Understanding the links between plant ranges and soil microbial diversity remains a major frontier in community and ecosystem ecology. Characterizing how plant-fungal symbioses vary across climatic and edaphic gradients can provide a preview of how the diversity and functioning of ecosystems might be altered by global change. For instance, ectomycorrhizal (EM) fungi can help ameliorate drought stress for some tree species and expand the range of environments in which they persist. In the Pacific Northwest, reduced and more variable precipitation regimes might disrupt these types of host-fungal relationships in ecologically and economically important tree species. We surveyed Populus trichocarpa root and soil fungi in 25 naturally occurring stands from CA to WA that span a 200-1700mm annual precipitation gradient to examine how rainfall shapes the taxonomic and functional diversity of their fungal communities. Here, we address three main questions: (I) does precipitation correspond with fungal community diversity and structure? (II) are EM fungi more abundant in drier versus wetter sites? (III) does the connectivity and modularity of fungal co-occurrence networks vary with precipitation?
Results/Conclusions
Precipitation was associated with substantial changes in P. trichocarpa fungal communities. Observed richness was slightly elevated in sites with lower levels of precipitation, and there was significant community turnover across the rainfall gradient. In addition, community dissimilarity was predicted by community cohesion, indicating that fungal responses to precipitation are partly influenced by the amount of connectedness between co-occurring taxa. As expected, EM root colonization was negative correlated with precipitation, and certain EM taxa peaked in relative abundance in the driest sites. Together, this suggests that P. trichocarpa might increasingly rely on their EM symbioses in a drier world. Finally, we grouped sites into three precipitation categories (dry = <600 mm; mid = 600-1200; wet = >1200mm annual rainfall) and filtered all but the 100 most prevalent taxa across sites to examine co-occurrence networks. Networks significantly differed in the amount of clustering and geometric distance between nodes, with a greater number of connections between taxa and more modules (i.e., unique cliques of taxa) in dry sites. These results may hold clues as to the form and function of Populus-fungal symbioses under future precipitation regimes, and have important management implications for the sustainability of Populus biofeedstock programs in the region.