An emerging frontier of ecosystem science in the face of climate change is to predict not only ecological responses to trends in climate (e.g. warming) but large-scale demographic events in response to extreme climate events in near real-time. Among the most extensive demographic events is tree die-off in response to drought and warming. Although much research has focused on what exceeds thresholds for tree mortality, yielding numerous predictive relationships, none of these hypotheses have been tested with near-term ecological forecasting. Further, field experiments that have attempted to impose drought have been largely ineffective at improving predictions or testing near-term forecasts due to the challenges of effectively mimicking a severe drought or simultaneous mortality of treated and controls plots. There is thus a critical need for an experiment that takes advantage of an extreme drought as opposed to imposing drought, with such an experiment testing multiple alternate hypotheses in a near-term ecological forecasting context. The aim of this project is to rapidly implement experimental and monitoring measurements concurrent with the emerging severe drought developing across the southwestern US to test multiple different predictions of tree mortality for piñon pine (Pinus edulis), the most intensively studied tree species for drought-induced mortality.
Results/Conclusions
Drought severity progressed for much of the southwestern US from Fall 2017 through early May 2018. By April of 2018 of the emerging drought, soil moisture was low (<8%; 0-12 cm depth) at our experimental study sites northwest of Flagstaff, AZ. Piñon pine already showed signs of water stress, including browning needles and reduced bud elongation. Piñon pine seedlings had the greatest amount of browned needles (39%) compared to saplings and adults (13% and 8%, respectively), suggesting the younger trees are experiencing greater water stress. There was only very minor (<5%) piñon pine mortality across all size classes by April of 2018, yet current drought conditions will likely result in widespread die-off if the drought persists. Near-term ecological forecasting is being used throughout this drought to predict how piñon pine mortality will vary overtime and across regions and size-classes. Through improved mortality predictions and advancements in near-term ecological forecasting that can encompass extreme ecological and demographic events like tree die-off, this research is a critical step towards developing near-term ecological forecasts that provide the information needed for rapid land management actions and account for longer-term implications such as carbon management.