Mixed-Severity Fire Fosters Heterogeneous Spatial Patterns of Conifer Regeneration in a Dry Conifer Forest

Sparkle Malone,Paula Fornwalt,Carolyn Sieg,Marin Chambers,Jose Iniguez,Mike Battaglia

doi:10.3390/f9010045

Abstract

We examined spatial patterns of post-fire regenerating conifers in a Colorado, USA, dry conifer forest 11–12 years following the reintroduction of mixed-severity fire. We mapped and measured all post-fire regenerating conifers, as well as all other post-fire regenerating trees and all residual (i.e., surviving) trees, in three 4-ha plots following the 2002 Hayman Fire. Residual tree density ranged from 167 to 197 trees ha−1 (TPH), and these trees were clustered at distances up to 30 m. Post-fire regenerating conifers, which ranged in density from 241 to 1036 TPH, were also clustered at distances up to at least 30 m. Moreover, residual tree locations drove post-fire regenerating conifer locations, with the two showing a pattern of repulsion. Topography and post-fire sprouting tree species locations further drove post-fire conifer regeneration locations. These results provide a foundation for anticipating how the reintroduction of mixed-severity fire may affect long-term forest structure, and also yield insights into how historical mixed-severity fire may have regulated the spatially heterogeneous conditions commonly described for pre-settlement dry conifer forests of Colorado and elsewhere.

Highlights

Many dry conifer forests of western North America were regulated by a relatively frequent mixed-severity fire regime [1,2,3]
Suitable sites were areas that burned with a heterogeneous mosaic of severities, that were on US Forest Service land that were accessible, and not impacted by post-fire logging or planting activities
We described non-spatial aspects of forest structure using tree density, diameter at breast height (DBH), tree height, basal area (BA), and quadratic mean diameter (QMD) [42,43]

Summary

Introduction

Many dry conifer forests of western North America were regulated by a relatively frequent mixed-severity fire regime [1,2,3]. The clustered spatial pattern commonly associated with historical dry conifer stands—where well-defined groups of trees and individual trees were interspersed in a matrix of treeless openings is further attributed to the relatively frequent historical fire regime [7,8,9]. This heterogeneity in historical forest structure is thought to be more resilient to drought and subsequent wildfires [1], yet little is known regarding the process responsible for this spatial structure. We Forests 2018, 9, 45; doi:10.3390/f9010045 www.mdpi.com/journal/forests

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