Research article

Historic range of variability in landscape structure in subalpine forests ofthe Greater Yellowstone Area, USA

Daniel B. Tinker1,2,*, William H. Romme3 and Don G. Despain4

1Department of Zoology, University of Wisconsin, USA; 2Department of Botany, University of Wyoming,Laramie, WY 82071, USA; 3Biology Department, Fort Lewis College, USA; 4USGS, Department of Biology,Montana State University, USA; *Author for correspondence (e-mail: tinker@uwyo.edu)

Received 8 January 2002; accepted in revised form 13 February 2003

Key words: Disturbance, Fire, Historic range of variability, Landscape structure, Lodgepole pine, Logging, Yel-lowstone

Abstract

A measure of the historic range of variability (HRV) in landscape structure is essential for evaluating currentlandscape patterns of Rocky Mountain coniferous forests that have been subjected to intensive timber harvest.We used a geographic information system (GIS) and FRAGSTATS to calculate key landscape metrics on two� 130,000-ha landscapes in the Greater Yellowstone Area, USA: one in Yellowstone National Park (YNP), whichhas been primarily shaped by natural fires, and a second in the adjacent Targhee National Forest (TNF), whichhas undergone intensive clearcutting for nearly 30 years. Digital maps of the current and historical landscape inYNP were developed from earlier stand age maps developed by Romme and Despain. Maps of the TNF land-scape were adapted from United States Forest Service Resource Information System (RIS) data. Key landscapemetrics were calculated at 20-yr intervals for YNP for the period from 1705-1995. These metrics were used tofirst evaluate the relative effects of small vs. large fire events on landscape structure and were then compared tosimilar metrics calculated for both pre- and post-harvest landscapes of the TNF.Large fires, such as those thatburned in 1988, produced a structurally different landscape than did previous, smaller fires (1705-1985). Thetotal number of patches of all types was higher after 1988 (694 vs. 340-404 before 1988), and mean patch sizewas reduced by almost half (186 ha vs. 319-379 ha). The amount of unburned forest was less following the 1988fires (63% vs. 72-90% prior to 1988), yet the number of unburned patches increased by nearly an order of mag-nitude (230 vs. a maximum of 41 prior to 1988). Total core area and mean core area per patch decreased after1988 relative to smaller fires ( � 73,700 ha vs. 87,000-110,000 ha, and 320 ha vs. 2,123 ha, respectively). No-tably, only edge density was similar (17 m ha−1 after 1988) to earlier landscapes (9.8-14.2 m ha−1).Three de-cades of timber harvesting dramatically altered landscape structure in the TNF. Total number of patches increasedthreefold (1,481 after harvest vs. 437 before harvest), and mean patch size decreased by � 70% (91.3 ha vs.309 ha). None of the post-harvest landscape metrics calculated for the TNF fell within the HRV as defined inYNP, even when the post-1988 landscape was considered. In contrast, pre-harvest TNF landscape metrics wereall within, or very nearly within, the HRV for YNP. While reference conditions such as those identified by thisstudy are useful for local and regional landscape evaluation and planning, additional research is necessary tounderstand the consequences of changes in landscape structure for population, community, ecosystem, and land-scape function.

© 2003 Kluwer Academic Publishers. Printed in the Netherlands.Landscape Ecology 18: 427–439, 2003. 427

Introduction

Forest fragmentation has become an important issuein management and conservation of landscapesthroughout the world (Franklin and Forman 1987;Saunders et al. (1991) and Noss and Csuti (1994), andothers). A symposium on fragmentation of forestedlandscapes in the Rocky Mountains of western NorthAmerica was held inpublished (1997), during whichthe current state of our understanding was summa-rized (Knight et al. 2000). A key conclusion of thatsymposium was that we lack a suitable “natural” ref-erence condition against which to compare land-scapes that have been altered by human disturbancessuch as logging or road building. Many high-eleva-tion areas in the Rocky Mountains have been sub-jected to extensive clear-cutting during the last half-century, but we do not know the extent to which thelandscape patterns produced in these areas do or donot resemble the kinds of landscape patterns thatwould exist under a natural disturbance regime. Thisquestion is further complicated by the fact that natu-ral landscapes are dynamic, and that landscape struc-ture and composition continually change in responseto disturbances (e.g., fires) and recovery (White et al.1999). Therefore, a measure of the historic range ofvariability (HRV) in landscape structure under a “nat-ural” disturbance regime is necessary to provide anobjective context for comparing and evaluating thecurrent and projected structure of modern anthropo-genic landscapes (Swetnam et al. 1999). This conceptof maintaining landscape structure within a dynamicHRV has emerged over the last decade as a land man-agement alternative to mimicking static landscapeconditions (Baker 1992; Mladenoff et al. 1993; Swan-son et al. 1994; Cissel et al. 1998, 1999; Landres etal. 1999). The objective of this study was to providea quantitative description of the HRV in landscapestructure for an unlogged forested subalpine land-scape in the Rocky Mountains that has been shapedby natural fires during the last three centuries, and toevaluate the current structure of a harvested landscaperelative to the reference landscape.

