Human and climatic influences on fire occurrence in California's North Coast Range, USA

Abstract

Outside of the immediate coastal environments, little is known of fire history in the North Coast Range of California. Fire scar specimens were collected from ponderosa pine (Pinus ponderosa C. Lawson), sugar pine (Pinus lambertiana Douglas), incense cedar (Calocedrus decurrens [Torr] Florin), and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) for seven plots in mixed-conifer forests from the Mendocino National Forest, California, USA. Five plots were on high ridges immediately adjacent to the Sacramento Valley (DRY plots). The other two plots were on mesic north facing slopes interior in the range (MESIC plots), and were separated from the Sacramento Valley by at least one to several ridge systems. These two plots were selected because they supported populations of rare lady's slipper orchids (Cypripedium fasciculatum [Kellogg ex S. Watson] and C. montanum [Douglas ex Lindl.]). We found that DRY plots had unusually short fire return intervals (FRI) compared to other areas in northwestern California. The median FRI for these plots ranged from 4.5 yr to 6 yr in comparison with a tenth percentile of 11 yr, grand median of 24 yr, and ninetieth percentile of 66 yr for FRIs from other mixed conifer plots (n = 109) in the region. In northwestern California, most fire scars have been found primarily at ring boundary (68%) and secondarily in latewood (23%) with few in earlywood (9%). In contrast, in the DRY plots 35% (88) of the fire scars were in earlywood with only 15% (39) at the ring boundary. Fire occurrence was associated with drought conditions in the year of fire, and with wet conditions three years before the fire year. Before ~1850, fires that scarred at least two trees on a site were quite frequent for the DRY plots while being less frequent and more variable on the MESIC plots. However, the MESIC orchid habitats burned with frequency and seasonality similar to mixed conifer forests in the Klamath Mountains of northern California. Fires were less frequent after ~1850, with fires ceasing on most plots shortly after 1900. We suggest that these unusually low FRIs and high incidence of fire scars in earlywood were due to the adjacency of the DRY plots to the hot, relatively dry Sacramento Valley grasslands that were likely influenced by the burning practices of Native Americans.