Fire and persistence of montane chaparral in mixed conifer forest landscapes in the northern Sierra Nevada, Lake Tahoe Basin, California, USA

Tree cover in the mixed conifer zone in the Sierra Nevada is often interrupted by large shrub fields, or stands of montane chaparral. Chaparral stands are thought to be associated with either poor site conditions, or locations that have experienced severe fire. Fires have been excluded from mixed conifer forests for nearly a century because of a management policy of suppressing fire. This study quantifies the intervals between fires, and the response of chaparral shrubs and trees to fire and fire exclusion in six montane chaparral stands. Quercus vaccinifolia and Arctostaphylos patula were the dominant shrubs on all sites and the last fires burned in the six sites between 1861 and 1882. The mean point fire return interval before fire suppression was 28 yr (range 16-40 yr), and fire frequency in chaparral was lower than in nearby forest. The age structure of shrubs and trees in the stands indicate that shrubs and trees regenerated immediately post-fire. Shrub and tree regeneration, however, were not confined to a brief post fire period, regeneration continued for at least another five decades. On our sites, exclusion of fire due to fire suppression has caused the average area of a chaparral stand to shrink by 62.4%. Chaparral has been replaced by forest and this vegetation change has reduced the heterogeneity of the mixed conifer forest landscapes in the Sierra Nevada.