Spatial heterogeneity and structural development pathways following mixed-severity fire in western larch/mixed-conifer forest

Background/Question/Methods

Wilderness areas embody a scientific resource because they are places where ecological processes are largely free from direct human interference. This is especially true for lightening-ignited fires, which are aggressively suppressed virtually everywhere outside of large wilderness areas. Since the 1980s managers have allowed many fires to burn with minimal interference in forests of the Bob Marshall Wilderness in northwestern Montana, USA. We used this contemporary active fire regime to investigate fire-effects and post-fire structural development in western larch (Larix occidentalis)/mixed-conifer forest. Our study system comprises mixed-conifer forest in the South Fork Flathead River valley within the BMW. We reconstructed pre-fire live tree populations and mapped all dead trees ≥20 cm dbh and all live trees ≥5 cm dbh within a randomly located 4.41 ha (210 m x 210 m) study area that burned in 2003. We sampled coarse woody debris (CWD) and tree regeneration within 49 contiguous 900 m² quadrats (30 m x 30 m) superimposed on our mapped study area. 

Results/Conclusions

Mortality of stems ≥20 cm dbh for all species combined was 50%, and for the three most abundant species, Larix occidentalis, Pseudotsuga menziesii, and Pinus contorta was 7%, 43%, and 88%, respectively. We identified four post-fire tree regeneration groups at the 900 m² quadrat scale that span a gradient from low densities of slow growing, shade-tolerant tree regeneration (RG1), to extremely high densities of very fast growing shade-intolerant species (RG4). The RGs were associated with different fire severities and residual live tree size structures at the 900m² quadrat scale. Percent mortality of trees ≥20 cm dbh increased, and density of live trees ≥5 cm dbh decreased from RG1 to RG4, but volume of CWD did not vary across RGs. The RGs and their associated residual stand structures assemble into four distinct post-fire structural development pathways, revealing some of the successional mechanisms through which fires generate and maintain structural complexity and spatial heterogeneity in mixed-conifer forests. Based on these results we suggest that forest restoration and ecological forestry in western larch/mixed-conifer forests will require different conceptual models and silvicultural systems from those appropriate to dry forests with regimes of frequent low-severity fires, or moist forests with regimes of infrequent high-severity fires.