Oomycetes during 120,000 years of temperate rainforest ecosystem development

The occurrence of plant-associated oomycetes in natural ecosystems and particularly during long-term ecosystem development is largely unknown, despite the importance of many oomycetes as plant pathogens. Using DNA sequencing from roots, we investigated the frequency and host relationships of plant-associated oomycete communities along a 120 000 year glacial chronosequence, comprising site ages with rapid compositional change (\"early succession\"; 5, 15, and 70 years old soil); relatively stable higher-diversity sites (\"mature\", 280, 500, 1000, 5000, 12000 years); and ancient, nutrient-limited soils with declining plant diversity and stature (\"retrogression\", 60 000, 120 000 years). Plant-associated oomycetes were frequent in all three early successional sites, occurring in 38 - 65% of plant roots, but rare (average 3%) in all older ecosystems. Oomycetes were highly host specific, and more frequent on those plant species that declined most strongly in abundance between ecosystem ages. The results support the particular importance of plant-associated oomycetes in early succession (up to 70 years). High host specificity and correlations of abundance of oomycete inside roots with declining plant species are consistent with oomycete-driven successional change.