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To improve the knowledge of ecosystem dynamics within frequent-fire forests and to develop targets for forest restoration, we dendrochronologically reconstructed four 1-ha plots within dry mixed-conifer forests in northern Arizona, USA. Forest densities in the study area increased from 139.8 live trees ha-1, 10.26 m² of basal area ha-1, and 14.9% canopy cover in 1879 (the assumed year of fire exclusion) to 1,116.8 live trees ha-1, 42.23 m² of basal area ha-1, and 55.3% canopy cover in 2014. Shade-tolerant species also became more prevalent. Initial increases in tree density occurred near the established overstory or randomly throughout each stand, rather than within canopy gaps. Tree spatial patterns were random or aggregated in 1879 and 88.3% of trees were isolated individuals or in groups of 2-4 trees. Sprouting hardwoods and shade-tolerant conifers were more likely than other tree species to have been members of groups, whereas shade-intolerant conifers were more likely to be isolated individuals. Relative shade tolerance and the reproductive strategies of component species contribute to fine-scale spatial patterns in mixed-species forests. This interaction between species silvics and fine-scale spatial patterns is an important consideration for management activities targeting heterogeneity and the natural ranges of variability in frequent-fire forests. [ABSTRACT FROM AUTHOR]

Wildfires in forest ecosystems produce landscape mosaics that include relatively unaffected areas, termed fire refugia. These patches of persistent forest cover can support fire-sensitive species and the biotic legacies important for post-fire forest recovery, yet little is known about their abundance and distribution within fire perimeters. Readily accessible 30-m resolution satellite imagery and derived burn severity products are commonly employed to characterize post-fire landscapes; however, coarse image resolution, generalized burn severity thresholds, and other limitations can constrain accurate representation of fire refugia. This study quantifies the abundance and pattern of fire refugia within 10 fires occurring in ponderosa pine and dry mixed-conifer forests between 2000 and 2003. We developed high-resolution maps of post-fire landscapes using semi-automated, object-based classification of 1-m aerial imagery, conducted imagery- and field-based accuracy assessments, and contrasted these with Landsat-derived burn severity metrics. Fire refugia area within burn perimeters ranged from 20% to 57%. Refugia proportion generally decreased with increasing Landsat-derived burn severity, but still accounted for 3–12% of areas classified as high severity. Patch size ranged from 1-m2 isolated trees to nearly 8000 ha, and median patch size was 0.01 ha—substantially smaller than a 30-m Landsat pixel. Patch size was negatively related to burn severity; distance to fire refugia from open areas was positively related to burn severity. Finally, optimized thresholds of 30-m post-fire normalized burn ratio (NBR) and relative differenced normalized burn ratio (RdNBR) delineated fire refugia with an accuracy of 77% when validated against the 1-m resolution maps. Estimations of fire refugia abundance based on Landsat-derived burn severity metrics are unlikely to detect small, isolated fire refugia patches. Finer-resolution maps can improve understanding of the distribution of forest legacies and inform post-fire management activities including reforestation and treatments. [ABSTRACT FROM AUTHOR]