Fuelbed heterogeneity, flammability, and restoration of historically fire frequent oak woodlands with fire

The role of fire in the maintenance of Quercus-dominated ecosystems is widely recognized. Variability in surface fuels is a mechanism rooted in the maintenance of stand structure. This study examined fuelbed heterogeneity, fire behavior, and young conifer mortality within Quercus garryana woodlands and grasslands in the Bald Hills of Redwood National Park, California, USA. Woodlands were stratified into five structural communities: grassland; oak savanna; oak cluster; oak woodland; and woodland invaded by Pseudotsuga menziesii (invaded woodland). Live and dead fuel moisture and fuel mass (Mg ha-1) in three fuel strata (herbaceous, litter, woody) were sampled in the summers of 2008 and 2009. Fire temperature maxima during prescribed burns were measured with pyrometers. Across woodland structural communities, results reveal herbaceous mass decreases markedly from a high in grassland (3.38 ± 0.19 Mg ha-1) to a low in invaded woodland (0.03 ± 0.03 Mg ha-1) (p 0.001), whereas leaf litter and woody fuel mass increase significantly along this gradient (p 0.001). Mean fire temperatures ranged from 74.7 °C in invaded woodland up to 207.9 °C in grassland, but did not differ significantly across structural communities. P. menziesii invasion ultimately dampens flammability through increased fuel moisture, replacement of herbaceous with woody fuels, and altered understory micro-climate. To prevent invasion by P. menziesii, managers use prescribed fire to kill fire-sensitive saplings. Post-fire mortality was monitored for 100 P. menziesii saplings following a prescribed burn. The fire resulted in 50 percent mortality of conifer seedlings and saplings, with strong patterns in the drivers of individual tree injury. Percent crown volume scorched was the most important factor affecting initial mortality status, and was related to cambium injury (r = 0.67). Prescribed fire is a valuable tool for the maintenance of oak woodlands and grasslands, but the achievement of desired fire effects in these ecosystems may depend on site-specific burning strategies to account for alterations in fuelbeds and flammability across structural communities.