Pre-Fire Treatments Have Persistent Effects on Post-Fire Plant Communities

Wildfires characterized by large areas of high severity are increasingly occurring in ponderosa pine (Pinus ponderosa P. & C. Lawson) forests of the Southwest to extents that are out of the natural range of variability. Managers are now routinely applying thinning and/or burning treatments to reduce fire severity. To investigate the effects of pre-fire treatments and fire severity on post-fire vegetation recovery, we re-measured established plots on the 2002 Rodeo-Chediski Fire on the White Mountain Apache Tribal (WMAT) lands eight years post-fire and the Apache-Sitgreaves National Forest (ASNF) nine years post-fire. On the WMAT lands we re-measured 70 plots stratified by fire severity (high, low) and pre-fire treatment (untreated, and cut/burned). We found significantly higher overall plant cover, exotic forb cover (although this was still low, <1%) and pine regeneration frequency in high severity areas, and highly significant overall differences in plant community composition and abundance between severity classes. Pre-fire treatment also influenced vegetation response within fire severity class. In particular, pine regeneration was more frequent in pre-fire treated areas than untreated areas for both severity classes, which we linked to a generally more open canopy in treated low severity fire areas and to a more heterogeneous neighborhood severity pattern in treated high severity areas. On the ASNF portion of the study, we re-measured 80 plots in paired pre-fire thinned sites, which were less severely burned, and pre-fire unthinned sites that were more severely burned. Plant community composition and abundance in thinned and unthinned areas were 3 converging nine years post-fire; however, persistent differences included significantly higher overall plant cover, as well as higher mean shrub cover in the untreated areas. A low exotic species response was observed on both study areas, but we did detect an increased frequency of some exotic species compared to the initial observations. Studies throughout the Southwest have documented varying rates of exotic species invasions, suggesting there is no clear pattern between fire severity and exotic species response. Although high severity fire can increase growing space for exotic species, post-fire management practices, on-site propagules and weather patterns may be the more important drivers of exotic response

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

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. © 2023 the Author(s)

Appendix C. Forest structure and live aboveground carbon stock at the end of the 100-year simulations.

Forest structure and live aboveground carbon stock at the end of the 100-year simulations

Appendix A. Summary of the Climate-Forest-Vegetation Simulator (Climate-FVS) key features.

Summary of the Climate-Forest-Vegetation Simulator (Climate-FVS) key features

Supplement 1. Species’ climate profile file.

<h2>File List</h2><div> <a href="ClimateProfile.csv">ClimateProfile.csv</a> (MD5: b59a0a24df004519f6546ee658306f64)</div><h2>Description</h2><div> <p>ClimateProfile.csv is a comma-separated text file containing the 'Species' Climate profile' data set for the studied stands obtained through the Get Climate-FVS Ready Data (<a href="http://forest.moscowfsl.wsu.edu/climate/customData/">http://forest.moscowfsl.wsu.edu/climate/customData/http://forest.moscowfsl.wsu.edu/climate/customData/</a>, accessed13 March 2012). Column definitions are:</p> -- TABLE: Please see in attached file. -- </div

Effects of postfire climate and seed availability on postfire conifer regeneration.

Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform postfire management in low elevation forests of California, USA, we collected 5-yr postfire recovery data from 1,234 study plots in 19 wildfires that burned from 2004-2012 and 18&nbsp;yrs of seed production data from 216 seed fall traps (1999-2017). We used these data in conjunction with spatially extensive climate, topography, forest composition, and burn severity surfaces to construct taxon-specific, spatially explicit models of conifer regeneration that incorporate climate conditions and seed availability during postfire recovery windows. We found that after accounting for other predictors both postfire and historical precipitation were strong predictors of regeneration, suggesting that both direct effects of postfire moisture conditions and biological inertia from historical climate may play a role in regeneration. Alternatively, postfire regeneration may simply be driven by postfire climate and apparent relationships with historical climate could be spurious. The estimated sensitivity of regeneration to postfire seed availability was strongest in firs and all conifers combined and weaker in pines. Seed production exhibited high temporal variability with seed production varying by over two orders of magnitude among years. Our models indicate that during droughts postfire conifer regeneration declines most substantially in low-to-moderate elevation forests. These findings enhance our mechanistic understanding of forecasted and historically documented shifts in the distribution of trees

Tamm Review: Reforestation for resilience in dry western U.S. forests

The increasing frequency and severity of fire and drought events have negatively impacted the capacity and success of reforestation efforts in many dry, western U.S. forests. Challenges to reforestation include the cost and safety concerns of replanting large areas of standing dead trees, and high seedling and sapling mortality rates due to water stress, competing vegetation, and repeat fires that burn young plantations. Standard reforestation practices have emphasized establishing dense conifer cover with gridded planting, sometimes called \u27pines in lines\u27, followed by shrub control and pre-commercial thinning. Resources for such intensive management are increasingly limited, reducing the capacity for young plantations to develop early resilience to fire and drought. This paper summarizes recent research on the conditions under which current standard reforestation practices in the western U.S. may need adjustment, and suggests how these practices might be modified to improve their success. In particular we examine where and when plantations with regular tree spacing elevate the risk of future mortality, and how planting density, spatial arrangement, and species composition might be modified to increase seedling and sapling survival through recurring drought and fire events. Within large areas of contiguous mortality, we suggest a “three zone” approach to reforestation following a major disturbance that includes; (a) working with natural recruitment within a peripheral zone near live tree seed sources; (b) in a second zone, beyond effective seed dispersal range but in accessible areas, planting a combination of clustered and regularly spaced seedlings that varies with microsite water availability and potential fire behavior; and (c) a final zone defined by remote, steep terrain that in practice limits reforestation efforts to the establishment of founder stands. We also emphasize the early use of prescribed fire to build resilience in developing stands subject to increasingly common wildfires and drought events. Finally, we highlight limits to our current understanding of how young stands may respond and develop under these proposed planting and silvicultural practices, and identify areas where new research could help refine them

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

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