Drylands frequently have the potential to support biological soil crusts (biocrusts – a soil surface community of cyanobacteria, mosses, and lichens, and other organisms), yet biocrusts are often compromised or missing altogether due to disturbance. There is a need to develop techniques to grow biocrusts ex-situ, so that they may be reintroduced to degraded drylands, boosting ecosystem function. This goal provides a rich context with which to investigate how different combinations of traits and interactions among species might influence overall productivity of the community being grown. Ecological theory would predict: 1. Communities with greater taxonomic or trait diversity are more likely to be more productive, 2. Facilitative interactions among community members are likely to enhance community productivity. In multiple greenhouse experiments, we attempted to grow a total of 6 moss species and 8 lichen species alone and in various combinations to test these predictions. We monitored percentage cover of all species, and their summed cover as an index of total productivity, through time.
We were able to grow all moss species to some extent, but all but one lichen species failed to grow. In one experiment, we found that two moss species with similar trait profiles discouraged one another’s growth when grown together compared to alone. However, the addition of a third species, a lichen with a fundamentally different set of traits including N-fixation, produced the greatest growth rates for all three species. These findings suggest either that intentional maximization of trait diversity (indirectly inducible by increasing species richness) will increase productivity, or that certain taxa may act as universal facilitators. We then grew five moss species in all possible combinations of 1, 2, 3, 4, and 5 species and produced very different results. We found that species richness of the sample was only a weak positive predictor of total biomass produced. Rather, the most influential factor was the presence of the two best growing species. These 2 species, one an early successional acrocarp, and the other a late successional pleurocarp, seemingly have few traits in common, and did not improve one another’s growth in combination. We conclude with a set of working hypotheses: 1. Mosses are more likely to compete with one another than facilitate each other and this effect is incompletely counter-balanced by taxonomic or trait diversity. 2. Competitive interactions can be overcome by the introduction of a species that acts as a universal facilitator.