Sharp Turn Ahead: Modeling the Risk of Sudden Forest Change in the
Western Conterminous United States
Abstract
Anthropogenic climate change is driving shifts in vegetation
communities. These shifts are projected to increase in frequency and
extent as climate change intensifies into the future. Trees are
long-lived and, in the absence of disturbance, can tolerate years to
decades of climate conditions for which they are maladapted. A
disturbance, however, can trigger a sudden shift in vegetation type
where “legacy” forests are maladapted. Identifying the level of forest
maladaptation to climate and likelihood of disturbance could help land
managers anticipate sudden vegetation shifts as they plan for the
future. As part of the NASA CMS Project, we implemented an Environmental
Evaluation Modeling System (EEMS) model to evaluate the risk for sudden
forest shift in the western conterminous United States. To determine
where forests will be maladapted under future conditions, we simulated
vegetation without fire using the MC2 dynamic global vegetation model
(DGVM) with 20 different climate scenarios. Vegetation type departure
between historical and future periods is used as a proxy for the risk of
vegetation shift due to maladaptation. The greater the departure, the
greater the risk. To limit our analysis to actual forested areas, we
used LandFire forest landcover from the U.S. Departments of Agriculture
and Interior. To quantify disturbance risk, the EEMS model uses a
disturbance dataset produced as part of the NASA CMS project. In
addition to quantifying the risk of sudden vegetation shift, the EEMS
model also provides a standalone data layer of forest maladaptation
useful for decision support as well as the spatial data layers for each
node in the EEMS model. To conclude, we discuss how the model can be
combined with other models – e.g. species distribution models and
economic models – to further inform land management decisions.