Tree spatial patterns in fire-frequent forests of western North America, including mechanisms of pattern formation and implications for designing fuel reduction and restoration treatments

Restoring characteristic fire regimes and forest structures are central objectives of many restoration and fuel reduction projects. Within-stand spatial pattern is a fundamental attribute of forest structure and influences many ecological processes and ecosystem functions. In this review we synthesize the available spatial reference information for fire-frequent pine and mixed-conifer forests in western North America; interpret this information in the context of restoration and fuel reduction treatment design; and identify areas for future research, including recommended approaches for quantifying within-stand tree spatial patterns. We identified 50 studies of tree spatial patterns in fire-frequent pine and mixed conifer forests, 25 of which documented spatial reference conditions. The characteristic structure of fire-frequent forests is a mosaic of three elements: openings, single trees, and clumps of trees with adjacent or interlocking crowns. This mosaic structure typically manifests at scales <0.4 ha, but sometimes extends to scales as large as 4 ha, particularly on sites with fire regimes that include both low- and moderate-severity fires. We documented preferential use of global pattern analysis techniques (90% of analyses) relative to local analysis techniques (10% of analyses). Ripley’s K statistic, an example of global spatial pattern analysis, was the most frequently used analytic technique (38% of analyses). These findings are important because global pattern analysis does not explicitly quantify spatial heterogeneity within a pattern, the very thing spatial reference studies seek to characterize and one of the core structural attributes treatments aim to restore. Based on these findings, we encourage managers to consciously adopt a view of forest structure that accommodates spatial heterogeneity within forest stands, and to use this conceptualization of forest structure to guide prescription development. Restoration prescriptions and marking guidelines that explicitly incorporate within-stand spatial heterogeneity—such as by specifying the numbers and sizes of openings and tree clumps, and the number of widely-spaced single trees to retain per unit area—will improve the likelihood of restoring characteristic forest structures and the ecological processes such structures support. We infer that the near-exclusive use of global pattern analysis has limited the quan- tity and usability of spatial reference information available to managers, has also likely limited the development and testing of novel ecological hypotheses about pattern-generating mechanisms. Consequently, we recommend that forest scientists change how they quantify tree spatial patterns by complimenting the traditional global analysis methods with local pattern analysis techniques, which quantify spatial heterogeneity within a study area.