseph Fourier, Grenoble, France2 Plant ecology Laboratory, The University of Montana, Missoula 59812 USA, Missoula, Montana, USA3 BIOGECO, UMR 1202 INRA - Université Bordeaux 1, Bordeaux, France
ABSTRACT- Theoretical models predict that the net outcome of biotic interactions among plants is the sum of co-occurring negative and positive interactions, with facilitation generally increasing in importance with increasing abiotic stress. However, species differ in fundamental niche optima, thus the intensity of stress is relative among species and important only in the context of these relative differences. We tested the hypothesis that the facilitative response of a species is relative to how much abiotic conditions deviate from the optimum conditions for that particular species (stress), and the competitive response ability of the species (i.e. its ability to tolerate the inhibitory effects of neighbors). In a field experiment, we examined the responses of three co-occurring species with different ecological optima, Bromus erectus, Brachypodium rupestre and Arrhenatherum elatius, to the alleviation of a primary limiting resource (water), and to biotic interactions in a mesoxeric grassland in eastern France. We found that A. elatius had a strong positive response to watering, the response of B. rupestre was moderately positive, and B. erectus did not respond significantly, suggesting that water stress was only important for the two first species. Most importantly, the net outcome of the interaction between each species and its neighbors depended on the degree of water stress it experienced in its natural habitat. For survival, in the control plots we found no significant interactions for B. erectus (not stressed) whereas B. rupestre and A. elatius (stressed species) were facilitated. Enhancing water availability suppressed facilitation of B. rupestre and A. elatius and led to competitive exclusion of B. erectus. In contrast to survival, there was no facilitation for growth in the control plots and competition intensity increased for all three species with watering. In our experiment, the competitive response of a species was inverse to its ability to tolerate stress, indicating a tradeoff between these components of the species response. A facilitative outcome appears to be a function of a species having both a low tolerance to a particular abiotic stress, and a strong competitive response ability.©
Key words: competition, facilitation, optimum, stress tolerance