The frequency and severity of droughts is increasing with climate change and causing an increase in forest drought-induced mortality (DIM). Because forests provide many services to society and represent a major terrestrial carbon sink, increased DIM will have profound ecological, social and economic consequences from local to global scales. Predicting when, where and which trees will die of drought remains a challenge, in part, because the processes leading to DIM and their interactions are neither fully understood nor fully represented in mechanistic models. Both impaired plant hydraulics and depletion of stored non-structural carbohydrates (NSC) have been implicated in DIM and empirical and modelling evidence indicate that most DIM occurs as a result of their interaction. However, the mechanistic nature of this interaction is not well understood, and, therefore, cannot be currently modelled. In the greenhouse, we have experimentally shown that NSC storage depletion has a direct negative effect on osmoregulation and plant water retention. These results are exciting because they provide a mechanistic link to quantify and model the interaction between NSC storage and plant hydraulics, which has been identified as a major challenge. However, we lack evidence that such effects occur in the field. Our greenhouse experiments also showed that plant carbohydrate depletion can spread through ectomycorrhizal networks and impair water relations. If a carbon allocation tradeoff exists between maintaining water relations and sustaining symbionts, this provides an opportunity to mechanistically incorporate symbiotic biotic agents into DIM models via their effects on NSC storage. In this exploratory research, we propose to test: 1) the mechanistic link between plant hydraulics and NSC storage in the field, 2) the potential for belowground fungal networks to affect this link and to influence forest vulnerability to drought. Such effects are expected to occur, for instance, when some -but not all- individuals in a network die and cease to supply carbon to the network, therefore increasing carbon demand from the network on surviving neighbors. We propose a field experiment in Pinus ponderosa saplings, where we manipulate carbon supply to the ectomycorrhizal (EM) network, water availability, and sapling connections to the EM network to test effects of NSC depletion on plant hydraulics, and potential carbon allocation tradeoffs between maintaining water relations and sustaining symbionts with consequences on forest vulnerability to drought. Our proposed research addresses two significant challenges for modelling forest responses to drought: how to quantify and model the interdependency between plant hydraulics and carbohydrate availability, and how to incorporate interactions with below ground symbiotic organisms. The data from this research will greatly improve the development of models of tree mortality, a global phenomenon with large consequences.