Long‐term vegetation responses to reintroduction and repeated use of fire in mixed‐conifer forests of the Sierra Nevada

Abstract

Abstract. Nearly a century of fire suppression has changed fundamental aspects of the structure and functioning of fire-adapted forests throughout the western U.S. Prescribed fire is increasingly used to restore forest structure and reduce surface fuels with limited consideration of its consequences for biological diversity. In this study, we used more than two decades of data from permanent plots in mixedconifer forests of Sequoia and Kings Canyon National Parks, California, to explore changes in plant diversity and abundance following reintroduction and repeated use of fire. Data on stand structure, fuel loading, fire severity and heterogeneity, and the richness and abundance of major growth forms were collected on 51 plots representing one of three treatments: control, first-entry burn, and second-entry burn. Understories showed distinct compositional changes over time in first-and second-entry burns. Burned plots supported more than twice as many species as controls 10 yr after treatment; first-entry plots showed a nearly threefold increase in richness by year 20. Burned plots supported four to five times as many shrub species as controls 5-10 yr after burning. Total plant cover (dominated by perennial forbs and shrubs) increased in first-entry plots, but did not differ from controls until 20 yr after treatment. Following secondentry, cover did not change through final sampling (year 10). Nonnative species were rare, occurring in only three plots at low abundance. Higher severity fires led to greater numbers of species and to greater plant cover. Species richness was not correlated with burn heterogeneity. Long-term observations suggest that reintroduction of fire in previously unmanaged forests can gradually enhance the diversity and abundance of understory species. Repeated burning-necessary to achieve structural and fuel-reduction objectives-does not appear to have a detrimental effect on plant diversity and may enhance the distributions of species that are adversely affected by fire exclusion. If fire is to play an important role in restoration, however, it will need to be maintained as a frequent and spatially dynamic process on the landscape.

Long-term effects of prescribed fire on mixed conifer forest structure in the Sierra Nevada, California

“…Live stem biomass declined throughout post-fire observations, although the rate of biomass loss slowed considerably by eight years post-fire. While high severity disturbances can change forests from net carbon accumulators to net carbon emitters for long periods (Cohen et al, 1996;Kashian et al, 2006;Dore et al, 2010), our results imply that low to moderate severity disturbances have milder effects on live biomass, likely stabilizing in a relatively short period of time (likely less than 10 years) (also see Hurteau and North, 2010;Webster and Halpern, 2010). Moreover, reductions in forest density following prescribed fire are expected to decrease risks of crown fires , potentially reducing the likelihood of massive carbon losses from future wildfires North et al, 2009).…”

“…This research effort will require understanding the mechanisms that drive differences in forest response to fire, and will need to consider the effects of climate, typical fire weather, alterations of fuel types and amounts, and physiological and allometric differences among dominant species. It will also become increasingly important to describe differences between first versus second-entry burns as restoration treatments continue at high priority sites (see Webster and Halpern, 2010). A mechanistic understanding of fire effects is critical, especially as climatic conditions change in ways that may preclude comparatively simple restoration prescriptions (Millar et al, 2007).…”

Effects of tree cutting and fire on understory vegetation in mixed conifer forests

“…Major changes to fire regimes, tree structure and composition, forest floor and light conditions, climate, and introduction of livestock and exotic species may all influence understory vegetation (Battaglia and Shepperd, 2007;Knapp et al, 2013). Except for rare forests such as those with natural fire regimes continued through the 1900s (mostly in Mexico; Minnich et al, 2000) or with managed active fire programs (e.g., Sequoia/Kings Canyon National Parks; Webster and Halpern, 2010), the key evolutionary process of low-and mixed-severity fire has been excluded after settlement (Heinlein et al, 2005;Baker et al, 2007;Falk et al, 2011). Fuel loads accrued during the 1900s support severe, stand-replacing fire regimes in many areas (Freeman et al, 2007;Crotteau et al, 2013;Fornwalt and Kaufmann, 2014).…”

Reintroducing fire into a ponderosa pine forest with and without cattle grazing: understory vegetation response

“…There is also a body of literature documenting the effects of wildfire and prescribed burning on understory abundance or diversity in the absence of cattle grazing , Metlen and Fiedler 2006, Knapp et al 2007, Laughlin and Fulé 2008, Webster and Halpern 2010. Evidence from these studies suggest that understory abundance or diversity can be enhanced by prescribed fire, although some report variable responses (Laughlin et al 2004, Laughlin 2006, Moore et al 2006).…”