Endomycorrhizal role for interspecific transfer of phosphorus in a community of annual plants

Phosphorus-32 applied to leaves of Plantago erecta in a serpentine annual grassland reached the shoots of about 20 percent of the close neighbors. Vesicular-arbuscular mycorrhizae connect the root systems of neighbors of different species and probably mediate nutrient transfers among them. Spatial patterns of transfer show that taxonomic affinity, distance from donor, and size of recipient do not serve as predictors of transfer and that models of transfer by simple diffusion are not appropriate. No alternative predictor was discovered. The results underscore the importance of belowground interactions in explaining neighbor effects, but the factors controlling nutrient transfer and its consequences for community structure appear complex.

Diversity-stability relationships in community ecology: re-examination of the portfolio effect

In plant communities, the portfolio effect, also called "statistical averaging effect", expresses the fact that stability in aggregate community properties such as biomass productivity generally rises with species diversity, simply because of the statistical averaging of the fluctuations in species' properties. This paper essentially upgrades the previous formulations of the portfolio effect, first developed by Doak and collaborators and then by Tilman. It uses a theoretical approach based on simple statistical relationships and some simplifying assumptions proposed by these authors. The new formulation presented extends and improves the previous relationships in the sense that it takes into account simultaneously a varying scaling power of the variance, the interaction effect between species, the heterogeneity in species productivity and interspecies correlated responses to the environment. It appears that the simple statistical averaging, as inferred from this formulation, does not necessarily lead to a positive correlation between species diversity and community stability.