Title: Monitoring Forest Response to Past and Future Global Change in Greater Yellowstone

Contact: Scott Powell, spowell@montana.edu

Funding: National Aeronautics and Space Administration (NASA)

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Conifer Forest Expansion Research Introduction and Overview:

     Biophysical Determinants of Conifer Expansion: The Greater Yellowstone Ecosystem (GYE) is characterized by steep abiotic gradients in climate, soils, and topography. These gradients influence and are influenced by landuse and disturbance regimes to shape the distribution of vegetation. Gross vegetation patterns in the GYE have been well documented (Despain, 1990). Xeric valley bottoms are dominated by grassland (e.g. Festuca idahoensis) and shrubland systems (e.g. Artemesia tridentata). Moving upslope, grasslands and shrublands give way to low density conifer savannas (e.g. Juniperus scopulorum, Pseudotsuga menzeisii), which grade into higher elevation, mesic conifer forests (e.g. Pinus contorta, Picea engelmanni). High elevation conifer forests (e.g. Abies lasiocarpa, Pinus albicaulus) are again patchy towards upper treeline, often dominated by krummholtz tree growth forms that give way to tundra and bare, rocky ridges. Steep abiotic gradients and static ecotones between conifer forest and non-forest initially led researchers to believe that edaphic and topographic factors were responsible for the long-term maintenance of vegetation boundaries (Loope and Gruell 1973). Our research however, indicates that far from being static, the boundaries between conifer forest and non-forest are highly dynamic, and conifer forests are rapidly increasing in extent and density across the GYE. These changes likely have significant consequences for multiple ecosystem processes, including biogeochemical cycling, natural disturbance regimes, and habitat provision.

     Since the time of European settlement, it has been widely observed that conifer forests have increased in extent and density across large areas of the GYE. These changes have been well documented by repeat historical photography (Meagher and Houston 1998; Gruell 1983). Some of these changes are attributable to forest regrowth following extensive fires prior to European settlement (Loope and Gruell 1973; Arno and Gruell 1983; Barrett and Arno 1982), but in many locations conifer forests have expanded into grasslands, shrublands, and hardwood ecosystems (Arno and Gruell 1986). Furthermore, many regions that previously supported low density, open-canopy conifer woodlands have increased in density (Arno et al. 1997). Only a few studies have attempted to quantify the rate of conifer expansion in small portions of the GYE. Simulation modeling of a watershed in the Centennial Mountains found that the area of conifer forest had increased from 15% to 51% between 1856 and 1996 (0.26% increase per year), largely at the expense of grasslands and shrublands (40% loss), and broadleaf forests (75% loss) (Gallant et al. 2003). This raises questions about the overall extent and rate of conifer expansion across the entire GYE.

       Apart from the overall extent and rate of conifer expansion in the GYE, it is uncertain if change is occurring systematically across all forest types or rather is confined to particular biophysical locations. Climate variability and change (Jakubos and Romme 1993), atmospheric change (Soule et al. 2003), fire suppression (Arno and Gruell 1986), and grazing regimes (Richardson and Bond 1991) have all been hypothesized as drivers of woody plant expansion. These drivers are complex and interacting, and likely influence the biophysical footprint of conifer expansion in the GYE. At the extent of the GYE, no studies have yet been undertaken to explore these fundamental interactions. This study will be the first to quantify the inherent variability in the rates of conifer expansion across the GYE and to establish relationships between the spatial distribution of this phenomenon and it’s biophysical determinants.

     Steep abiotic gradients and the diversity of forest types in the GYE suggest that the observed pattern of conifer expansion underlies the relative influence of drivers. Soil moisture and temperature, as influenced by climate, soil type, and vegetation, are widely regarded as key limiting factors for conifer seedling establishment (Patten 1963). These biophysical conditions ultimately dictate the dynamic trajectory of a site. In the absence of suitable soil conditions for seedling establishment, conifer expansion is unlikely to occur. Research indicates that on sites susceptible to drought, conifer expansion is more likely triggered by cooler and wetter conditions, while on mesic sites, conifer expansion is correlated with a warmer and drier climate (Miller and Halpern 1998; Butler 1986; Jakubos and Romme 1993). This suggests that drivers of conifer expansion like fire suppression, grazing, and climate variability might have variable effects as influenced by the biophysical environment.

