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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.

A combination of forest thinning followed by prescribed burning is widely applied in the western United States to increase ecosystem resistance and resilience to disturbances. Understory plant community responses may be driven by both management treatments and climatic factors. Thus, responses to treatments during a 20‐year megadrought have implications for the role of management in fostering adaptive capacity to climate change.We used a network of five sites (600 plots) spanning an environmental gradient in ponderosa pine (Pinus ponderosa) forests of the American Southwest, an ecosystem that is broadly distributed and actively managed throughout the western United States. We used repeated long‐term monitoring data to quantify plant community responses to treatment 1–5‐, 6–10‐ and >10‐year post‐implementation. Specifically, we focussed on the effects of treatment and abiotic conditions on native and non‐native plant cover and species richness and the proportion of native species with northern (cool‐mesic) biogeographic affinities.Overall, thinning and prescribed burning nearly doubled native cover and increased native species richness by about 50% relative to untreated controls. These effects persisted for over a decade after treatment, even under the influence of significant and persistent drought. Cover and richness were also greater on intermediate to wet sites. Finally, native species with northern biogeographic affinities were reduced for up to 5 years after treatment relative to those with southern (warm‐xeric) affinities, and in dry years, indicating that both management and interannual climate variability may foster shifts to plant communities that are more resilient to a warming climate.Synthesis and applications. In ponderosa pine forests of the American Southwest, tree thinning followed by prescribed burning will generally promote restoration goals of increasing resilience to climate change by enhancing the diversity and abundance of native understory plant species, even during a persistent 20‐year megadrought. [ABSTRACT FROM AUTHOR]

Management of many dry conifer forests in western North America is focused on promoting resilience to future wildfires, climate change, and land use impacts through restoration of historical patterns of forest structure and disturbance processes. Historical structural data provide models for past resilient conditions that inform the design of silvicultural treatments and help to assess the success of treatments at achieving desired conditions. We used dendrochronological data to reconstruct nonspatial and spatial forest structure at 1860 in fourteen 0.5 ha plots in lower elevation (~1900-2100 m) ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) forests across two study areas in northern Colorado. Fires recorded by trees in two or more plots from 1667 to 1859 occurred, on average, every 8-15 years depending on scale of analysis. The last fire recorded in two or more plots occurred in 1859. Reconstructed 1860 stand structures were very diverse, with tree densities ranging from 0 to 320 trees-[ha.sup.-1], basal areas ranging from 0.0 to 17.1 [m.sup.2] x [ha.sup.-1], and quadratic mean diameters ranging from 0.0 to 57.5 cm. All trees in 1860 were ponderosa pine. Trees were significantly aggregated in 62% of plots in which spatial patterns could be estimated, with 10% to 90% of trees mainly occurring in groups of two to eight (maximum, 26). Current stands based on living trees with a diameter at breast height of ≥4 cm are more dense (range, 175-1010 trees-[ha.sup.-1]) with generally increased basal areas (4.4 to 23.1 [m.sup.2] x [ha.sup.-1]) and smaller trees (quadratic mean diameters ranging from 15.7 to 28.2 cm) and contain greater proportions of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and Rocky Mountain juniper (Juniperus scopulorum Sarg.). This is the first study to provide detailed quantitative metrics to guide restoration prescription development, implementation, and evaluation in these and similar ponderosa pine forests in northern Colorado. Key words: ecological restoration, ponderosa pine, dendrochronology, stand structure, spatial pattern. L'amenagement de plusieurs forets seches de coniferes dans l'ouest de l'Amerique du Nord vise surtout a favoriser la resilience face aux feux de foret, au changement climatique et aux impacts de l'utilisation des terres dans l'avenir par la restauration de la structure et des processus de perturbation passes de la foret. Les donnees historiques sur la structure fournissent des modeles des conditions passees de resilience qui contribuent a l'elaboration des traitements sylvicoles et aident a evaluer si les traitements reussissent a creer les conditions souhaitees. Nous avons utilise des donnees dendrochronologiques pour reconstituer la structure spatiale et non spatiale de la foret en 1860 dans 14 placettes echantillons de 0,5 ha dans les forets de pin ponderosa (Pinus ponderosa Douglas ex Lawson & C. Lawson) situees a basse altitude (~1900-2100 m) dans deux aires d'etude du nord du Colorado. Les feux detectes chez les arbres dans deux placettes echantillons ou plus de 1667 a 1859 sont survenus en moyenne a tous les 8 ans a 15 ans selon l'echelle utilisee pour l'analyse. Le dernier feu detecte dans deux placettes echantillons ou plus est survenu en 1859. La structure reconstituee des peuplements de 1860 etait tres diversifiee: la densite des arbres variait de 0 a 320 tiges-[ha.sup.- 1], la surface terriere de 0,0 a 17,1 [m.sup.2] x [ha.sup.-1] et le diametre quadratique moyen ([D.sub.g]) de 0,0 a 57,5 cm. Tous les arbres de 1860 etaient des pins ponderosa. Les arbres etaient significativement regroupes dans 73 % des placettes echantillons dans lesquelles les configurations spatiales pouvaient etre estimees: 40 % a 93 % des arbres etaient en groupes d'au moins 2 a 25 arbres. Sur la base des arbres avec un diametre a hauteur de poitrine ≥4 cm, les peuplements actuels sont plus denses (175 a 1010 tiges-[ha.sup.-1]), ont une surface terriere plus elevee (4,4 a 23,1 [m.sup.2] x [ha.sup.-1]) et de plus petites tiges ([D.sub.g] = 15,7 a 28,2 cm); ils contiennent une plus grande proportion de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) et de genevrier saxicole (Juniperus scopulorum Sarg.). C'est la premiere etude qui fournit des mesures quantitatives detaillees pour appuyer l'elaboration, la mise en oeuvre et l'evaluation des prescriptions pour la restauration de ces forets de pin ponderosa et d'autres forets semblables dans le nord du Colorado. [Traduit par la Redaction] Mots-cles: restauration ecologique, pin ponderosa, dendrochronologie, structure des peuplements, configuration spatiale., 1. Introduction Forest management in many dry pine and mixed-conifer forests of western North America is increasingly focused on promoting ecological resilience through restoration of forest structural complexity and natural [...]

