Physical stress and diversity-productivity relationships: The role of positive interactions

if environmental stress provides conditions under which positive relationsh ips between plant species richness and productivity become apparent, then s pecies that seem functionally redundant under constant conditions may add t o community functioning under variable conditions. Using naturally co-occur ring mosses and liverworts, we constructed bryophyte communities to test re lationships between species diversity(1, 2, 4, 8, 16, 24, or 32 species) an d productivity under constant conditions and when exposed to experimental d rought. We found no relationship between species richness and biomass under constant conditions, However, when communities were exposed to experimenta l drought, biomass increased with species richness. Responses of individual species demonstrated that facilitative interactions rather than sampling e ffects or niche complementarity best explained results-survivorship increas ed for almost all species, and those species least resistant to drought in monoculture had the greatest increase in biomass. Positive interactions may be an important but previously underemphasized mechanism linking high dive rsity to high productivity under stressful environmental conditions.