Wildfire has long been an important and complex disturbance agent in forests dominated by ponderosa pine in the western United States. However, many recent fires have burned with increased severity across large, contiguous areas, resulting in vast expanses with no surviving overstory trees. This trend of more severe fires is thought to be the result of greater forest density and homogeneity due to fire suppression and other land use activities since Euro-American settlement, as well as drier conditions and earlier and longer fire seasons.
A multitude of environmental factors likely influence whether ponderosa pine can regenerate in a given location following uncharacteristically severe fire. Ponderosa pine relies on seeds produced by surviving trees to regenerate, yet its seeds generally disperse only short distances. Thus, regeneration success may be closely tied to the proximity of surviving trees.
Elevation and aspect may further influence whether seeds germinate and whether regenerating trees survive, due to their influence soil moisture and temperature. Ectomycorrhizal (EM) fungi can also influence the survivorship of regenerating trees by enhancing pathogen resistance and improving water and nutrient acquisition.
Understanding how regeneration patterns relate to environmental factors such as these can help managers anticipate natural recovery within fire perimeters, and determine whether post-fire management activities, such as tree planting, are needed to maintain a forested condition.
Researchers are exploring regeneration spatial patterns by collecting the geographic location of seedlings, and their results can suggest how sampling methods influence the detection probability of post-wildfire regeneration. Project objectives are to:
Understand the spatial patterns of ponderosa pine and aspen regeneration following large wildfires;
Determine if fire severity and distance from mature forest influence post-wildfire ectomycorrhizal abundance and community composition;
Understand if niches of post-fire ponderosa pine regeneration and seedling growth differ across fire severities; and
This research is being conducted in burn scars in Arizona, Colorado, Wyoming, and South Dakota that experienced severe wildfires. Fires include the Pumpkin Fire in Arizona, Rodeo-Chediski Fire in Arizona, Hayman fire in Colorado, and Jasper Fire in South Dakota. Fires burned at least 10 years ago to allow ample time for tree regeneration to occur. Three complimentary approaches are being used:
Researchers are using data from over 600 100-m2 plots distributed across Colorado, Wyoming, and South Dakota to examine coarse-scale patterns of post-fire regeneration across this broad region. Specifically, they are quantifying how regeneration in severely burned areas compares to that in unburned areas and areas that burned less severely, and how regeneration in severely burned areas is related to environmental factors like distance from surviving trees, elevation, and the density of understory vegetation.
Finer-scale patterns of post-fire tree regeneration in severely burned areas are being examined in 30 40,000-m2 plots located in Arizona, Colorado, and South Dakota. In these plots, researchers are mapping all regenerating trees and exploring their spatial distribution to examine whether facilitative or competitive interactions are occurring, the role of seed dispersal, and the role of slope, aspect, elevation, etc.
Data from 360 1-m2 regeneration-centric plots within severely burned portions of the Pumpkin Fire, Arizona, are helping researchers characterize regeneration niches – that is, the environmental factors associated with ponderosa pine regeneration and growth at the microsite scale. In each plot, researchers measured topographic attributes including slope and aspect, substrate attributes including litter, rock, bare ground, and understory vegetation cover, EM fungi attributes, and soil physical and chemical attributes.
Researchers also set up an experiment to understand if EM abundance, inoculum potential, and community composition differ with distance from mature forest and fire severity. They are exploring if EM abundance, inoculum potential, and community composition influence seedling growth.
Researchers are exploring how plot size influences seedling detection by using spatial data to simulate optimal combinations of quadrat size and sample size. They are using a similar method to develop sampling protocols for accurately estimating seedling density.
Preliminary results suggest dense regeneration of oak and juniper in high-severity burn areas on the Rodeo-Chediski Fire. Ponderosa pine regeneration is aggregated near the forest edge on the Pumpkin Fire but more randomly dispersed (>200 meters beyond the edge) in high-severity burn areas.
Please see the “Publications” section for more information on findings from completed portions of the project.