At the heart of the questions addressed by thisstudy are issues relating to the similarities or differ-ences between landscapes that have been altered bymodern anthropogenic disturbances such as loggingand road-building, and those that are naturally patchyas a result of fires, insect outbreaks, and other naturaldisturbance processes (Knight et al. 2000). Noss andCsuti (1994) suggested that naturally patchy land-

scapes have rich internal patch structure, with lowcontrast between edges of adjacent patch types. Incontrast, fragmented landscapes exhibit relativelysimple patch structure and high-contrast edges. Large,infrequent disturbances such as wildfires play an im-portant role in the creation of naturally patchy forestlandscape patterns (Veblen 2000; White et al. 1999;Turner and Dale 1998; Foster et al. 1998; Turner etal. 1995). Clearcut timber harvesting also alters land-scape pattern, but previous studies have suggestedthat intensive clearcutting may produce a qualita-tively different landscape structure than wildfires(Hansen et al. (1991) and Mladenoff et al. (1993),Reed et al. (1996a, b); Wallin et al. (1996) and Tinkeret al. (1998); Tinker and Baker (2000)).

The development of landscape structure analysisprograms such as FRAGSTATS (McGarigal andMarks 1995) and r.le (Baker and Cai 1992) has pro-vided researchers and managers an objective methodby which to quantify the effects of changes in land-use practices on the pattern and structure of forestedlandscapes. Until recently, however, few studies haveused a range of historic landscape structures existingover a long time period as a dynamic reference con-dition by which to compare human-altered land-scapes. For example, previous studies in the RockyMountains (Reed et al. (1996a,b); Tinker et al.(1998)) identified forest fragmentation effects fromclearcutting and road building, but did not place cur-rent landscape structures within a long-term histori-cal reference framework. Mladenoff et al. (1993)compared the structure of a fragmented upper-Mid-western landscape to that of an intact, old growth for-est, but no measure of historic variability was in-cluded. Most recently, studies in the PacificNorthwest (Cissel et al. 1999; Wallin et al. 1996) us-ing historical reference landscapes representing atemporal range of several centuries, have shown thatquantitative measures of landscapes affected by bothactual and simulated clearcut timber harvesting mayfall outside the range of variability of historic land-scape structure. However, major differences betweenPacific Northwest forests and Rocky Mountain forestswith respect to climate, topography, fire regime, land-use history, harvest rotation, and regeneration meth-ods make it difficult to extrapolate findings from oneregion to another.

HRV in landscape structure for many terrestrialecosystems may be quite broad. In particular, largeinfrequent disturbances, such as the fires that burnedin Yellowstone National Park (YNP) in 1988, may

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have qualitatively different effects on landscape struc-ture than smaller, more frequent disturbances(Romme et al. 1998). Intense crown fires rarely burnan entire area, but create a spatially and temporallyheterogeneous landscape mosaic of intensely burned,moderately burned and unburned patches (Turner andRomme 1994). The fire regime in subalpine portionsof YNP during the last several centuries has beencharacterized by large, severe fires that burn extensiveareas every 100-300 years, followed by smaller, morefrequent fires during the interim between large fires(Romme (1982) and Romme and Despain (1989); butsee Millspaugh and Whitlock (2000) for an evenlonger temporal perspective on Yellowstone’s fire re-gime and climatic interactions). It is this dynamicrange of landscape pattern and structure that we haveattempted to estimate in this study. The HRV for thelast 300 years that we define for YNP may provide asuitable reference condition for evaluating subalpineforests elsewhere in the Rocky Mountains. We com-pared the long-term variation in the Yellowstone land-scape with changes produced in the last 40 years inthe nearby Targhee National Forest (TNF) by inten-sive clearcut logging. The TNF is representative ofmany areas dominated by lodgepole pine forests inthe Rocky Mountains, where timber harvest has beena management emphasis throughout the last half-cen-tury.

We addressed four specific questions: 1) What wasthe range of variation in landscape metrics such asmean patch size, number of patches, edge density, ortotal core forest area in the Yellowstone landscapefrom 1705 to 1988? 2) What were the relative impactsof large versus small fires on landscape metrics inYellowstone? 3) How did landscape metrics changein the Targhee National Forest following three de-cades of clearcut timber harvesting? 4) Do post-har-vest landscape metrics in the Targhee National Forestfall within the historic range of variability as charac-terized by the natural fire regime in Yellowstone Na-tional Park?

Methods

Study areas

The Greater Yellowstone Area (GYA), centered onYNP, occupies approximately 12 million ha in north-western Wyoming, eastern Idaho, and southwesternMontana, USA (N 45°0�0�, W 111°0�0�; Figure 1).

Our study area within YNP corresponds to a foreststand-age map created by W.H. Romme and D.G. De-spain (unpublished data) in 1985 of an approximately130,000-ha region of the YNP subalpine plateaus. Theelevation ranges from 2,200-m near the westernboundary of YNP to over 2,500-m along the Conti-nental Divide near the central portion of the park. Thedominant tree species is lodgepole pine (Pinus con-torta Dougl. Ex Loud var. latifolia Emgelm. ex Wats),with lesser amounts of subalpine fir (Abies lasiocarpa(Hook.) Nutt.) and Engelmann spruce (Picea engel-mannii Parry ex Engelm.) and whitebark pine (Pinusalbicaulis Engelm.).

For the TNF, we selected a portion of the Forestadjacent to the western boundary of YNP. Lodgepolepine forests also dominate the TNF study area, al-though some Douglas fir (Pseudotsuga menziesii[Mirb.]) occurs at the lowest elevations in the west-ern portion of the study area. The elevation averagesnear 2,200-m, and, while it drops slightly from eastto west, is relatively consistent across the study land-scape. Intensive commercial timber harvesting,largely clearcutting, began in the early 1960s and con-tinued until 1991, after which little or no clearcutharvesting has occurred (Judy Warrick, TNF, personalcommunication). The TNF study area is approxi-mately 133,000-ha in extent (comparable to the130,000-ha YNP study area) and was selected be-cause of its juxtaposition to YNP, as well as its simi-larity to the YNP study area with regard to geologicsubstrate, elevation, climate, and forest type (see be-low for complete description of how the study areawas delineated).

Map development

The base map used for analysis of the Yellowstonelandscape was a digital raster coverage created in theGRASS (USA-CERL, published (1997)) GeographicInformation System by W.H. Romme and D.G. De-spain in 1987. This map depicts stand age (i.e., timesince the last stand-replacing fire) for all forestpatches within a 128,840-ha study area in the centraland southern portion of YNP (Figure 2). The standage map was developed through extensive fieldwork,expanding from the more limited stand age map re-ported in Romme (1982) and using the same methodsreported in that earlier study. Forest patches that ap-peared to represent different stand ages were identi-fied on aerial photographs. Each patch > 4 ha in sizewas then located in the field and sampled by collect-

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ing 5-10 increment cores from dominant canopylodgepole pine, as well as cross-sections of fire scarswhere available. Very large patches (> 500 ha) weresampled in two or more places. Approximate dates ofpast fires were determined from the ages of the oldesttrees, and fire years were pinpointed with fire scarswhenever possible (Romme 1982). After dating all ofthe forest patches, the aerial photo map was convertedinto a stand age map. Some of the original patchesvisible on aerial photography were merged in the fi-nal stand age map, because they represented standsthat had originated after the same fire but had devel-oped different tree densities, perhaps because of dif-ferent initial postfire conditions (Turner et al. 1997).In addition, some large patches on the aerial photog-raphy were divided into two or more patches of dif-ferent stand ages, based on tree age samples andboundaries visible in the field but not in the aerialphotos. Fieldwork was conducted from 1977-1987,and thus the map represented the stand age mosaic,as it existed just before the extensive 1988 fires. Forthis study, we converted the original GRASS rastercoverage to an Arc/Info (ESRI 1995) GRID coveragein Arc/Info.

The majority of the study area was covered bylodgepole pine prior to 1988, and each polygon on themap contained attribute information representing theage of the stand (time since the last lethal fire) in 1985(Romme 1982). The pixel size of this and all subse-quent raster maps developed from the base map was50-m. However, the minimum mapping unit of theoriginal stand age map was 4-ha, so we used 4-haresolution for all analyses of landscape structure. Ac-cordingly, all polygons less than 4-ha in size were re-placed with the value of the majority of the surround-ing pixels. This was done by first removing most ofthe single-pixel polygons (0.25-ha) using the MA-JORITYFILTER process in the GRID module of Arc/Info (ESRI 1995). The raster coverage was then con-verted back to a vector map, and all other polygonssmaller than 4-ha were removed manually usingARCEDIT. The corrected polygon coverage was thenconverted back to a raster coverage in ARC/INFO.Finally, the raster coverage was converted to anASCII file for analysis in FRAGSTATS.

To define the HRV for the Yellowstone study area,we first calculated landscape metrics at 20-year inter-vals over the last 280 years preceding 1985 (the dateof our stand age base map). To create the 1965 map,

Figure 1. Locations of study areas in Yellowstone National Park (130,000-ha) and Targhee National Forest (133,000-ha) in Wyoming andIdaho, USA. YNP Study Area corresponds to stand age map developed by Romme and Despain in 1985 (see Figure 2 for more detail).

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we subtracted 20 years from the age of each polygonin the 1985 base map, creating a map that containedthe ages of each polygon in 1965. If, following thereclassification, any of the resulting polygons had avalue of zero or less (because the stand had burnedand been re-initiated during the interval from 1965-1985) we assigned an age of 350 years to those poly-gons. We generally did not know the actual age ofstands at the time they burned, but this arbitrary valuewas used to assure that the stand would remain in the“forested/unburned” class (see below) during the re-mainder of the 1705-1985 period of analysis. Thisapproach introduced some error because a few standsprobably burned more than once between 1705 and1985, and we accounted only for the most recentburn. However, the amount of area that actuallyburned twice between 1705-1985 probably was verysmall, because stands < 250-yr old have a very lowprobability of burning in Yellowstone’s subalpine for-

ests, except under severe conditions like 1988 (Ren-kin and Despain 1992). Such extreme burning condi-tions as 1988 probably did not occur in theYellowstone study area between the 1730s and 1985(Romme and Despain 1989).

This reclassification was then repeated on thenewly created 1965 map to create the 1945 landscapemap, and so on until all maps from 1705 through1985 had been created. We chose 1705 as the oldestmap to use for our analysis because stands that initi-ated during the early 18th century were the oldeststands whose ages were confidently estimated byRomme and Despain’s (1989) study. Also, the early1700s appear to have been characterized by large-scale fires under extreme burning conditions similarto those of 1988.

The polygons in each stand age map were then re-classified as either unburned forest, burned forest, ornon-forest. Any grid-cell with a stand-age value of 20

Figure 2. Study area in Yellowstone National Park. This map of lodgepole pine stand ages (years) in 1985 was developed by W.H. Rommeand D.G. Despain in 1985 using aerial photographs and extensive field sampling as described by Romme (1982) and Heinselmann (1973).

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years or greater was considered to be unburned for-est; all grid-cells with a value less than 20 were con-sidered burned forest. This 20-yr threshold was basedon field observations in YNP over the last 20 years,and represents an approximate age at which high edgecontrast between burned and unburned stands is ex-pected to disappear as young pine stands re-establishin burned areas. The non-forest portion of the land-scape consisted of various other cover types such asmeadows or riparian areas that were reclassified intoa single cover type, which did not change during theanalysis period (1705-1985).

In addition to creating landscape maps for the pe-riod 1705-1985, we created a 1995 map to capture theeffects of the large-scale fires of 1988. We began byreclassifying a burn severity map developed from anOctober 2, 1988 LANDSAT image into two classes:stand-replacing fire (crown or severe surface fire) or“unburned” (unburned or light surface fire that did notkill the canopy or initiate stand regeneration). The4-ha resolution of the 1985 stand age map was thenapplied to the reclassified burn severity map. This re-classified, adjusted resolution coverage was mergedwith the 1985 stand-age map, creating a new 1995coverage consisting of burned, unburned, and non-forest polygons, similar to the historic sequence ofYNP maps. Each historic map thus produced (for1705-1985 plus 1995) was converted into an ASCIIfire and analyzed for landscape structure usingFRAGSTATS.

For the TNF, we obtained a 1991 ARC/INFO digi-tal coverage from the United States Forest Service(USFS) Region Two office, depicting the vegetationcover of the entire TNF. The coverage originated aspart of the USFS RIS data base, and was developedfrom 1:24000-scale aerial photographs and LAND-SAT thematic mapper data obtained at a resolution of30-m, as well as individual stand exams. The spatialresolution for the stand exams was 30 meters (USFSUnpublished Metadata). To analyze a landscape simi-lar in extent to the YNP study area, only a portion ofthe TNF coverage was used. We therefore selected theregion of the TNF that is adjacent to the westernboundary of YNP because of similarities in geologicsubstrate, elevation, climate, and cover type. The for-ests of this part of the TNF are dominated by lodge-pole pine, as is the study area in YNP.

To reduce the extent of the TNF map, which wasinitially much larger than the YNP study area, we be-gan by clipping the area that shares a common bound-ary with YNP into randomly-chosen, consecutively

smaller regions using a clip coverage created in ARC/INFO, until the study area extent was similar to theYNP study area. Other than the forest boundary, therewere no other human or natural landforms or bound-aries that could be used to delineate the TNF studyarea. This resulted in a somewhat trapezoidal-shapedregion bordering the western boundary of YNP. Inaddition, the clipping process in ARC/INFO dissectedmany of the polygons along the western and southernedges of the newly created map perimeter, creatingunnatural, linear boundaries. Therefore, the small,outermost perimeter polygons containing the linearboundaries were removed in ARCEDIT to create amore natural study area boundary, without signifi-cantly reducing the extent of the map. The resultingfinal coverage was approximately 133,000-ha in ex-tent. Because we were interested only in forested andclearcut polygons, the map was classified based onU.S. Forest Service RIS data into five vegetationtypes: non-forest, recently clearcut with no regenera-tion, recently clearcut with seedling establishment(one-foot in height, or less), saplings 1� to 3� diam-eter, and mature forest (poles 3-7� diameter and ma-ture trees > 7� diameter) (Figure 3).

We also recreated the pre-harvest forest landscapestructure (ca. 1950) to quantify the effects of threedecades of clearcut timber harvesting on the TNFlandscape. We assumed that all polygons that wereeither non-regenerating clearcuts or regeneratingseedlings in 1991 had been clearcut during the previ-ous 30-40 years. The other forest types were all ag-gregated into a mature forest class. None of our TNFstudy area has burned since 1950, although other por-tions of the TNF did burn in 1988. We thus created amap of mature forest, non-forest (meadows, etc.), andrecent clearcut forests, both before and after the in-tensive clearcutting of the 20th century. The minimumpolygon resolution of 4-ha also was applied to bothTNF maps. All of the TNF landscape maps were con-verted to ASCII format.

FRAGSTATS analysis

Landscape metrics for all maps were calculated usingthe raster version of FRAGSTATS (McGarigal andMarks 1995). A 50-m depth-of-edge influence wasused to calculate all edge and core habitat relatedmetrics. The edge-contrast weightings used are in-cluded in Table 1. We chose number of patches, meanpatch size and patch size standard deviation, and edgedensity as useful metrics for describing the entire

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landscape. We used number of patches, percent oflandscape, mean patch size and patch size standarddeviation, edge density, total core area, and mean corearea per patch as meaningful indicators of class-levellandscape pattern and function (Cissel et al. 1999).

Results

The Yellowstone landscape from 1705 to 1988

The range of values in landscape metrics from 1705-1985 as shown in the left column in Table 2 and thecorresponding values after the 1988 fires are in theright column. The number of patches of all patchtypes across the Yellowstone landscape ranged from340-404 during 1705-1985, but nearly doubled fol-lowing the 1988 fires to 694. Similarly, mean patchsize remained relatively comparable during the pre-1988 years (319-379 ha), but was almost 50% smallerafter 1988 (186 ha). Patch size standard deviation de-creased almost two-fold after 1988 relative to the pre-vious 283 years, suggesting that patches across theYNP landscape had become more similar in size afterthe large fires. Edge density only increased from9.8-14.2 m ha−1 before 1988 to over 17 m ha−1 in1988.

The amount of unburned forest in YNP rangedfrom 72-90% prior to 1988, but was reduced to 63%following the 1988 fires. Notably, in 1988 the per-centage of the landscape composed of burned forestincreased from a maximum of 18% during the previ-ous 283 years to 27% (Table 2). Approximately 10%of the landscape was classified as non-forest (e.g.,meadows) in all landscape maps from 1705-1995, and

Figure 3. Study area in the Targhee National Forest in eastern Idaho. The eastern boundary of the study area is adjacent to YellowstoneNational Park’s western boundary.

Table 1. Contrast edge weightings used during ?? of YNP and TNFlandscapes. Values range from 0-1, with zero being the lowest edgecontrast between two patches, and one being the highest edge con-trast between two patches. