Fire history of a mixed conifer forest on the Mogollon Rim, northern Arizona, USA

It has been suggested that large, high-severity fires historically structured warm-dry mixed conifer forests in the American South-west. To test this, we reconstructed fire regime characteristics of an 1135-ha (11.3km(2)) mixed conifer landscape in northern Arizona using complementary approaches. We analysed composite fire intervals, point fire intervals, natural fire rotation, landscape characteristics and forest age structure. Composite analysis of cross-dated fire scars from 133 trees indicated a mean fire interval (MFI) of 2.0-8.5 years between 1670 and 1879. Frequent fires halted abruptly after 1879. Mean point fire interval (MPFI) was 11.8 years and ranged 2-61 years. Mean fire rotation was 14.4 years. Density of most occurring tree species increased dramatically after fire regime disruption, with south-western white pine (Pinus strobiformis) and white fir (Abies concolor) showing large numerical gains. Tree establishment patterns compared with widespread fire dates did not suggest historical high-severity fires at the site level. Although strong evidence of high-severity fire at finer scales was lacking, spatial locations of 'young' plots suggested the possibility of historical high-severity disturbances <= 25 ha in size. The historical fire regime on this landscape was one of high-frequency, low-severity fires. Current conditions call for restoration of forest structure and function.