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Table 2 The following table gives a summary of all long-term studies we found that quantified changing fuel loads and/or forest cover over time. Some of these studies are not included in Fig. 1 if there were other studies in the same location that provided the same information or covered a longer period of time

From: Trends in western USA fire fuels using historical data and modeling

Title Author and year published Region Veg type Years of study Disturbance during study Summary URL
Landscape Changes in the Southwestern United States: Techniques, Long-term Data Sets, and Trends Allen et al. 1998 Southwest USA Multiple 1899–1977 ? Juniper expansion can be observed from repeat photos
Long Term Vegetation Dynamics of Ponderosa Pine Forests Bakker 2005 (Dissertation) Northern AZ Ponderosa Pine 1941–2004 Grazing on some plots Overstory cover increased while understory decreased.
Periodic Remeasurement of the Gus Pearson Natural Area Fule et al., 2001 (Report) Gus Pearson, AZ Ponderosa pine 1920–2000 No harvesting General trend toward declining growth. Increased mortality since 1945.
Comparing Tree-Ring Chronologies and Repeated Timber Inventories as Forest Monitoring Tools Biondi 1999 Gus Pearson, AZ Ponderosa pine 1920–1990 No harvesting Decreased growth rates and higher stand density
Twentieth-century changes in forests of the Sierra Nevada Bouldin 1999 Northern Sierra Nevada, CA Conifer forests 1935–1992 Fire, drought mortality Used a large number of forestry plots to show that density of small trees has increased greatly, with decreases in large tree density. Standing dead tree densities are higher than before.
Stand dynamics and topographic setting influence changes in live tree biomass over a 34-year permanent plot record in a subalpine forest in the Colorado Front Range Chai et al. 2019 CO Subalpine forest 1982–2016 insects and pathogens Despite increased mortality rates over time, there was also a trend of increasing biomass in live trees.
Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests Collins et al. 2011 Sierra Nevada, CA Conifer trees, forb and shrub cover 1911–2007 Fire on some plots Re-sampled a 1911 timber inventory. Areas with no fire or low severity fire had higher tree density and canopy cover relative to 1911.
Changing forest structure across the landscape of the Sierra Nevada, CA, USA, since the 1930s Dolanc et al. 2014 Central-Northern Sierra Nevada, CA Conifer forests 1930s–2000s Fire in some areas Used plot data from VTM and FIA to look at changes in forest structure over time (used averages w/in similar regions, not remeasurements of identical plots). Found similar results as Bouldin (above)
Climate, environment, and disturbance history govern resilience of western North American Forests Hessburg et al. 2019 Western USA Multiple 1925–2008 (and other time ranges) multiple Review of forest structure change across western USA, including repeat photography.
Spatiotemporal Variability of Wildland Fuels in US Northern Rocky Mountain Forests Keane 2016 Rocky Mountains (MT, ID) All types of fuels 1993–2004 no disturbance There is high spatiotemporal variability in fuel accumulation, especially for larger fuels.
Long-term surface fuel accumulation in burned and unburned mixed-conifer forests of the Central and Southern Sierra Nevada Keifer et al. 2006 Sierra Nevada, CA Fuel loads 1971–2003 Fire on some plots Fuel load increased over time in unburned plots. In burned plots, 31 years post-fire fuel loads were higher than pre-fire.
A Half Century of Change in Alpine Treeline Patterns at Glacier National Park, Montana, U.S.A. Klasner and Fagre 2002 MT High-altitude forests (near treeline) 1945–1991 (some photos from 1927 to 1997) Road/trail construction/maintenance Forest homogeneity increased due to greater area with trees and greater tree density within existing patches.
Change in Vegetation Patterns Over a Large Forested Landscape Based on Historical and Contemporary Aerial Photography Lydersen and Collins 2018 Sierra Nevada, CA Conifer Forest (via aerial photos) 1941–2005 Fire in some areas The amount of area with dense forest cover increased, and continuous patches of dense forest grew larger.
