Numerous factors influence the establishment and growth of tree seedlings in high-severity burned patches. Understanding spatial patterns and environmental conditions influencing ponderosa pine and aspen regeneration post-wildfire can help managers monitor natural recovery. Heavy fuel loads combined with climate changes such as drought and increasing temperatures are altering fire regimes in ponderosa pine forests. These forests historically experienced frequent low-severity fires, but recently, wildfires have increased in size and severity. There is concern for the future sustainability of ponderosa pine forests in large burn patches that kill seed-producing trees, which can prevent or significantly delay post-fire pine regeneration.
Tree regeneration within high-severity patches can be influenced by a number of factors including post-wildfire microhabitat, distance to seed source, soils properties, plant and mycorrhizal fungi communities, and time since fire. Ectomycorrhizal (EM) fungi can also influence the performance and survivorship of regenerating seedlings by enhancing pathogen resistance and improving acquisition of water and nutrients. Understanding the spatial patterns and environmental conditions that influence growth and survival of post-wildfire regenerating ponderosa pine and aspen trees can help managers monitor natural recovery or assist with management prescriptions.
Researchers from the Rocky Mountain Research Station, Northern Arizona University, and Colorado State University are quantifying post-fire conifer regeneration in multiple large wildfires where pre-fire forests were dominated by ponderosa pine, and they are examining how regeneration is governed by distance from seed source and other factors. Specific 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 EM abundance and community composition;
understand if post-fire seedling densities, growth, and micro-habitat differ across fire severities; and
determine if plot size influences seedling detection probability when sampling post-wildfire tree regeneration.
Post-fire seedlings can be rare or their distributions can be aggregated, leading to heterogeneity in detection probabilities when assessing regeneration patterns. By collecting the geographic location of each seedling, this study will help managers understand regeneration spatial patterns and the influence of sampling methods on the detection probability of post-wildfire regeneration.
This research is being conducted in areas where large wildfires have occurred in Arizona and Colorado. All fire sites are 10 years old or older, allowing ample time for conifer regeneration to occur. Three complimentary approaches are being used:
Examining post-fire regeneration in replicated 200 X 200 m plots of moderate/mixed-severity burned areas and in the edge and interior (200 m from the unburned forest edge) of high-severity burned areas in the 2000 Pumpkin Fire (photo, right) and the 2002 Rodeo-Chediski Fire. Researchers are quantifying regeneration patterns, distance to seed-source trees, and size of openings by collecting the spatial location, height, species, and diameter at breast height of all seedlings, saplings (<1.5 m tall), and closest seed-source trees to each seedling.
Collecting post-fire tree regeneration and other data in 100 m2 plots that are distributed every 25 to 50 m along transects. Transects are anchored 50 m inside live forest edges and extend out into the high severity burn areas up to 250 m into the burn scar of the 2002 Hayman Fire in Colorado.
Randomly sampling 360 1- X 1-m seedling-centric quadrats across different fire severities within the larger plots on the Pumpkin Fire to determine the environmental factors most strongly associated with ponderosa regeneration and growth. In each seedling quadrat, researchers measured aspect, percent slope, and overstory cover. They measured ground cover variables including total plant, litter, rocks, moss, bare ground, vegetative form (herbaceous plants, graminoids, and shrubs), exotic plants, and individual plant species cover, and they measured litter depth and soil physical and chemical properties.
To understand if EM abundance, inoculum potential, and community composition differ with distance from mature forest, researchers set up a mycorrhizal inoculum potential experiment along gradients of disturbance caused by both fires.
The researchers are using a simulation study to explore the impact of spatial methods on detection probability of post-wildfire regeneration. They are estimating optimum plot size by using collected spatial data to simulate regeneration at a 1- X 1-m step increment and to set an optimal plot size at the point where 95 percent of the plots contain a “1” (1 = proportion of plots X plot size for true occupancy). They will construct surfaces from which optimal combinations of quadrat size and sample size can be assessed to reach optimal sampling effort. A similar method will be used to determine optimal methodology to accurately estimate seedling density.
The research team completed field work on the Pumpkin Fire and Hayman fire, and will complete work on the Rodeo-Chediski Fire in Fall 2017. Preliminary results reveal ponderosa pine regeneration densities were lower farther from forest edges, and displayed spatial aggregation in both the edge and interior plots on both wildfires. Native sprouting trees dominated tree regeneration on the Rodeo-Chediski Fire, but they did not influence ponderosa pine spatial locations or height. At this stage of development, these heterogeneous patches, characterized by drought-tolerant sprouting species or low pine densities, could be more resilient to climate change and severe wildfires than the overly-dense ponderosa pine forests that were present before the wildfires.