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Reforestation activities such as tree planting are important management tools to offset carbon emissions and restore forest ecosystem integrity. Severe wildfire activity, a key driver of forest loss, is increasing throughout the western United States (US) and creating an immense backlog of areas needing reforestation. Major financial investments and recent policy changes are expected to accelerate rates of tree planting, yet the broad-scale impact and efficacy of post-fire planting activities remain poorly understood. We quantified the outcomes of recent (1987–2022) post-fire plantings in the US Interior West using remotely sensed estimates of forest cover change and in-situ survival records (69,245 seedlings) spanning 297 unique fire events. Overall, planted areas gained forest cover 25.7 % more rapidly than environmentally similar, unplanted sites in the same fires, and planted seedling survival averaged 79.5 % (SD = 23.2 %) after one growing season. However, the effects of planting were highly variable over time and across environmental gradients. Forest cover gain and planted seedling survival were typically highest in cold, wet areas and when planting was followed by wetter-than-average years. Planting season also shaped outcomes, with late summer or fall plantings performing best on warm, dry sites, and spring plantings performing best in cold, wet areas. Forest cover gain was fastest in planting units that burned at low to moderate severity and had > 20 % post-fire forest cover in the surrounding area. Nearly half of all plantings were completed in such areas, where natural regeneration processes are most likely to promote forest recovery even without intervention. Here, we demonstrate that tree planting can enhance post-fire forest recovery rates at broad scales, though its effects are dependent on a range of environmental and operational factors. Our results help inform realistic expectations of planting outcomes, an issue of global relevance as such projects expand to achieve restoration and climate mitigation goals. • Tree planting is commonly used to achieve restoration and climate mitigation goals • Areas with post-fire tree planting show 25.7 % faster rates of forest regrowth • Survival of planted seedlings averaged 79.5 % after one growing season • Forest regrowth and seedling survival varied widely across the Interior West • Outcomes of post-fire plantings are likely to differ across environmental gradients [ABSTRACT FROM AUTHOR]

Changes in forest structure and shifts in tree species composition have occurred globally due to climate change and altered disturbance regimes. With climate trending toward warmer and drier conditions, these altered forest communities may reorganize in diverse and unpredictable ways. This is especially true in mountain environments where a range of vegetation types and abiotic conditions coexist. In this study, we used long-term permanent plot data from a site spanning broad environmental gradients to assess regeneration and mortality patterns in populations of aspen (Populus tremuloides). The study site, located on the San Francisco Peaks, Arizona, USA, is near the hot, dry edge of the species' range and has experienced compounding pressure from extreme drought, chronic ungulate browsing, and wildfire in the past two decades. Over a 20-year study period, spanning one of the most prolonged drought periods in at least 1200 years, aspen overstory mortality averaged 42 % and was most common in smaller, younger trees and at lower elevations. Aspen regeneration density increased 13 % and was found in a greater proportion of study sites. However, we observed a noticeable lack of stems in the tallest regeneration size class (>200 cm) and the smaller tree size class (2.5–15 cm in diameter), potentially indicating a demographic bottleneck whereby few trees are recruiting into the overstory. Likewise, prolific aspen suckering occurred after a 2001 wildfire, although regeneration density eventually decreased to pre-fire levels, with <1 % of individuals reaching heights >200 cm. Aspen regeneration densities showed the greatest increases in cool, wet sites and beneath open forest canopies. Disturbances function as catalysts for aspen regeneration, but persistence of aspen stands depends on recruitment of stems into overstory size classes, a process that is limited, particularly on lower and more exposed sites. • Multi-decade evidence of aspen changes across mountainous environment. • Significant aspen mortality especially in young and lower-elevation trees. • Despite increased aspen regeneration, notable lack of recruitment into mature size classes. • Aspen regeneration showed the greatest increases in cool, wet sites and beneath open forest canopies. [ABSTRACT FROM AUTHOR]

Across the western United States, large, severe wildfires in montane forests are creating treeless patches that can fail to reforest naturally due to a lack of seed sources and a warming climate. Nursery-grown tree seedlings are commonly planted by land managers into these areas to promote forest recovery and resilience, but uncertainty exists about what influences their survival across the landscape. We obtained survival monitoring data that had been collected one growing season after planting for 5656 tree seedlings, which were distributed across nine montane wildfires and four National Forests in Colorado, USA. We used these data to examine how seedling survival varied across a range of factors, including long-term average climatic conditions, post-planting weather conditions, and locations and species of seed lot collection. We found that survival after one growing season averaged 80 % across all plots but ranged from 0 % to 100 %. Survival was greater at cooler, wetter sites and at sites planted in warmer, wetter years. Survival was also greater for ponderosa pine than for Douglas-fir, and when the locally collected seed lot used to produce the seedlings came from a site that was lower in elevation than the planting site. Our results suggest that the location and timing of tree planting, and the plant materials used, play critical roles in planting success in Colorado montane wildfires, and should help land managers optimize post-fire planted seedling survival under both current and future climatic conditions. • Monitoring data were used to examine drivers of planted tree seedling survival. • Data were collected after one growing season in nine montane Colorado wildfires. • Survival was greater for ponderosa pine seedlings than for Douglas-fir seedlings. • Survival was greater at cool, wet sites and at sites planted in warm, wet years. • Survival increased when seed sources were lower in elevation than planting sites. [ABSTRACT FROM AUTHOR]

