Initial tree regeneration responses to fire and thinning treatments in a Sierra Nevada mixed-conifer forest, USA

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Abstract

Fire is a driver of ecosystem patterns and processes in forests globally, but natural fire regimes have often been altered by decades of active fire management. Following almost a century of fire suppression, many western U.S. forests have greater fuel levels, higher tree densities, and are now dominated by fire-sensitive, shade-tolerant species. These fuel-loaded conditions can often result in high-intensity crown fires replacing historic low- to moderate-intensity fire regimes. In the mixed-conifer forests of the California Sierra Nevada, thinning and prescribed fire are widely used to reduce fuels and shift future stand composition from shade-tolerant species to more fire-resistant pines (Pinus lambertiana and Pinus jeffreyi) that were historically more abundant. The impacts of these treatments, however, on forest regeneration composition and abundance are unclear. We examined the effects of prescribed fire and common thinning treatments (understory and overstory thinning) on microsite conditions, seed rain, and tree regeneration in an old-growth, mixed-conifer forest in the Sierra Nevada, California, USA. Treatments significantly altered environmental conditions, but there was substantial variation and overlap in conditions among treatments. Seed rain of shade-tolerant Abies concolor and Calocedrus decurrens was 5–26 times greater than P. jeffreyi and P. lambertiana, creating inertia in efforts to shift stands towards increased pine abundance. Survival of Pinus germinants was greatest in burned microsites. The burn-overstory thin treatment had both the highest mortality of advanced A. concolor and C. decurrens regeneration and the greatest increase in pine regeneration. Species occupied microsites gradating from low light/high moisture to high light/low moisture in the order: C. decurrens, A. concolor, P. lambertiana, and P. jeffreyi. Results suggest prescriptions may need to thin mature A. concolor and C. decurrens to significantly reduce their seed rain, create an abundance of burned open microsites, or plant Pinus seedlings to shift regeneration composition in treated stands.

Introduction

Fire is an important disturbance agent in many forests worldwide, shaping ecosystem patterns and processes (Naveh, 1974, Philips, 1974, Wein and MacLean, 1983, Gill et al., 1990, Coutinho, 1990, Agee, 1993). Natural fire regimes have been altered in many forest ecosystems from decades of active fire management (Cooper, 1960, Linder and Östlund, 1998, Fulé and Covington, 1998, Ward et al., 2001, Vigilante and Bowman, 2004, North et al., 2007). Fire exclusion is believed to be especially important in forests that were previously characterized by frequent, low- to moderate-intensity surface fires such as southwestern ponderosa pine, Sierra Nevada mixed-conifer, and central hardwood forests of North America (McKelvey et al., 1996, Covington, 2000, Shang et al., 2007). Fire exclusion in these forest types can result in greater canopy cover and density of shade-tolerant trees, higher fuel loads, and increased fuel continuity, increasing the potential for high-intensity stand replacement fires (Kilgore, 1973, Parsons and DeBenedetti, 1979, McKelvey et al., 1996, Weatherspoon and Skinner, 1996, North et al., 2005, Shang et al., 2007).

In response, federal and state forest agencies have increasingly focused on reducing forest fuels (SNEP, 1996, HFRA, 2003, California Board of Forestry, 2004, SNFPA, 2004). Thinning and/or prescribed fire have become common management tools in fire-suppressed forests to reduce the probability of high-intensity, stand-replacing wildfires (Graham et al., 2004, SNFPA, 2004, Agee and Skinner, 2005). In Sierran mixed-conifer forests a common secondary objective is to promote a composition change from fire-sensitive, shade-tolerant firs and incense-cedar (Abies sp. and Calocedrus decurrens), toward more fire-resistant, shade-intolerant species (Pinus sp.) which were more abundant prior to fire suppression (Stephens, 2000, SNFPA, 2004, North et al., 2007). There has been little research, however, on the effects of these restoration treatments on mixed-conifer regeneration dynamics which will affect future forest composition and fire resilience.

