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Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration.

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. [ABSTRACT FROM AUTHOR]

Shifting fire regimes alter forest structure assembly in ponderosa pine forests and may produce structural heterogeneity following stand-replacing fire due, in part, to fine-scale variability in growing environments. We mapped tree regeneration in eighteen plots 11 to 15 years after stand-replacing fire in Colorado and South Dakota, USA. We used point pattern analyses to examine the spatial pattern of tree locations and heights as well as the influence of tree interactions and topography on tree patterns. In these sparse, early-seral forests, we found that all species were spatially aggregated, partly attributable to the influence of (1) aspect and slope on conifers; (2) topographic position on quaking aspen; and (3) interspecific attraction between ponderosa pine and other species. Specifically, tree interactions were related to finer-scale patterns whereas topographic effects influenced coarse-scale patterns. Spatial structures of heights revealed conspecific size hierarchies with taller trees in denser neighborhoods. Topography and heterospecific tree interactions had nominal effect on tree height spatial structure. Our results demonstrate how stand-replacing fires create heterogeneous forest structures and suggest that scale-dependent, and often facilitatory, rather than competitive, processes act on regenerating trees. These early-seral processes will establish potential pathways of stand development, affecting future forest dynamics and management options. [ABSTRACT FROM AUTHOR]

The Mexican spotted owl (Strix occidentalis lucida) was listed as a threatened species in 1993, primarily because of concerns over the loss of late seral forest habitat to timber harvest and wildfire. A recovery plan prepared for this owl subspecies explicitly assumed that nesting (and/or roosting) habitat was a primary factor limiting distribution of Mexican spotted owls and provided four desired conditions for identifying and managing potential owl nesting/roosting habitat in forested habitat. We used data collected at nest sites of Mexican spotted owls in the Sacramento Mountains, New Mexico, to evaluate how well these desired conditions and associated forest structural attributes described nesting habitat in this area. All nest sites included in our analyses successfully fledged young during the study. These nest sites generally featured higher levels of the structural attributes included in the desired conditions (total basal area, density of trees > 46 cm in dbh, percentage of basal area in trees 30 - 46 cm dbh, and percentage of basal area in trees > 46 cm dbh) than the surrounding stand, yet only 46 - 87% of sampled nest sites met single desired conditions and only 22% met all four conditions simultaneously. The best generalized linear models using combinations of these four structural attributes plus canopy cover to distinguish between nest sites and random sites within owl home ranges all contained canopy cover and percentage of basal area in trees > 46 cm dbh. Relative importance values were high for both of these attributes (1.000 and 0.983, respectively), and confidence intervals around parameter estimates included zero for all other attributes. The present combination of four desired conditions did not consistently identify nesting habitat in this area, required managing for levels of structural attributes that were greater than levels typically observed at successful owl nest sites, and did not include canopy cover, which was the single best predictor in the Sacramento Mountains. We recommend revising the desired conditions in the Sacramento Mountains to emphasize canopy cover and some attribute measuring the large tree component. We also recommend repeating this assessment in other geographic areas to determine how well the desired conditions for those areas describe nesting habitat for owls. [ABSTRACT FROM AUTHOR]