Repeated wildfres alter forest recovery of mixed-conifer ecosystems

Abstract

Most models project warmer and drier climates that will contribute to larger and more frequent wildfres. However, it remains unknown how repeated wildfres alter post-fre successional patterns and forest structure. Here, we test the hypothesis that the number of wildfres, as well as the order and severity of wildfre events interact to alter forest structure and vegetation recovery and implications for vegetation management. In 2014, we examined forest structure, composition, and tree regeneration in stands that burned 1-18 yr before a subsequent 2007 wildfre. Three important findings emerged: (1) Repeatedly burned forests had 15% less woody surface fuels and 31% lower tree seedling densities compared with forests that only experienced one recent wildfre. These repeatedly burned areas are recovering differently than sites burned once, which may lead to alternative ecosystem structure. (2) Order of burn severity (high followed by low severity compared with low followed by high severity) did influence forest characteristics. When low burn severity followed high, forests had 60% lower canopy closure and total basal area with 92% fewer tree seedlings than when high burn severity followed low. (3) Time between fires had no effect on most variables measured following the second fire except large woody fluels, canopy closure and tree seedling density. We conclude that repeatedly burned areas meet many vegetation management objectives of reduced fuel loads and moderate tree seedling densities. These differences in forest structure, composition, and tree regeneration have implications not only for the trajectories of these forests, but may reduce fire intensity and burn severity of subsequent wildfres and may be used in conjunction with future fire suppression tactics.