Topographic information improves simulated patterns of post‐fire conifer regeneration in the southwest United States

Abstract

The western United States is projected to experience more frequent and severe wildfires in the future due to drier and hotter climate conditions, exacerbating destructive wildfire impacts on forest ecosystems such as tree mortality and unsuccessful post‐fire regeneration. While empirical studies have revealed strong relationships between topographical information and plant regeneration, ecological processes in ecosystem models have either not fully addressed topography‐mediated effects on the probability of plant regeneration, or the probability is only controlled by climate‐related factors, for example, water and light stresses. In this study, we incorporated seedling survival data based on a planting experiment in the footprint of the 2011 Las Conchas Fire into the Photosynthesis and EvapoTranspiration (PnET) extension of the LANDIS‐II model by adding topographic and an additional climatic variable to the probability of regeneration. The modified algorithm included topographic parameters such as heat load index and ground slope and spring precipitation. We ran simulations on the Las Conchas Fire landscape for 2012–2099 using observed and projected climate data (i.e., Representative Concentration Pathway 4.5 and 8.5). Our modification significantly reduced the number of regeneration events of three common southwestern conifer tree species (piñon, ponderosa pine, and Douglas‐fir), leading to decreases in aboveground biomass, regardless of climate scenario. The modified algorithm decreased regeneration at higher elevations and increased regeneration at lower elevations relative to the original algorithm. Regenerations of three species also decreased in eastern aspects. Our findings suggest that ecosystem models may overestimate post‐fire regeneration events in the southwest United States. To better represent regeneration processes following wildfire, ecosystem models need refinement to better account for the range of factors that influence tree seedling establishment. This will improve model utility for projecting the combined effects of climate and wildfire on tree species distributions.

Restoring frequent fire to dry conifer forests delays the decline of subalpine forests in the southwest United States under projected climate

“…Simultaneously, increasing aridification is making forests more flammable as dead fuels become increasingly available to burn (Goodwin et al, 2020(Goodwin et al, , 2021Juang et al, 2022). High-severity fire combined with warmer and drier conditions is already decreasing conifer regeneration in the western United States (Stevens-Rumann et al, 2018) and projections under future climate show limited conifer regeneration following high-severity wildfire in southwestern US forests (Jung et al, 2023;Keyser et al, 2020). A greater increase in high-severity fires occurring at high elevation in mixed-conifer forests compared with lower elevations is also expected, leading to high mortality of the dominant species, Engelmann spruce and subalpine fire, which are not adapted to fire (Cassell et al, 2019;Flatley & Fulé, 2016;Remy et al, 2021).…”

Mortality thresholds of juvenile trees to drought and heatwaves: implications for forest regeneration across a landscape gradient

“…Even with successful seedling establishment, post-disturbance tree survival can be precarious for multiple years (White, 1985), and survival may be further impeded by subsequent climate extremes (Kemp et al, 2019;Urli et al, 2023). Survival of juvenile trees can be highly sensitive to temperature-related factors of microclimate, such as vapor pressure deficit (Crockett and Hurteau, 2022) and heat load index (Marsh et al, 2022;Jung et al, 2023). Previous studies have addressed seedling recruitment under changing climate (Davis et al, 2019;Kemp et al, 2019;Bailey et al, 2021;Nelson et al, 2021;Stewart et al, 2021), yet few have examined drivers of mortality in subsequently established juvenile trees (e.g., Rother and Veblen, 2016).…”