Document: MEN-3-68-39

Forest litter densities under different canopy species in the mixed conifer forest of the southern Sierra Nevada-a factor affecting ground fire spread?

MENNING, K.M.* ¹, J.J.BATTLES ¹, T.L.BENNING ¹ and N.L.STEPHENSON ²

University of California, Berkeley, CA 94720, USA ¹
United States Geological Survey ²

Abstract:
The mixed conifer forest in the southern Sierra Nevada is a mid-elevation montane forest with a high basal area (70 m²/ha), dominated by long-lived conifers. Historically, this forest was regularly disturbed by frequent, low-severity fires with ground and surface fuels as important vectors of fire spread. The mixed conifer forest is characterized by a variable species mix of two to five conifers and several hardwoods. The canopy species diversity in this forest is expected to result in a similarly variable litter base. Earlier studies of forest structure and fire history in the area revealed that topographic factors are important determinants of both forest structure and fire history patterns. However, the specific link between variability in forest composition and in fire regimes has not been identified. To address one possible explanatory factor we examined the density of the forest litter mat across 10,000 ha of the mixed conifer forest in the Mineral King watershed, California, to determine if the dominant canopy species would affect litter density and could thereby help regulate fire behavior. Dense litter mats, for example, retain more moisture and have low air flow, resulting in slow fire spread. We sorted our plots into those dominated by red fir (Abies magnifica), white fir (A. concolor), sequoia or incense-cedar (Sequoiadendron giganteum and Calocedrus decurrens), and pine species (Pinus ponderosa, P. jeffreyi, P. monticola, P. contorta). Red fir litter was the densest at 0.11 g/cm³ (n = 30), followed by pine (0.10 g/cm³, n = 14), white fir (0.09 g/cm³, n =57), and least dense, sequoia and cedar (0.084 g/cm³, n = 14). Significant differences in litter density were observed between red fir and white fir (p = 0.005), and between red fir and sequoia/cedar (p = 0.003). The high density of pine litter in comparison to white fir litter was unexpected. We had anticipated that pine litter, composed largely of long needles, would have the lowest density. Red fir had the highest variability (by coefficient of variation) in litter density, which may be an important result for explaining patchy fire behavior in that vegetation type.

Keywords: fire ecology, forest ecology, disturbance, litter

This abstract is being presented at: 1:45 PM in session:
Oral Session #35: Fire Ecology.