Separating the Effects of Wildfires from Climate in Growth of Ponderosa Pine (Pinus ponderosa Douglas ex. C. Lawson), Central Idaho, U.S.A.

Scientists use climate proxies, such as tree rings, to extend the climate record back in time, adding to the growing body of knowledge of past climate change. Tree rings provide a high-resolution proxy of climate. Many of the reconstructed climate records for the western U.S. use ponderosa pine (Pinus ponderosa Douglas ex. C. Lawson), a fire-adapted species that grows in areas prone to frequent fires. Such a disturbance as fire can introduce noise to climate reconstructions by causing growth releases or suppression following a fire event. My objective was to determine whether fire damage causes a quantifiable change in growth patterns of affected trees and whether the affected trees experience the changes in the same way.

Increment cores and cross-sections were collected from living and dead ponderosa pine trees in Payette National Forest, central Idaho, U.S.A. One chronology was developed from a stand of ponderosa pine showing no evidence of frequent fire while two chronologies, one from cores collected mostly from trees without visible fire damage and one from cross-sections of fire-scarred trees, snags, and stumps, were developed from a separate cluster of three subsites affected by frequent fire.

The mean fire-free interval for all fire subsites was 7.38 years. The mid-1600s to mid-1900s were characterized by nearly continuous fire activity at the fire subsites. The results from superposed epoch analysis (SEA) showed that tree growth is significantly lower than average the year of a fire event. The fire site chronologies showed slightly suppressed growth for 3 years after a fire. Significantly (p < 0.05) below average growth occurred after large fires in the fire site cross-section chronology. The difference chronologies indicated that fire does not cause a systematic change in tree growth and any added signal is comparable to other noise in the chronologies. Analyses using the computer program OUTBREAK showed that some fire years appeared to be followed by growth suppressions while others are not, regardless of fire size. Analyses of the chronologies and the fire history of the subsites indicated that no statistically significant systematic signal is introduced into the tree-growth patterns by fire events.