Regeneration patterns in old-growth red fir–western white pine forests in the northern Sierra Nevada, Lake Tahoe, USA

Red fir (Abies magnifica) forests in the Sierra Nevada are known to demonstrate both shade tolerant and disturbance related regeneration making it difficult to understand the role of disturbances in the regeneration dynamics of the forests. Four stands with different structural characteristics were selected for intensive sampling in order to capture the observed range of structural variability (e.g. composition, age, size and spatial pattern) in an old-growth red fir–western white pine (Pinus monticola) forest in the northern Sierra Nevada. We used detailed stem mapping, stand structural analysis and cross-dated fire scar samples to identify the relationships between disturbances and stand structure. All trees >5 cm dbh within four 0.5-ha plots were aged and mapped. The species composition of the plots was similar but the density and basal area of the tree populations varied among the plots. Red fir density and basal areas are greater than that of western white pine. The age structure indicated continuous, but variable recruitment and there were few seedlings and saplings. The mean point fire return interval was 76 years (range 25–175 years) for the 400-ha study area. Most fires scarred only single samples suggesting that burns were small and patchy, but pulses of recruitment suggest that some fires were moderate in severity. Regeneration pulses coincided with the dates of several fires (e.g. 1636, 1770). Moran's I, a measure of spatial autocorrelation, indicated that red fir and western white pine exhibited positive spatial autocorrelation at short (3–12 m) and intermediate (36–75 m) distances. Groups of similar age trees were spatially discrete and groups of different ages tended to overlap, resulting in an all aged forest. The spatial pattern of tree ages and the record of disturbance indicate that infrequent moderate severity fires have a lasting influence on the structure and development of old-growth red fir forests.