Disease is an important mechanism in maintaining local plant diversity, but how and why cumulative pathogen pressure varies across landscapes is not well understood. Here we define cumulative pathogen pressure (CPP) as the collective pathogen-induced mortality across plant species. Landscape fragmentation is globally pervasive; it is critical to understand if the same mechanisms that maintain diversity in continuous habitat are also applicable in fragments. We examined seed bank diversity, fungal-mediated seed mortality, and seed-borne fungal diversity in an experimentally fragmented landscape in Lawrence, Kansas. We buried seeds of six prairie plant species for a year in three locations: small patches, edges of large patches, and centers of large patches. Half of the seeds in each location were treated with fungicide. We quantified germination success for exhumed seeds and from a subset of buried seeds, we cultured fungi for identification using ribosomal DNA sequencing. If plant-pathogen interactions influence landscape-scale patterns of plant community structure in fragments, we expected to see: 1) patch size differences in the diversity and composition of seedbanks and above-ground plant communities; 2) patch size differences in pathogen-induced seed mortality; 3) host-specific fungi; and 4) patch size differences in fungal community composition.
Results/Conclusions
From surveying vegetation and germinating seeds in over 300 plots across the landscape, we now know that seed bank composition and above-ground plant diversity differ between large and small fragments. Additionally, seed germination was significantly lower for seeds buried in small patches; the magnitude of the location effect depended on plant species (species*location; F=2.2, p=0.01). Whether landscape position predicted the difference in germination between fungicide treatments depended on species (species*location; F=1.91, p=0.04). Importantly, landscape locations where seeds had the lowest germination rates were also the locations where fungicide increased germination most. As predicted, fungal community structure differed among large and small patches (F=2.104, p<.0019). We also found that fungi show host preferences—a necessary condition for negative feedbacks (F=4.028, p< .0001). Most recently, we have shown that a generalist isolate of Fusarium, identified as an indicator phylotype for fungal communities in small patches (Indicator Species Analysis; p=0.001) grows more quickly at high temperatures characteristic of small patches. Overall, landscape fragmentation clearly influences interactions between plants and detrimental fungi.