Combining national forest inventory data with spatial indicators of water availability to assess forest health vulnerability

Forests cover approximately one-third of the total surface area of the United States; thus, there is great interest in monitoring the state of forests nationwide as they respond to shifting disturbance regimes, climate change and management activities. Past studies suggest reduced growth rates and increased mortality of trees under drought conditions; such relationships can be used along with spatial information on soils and topography to identify areas of concern for forest managers. Understanding where forests are (and are not) doing well is the first step to establishing a framework for predicting forest health responses to future climate change and increasing drought frequency. For this work, we calculated growth and mortality rates of tree species of high concern for forest health decline from the US Forest Service's Forest Inventory and Analysis database and related these rates to physiographic characteristics that we expected would be correlated to tree health, with particular emphasis on variables indicative of water availability. Soil drainage and productivity indices, along with topographic variables such as aspect, slope, and elevation, were used to model forest growth and mortality rates of key tree species across the US. Growth and mortality rates were compared to baseline rates at an ecoregion level to identify areas within each ecoregion with high susceptibility to further decline under future drought and/or insects and disease. Preliminary results pinpoint a few areas of concern for key forest species. For example, in the Southern Rockies, ponderosa pine has restricted growth on south-facing aspects compared to north-facing aspects, and the effect is particularly pronounced in lower elevation forests where we expect large impacts of climate change on water availability. On the other hand, high growth rates of Engelmann spruce in the Northern Rockies in moist soils at lower, warmer elevations could increase vulnerability to future spruce beetle outbreaks. Our results are anticipated to help inform hazard and risk mapping of US forest threatened by drought and cascading threats such as insects and diseases.