Forest structure and pattern vary by climate and landform across active-fire landscapes in the montane Sierra Nevada

Restoration of fire-dependent forests is often guided by reference conditions from forests with an active fire regime, thought to be resilient to current and future disturbances and stresses. Reference conditions are usually based on historical data or reconstruction, which greatly limits the scale and completeness of data that can be collected. In the Sierra Nevada of California, large areas with reintroduced active fire regimes coupled with extensive lidar data coverage provide the unique opportunity to develop a contemporary regional reference condition dataset across a wide gradient of biophysical conditions. We developed this dataset with a focus on three questions: (1) What is the geographic and environmental distribution of restored active-fire forest areas in the Sierra Nevada mixed-conifer zone? (2) What are the ranges of variation in forest structure and spatial patterns across reference areas? And (3) How do stand density, tree clumping, and canopy opening patterns vary by topography and climate in reference areas? We analyzed fire history and environmental conditions over 10.8 million ha, including 3.9 million ha in the Sierra Nevada mixed-conifer zone, and found 30,377 ha of restored active-fire areas. Although reference areas were distributed throughout the Sierra Nevada they were more abundant on National Park lands (81% of reference areas) than National Forest lands and were associated with higher lightning strike density. Lidar-measured ranges of variation in reference condition structure were broad, with tree densities of 6-320 trees ha(-1) (median 107 trees ha(-1)), basal area of 0.01-113 m(2) ha(-1) (median 21 m(2) ha(-1)), average size of closely associated clumps of trees from > 1 to 207 trees (median 3.1 trees), and average percent of stand area > 6 m from the nearest canopy ranging from 0% to 100% (median 5.1%). These ranges correspond well with past studies reporting density and spatial patterns of contemporary and historical active-fire reference stands in the Sierra Nevada, except this study observed greater total variation due to the much greater spatial extent of sampling. Within the montane forest zone, reference areas at middle elevations had lower density (86 vs. 121 trees ha(-1)), basal area, (13.7 vs. 31 m(2) ha(-1)), and mean clump size (2.7 vs. 4.0 trees) compared to lower- and higher-elevation reference areas, while ridgetops had lower density (101 vs. 115 trees ha(-1)), basal area (19.6 vs. 24.1 m(2) ha(-1)), and mean clump size (3.0 vs. 3.3 trees) and more open space (7.4% vs. 5.1%) than other landforms. Many of the relationships between physiography and reference structure were context-dependent, suggesting that management practices should create heterogeneous forest structure congruent with local climatic and topographic factors influencing stand conditions.