• We studied understory plant responses to forest restoration in Colorado, USA. • Restoration increased native plant cover and richness 4–6 years post-treatment. • Restoration increased non-native plant cover and richness, but they were still low. • Open overstory conditions predicted high native understory plant responses. • Average site water stress poorly predicted native understory plant responses. Restoration efforts are underway in dry conifer forests across the western United States to increase their resilience to wildfire and other disturbances. Because such treatments typically decrease overstory density and homogeneity, they can also drive changes in the understory plant community. Past studies of post-treatment changes in understories have found variable results over short time frames and across regions, highlighting the need to study longer-term, region-specific responses. We investigated whether mechanical restoration treatments benefited understory plants in dry conifer forests of the Colorado Front Range, and what biotic and abiotic variables modified understory plant responses in treated areas. We analyzed data collected 1–2 years pre-treatment, 1–2 years post-treatment, and 4–6 years post-treatment in 168 plots, which were distributed across 8 sites and 16 pairs of treated and nearby untreated areas. Treatments were implemented by removing trees with heavy machinery or hand tools (i.e., thinning). By the time treatments were 4–6 years old, native understory plant cover was 1.7 times higher in treated compared to untreated plots, and native richness was 1.1 times higher. Heightened cover and richness values in treated plots were not driven by a single native plant functional group, but by a large portion of the native community; long-lived, graminoid, vegetatively spreading, and non-vegetatively spreading plants all had higher cover in treated plots, while short-lived, long-lived, forb, graminoid, and non-vegetatively spreading plants had higher richness in treated plots. Non-native plants showed 3.1 times higher cover and 4.4 times higher richness in treated compared to untreated plots at 4–6 years post-treatment, but were present at very low levels (e.g., ≤ 0.5% mean cover in either treatment). Greater native plant cover and richness at 4–6 years post-treatment were associated with lower overstory basal areas that resembled 19th-century forest structural conditions for the landscape. Contrary to expectations, a long-term measure of moisture availability (i.e., 30-year average climatic water deficit) was not a strong predictor of native cover or richness in treated plots 4–6 years post-treatment; rather, they were better predicted by moisture availability during the spring months prior to sampling. Overall, the consistent and enduring stimulation of cover and richness of native understory plants after mechanical treatments, with only limited invasion from non-native species, illustrates the important benefits of ongoing restoration activities in dry conifer forests of the Colorado Front Range. [ABSTRACT FROM AUTHOR]

Natural ranges of ecosystem variability (NRVs) describe the array of conditions and processes common to relatively intact or natural ecosystems, and are used to help land managers plan ecological restoration efforts and better understand ecosystem dynamics. In an effort to describe the variation in forest reference conditions and NRVs related to forest structure and spatial pattern across the American Southwest (Arizona and New Mexico, USA) prior to widespread fire exclusion (ca. late 1800s), we used historical inventory data and dendrochronology to reconstruct 33 Pinus ponderosa- dominated sites (0.8–3.2 ha in size) throughout the region. Though the historical stand densities, species compositions, tree spatial patterns, and modern increases in density varied widely among sites, site-level differences could partially be explained by abiotic factors and by forest type. Soil parent material and US Forest Service Terrestrial Ecosystem Units (TEUs) were important predictors of historical stand density, and species composition was a significant predictor of spatial pattern on our sites, with sites dominated by Pinus ponderosa and Quercus gambelii showing more distinct clustering of overstory trees. Reconstructed stand density (live trees ha −1 ) was also correlated with the proportion of trees found in groups. Lastly, the sites showing the smallest increases in stand density tended to be in the warmest, driest locations studied, suggesting that densities in these types of sites may be climatically limited and have changed comparatively less since the late 1800s. [ABSTRACT FROM AUTHOR]

