Tree regeneration following wildfires in the western US: a review

Abstract

Background: Wildfires, like many disturbances, can be catalysts for ecosystem change. Given projected climate change, tree regeneration declines and ecosystem shifts following severe wildfires are predicted. We reviewed scientific literature on post-fire tree regeneration to understand where and why no or few trees established. We wished to distinguish sites that won't regenerate to trees because of changing climate from sites where trees could grow post fire if they had a seed source or were planted, thus supporting forest ecosystem services for society and nature, such as timber supply, habitat, watershed protection, and carbon storage. Results: Our literature review showed that little to no post-fire tree regeneration was more common in lowelevation, dry forest types than in high-elevation forest types. However, depending on the region and species, low tree regeneration was also observed in high elevation, moist forests. Regeneration densities varied by species and seedling densities were attributed to distances to a seed source, water stress or precipitation, elevation, slope, aspect, and plant competition. Our findings provide land managers with two primary considerations to offset low tree regeneration densities. First, we supply a decision support tool of where to plant tree seedling in large high severity burned patches. Second, we recommend possibilities for mitigating and limiting large high severity burned patches to increase survival of trees to be sources of seed for natural regeneration. Conclusions: Few or no tree seedlings are establishing on some areas of the 150+ forest fires sampled across western US, suggesting that forests may be replaced by shrublands and grasslands, especially where few seed source trees survived the wildfires. Key information gaps on how species will respond to continued climate change, repeated disturbances, and other site factors following wildfires currently limit our ability to determine future trends in forest regeneration. We provide a decision tree to assist managers in prioritizing post-fire reforestation. We emphasize prioritizing the interior of large burned patches and considering current and future climate in deciding what, when, and where to plant trees. Finally, managing fires and forests for more seed-source tree survival will reduce large, non-forested areas following wildfires where post-fire management may be necessary.

Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests

“…The proportion and spatial configuration of fire severity in fire‐prone forests are key determinants of their long‐term persistence (Stevens et al ; Steel et al ). Lower‐severity fire or scattered patches of higher‐severity fire reduce the risk of conversion to a non‐forest vegetation type (Kemp et al ; Stevens‐Rumann et al ; Walker et al ), while prospects for forest regeneration are bleaker when high‐severity patch sizes are much larger than the natural range of variation for the system (Miller & Safford ; Stevens et al ; Stevens‐Rumann & Morgan ). Thus, the forest‐structure‐mediated feedback between past and future fire severity underlies the resilience of the Sierra Nevada yellow pine/mixed‐conifer system.…”

“…Most dry coniferous tree species in frequent-fire forests did not evolve mechanisms to protect propagules (e.g. seeds, buds/stems that can resprout) from high-severity fire, so recruitment in large patches with few or no surviving trees is often limited by longer-distance dispersal of tree seeds from unburned or lower-severity areas (Welch et al 2016;Stevens-Rumann & Morgan 2019). In the Sierra Nevada, the absence of tree seeds after severe wildfire can lead to forest regeneration failure as resprouting shrubs outcompete slower-growing conifer seedlings and provide continuous cover of flammable fuel that makes future highseverity wildfire more likely (Collins & Roller 2013;Coppoletta et al 2016), though this pathway does not materialise in forests with a slower post-fire vegetation response (Prichard & Kennedy 2014;Stevens-Rumann et al 2016).…”

Still standing: Recent patterns of post-fire conifer refugia in ponderosa pine-dominated forests of the Colorado Front Range

“…Mixed stands of coniferous and/or angiosperm trees with ponderosa pine are expected to vary in post-fire regeneration depending on species-specific regeneration processes and gradients of fire severity and climate [28,48]. Sprouting woody shrubs, such as Gambel oak, and suckering aspen stands have shown areas of abundant post-fire regeneration in our study region [28,[49][50][51].…”

Limitations to recovery following wildfire in dry forests of southern Colorado and northern New Mexico, USA

“…If reforestation of recently burned areas remains an important goal, relatively cool/wet sites should be prioritized first because seedling survival in these areas is likely to be greatest, now and into the future. Within these wetter areas, planting units should be limited to locations where available seed sources are most distant (Stevens‐Rumann and Morgan ). The statistical analyses and spatial models presented here help to locate these types of sites in heterogenous burned landscapes by identifying areas that are climatically suitable, but in which recovery is limited by the proximity to live conifers.…”