Fire regimes, forest change, and self‐organization in an old‐growth mixed‐conifer forest, Yosemite National Park, USA

Abstract

Fire is recognized as a keystone process in dry mixed-conifer forests that have been altered by decades of fire suppression, Restoration of fire disturbance to these forests is a guiding principle of resource management in the U.S. National Park Service. Policy implementation is often hindered by a poor understanding of forest conditions before fire exclusion, the characteristics of forest changes since excluding fire, and the influence of topographic or self-organizing controls on forest structure. In this study the spatial and temporal characteristics of fire regimes and forest structure are reconstructed in a 2125-ha mixed-conifer forest. Forests were multi-aged, burned frequently at low severity and fire-return interval, and forest structure did not vary with slope aspect, elevation, or slope position. Fire exclusion has caused an increase in forest density and basal area and a compositional shift to shade-tolerant and fire-intolerant species. The median point fire-return interval and extent of a fire was 10 yr and 115 ha, respectively. The pre-Euro-American settlement fire rotation of 13 yr increased to 378 yr after 1905. The position of fire scars within tree rings indicates that 79% of fires burned in the midsummer to fall period. The spatial pattern of burns exhibited self-organizing behavior. Area burned was 10-fold greater when an area had not been burned by the previous fire. Fires were frequent and widespread, but patches of similar aged trees were < 0.2 ha, suggesting small fire-caused canopy openings. Managers need to apply multiple burns at short intervals for a sustained period to reduce surface fuels and create small canopy openings characteristic of the reference forest. By coupling explicit reference conditions with consideration of current conditions and projected climate change, management activities can balance restoration and risk management.

Socioecological transitions trigger fire regime shifts and modulate fire–climate interactions in the Sierra Nevada, USA, 1600–2015 CE

“…The scale of the effects of Sierra Nevada Native American burning is disputed (17,34), but augmented ignitions would reduce fuels and fuel continuity across forest landscapes. Fire in Sierra Nevada lower montane forests is regulated by the dynamics of the burn-patch mosaic (15,35). Burn patches constrain fire spread until sufficient fuels build up in a burned patch so it can burn again.…”

Landscape‐scale modeling of reference period forest conditions and fire behavior on heavily logged lands

“…Presumably, moving highly altered forests towards conditions within the historical range of variability should increase their resilience to wildfire and reduce the risk of unexpected outcomes such as vegetation type conversions caused by unusually severe wildfire related to high fuel loads from logging and or fire exclusion (Weatherspoon and Skinner 1996, Landres et al 1999, Savage and Mast 2005, Odion et al 2010. Consequently, identifying reference conditions is a key step in the ecosystem managementrestoration planning process but it is a particularly challenging task in locations where most if not all of the pre-settlement forests have been removed by logging and other types of human activity (Landres et al 1999, Swetnam et al 1999, Scholl and Taylor 2010, Wiens et al 2012.…”

“…Quantitative data on forest reference conditions can be developed from various sources including early land surveys and forest reconstructions using dendroecology (Fulé et al 1997, North et al 2007, Scholl and Taylor 2010, Collins et al 2011). In the western USA, late-19th century General Land Office (GLO) surveys have been used to estimate forest structure before areas were logged or cleared for grazing or other uses.…”

“…Estimates of basal area, species composition, and diameter-class distributions required pooling data across even larger areas (.7.8 km 2 ) with the least accurate estimates for forest size structure (Williams and Baker 2011). Spatial heterogeneity in structural attributes (density, size, age, basal area), which is a hallmark of many ponderosa pine and dry mixed conifer forests (Cooper 1960, White 1985, Youngblood et al 2004, Beaty and Taylor 2007, Scholl and Taylor 2010, Larson and Churchill 2012 occurs mainly at small spatial scales (,0.4 ha; Larson and Churchill 2012) and is not well captured by small point samples from aggregated samples (Bouldin 2008, Hanberry et al 2011). This could lead to misguided goals and objectives for restoration of forest structural conditions typical of forests with an intact fire regime.…”

“…Comparisons of survey data with current conditions in the same locations have been used to identify forest change caused by climate change, logging, fire suppression, or other land use practices (Scholl and Taylor 2010, Collins et al 2011, Dolanc et al 2013. However, detailed forest survey data on presettlement forest conditions is sparse in most v www.esajournals.org forested ecosystems.…”