Interactions between tree refugia and high severity fire patches in dry conifer forests of the Interior West

Background/Question/Methods

Wildfire is a complex landscape process with great uncertainty in whether trends in size and severity are shifting trajectories for ecosystem recovery that is outside of the historical range of variability. Considering that wildfire size and severity is likely to increase into the future with drier climate, it is important that we understand wildfire effects and ecosystem recovery. In this study we explore patterns in legacy tree islands within wildfire perimeters throughout the Rocky Mountains and Black Hills in dry mixed conifer forests using aerial imagery to better recognize where legacy tree islands exist within fire perimeters and to make inferences about how large (~ >900 ha) high severity patches recover from wildfire. We hypothesize that the frequency of small legacy tree islands will be positively correlated with the total area burned by high severity stand replacing fire. We would expect larger wildfires to have both a greater percentage of area burned by high severity stand replacing fire and a higher relative frequency of smaller tree islands.

Results/Conclusions

The objective of this research was to characterize the effects of wildfire through an analysis of the distribution of legacy tree refugia. Our results suggest that large stand replacing patches, created by high severity fire, accounted for >20% of of area burned. Single surviving trees and small tree islands occurred more frequently under high severity classes compared to moderate and low severity which coincide with >80% of legacy tree islands. Our results also show a positive relationship between wildfire size, percentage of area burned by high severity stand replacing fire, and the relative frequency of smaller tree islands. These results have serious implications for ecosystem recovery. Regeneration distances required to initiate forest recovery far exceed 1.5 canopy height or 200 m, distances where the vast majority of regeneration is likely to arise. The size of stand replacing patches and the distance from potential seed sources indicate that large patches of stand replacing fire produce uneven aged forest. Some studies have suggested that large patches of stand replacing fire are natural on the landscape due to evidence of large patches of even aged trees that regenerated within wildfire perimeters. This is not supported by our results which indicate that large patches of stand replacing fires are likely to lead to uneven aged forest, very long recovery times, and great uncertainty in the structure of forest in Western dry conifer forests.