The effects of conifer encroachment and overstory structure on fuels and fire in an oak woodland landscape

Abstract

The role of fire in the maintenance of oak-dominated ecosystems is widely recognized. Fire exclusion results in structural and compositional shifts that alter fuel bed composition and structure, together influencing fire behavior and effects. To clarify the influence of over story structure on fuels and fire intensity in oak woodlands and savannas, we examined fuel beds across a gradient from open grassland to Douglas-fir- (Pseudotsuga menziesii (Mirb.) Franco) invaded Oregon white oak (Quercus garryana Douglas ex Hook.) woodland in the Bald Hills of Redwood National Park, California, USA. Herbaceous mass decreased markedly from a high in grasslands (3.38 Mg ha -1) to a low in invaded woodlands (0.03 Mg ha -1), whereas leaf litter and woody fuel mass increased substantially along this gradient. Mean fire temperatures at 30 cm height ranged from 74.7 °C in invaded woodland up to 207.9 °C in grassland. Highly flammable grassland and savanna communities maintain heavy herbaceous mass, but low woody mass, favoring quick-spreading, relatively high-intensity fires. The encroachment of Douglas-fir into grasslands and oak-dominated communities dampens flammability through changes in fuelbed composition and structure (e.g., the replacement of herbaceous fuels with woody fuels), underscoring the necessity for ecological restoration efforts that focus on fuel bed structure in addition to other common restoration goals.