Quantifying spatial patterns of tree groups and gaps in mixed-conifer forests: Reference conditions and long-term changes following fire suppression and logging Lydersen et al. 2013 Sierra Nevada, CA Conifer trees 1929–2008 Logging, all plots 1929 (pre-logging) was treated as a reference condition, with 2007/8 showing regrowth ~ 80 years after logging. Canopy cover was higher in 2007/8 compared to 1929.
Forest vegetation change and surface hydrology following 47 years of managed wildfire Stevens et al., 2020a Sierra Nevada, CA Conifer trees, shrubs 1970–2017 Fire on some plots Slight increase in total tree density; decrease in density of large trees. Increase in the number of plots with shrub presence.
Southern Cascades Bioregion. Ch. 12 of “Fire in California’s Ecosystems” Skinner and Taylor, 2018 So. Cascades, CA Conifer forests 1925–1993 None Shows increasing tree density, high levels of recruitment, and some mortality of large trees, as well as increased litter cover, using repeat photography.
Fire regimes and forest changes in mid and upper montane forests of the southern Cascades, Lassen Volcanic National Park, California, U.S.A. Taylor 2000 So. Cascades, CA Conifer forests 1925–1993 None Repeat photography and tree ring records show increased density in Jeffrey Pine and White Fir forests, but little change in red fir forests. Photos also show increased litter cover.
Long-term response of old-growth stands to varying levels of partial cutting in the Eastside Pine Type Dolph et al. 1995 East side of Cascades, CA Conifer forests 1938–1991 None Repeat measurements show increased density of small trees, but 6% reduction in “sawtimber component”, leading to an overall 13-32% increase in volume of trees over 3.6” DBH.
Twenty-year change in aspen dominance in pure aspen and mixed aspen/conifer stands on the Uncompahgre Plateau, Colorado, USA Smith and Smith 2005 CO Aspen and conifers 1979–2003 None? Aspen density remained stable or decreased, while conifer basal area increased over the 20 year study period.
Vegetation Change and Park Purposes in the High Elevations of Yosemite National Park, California Vale 1987 Yosemite, CA High-altitude forests (near treeline) 1900–1985   Forests at the upper forest line have increased in density, meadows have been encroached by trees
Widespread Increase of Tree Mortality Rates in the Western United States van Mantgem et al. 2009 Western USA (CA, OR, WA, ID, CO, AZ) Forests > 200 years old ~ 1981–2004 (first measurement dates ranged from 1955 to 1994) Undisturbed Mortality increased across size classes and elevation ranges. Mean tree density and basal area in the study plots declined slightly during the study period.
Comparison of Historical and Contemporary Forest Structure and Composition on Permanent Plots in Southwestern Ponderosa Pine forests Moore et al., 2004 AZ and NM Ponderosa pine 1909–1999 Logging, all plots Stand density increased greatly; tree diameters shifted toward smaller size classes.
Drought-induced shift of a forest–woodland ecotone: Rapid landscape response to climate variation Allen and Breshears 1998 New Mexico Ponderosa pine 1935–1975 drought ponderosa pine forest receded quickly during a drought
Spatial Patterns of Pinyon–Juniper Woodland Expansion in Central Nevada Weisberg et al. 2007 Central NV Pinyon-juniper woodlands 1966–1995 ? Analysis of aerial photos showed an 11% increase in woodland area.
Post-1900 Mule Deer Irruptions in the Intermountain West: Principle Causes and Influences Gruell 1986 Inter-mountain West (ID, MT, NV, UT, WY) Various woody plants 1868–1982 Grazing and Fire Suppression Widespread succession from grass dominance to trees and shrubs due to grazing (Reducing competing grass cover) and fire suppression.
