Ectomycorrhizal networks propagate carbon deficit and drought stress among trees

How plants allocate carbon determines their productivity, responses to stress, and interactions with other organisms. A substantial amount of plant carbon is stored as non-structural carbohydrates (NSC), which sustain turgor via osmoregulation and fuel metabolism when carbon is limited. NSC also support root-colonizing mycorrhizal fungi, thus we hypothesized that under carbon-limiting conditions such as drought, a trade-off between feeding mycorrhizal fungi and maintaining turgor may arise. We reduced carbon allocation to ectomycorrhizal (EcM) networks by girdling Pinus ponderosa trees exposed to drought or ambient conditions and manipulated putative fungal connections between trees by trenching. We show that, in droughted plots, trees putatively connected to girdled trees by EcM networks had 33 % less needle NSC and >10% less turgor than those connected to ungirdled trees. Trees disconnected from the mycorrhizal network by trenching had increased NSC likely from the increased water availability with girdling, but these gains were offset in the presence of networks. Our results demonstrate that the increased carbon demand by EcM fungi in response to reduced carbon inputs from some trees can deplete NSC in neighboring trees via shared mycorrhizal networks. At least in the short term, allocation trade-offs under carbon-limiting conditions may expose networked trees to carbon deficits. This may increase vulnerability to drought, which may be particularly acute given shifts in climate.