     In mesic, high elevation, subalpine forests, with fire return intervals up to hundreds of years long (Arno 1980), it is unlikely that modern fire suppression efforts have been in effect long enough to produce significant changes in forest structure and composition. Here, conifer expansion is more often associated with climate variability that facilitates the establishment, survival, and growth of conifer seedlings (Jakubos and Romme 1993). Alternatively, a number of case studies have suggested that fire regimes have been especially altered in xeric, low elevation forests (Houston 1973; Arno and Gruell 1983; Arno and Gruell 1986; Dando and Hansen 1990). Historic fire return intervals in Douglas-fir forests in southwestern Montana were estimated to be between 25-40 years (Arno and Gruell 1983; Arno and Gruell 1986), partially attributable to frequent Native American burning (Barrett and Arno 1982). Frequent, low-intensity fires that swept through the understory of open Douglas-fir stands maintained lower densities of trees by killing young conifer seedlings and saplings. With a significant reduction in fire frequency and extent in the 20th century, the probability of conifer survival increased and forests expanded their range and increased in density.

     In addition, livestock grazing, as a de facto form of fire suppression has profoundly shaped the dynamics between trees and grasses in the northern Rocky Mountains, especially in lower elevation forests. Intermediate grazing levels are most closely associated with extensive conifer expansion, as higher levels can be destructive to conifer seedlings and lower levels permit competitive growth of herbaceous vegetation (Butler 1986). As succession proceeds from grasslands to shrublands to forests, significant microsite ameliorations occur, reducing soil surface temperatures, increasing availability of soil moisture, protecting conifer seedlings from livestock trampling and wind, and increasing soil organic matter from litter accumulation (Patten 1963; Sindelar 1971). In Douglas-fir forests of southwestern Montana, sagebrush expansion typically precedes Douglas-fir expansion, and sagebrush act as “nurse plants” in improving the microsite environment for continued forest succession (Sindelar 1971).

     Biophysical variation in the rates, extent, and drivers of conifer expansion have important consequences for understanding the interactions between forest dynamics and ecosystem processes such as carbon sequestration, fuel loading and fire behavior, habitat provision, forage production, hydrologic cycling, and insect and pathogen outbreaks. For informed decision making, it is important for land managers to have accurate information about dynamic landscapes.

Spectral and Biophysical Modeling of Conifer Expansion:

     Accurate land cover classifications for large regions are necessary for land managers to make informed decisions. Only a few high spatial resolution vegetation classifications have been completed for the GYE. The USGS produced a national land cover data set in 1992 that included the GYE (National Land Cover Data, 1992), but the discrete vegetation classes lacked important local detail. The USGS/Biological Resources Division Gap Analysis Program (GAP reference) used 30-m Landsat TM data to map vegetation of the 3 GYE states (Idaho, Montana, and Wyoming), but no effort was made to edge-match the individual maps. Portions of the GYE were mapped from air photos by individual National Forests of the GYE (USFS reference), and satellite imagery was used to map small portions of the GYE, namely within Yellowstone National Park (Jakubauscus 1996; Turner et al. 1994). Parmenter et al. (2003) were the first to accurately classify the entire GYE at a 30-m spatial resolution. Their hierarchical vegetation classification scheme was based upon discrete classes, some consisting of mixtures of two separate classes (e.g. conifer/herbaceous). Their study was also the first to accurately map vegetation change in the GYE between 1975-1995. However, because the mixed classes contained high biophysical variability, they were unable to depict change that did not involve transitions between discrete classes. As such, increases in conifer stem densities were often undetected. Our previous research (Powell and Hansen, in prep.) indicated that conifer expansion was a rapid and widespread dynamic in the GYE. Our objective in this study is to improve upon the classification of mixed classes through the use of continuous classification. Improvement of these classes will enable improved quantification of the dynamics of conifer expansion.

     Continuous classification of biophysical variables often presents a more realistic interpretation of vegetation cover, and are more easily integrated into a wider variety of research and land management activities. Whereas discrete classifications imply fixed edges between classes, continuous classifications are suggestive of more dynamic landscapes. Classifications of continuous biophysical variables such as leaf area index (LAI) are increasingly common in the literature (Cohen et al. 2003; White et al. 2003) and are often relied upon to drive biogeochemical cycling models. Continuous classifications of canopy cover have been performed using a variety of techniques, including multiple regression (Lawrence and Ripple 1998), reduced major axis regression (Larsson 1993; Cohen et al. 2003), and spectral mixture analysis (Sabol et al. 2002). In this study, we present a method for continuous mapping of percent conifer cover across the GYE for 1985. Then, based upon our previous study of the extent and rates of change by cover type and biophysical setting (Powell and Hansen in prep.), we present a method of change prediction to extrapolate percent conifer cover to 1999. The results of this study will enable accurate quantification of the extent, rates, and spatial distribution of percent conifer change across the GYE, and facilitate analyses of the consequences of conifer expansion for carbon sequestration, biodiversity, and fire behavior.