The 2002 Hayman Fire burned with mixed-severity across a 400-ha dry conifer study site in Colorado, USA, where overstory tree and surface cover attributes had been recently measured on 20 0.1-ha permanent plots. We remeasured these plots repeatedly during the first post-fire decade to examine how the attributes changed through time and whether changes were influenced by fire severity. We found that most attributes were temporally dynamic and that fire severity shaped their dynamics. For example, low-severity plots experienced a modest reduction in live overstory density due to both immediate and delayed tree mortality, and no change in live overstory basal area through time; in contrast, high-severity plots experienced an immediate and total loss of live overstory density and basal area. Large snag density in low-severity plots did not vary temporally because snag recruitment balanced snag loss; however, in high-severity plots large snag density increased markedly immediately post-fire and then declined by about half by post-fire year ten as snags fell. Mineral soil cover increased modestly immediately post-fire in low-severity plots and substantially immediately post-fire in high-severity plots, but changed little in ensuing years for either severity class. By incorporating pre-fire and repeatedly-measured post-fire data for a range of severities, our study uniquely contributes to the current understanding of wildfire effects in dry conifer forests and should be of interest to managers, researchers, and others. [ABSTRACT FROM AUTHOR]

The 2010 Church's Park Fire burned beetle-killed lodgepole pine stands in Colorado, including recently salvage-logged areas, creating a fortuitous opportunity to compare the effects of salvage logging, wildfire and the combination of logging followed by wildfire. Here, we examine tree regeneration, surface fuels, understory plants, inorganic soil nitrogen and water infiltration in uncut and logged stands, outside and inside the fire perimeter. Subalpine fir recruitment was abundant in uncut, unburned, beetle-killed stands, whereas lodgepole pine recruitment was abundant in cut stands. Logging roughly doubled woody fuel cover and halved forb and shrub cover. Wildfire consumed all conifer seedlings in uncut and cut stands and did not stimulate new conifer regeneration within four years of the fire. Aspen regeneration, in contrast, was relatively unaffected by logging or burning, alone or combined. Wildfire also drastically reduced cover of soil organic horizons, fine woody fuels, graminoids and shrubs relative to unburned, uncut areas; moreover, the compound effect of logging and wildfire was generally similar to wildfire alone. This case study documents scarce conifer regeneration but ample aspen regeneration after a wildfire that occurred in the later stage of a severe beetle outbreak. Salvage logging had mixed effects on tree regeneration, understory plant and surface cover and soil nitrogen, but neither exacerbated nor ameliorated wildfire effects on those resources. [ABSTRACT FROM AUTHOR]

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]