It is difficult to predict how fuel reduction treatments will affect regeneration in Sierran mixed-conifer forests. These forests have a spatially variable patch structure that interacts with thinning and fire to significantly change microsite conditions and competing vegetation (Wayman and North, 2007). Understory tree abundance and resource conditions differ between undisturbed mixed-conifer's three main vegetation patch types: open-canopy, closed-canopy, and shrub patches (North et al., 2002, Gray et al., 2005). Reductions in canopy cover can reduce regeneration of some species through exposure to high levels of direct solar radiation, resulting in desiccation (Fowells and Stark, 1965, McDonald, 1976). However reducing stem density with mechanical thinning can increase soil moisture availability, ameliorating exposure effects if seedlings are able to establish sufficient root depth before surface layers dry out (Haig et al., 1941). Disturbance of intact litter layers can promote seedling establishment under high insolation, since intact litter can dry out rapidly and reach high surface temperatures, causing high mortality of tree germinants (Isaac, 1943, Gray and Spies, 1997). Thinning and/or prescribed fire also impact shrub composition and abundance. Shrubs can provide protective cover facilitating regeneration, or compete with and suppress regeneration, depending on the ecosystem, shrub species, and tree species of interest (Tappeiner and Helms, 1971, Lanini and Radosevich, 1986, Gomez-Aparicio et al., 2004). To examine how fire and thinning treatments will affect mixed-conifer regeneration, inter- and intra-treatment changes in microsite conditions, seed abundance, seedling establishment, and seedling survival need to be followed to infer abiotic and biotic influences.

The objectives of this study were to investigate understory tree mortality and regeneration in response to burning and thinning treatments for the dominant tree species in a Sierra Nevada mixed-conifer forest: white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), incense-cedar (Calocedrus decurrens (Torr.) Florin), sugar pine (Pinus lambertiana Dougl.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf). All overstory species reproduce exclusively by seed. We examined tree regeneration through its early life-cycle phases including seed quantity, germination, and establishment. We measured fixed plots before and after thinning and prescribed fire treatments, and quantified regeneration composition and abundance in relation to (1) seed production, (2) sown seedling germination and early survival in controlled microsites, (3) the distribution and frequency of understory tree mortality and natural regeneration, and (4) relationships between microsite conditions, burning and thinning treatments, and post-treatment regeneration. Our previous studies of regeneration in untreated stands (Gray et al., 2005) found all overstory species (except for P. jeffreyi) had greater regeneration under shadier conditions, suggesting increased understory light via canopy cover reductions may not promote seedling regeneration of both pine species (P. lambertiana and P. jeffreyi). We hypothesize that (1) burning would kill most established seedlings and saplings but would promote all species regeneration by reducing litter and shrub cover; (2) in all burning and thinning treatment combinations, seed rain for shade-tolerant A. concolor and C. decurrens will be much greater than pine seed rain, but total seed rain by species will be proportionally reduced by basal area removal; (3) germination and survivorship of sown seed will be highest for shade-tolerant species (Abies sp. and Calocedrus sp.) in closed-canopy conditions and understory thin treatments, while pine germination and survivorship will be highest in open conditions and overstory thins; (4) thinning would primarily affect resource levels, with shade-tolerant (A. concolor and C. decurrens) natural regeneration favored by higher soil moisture and shade conditions in light thinning (understory) treatments, and inhibited by high light levels in heavy thinning (overstory) treatments.

Section snippets

Description of study site

The study was conducted in an unmanaged, old-growth forest at the Teakettle Experimental Forest (hereafter Teakettle) approximately 80 km east of Fresno, CA (36°58′N, 119°02′W) in the Sierra National Forest. Elevation within the study area ranges from 1900 to 2600 m. Common soils are well-drained Dystric and Lithic Xeropsamments of loamy sand to sandy loam textures derived from granitic rock (USDA Forest Service and Soil Conservation Service, 1993), and exposed granitic rock is common throughout

Stand structure and environmental variables

Total live tree basal area reductions by treatment were as follows: B-NT, 7%; NB-UT, 32%; B-UT, 42%; NB-OT, 61%; B-OT, 69%, with the largest reductions attributed to A. concolor (Table 1). All treatments except B-NT greatly reduced densities of trees less than 75 cm DBH. Understory thins and unthinned plots had no major reductions in large tree (>75 cm DBH) stem densities, while overstory thinning reduced large tree densities (see North et al., 2007). For saplings (trees ≥50 cm tall and <5 cm DBH),

Discussion

Increasing the forest's pine composition after almost a century of fire suppression may be difficult because shade-tolerant A. concolor and C. decurrens are now large co-dominants that survive moderate fuels treatments and are prolific seed producers. Although P. jeffreyi and P. lambertiana had high germinant survival, A. concolor and C. decurrens seed rain and seedling frequency were up to an order of magnitude higher than the pines. Treatments significantly altered understory light, soil

Acknowledgments

Support for portions of this project was provided by the USFS/USDI Joint Fire Sciences Program, the Sierra Nevada Research Center of the USFS Pacific Southwest Research Station, the USFS Pacific Northwest Research Station, the Andrew Mellon Foundation, and the California State University Agricultural Research Initiative. We wish to thank Jim Innes, the 2000–2005 Teakettle field crews, and Ryan Lopez for their help with field work. We also thank three anonymous reviewers for their helpful

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