Present and past old-growth forests of the Lake Tahoe Basin, Sierra Nevada, US

We described 38 relictual old-growth stands - with data on the mortality, regeneration, floristic richness, fuel load and disease incidence in our study area in the Tahoe Basin of California and Nevada. The stands are within the lower and upper montane zones (1900-2400 in a.s.l.) and they are rare, occupying < 2% of the land in the Basin's watershed. Correlation matrices and ANOVAs of forest types and conifer species with environmental gradients revealed significant relationships with elevation, distance east of the Sierran crest, slope aspect, annual precipitation, date of complete snow melt, litter depth and degree of soil profile development. Pathogens, parasites and wood-boring insects were present on 23% of living trees; 16% of all trees were dead. We compared these stands to a reconstruction of pre-contact Basin forests and to ecologically analogous old-growth forests of Baja California that have never experienced fire suppression management. Currently, overstorey trees (> 180 yr old) in the Basin stands have ca. 33% cover, 54 m(2).ha(-1) basal area and 107 individuals.ha(-1), values very similar to reconstructions of pre-contact Basin forests and to modern Baja California forests. Understorey trees (60-180 yr old), however, are several times more dense than historic levels and species composition is strongly dominated by A. concolor, regardless of the overstorey composition. The ratio of Pinus : Abies has increased - and the age structure of extant stands predicts that it will continue to increase - from approximately 1:1 in pre-contact time to 1:7 within the next century. Disease incidence and mortality in Baja forests were lower. Although we quantitatively defined current Basin old-growth forests - in terms of stand structure - we realize that our definition will differ from that of both past and future old-growth forests unless management protocols are changed.