IDENTIFYING FOREST REFERENCE CONDITIONS ON EARLY CUT‐OVER LANDS, LAKE TAHOE BASIN, USA

Pre‐Euro‐American settlement forest structure and fire regimes for Jeffrey pine–white fir, red fir–western white pine, and lodgepole pine forests in the Lake Tahoe Basin (California and Nevada, USA) were identified using stand structural analysis and fire scars based on measurements of stumps of trees that were cut in the 19th century. Comparisons of the presettlement reference with contemporary conditions were then used to determine how and why contemporary conditions deviate from presettlement conditions and to guide ecological restoration. Contemporary forests varied in different ways compared to the presettlement reference. Contemporary Jeffrey pine–white fir forests have more and smaller trees, more basal area, less structural variability, and trees with a more clumped spatial distribution than presettlement forests. The mean presettlement fire‐return interval for Jeffrey pine–white fir forests was 11.4 yr, and most (>90%) fires burned in the dormant season; no fire was recorded in the study area after 1871. Similar differences were identified in the structural characteristics of contemporary and presettlement red fir–western white pine and lodgepole pine forests. However, 19th‐century logging changed the composition of red fir–western white pine forests, and these forests now have more lodgepole pine than red fir or western white pine. Comparison of contemporary forests with the presettlement reference suggests that restoration treatments in Jeffrey pine–white fir forests should include: (1) density and basal‐area reduction, primarily of smaller diameter trees; (2) reintroduction of frequent fire as a key regulating disturbance process; and (3) increasing structural heterogeneity by shifting clumped tree distributions to a more random pattern. Restoration treatments in red fir– western white pine forests should include: (1) a shift in species composition by a density and basal‐area reduction of lodgepole pine; and (2) increasing structural heterogeneity by shifting tree distributions to a more random pattern. In lodgepole pine forests the restoration emphasis should be: (1) a density and basal‐area reduction of small‐diameter trees; and (2) an increase in structural heterogeneity that shifts tree spatial patterns from clumped to a more random distribution. Reintroduction of fire as a regulating process into high‐elevation red fir–western white pine and lodgepole pine forests can be viewed as a longer‐term restoration goal. The method for quantifying presettlement reference conditions from stumps complements, or is an alternative to, methods based on dendroecology or repeat photography.