A space-for-time assessment of drought vulnerability for ectomycorrhizal fungi of seasonally dry, old-growth Pseudotsuga menziesii ecosystems.

Douglas-fir (Pseudotsuga menziesii) forests of British Columbia are typically exposed to summer droughts, which are likely to increase in magnitude with climate warming. We examined the ectomycorrhizal fungi (EMF) of seasonally dry coastal and interior P. menziesii old-growth stands on contrasting (mesic vs. dry) soils to better understand their vulnerability to a reduction in soil water-holding capacity (as a proxy for rising climatic moisture deficits [CMD]). Coastal stands (CMD = 330) had considerably higher EMF species richness than interior stands (95 vs. 53 spp., on average), which was mirrored by a similar disparity in sporocarp fruiting. Interior forests (CMD = 402) were further differentiated from coastal stands by a greater proportion of ascomycete species and long-distance rather than contact mantle exploration types. We also found that pioneer fungi (Wilcoxina, Rhizopogon and Suillus) dominated interior stands (68% of read depth, on average), an unusual attribute for old-growth forests. Only the coastal stands demonstrated a clear decline in EMF species richness on dry vs. mesic soils, along with a weakly significant disparity in community composition. Increasing CMD as projected for the coast could result in species loss, based on this space-for-time substitution, although the sizeable EMF community of dry soils ([Formula: see text] 80 spp. per plot) may provide adequate functional resilience under climate change, at least in the near term. The smaller, more stable cohort of EMF species (dominated by pioneer fungi) found in the interior may represent the ultimate trajectory of EMF communities with P. menziesii under continuous warming.