Fire regimes and forest changes in mid and upper montane forests of the southern Cascades, Lassen Volcanic National Park, California, USA

Aim Spatial and temporal variation in fire regime parameters and forest structure were assessed. Location A 2630-ha area of mid- and upper montane forest in Lassen Volcanic National Park (LVNP). Methods Two hypotheses were tested concerned with fire-vegetation relationships in southern Cascades forests: (1) fire regime parameters (return interval, season of burn, fire size, rotation period) vary by forest dominant, elevation and slope aspect; and (2) fire exclusion since 1905 has caused forest structural and compositional changes in both mid- and upper montane forests. The implications of the study for national park management are also discussed. Results Fire regime parameters varied by forest compositional group and elevation in LVNP. Median composite and point fire return intervals were shorter in low elevation Jeffrey pine (Pinus jeffreyi) (JP) (4-6 years, 16 years) and Jeffrey pine-white fir (Abies concolor) (JP-WF) (5-10 years, 22 years) and longer in high elevation red fir (Abies magnifica)- western white pine (Pinus monticola) (RF-WWP) forests (9-27 years, 70 years). Median fire return intervals were also shorter on east-facing (6-9 years, 16.3 years) and longer on south- (11 years, 32.5 years) and west-facing slopes (22-28 years, 54-years) in all forests and in each forest composition group. Spatial patterns in fire rotation length were the same as those for fire return intervals. More growing season fires also occurred in JP (33.1%) and JP-WF (17.5%) than in RF-WWP (1.1%) forests. A dramatic decline in fire frequency occurred in all forests after 1905. Conclusions Changes in forest structure and composition occurred in both mid- and upper montane forests due to twentieth-century fire exclusion. Forest density increased in JP and JP-WF forests and white fir increased in JP-WF forests and is now replacing Jeffrey pine. Forest density only increased in some RF-WWP stands, but not others. Resource managers restoring fire to these now denser forests need to burn larger areas if fire is going to play its pre-settlement role in montane forest dynamics.