Spatial and temporal variation of ®re regimes in a mixed conifer forest landscape, Southern Cascades, California, USA

Aim In this study, we evaluated the ®re-forest mosaic of a mixed conifer forest landscape by testing the hypothesis that pre-®re suppression ®re regime parameters vary with species composition (tree species), and environment (i.e. slope aspect, slope position, elevation). Location Our study was conducted in the 1587 ha Cub Creek Research Natural Area (CCRNA), Lassen National Forest, CA, USA. Methods We quanti®ed the return interval, seasonal occurrence, size, rotation period, and severity of ®res using dendroecology. Results Slope aspect, potential soil moisture, forest composition, and ®re regime parameters in our study area co-vary. Median composite and point ®re return intervals (FRI) were longest on higher, cooler, more mesic, north-facing (NF) slopes covered with white ®r (Abies concolor), Douglas ®r (Pseudotsuga menziesii)±white ®r, and red ®r (A. magni®ca)±white ®r forests, shortest on the dry, south-facing (SF) slopes covered with ponderosa pine (Pinus ponderosa)±white ®r forests and intermediate on west-facing slopes dominated by white ®r±sugar pine (P. lambertiana)±incense cedar (Libocedrus decurrens) forests. The spatial pattern for length of ®re rotation (FR) was the same as that for FRI. Fires in CCRNA mixed conifer forests occurred mainly (90%) in the dormant season. Size of burns in CCRNA mixed conifer forests were generally small (mean ˆ 106 ha), however, during certain drought years widespread ®res burned across fuel breaks and spread throughout the watershed. Fire severity was mainly high on upper slopes, low on lower slopes and moderate and low severity on middle slopes. Patterns of ®re severity also varied with slope aspect. Fire frequency decreased dramatically in CCRNA after 1905. Conclusions In CCRNA, ®re regime parameters [e.g. FRI, ®re extent, FR, ®re severity] varied widely with species composition, slope aspect and slope position. There was also temporal variation in ®re extent with the most widespread ®res occurring during drought years. The important contributions of topography and climate to variation in the ®re regime indicates that exogenous factors play a key role in shaping the ®re-forest structure mosaic and that the ®re-forest structure mosaic is more variable, less predictable and less stable than previously thought. Finally, some characteristics of the ®re regime (i.e. ®re severity, season of burn) in CCRNA are different than those described for other mixed conifer forests and this suggests that there are geographical differences in mixed conifer ®re regimes along the Paci®c slope.