Consequences of Conifer Expansion for Carbon Source/Sink Dynamics:

     Documented increases in atmospheric carbon dioxide and hypothesized linkages to climate warming have resulted in increased research attention on global carbon budgeting. Since the start of the industrial revolution, the atmospheric concentration of carbon dioxide has increased by approximately 31 +/- 4% and the global mean surface temperature has risen by 0.6 +/- 0.2oC over the 20th century (IPCC 2001). Quantified emissions, or sources of carbon dioxide remain larger than quantified reservoirs, or sinks of carbon dioxide, essentially resulting in an undiagnosed “missing sink”. Current estimates of the size of the northern hemisphere terrestrial sink range between 1-2 Pg C/yr., but the temporal and spatial dimensions, as well as the drivers of the sink remain uncertain (Pacala et al. 2001).

     Carbon sinks attributed to conifer expansion are hypothesized to account for some fraction of the “missing sink” in global carbon budgets (Schimel 2002; Houghton 1999). By increasing biomass, expanding conifer forest ecosystems store more carbon and uptake more carbon dioxide than their former grassland or shrubland counterparts (Hansen et al. 2000). Likewise, as conifer woodlands increase in density, more carbon is stored (Covington and Moore 1994). Current regional, national, and global carbon accounting systems retain large uncertainty with regards to the influence of conifer expansion and fire exclusion on carbon uptake (Houghton et al. 1999). At the national level, few carbon budgeting studies have attempted to account for conifer expansion, and to date, only inventory based approaches have produced reliable estimates (Houghton et al. 1999; Houghton et al. 2000; Pacala et al. 2001). Acknowledging substantial uncertainty, Pacala et al. estimated that woody encroachment into non-forest ecosystems accounted for approximately 0.12 - 0.13 Pg C year-1 in the conterminous U.S., a substantial portion of the estimated 0.37 - 0.71 Pg C year-1 total carbon sink (Pacala et al. 2001). Higher accuracy quantification of the contribution of woody encroachment to carbon uptake has generally been confined to small, local scale analyses (Asner et al. 2003).

     What is known is that conifer expansion into grasslands and shrublands, and the in-filling of low density coniferous forests result in increased pools of aboveground carbon stores. Researchers simulated basal area increases in open ponderosa pine/Douglas-fir forests over 100+ years of fire exclusion, resulting in accumulation of 2,500 kg C ha1 year –1 in biomass (Keene et al. 1990). Other estimates of carbon accumulation from conifer expansion into non-forested ecosystems range from 650 kg C ha-1 year –1 in arid regions of the inland northwest to 1,400 kg C ha-1 year –1 in mesic locations of the western north central plains (Houghton et al. 2000). But these estimates are generalized across extensive regions with high biophysical variation. This study is designed to greatly improve upon these estimates by explicitly accounting for variation in forest type and biophysical setting.

     Wildfire is a driving ecological force for GYE forests. As variable as the vegetation types are across the study area, so too are the historic fire regimes, in terms of their frequency and intensity. Historic fire regimes ranged from frequent, low intensity fires in low elevation forests to infrequent, high intensity fires in higher elevation subalpine forests. As a consequence of effective 20th century fire suppression, grazing, and logging, many forest systems across the Greater Yellowstone Ecosystem have experienced reductions in the frequency and extent of wildfire. The temporal duration of forest carbon sinks therefore is in question, as a result of high fuel accumulation and risk of intense wildfire. This is particularly evident in lower elevation Douglas-fir forests where the fire return interval was historically frequent, and the rate of conifer expansion is high (up to 0.43% per year). In the absence of fire, the accumulation of dead and live biomass has potentially resulted in a higher risk of intense wildfire and thus potentially a loss of stored carbon (Sampson and Clark 1996). Many such locations are well outside their historical range of variability with respect to forest density, extent, and biomass. In historically high frequency, low intensity forest systems that are sequestering high levels of carbon, the release of carbon associated with higher intensity fires potentially offsets the gains associated with carbon sequestration (Sampson and Clark 1996). Therefore, it is critical for land managers to understand where the most rapidly changing places are on the landscape, and what the implications of these changes are for carbon storage and risk of wildfire.

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Last modified  June 8, 2004