PARENT SESSION
Tuesday, August 8, 1:30-5:00 pm
COS 35 - Plant species interactions I: competition
L-12, Lobby Level, Cook Convention Center
Presiders: K Wiens

Fertilization promotes overyielding and species coexistence among graminoids in subalpine grasslands.

Gross, Nicolas^*,1, Katharine, Suding², Catherine, Roumet³, Sandra, Lavorel⁴, ¹ Université Grenoble 1, Grenoble, isere, France² University of California at Irvine, Irvine, California³ Centre national de la recherche Scientifique, CEFE, Montpellier, Herault, France⁴ Université Grenoble 1, Grenoble, isere, France

ABSTRACT- Functional diversity can determine ecosystem processes and species coexistence. Several studies have shown that increasing functional diversity leads to an augmentation of ecosystem productivity, usually termed overyielding. But mechanisms that drive overyielding remain unclear especially in the absence of N-fixing plants. Fertilized subalpine grasslands show a high functional diversity and a strong coexistence between conservative and exploitative species. We hypothesize that overyielding can occur among these two functional groups in fertilized conditions (1) by complementarity for resources used among functional groups or (2) by facilitation. We conducted a competition experiment manipulating both functional group identity and species density along a gradient of fertilization. We used three different species per functional group and grew species alone, in intra and in interfunctional mixtures. After two years, we observed a strong overyielding between functional groups with fertilization. Overyielding was driven by a diminution of competition intensity in the interfunctional mixtures. These results indicate that there is complementarity among conservative and exploitative graminoids in high nutrient conditions. Both complementarity for light and augmentation of nutrient use efficiency as functional diversity increased can explain overyielding. These results are in contradiction to general competition models which predict exclusion of conservative species by exploitative species when nutrients are abundant. But complementarity among these functional groups can explain the high functional diversity observed in fertilized subalpine grasslands. Future experimentations which clearly identify mechanisms responsible of overyielding will give us strong insight into the relationship between functional diversity, ecosystem properties and species coexistence.

Key words: overyielding, species interaction and niche differentiation, dominant graminoids, subalpine grassland