• We surveyed ca. 15 years of cone production in 275 ponderosa pine trees (49 sites). • Weather-drivers of mast years vary widely among individuals and populations. • Cone production was more sensitive to summer precipitation at hotter, drier sites. • Masting behavior was strongly associated tree size, age, and neighborhood density. • Large, old, open grown trees produced more cones, with less variability. Masting describes the spatiotemporal variability in seed production by a population of plants. Both abiotic and biotic factors drive masting, but the importance of these factors can vary among individuals and populations. To better understand how a changing climate, altered disturbance regimes, or novel management strategies might affect future seed production, we quantified the joint influence of multiple factors on annual cone production in a widespread conifer species, Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). We reconstructed individual-level annual cone production across climatic gradients using the cone abscission scar method, and explored the causes and drivers of masting in this species. Site-level responses between weather and masting were highly variable, notably differing in the strength of their response to either summer or spring weather. In addition, the relationship to summer precipitation during cone initiation, a primary driver of annual seed production in this species, was strongest at hotter and drier sites. Additionally, we found that masting was strongly influenced by tree- and stand-level factors such as diameter, age, and local neighborhood density, all of which were associated with the mean, interannual variability, and between-tree synchrony of cone production at the individual-level. Larger and older trees produced more cones, more frequently, and with less synchrony than smaller and younger trees. Open grown trees experiencing lower levels of neighborhood competition also produced more cones with less interannual variability, but with higher between-tree synchrony. Because tree- and stand-level traits appear to regulate seed production more strongly than climate or weather in this species, management interventions targeting these factors could be powerful tools to manage future tree recruitment. Thus, current efforts to reduce stand density and conserve large trees in some ponderosa pine forests may enhance tree-level seed production and reduce variability in seed crops among years. [ABSTRACT FROM AUTHOR]

• Trailing-edge forests in the western US are vulnerable to climate impacts. • Annual maps of land cover revealed minor declines in total forest cover since 1985. • Pine-oak cover increased since 2000, but other conifer-dominated forests decreased. • Wildfire and drought were major drivers of forest change. Climate change is altering the distribution of woody plants by influencing demographic processes and modifying disturbance regimes. Trailing-edge forests may be particularly vulnerable to these effects because they exist at warm, dry margins of tree distributions. To better understand recent climate-driven changes in trailing-edge forests, we used Landsat time series and 1558 field reference plots to develop annual land cover maps from 1985 to 2020 in two large, biodiverse landscapes in central Arizona, USA. We then combined annual land cover maps with tree ring records and spatial data describing interannual climate, terrain, bark beetle (Curculionidae: Scolytinae) activity, wildfire, and harvest to quantify drivers of forest change. Throughout the two landscapes, forest extent declined by 0.3 % and 0.8 % from 1985 to 2020. However, considerable variation occurred within the study period, with abrupt (ca. 1–2 years) declines in forest extent followed by gradual (ca. 10 years) recovery on each landscape. Pinyon-juniper (Pinus edulis, Pinus monophylla , and/or Juniperus spp.) cover increased from 1985 to ca. 2000 but declined after 2000, a period of extreme drought and regional tree die-off. In contrast, pine-oak (Pinus ponderosa and Quercus spp.) cover increased from 2000 to 2020, primarily due to declines in ponderosa pine and mixed conifer cover over the same period. Wildfire was a key driver of transitions from forest to non-forest cover in our study area, with the occurrence of multiple compounded drought years playing an important role in unburned areas. By driving transitions to alternative forest types or non-forest cover, disturbance and drought will increasingly shape forest dynamics and ecosystem transformations throughout the southwestern US. [ABSTRACT FROM AUTHOR]

• Recently-burned area in southwestern US forests is increasing. • Landscape context, values, and future trajectories of change can inform management. • Postfire forest patches can be managed for resilience to future fires. • Novel reforestation practices may help adapt to future climate and fire in some areas. The increasing incidence of wildfires across the southwestern United States (US) is altering the contemporary forest management template within historically frequent-fire conifer forests. An increasing fraction of southwestern conifer forests have recently burned, and many of these burned landscapes contain complex mosaics of surviving forest and severely burned patches without surviving conifer trees. These heterogeneous burned landscapes present unique social and ecological challenges. Severely burned patches can present numerous barriers to successful conifer regeneration, and often contain heavy downed fuels which have cascading effects on future fire behavior and conifer regeneration. Conversely, surviving forest patches are increasingly recognized for their value in postfire reforestation but often are overlooked from a management perspective. Here we present a decision-making framework for landscape-scale management of complex postfire landscapes that allows for adaptation to a warming climate and future fire. We focus specifically on historically frequent-fire forests of the southwestern US but make connections to other forest types and other regions. Our framework depends on a spatially-explicit assessment of the mosaic of conifer forest and severely burned patches in the postfire landscape, evaluates likely vegetation trajectories, and identifies critical decision points to direct vegetation change via manipulations of fuels and live vegetation. This framework includes detailed considerations for postfire fuels management (e.g., edge hardening within live forest patches and repeat burning) and for reforestation (e.g., balancing tradeoffs between intensive and extensive planting strategies, establishing patches of seed trees, spatial planning to optimize reforestation success, and improving nursery capacity). In a future of increasing fire activity in forests where repeated low- to moderate-severity fire is essential to ecosystem resilience, the decision-making framework developed here can easily be integrated with existing postfire management strategies to optimize allocation of limited resources and more actively manage burned landscapes. [ABSTRACT FROM AUTHOR]