Vegetation differences in desert shrublands of western Utah’s Pine Valley between 1933 and 1989 Yorks et al. 1992 Western UT Desert shrubland 1933–1989 Grazing Canopy cover of grasses greatly increased, with some increase in shrub cover. Density of shrubs decreased (shrubs/m2)
Changes in pinyon-juniper woodlands in western Utah’s Pine Valley between 1933-1989 Yorks et al. 1994 Western UT Pinyon-juniper woodlands 1933–1989 Grazing Tree % cover decreased while density increased, due to shift in tree cover from juniper to pinyon (narrower crown). Shrub and grass % cover increased. Shrub density increased.
A century of vegetation change in the San Juan Mountains, Colorado: An analysis using repeat photography Zier and Baker 2006 CO Conifer and aspen forests, grasslands ~1900–~2000 Varying disturbances Both conifers and deciduous trees increased in extent, partially as recovery from disturbances. There was some encroachment of trees into grass/shrublands.
Fire and vegetative trends in the Northern Rockies: interpretations from 1871-1982 photographs Gruell 1983 Northern Rockies, MT Forest 1871–1982   Woody vegetation increased due to suppressed wildfire
ENVIRONMENTAL CHANGE IN GLACIER NATIONAL PARK, MONTANA: AN ASSESSMENT THROUGH REPEAT PHOTOGRAPHY FROM FIRE LOOKOUTS Butler and DeChano 2001 MT Montane forest 1935–1990s Avalanches, glacial recession, anthropogenic developments Increased forest density and extent likely due to a combination of fire suppression and climate change (including receding glaciers)
Fire’s Influence on Wildlife Habitat on the Bridger-Teton National Forest, Wyoming - Volume I: Photographic Record and Analysis Gruell 1980 WY Multiple tree and shrub types 1872–1975 Some fires, mostly prior to 1941 Increases in conifers and sagebrush
100,000 Trees Can’t Be Wrong: Permanent Study Plots and the Value of Time Duncan 2004 (FS publication for managers) Pacific NW (OR, WA) Multiple forest types 1910–2002 Varying disturbances There is a network of long term forestry plots in OR and WA, managed by the PNW region of the USFS. Biomass accumulation can continue even after 80 years post-disturbance.
Thirty-six years of tree population change in an old-growth Pseudotsuga- Tsuga forest Franklin and DeBell 1988 Cascade Range, WA Conifers 1947–1983 Undisturbed Old growth forest showed slight decline in tree density. The diameter distribution shifted upward, and 22% of the original stems died during the study period (though this was almost matched by recruitment)
Carbon stocks and accumulation rates in Pacific Northwest forests: role of stand age, plant community, and productivity Gray et al., 2016 Pacific NW (OR, WA, CA, ID) Live and dead trees, many types 1993–2007 disturbance on some plots Older trees accumulate C slower, but forests still have net C increase until ~ 400 years old when high mortality outweighs growth
Input and decay of coarse woody debris in coniferous stands in western Oregon and Washington Sollins 1982 Pacific NW (OR,WA) Coarse woody debris 16–46 year span Undisturbed Tree mortality resulted in dry-matter transfer of 1.5–4.5 Mg•ha − 1•year− 1 of boles and branches to the forest floor and 0.3–1.3 Mg•ha − 1•year− 1 of large-diameter roots directly to the mineral soil. , with decay rates slower than accumulation.
Historical and current landscape-scale ponderosa pine and mixed conifer forest structure in the Southern Sierra Nevada Stephens et al. 2015 Southern Sierra Nevadas, CA Ponderosa pine and mixed conifer forest 1911–2005 Very little disturbance Compared tree density and canopy cover measured by 1911 surveys and modern FIA inventories in same area, though not the same exact plot locations. Found increases in tree density, fir dominance, and canopy cover.
Vegetation and Soil Condition Changes on a Subalpine Grassland in Eastern Oregon Strickler 1961 Wallowa Mountains, OR Grasslands/shrubs 1938–1956 Grazing Total biomass and veg. cover increased over the 20 years due to improved range management.