Climate, Fire, and Vegetation Change in Yosemite National Park
On the Sierra Haute Route, California
Purpose: This research purpose was to describe quantitatively the changes that have occurred in the forests of Yosemite National Park between 1936 and 1999, to model how climate contributes to fire frequency, fire severity, and changes in vegetation, and to infer how Yosemite forests may change in the 21st century. Because Yosemite is a large landscape (3,027 km2) that has been in protected status since 1890, it is an ideal location to study forest dynamics and fire.
Methods: I investigated fire and vegetation change in Yosemite using two types of data; vegetation data from 655 historic plots (1932 – 1936) and from 207 modern plots (1988 – 1999) as well as fire record data from field and satellite monitoring. I compared the historic and modern distribution and abundance of large-diameter trees (over 92 cm in diameter for most species). I also quantified fire severity for the 103 fires >40 ha using 23 years of satellite data. Fire severity was determined by analyzing differences in pre-fire and post-fire Landsat Thematic Mapper satellite images. I also used topography, temperature, precipitation, soil water capacity, spring snowpack, and lightning strike data in analyses.
Results: Between the mid-1930s and the 1990s, the density of large-diameter trees in Yosemite National Park declined 24%. The decrease was apparent in all coniferous forest types. Declines were higher in subalpine and upper montane forest types, and less in lower montane forest types. Although the density of large-diameter trees declined, reintroducing fire in the ponderosa pine forests better preserved the species and densities of large-diameter trees present in the 1930s. Ponderosa pine still dominated the large-diameter component of plots that had burned. However, in unburned plots, the large-diameter component consisted of white fir, incense cedar, and valley live oak. Climate-induced summer drought has not changed much between the “Little Ice Age” (circa 1700) and present, but modeled climate change suggests that summer drought will increase significantly for all coniferous species by 2050.
Decreased spring snowpack exponentially increased the number of lightning ignitions. The principal mechanism whereby snowpack mediated lightning ignitions was through seasonal landscape flammability (lightning ignitions divided by lightning strikes). A secondary mechanism was fewer lightning strikes in years with snowpack deeper than average. The proportion of the landscape that burned at higher severities and the complexity of higher severity burn patches increased with the log10 of annual area burned. Using one climate forecast for snowpack changes, I project that the annual number of lightning ignitions in Yosemite National Park from 2020 to 2049 will be 19.1 % higher than between 1984 and 2005 and that the annual area burned at high severity will increase 21.9 %.
Significance: Large-diameter trees are important constituents of forest ecosystems. The decreases in Yosemite National Park, a large protected area, suggest that cumulative human effects on the ecosystem (through fire exclusion and climate change) have been high. If forests remain unburned for many years, restoring pre-settlement composition and structure may require a very severe fire followed by several centuries of post-fire forest growth. The implications of large-diameter tree decline are twofold: (1) preserving the largest specimens of tree species will become increasingly important, and (2) fire must be used to restore these ecosystems before they develop a non-characteristic composition. A comparison of summer drought in past, present and future climate scenarios suggests that the climate-driven portions of large-diameter tree decline may accelerate. Furthermore, climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions about fire may be understated – future fires may be more severe, and post-fire recovery may take longer.
Climate, fire, and vegetation change in Yosemite National Park
Forests in western North America have changed radically since the beginning of Euro-American settlement, but few studies based on vegetation surveys have quantitatively examined the changes in the distribution and abundance of the largest trees. Studies of fire have attributed recent increases in annual area burned and fire size to a warming climate, but have minimized the issue of fire severity and left ignition frequency unaddressed. I combined satellite measurements of fire severity with data from a network of 655 historical (1932 – 1936) and 208 modern (1988 – 1999) vegetation plots to analyze large-diameter trees and fire in Yosemite National Park (3,027 km2). I modeled tree water relations to partially explain tree species’ distributions in Yosemite and to compare how past climate, present climate, and expected future climate may be contributing to forest composition. As a large area with relatively few recent human impacts, Yosemite is ideal for examining the dynamics of large-diameter trees, the frequency of lightning-ignited fire, and the patterns of fire severity. Large temperature and precipitation gradients extend beyond the range of any single coniferous species, allowing characterization of species distributions with respect to both physical and biophysical variables.
Studies of forest change in western North America often focus on the increasing densities of small-diameter trees rather than on changes to forests’ large tree component. Large trees are thought to be resilient to forest change, but few studies examine this assumption. I combined data from 655 historical (1932-1936) and 208 modern (1988-1999) vegetation plots to analyze the demography of large-diameter trees of 14 species in Yosemite National Park (3,027 km2). Between the mid-1930s and the 1990s, large-diameter tree density in Yosemite National Park declined 24%. The decrease was apparent in all coniferous forests. Declines were greater in subalpine and upper montane forests (57.0% of park area), and less in lower montane forests (15.3% of park area). Densities of large-diameter Pinus albicaulis (whitebark pine), Quercus kelloggii (California black oak), and Quercus chrysolepis (canyon live oak) increased, but large-diameter densities declined for eleven other common species. Three general patterns emerged. For Pinus ponderosa (ponderosa pine), Pinus jeffreyi (Jeffrey pine), and Pinus lambertiana (sugar pine), proportional decreases in large-diameter trees were greatest at lower elevations. For Abies concolor (white fir), Abies magnifica (red fir), Pinus contorta (lodgepole pine), and Pinus monticola (western white pine), proportional decreases were approximately uniform throughout the range. For Quercus chrysolepis, Quercus kelloggii, Calocedrus decurrens (incense cedar), Pinus albicaulis, and Pseudotsuga menziesii (Douglas-fir), increases in density of large-diameter trees occurred in the higher-elevation portions of their ranges. The proportion of plots with large-diameter trees changed little; declines were due to decreasing density within plots. Large-diameter tree richness and evenness generally decreased. Although overall density of large-diameter trees declined, the reintroduction of fire to plots in the Pinus ponderosa mixed coniferous forests did not decrease the density of large-diameter trees compared to areas unburned since 1936. In plots unburned since 1936, the large-diameter component was predominantly Abies concolor, Calocedrus decurrens, and Quercus chrysolepis, whereas Pinus ponderosa dominated the large-diameter component of burned plots. The declines in large-diameter trees are consistent with increased moisture stress due to increasing overall tree density and with climate that has warmed since the establishment of today’s large-diameter individuals. Modeled changes to annual soil moisture deficit in past (circa 1700), present (1971 – 2000 climatological means) and future (2020 – 2049 estimates) climate scenarios suggest that the climate-driven portion of changes in forest structure will accelerate.
Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focused only on large fires and have left the issues of ignition frequency, fire severity, and fire spatial complexity unaddressed. I examined the relationship between decreasing snowpack and the ignition, size, and complexity of all fires that occurred in Yosemite National Park between 1984 and 2005. During this period, 1,870 fires burned 77,718 ha. Decreased spring snowpack exponentially increased the number of lightning ignitions (P < 0.001). The principal mechanism whereby snowpack mediated lightning ignitions was through seasonal landscape flammability. A secondary mechanism was fewer lightning strikes in years where April 1st snowpack had higher water content. I quantified fire severity for the 103 fires >40 ha with satellite fire severity indices using 23 yr of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities increased with the log10 of annual area burned (P < 0.001). Spatial complexity of high severity and moderate severity burn patches increased with the log10 of annual area burned (P = 0.007 and P = 0.002, respectively), but the spatial complexity of low severity burn patches was not related to area burned. Future climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions about fire may be understated – future fires may be more severe, and post-fire recovery may take longer. Using one forecast for snowpack, I project that the number of lightning ignitions in Yosemite National Park from 2020 to 2049 will be 19.1% higher than between 1984 and 2005 and that the annual area burned at high severity will increase 21.9%.
Models of water relations have been used to explain tree species distribution, but these models generally assume flat terrain and uniform soil water storage making them less useful in mountainous terrain. I extended a Thornthwaite-type model by considering differences in water demand based on the local slope and aspect. I used plot physical parameters (n = 655) to derive annual water balance. I calculated annual actual evapotranspiration (AET) as a proxy for productivity and annual soil moisture deficit (Deficit) as a proxy for summer drought and used those values to describe distributions for 17 tree species over the 2,300 m elevation gradient in Yosemite. I combined vegetation plot data with precipitation, temperature, soil water capacity, slope, aspect and latitude data sets of different resolutions to develop spatially explicit environmental templates – a potential improvement over models where all input parameters have uniform grid size. I calculated tree species biophysical envelopes over broad ranges of environmental gradients – in this study a range of 31.0 cm for soil water capacity, 33.4°C for July mean temperatures, and 918 mm yr-1 for annual precipitation were noted. As climate change involves complex interrelations between changes in temperature and precipitation, models using AET and Deficit may allow more precise predictions of species range shifts. I used present climatological averages, reconstructions of past climate, and climate projections to evaluate changes in the water balance for locations where trees of each of 17 species are now present. Changes in water balance from 1700 (“Little Ice Age”) to the present are relatively small, but changes between the present and expected climate in 2050 (1.5°C warmer) are large and significant for most species.
Marked changes in density and species composition of large-diameter trees have occurred between the mid-1930s and the 1990s. Climate, fire, and fire exclusion have all played a role in these forest changes, and the respective roles of each have varied in importance through time. Many agents of mortality for large-diameter trees (drought, pathogens, and insects) act slowly and potentially in response to acute processes such as fire, and mortality rates differ among species. Understanding current conditions and decadal or longer trends in vegetation is critical for land managers seeking to preserve late-successional forests. The results of this research imply seven recommendations for landscape ecologists and land managers: 1) Establish long-term permanent-plot networks to examine changing rates, causes, and consequences of tree mortality, 2) Extend climatological monitoring and soil mapping to improve calculation of plant water relations, 3) Use April 1st snow water equivalent in fire planning, 4) Consider the preservation of large-diameter trees in management plans, 5) Anticipate continued declines in large-diameter trees by protecting mature stands from high-severity fire, 6) Accelerate wildland fire use and prescribed burning, and 7) Preserve landscape patchiness in fire operations.
My dissertation committee members were: Jerry F. Franklin (Chair), James K. Agee, Alan R. Gillespie, Charles B. Halpern, Thomas M. Hinckley, Donald McKenzie, Gerard H. Roe (GSR), and Jan W. van Wagtendonk.
What is the temperature in Yosemite National Park?
Elevations in Yosemite range from about 600 m to almost 4000 m. This gives rise to considerable variation in temperature. Unfortunately, the only weather stations are at low elevations. This set of temperature figures is derived from calculations made by the PRISM project at Oregon State University. These data represent climatological averages over the 1971–2000 period, so the range of temperatures might be much greater on any particular day.