COS 148-2 - Multidecadal understory vegetation dynamics in a type-converted ponderosa pine forest

Abstract: Recent increases in fire activity have placed greater attention on how society responds to fires, and the long-term implications of management decisions. Changes in disturbance regimes and climate conditions are leading to post-fire vegetation communities that diverge strongly from pre-fire communities, e.g., type conversions from forest to non-forest ecosystems or invasions of non-native species. Dry mixed conifer and ponderosa pine forests of the western United States are at a heightened risk for type-conversion following combined drought and high severity fire. Post-fire aerial seed treatments are widely used to reduce the potential for adverse fire effects on ecosystem processes, i.e., preventing erosion or prohibiting the establishment of unwanted species, but still relatively little is known about the ecology of type-converted forests, or how seed treatments affect native vegetation recovery over the long-term. In this talk, we explore the drivers and dynamics of vegetation type conversion in a former ponderosa pine forest. We documented a 23-year period of change in understory vegetation dynamics at 49 transects following the 1996 Dome fire and subsequent reburn in the 2011 Las Conchas fire in northern New Mexico. Mixed models were used to relate vegetation dynamics over time to climate, management history, and previous burn severity. Overtime, conifer regeneration decreased while shrub and grass cover increased markedly. Seeded plots showed initial, modest increases in total plant cover in the first two years post-fire, favoring non-native seeded grasses (annual rye, mountain brome, and slender wheatgrass) at the expense of native grass and forb cover. However, seeded species declined in abundance in later years, and smooth brome, a contaminant in the seed mix, became dominant at most plots. Reburning led to rapid recovery of smooth brome populations while reducing native-grass cover by half, suggesting these changes may be long lasting, and further management may be required to control this species. Here, we show that wildfires and climate change are reshaping forests, and management actions may have lasting effects on ecosystem reorganization. Similar changes are likely ongoing in many dry conifer forests throughout the southwest United States.