Historical and current fire regimes in ponderosa pine forests at Zion National Park, Utah: Restoration of pattern and process after a century of fire exclusion

Abstract

Plant community resilience in frequent-fire forests of western North America has been compromised by over a century of fire exclusion. However, there are several western National Parks where fires have been reintroduced over the past several decades, including Zion National Park in southwest Utah. Here we reconstruct historical components of fire regimes in ponderosa pine dominated forests at Zion and compare these to recent fire frequency, current plant community and fuels structure, and potential fire behavior. Historical fires burned every 9–10 years on average up until 1879, when fires ceased contemporaneous with introduction of Euro-American livestock grazing and timber harvest in upland forests. Abundant tree regeneration occurred after fire exclusion, with tree density averaging 45 trees ha−1 in reconstructed 1880 forests versus 106 trees ha−1 today. Intervals between recent (since 1988) wildfires and prescribed fires in these same stands ranged from 7 to 13 years, similar to historical fire timing. Depending on whether plots had burned from zero to three times in recent fires, we found significant differences in canopy base heights (increased), duff and litter depths (decreased), and percent cover of grass and forbs (increased), but not tree density, tree basal area, shrub height, shrub cover, or woody fuels. Combined effects of recent fires on overstory and understory structure resulted in a significant difference in likelihood of crown fire occurrence, declining from a mean of 58% in plots with no fire since 1879 to 13% in plots with three fires since 1988. Significant effects were generally seen after two or three fires, suggesting it is the reintroduction of the fire regime and not just individual fire events that restore resiliency. Overall, effects of recent fires are building on the latent resiliency of ponderosa pine forests at Zion National Park, although questions remain about extent and future dynamics of oak and manzanita shrubfields that occupy similar environmental settings, along with a general lack of ponderosa pine regeneration across all plots.