Density‐dependent disease, life‐history trade‐offs, and the effect of leaf pathogens on a suite of co‐occurring close relatives

In a coastal prairie in California, we studied the impact of leaf diseases on the performance of 17 co‐occurring species of Trifolium and Medicago (“clovers”). We transplanted clovers in randomized arrays into the natural prairie community in 3 years of common garden experiments. We quantified infection rates by isolating fungi from leaves, and we measured disease severity as per cent leaf area damaged. In a fungicide experiment, we measured the impact of infection on biomass and survival. We assessed whether disease on transplants was positively related to natural abundance of that species in the surrounding community, which we monitored over 5 years. We assessed life‐history trade‐offs by testing whether more rapidly growing species were more susceptible to pathogens.

Rank abundance of clover species was stable over 5 years despite marked environmental fluctuations. Across hosts, fungal infection was not linearly related to density, although transplants of species that were locally absent showed lower and more variable infection. Disease severity was greater for more abundant species, but in only 1 of 3 years, and response to fungicide was not stronger in more abundant species. As predicted by life‐history trade‐offs, faster‐growing species experienced greater fungal infection. However, the impact of that infection was less negative, not more negative, on faster‐growing species.

Our results suggest that life‐history trade‐offs in plant–pathogen interactions may influence equalizing mechanisms among species in this guild, but the combined effects of greater infection with greater tolerance may limit rather than promote coexistence. We also found modest evidence that density‐dependent disease may contribute to stabilizing mechanisms. Lack of host specificity, rapid evolution of host use and temporal variation in climatic conditions may all influence the role that pathogens play in coexistence of these closely related plants.