How does past wildfire impact vulnerability to forest die-off?

Abstract:

Forests provide ecosystem services such as carbon sequestration and recreation but are at risk from disturbances including drought-induced die-off and wildfire. Current estimates of vulnerability to forest die-off integrate the observed vulnerabilities of forest types and stand characteristics, but do not account for the ability of past disturbance, such as wildfire or die-off, to modulate future disturbances of the same type. In addition, little is understood about how past disturbances can control future disturbances of a different type, such as how past wildfire controls future forest die-off. To understand how past wildfire interacts with vulnerability to future forest die-off, we asked: 1.) How does stand age and time since wildfire disturbance impact vulnerability to drought induced forest die-off? 2.) How does the severity of the most recent wildfire impact vulnerability to die-off? We constructed a chronosequence of California forests by combining geospatial information on historical fire with Landsat satellite remote sensing data to estimate stand age. We used Landsat to calculate the difference between the Normalized Difference Moisture Index (NDMI) before and after the 2012-2015 warm drought in California as a proxy for forest die-off. Increasing stand age was a moderately strong predictor of increased forest die-off across all conifer forest types. Stand age was a stronger predictor of die-off in mixed conifer forests, and weaker predictor of die-off in jeffrey pine and pinyon-juniper forests. Incorporating wildfire disturbance history would improve estimates of vulnerability to forest die-off and could be useful for prioritizing land management investment to preserve ecosystem services.