Topography and fire legacies drive variable post-fire juvenile conifer regeneration in eastern Oregon, USA

Spatial and temporal drivers of post-fire tree establishment and height growth in a managed forest landscape

“…Abundant, natural post-fire tree establishment and fast height growth rates of juvenile trees in most plots imply that dry to moist mixed conifer forests are rapidly recovering following the School Fire. High abundances of post-fire regeneration, similar to those observed in the School Fire, were also found following multiple fires in the southern Blue Mountains (Downing et al 2019;Boag et al 2020). However, we did observe very low abundances of post-fire regeneration (<60 stems ha −1 ) 16 years following high-severity fire in plots (4%) near the forest-grassland ecotone where forests in the Blue Mountains and western US appear to be more vulnerable to conversion from forest to non-forest or require longer periods to recover (Downing et al 2019;Stevens-Rumann and Morgan 2019;Boag et al 2020).…”

“…The annual precipitation is ~700mm, and the mean monthly maximum temperatures are ~18°C in July and August and the mean monthly minimum temperatures are −0.4°C in December and January 1 Predictor variables and expected direction of effect in analyses of annual post-fire establishment, post-fire juvenile stem density, and annual height growth (HG) increment with median and range of plot data. Expected effects ("+" positive, "-" negative, "NT" not tested) were based on prior studies (see superscript numbers and footnotes) in the Blue Mountains ecoregion or neighboring ecoregions a (Boag et al, 2020) b (Downing et al, 2019) c (Povak et 2013, and global climate models project temperatures will increase an additional 2.4 to 3.1 °C by 2050 relative to 1970-1999 (Halofsky et al 2018). Annual precipitation has not significantly changed, and model projections do not agree on the direction of change in future annual precipitation (Clifton et al 2018).…”

“…To assess the likelihood for post-fire recovery to forest (i.e., resilience), we compared post-fire stem densities to local silvicultural stocking recommendations and pre-fire stem densities. On lands managed by the USDA Forest Service in the Blue Mountains, minimum stocking recommendations range from ~60 trees ha −1 in lower elevation and drier forest types to >350 trees ha −1 in higher elevation and cooler-moister forest types (Powell 1999), and these stocking rates were used in similar post-fire regeneration studies of dry to moist mixed conifer forest types in the PNW (Povak et al 2020;Boag et al 2020). Specifically, we compared postfire stem density (sum of juveniles and surviving trees) for each plot to four post-fire stem density thresholds:…”

“…Increased fire activity is expected to catalyze changes in dry to moist mixed conifer forest communities, such as decreases in forested area, lower canopy cover within forested areas, and shifts toward more drought-tolerant species (Kerns et al 2018). Researchers have identified key limitations to the densities of post-fire seedling establishment (e.g., seed availability and moisture deficit) in the Blue Mountains (Downing et al 2019;Boag et al 2020), but the following aspects of post-fire ecosystem recovery have yet to be explored: (1) the influence of interannual climate on seedling establishment for multiple conifer species (but see Hankin et al 2019 in northern Rockies), (2) drivers of the spatial and temporal variability in annual height growth (but see Littlefield 2019 in the eastern Cascade Mountains), (3) shifts in post-fire tree composition and structure over time, and (4) the implications of post-fire management practices for regeneration (but see Povak et al 2020for effects of salvage logging in neighboring ecoregions).…”

Not Only Severe Events: Moderate Dry Periods Impact the Hydraulic Functioning and Survival of Planted Ponderosa Pine Seedlings

“…Forest recovery following wildfire is declining and becoming increasingly challenging to achieve through management interventions that seek to replicate past post-wildfire regeneration patterns [7,49,50]. The unfavorability of post-wildfire landscapes for tree recovery (when seed sources are available) has been well-documented [5,14,51], and recent studies point to the further impact of climate change in making post-wildfire environments inhospitable for recovery, both across broad burned areas and within specific microenvironments such as south-facing aspects [1,4,7,49,52]. Additionally, high soil temperatures and complete removal of the litter layer during high severity wildfire are associated with a large decline in post-fire soil water infiltration [53,54].…”