PS 3-41 - Post-fire plant community response in subalpine mixed conifer and ponderosa pine forests

Abstract: Vegetation type conversion is one of the most pressing issues facing land managers today. Specifically in high elevation forests, a combination of decades of fire suppression, outlawing of indigenous burnings, and climate change, has resulted in higher fuel loads. The accumulation of vegetative material over time can create conditions that alter the typical fire regime of an area. Historically, high elevation conifer forests experienced small, low severity fires at regular intervals. Now, due to the factors described above, researchers predict an increase in frequency and intensity of wildfires which may alter the forest plant community composition and overall structure. This has led to growing concerns on how positive fire-vegetation feedbacks are further altering the vegetation and leading to loss of forest through land conversion.

Here, we explore the post fire successional trajectory of secondary growth subalpine mixed conifer and ponderosa pine forests in the Valles Caldera National Preserve (VCNP) in Northern New Mexico, USA through the following questions:

1. Do burned sites significantly differ from unburned sites?


2. Are forests converging with nearby established upper montane valley grasslands?


3. Which species are driving the changes (if any) between burned and unburned forest sites?

After the 2011 Las Conchas fire, the National Park Service installed 36 monitoring sites in three eco-units: Ponderosa Pine (PP), Mixed Conifer (MC) and Mountain Valley Grassland (MV). Each eco-unit has 6 burned and 6 unburned control sites. Surveys of each site are completed using the line-point intercept method and individual plants are identified to species. Data was collected annually from 2011-2015 and in 2022. Nonmetric multidimensional scaling with Bray-Curtis dissimilarity was used to visualize the community composition over time. Permutational multivariate analysis of variance followed by pairwise comparisons were utilized to evaluate the dissimilarity between treatment each year. Species were evaluated using SIMPER.

Preliminary findings show forest understory plant communities are relatively resilient to high severity fire. Through two way repeated measures ANOVA, no significant difference was shown between forest treatments 11 years post fire in species richness or evenness. Graminoid and forb abundance, and total cover did significantly increase overtime in the burned sites, as did the abundance of nonnative species. Based on pairwise comparisons, both burned MC and PP were consistently more similar to the unburned forests than to grasslands. Our results suggest that while these forests are not transitioning into grasslands, the absence of tree cover is leading to a difference in species composition.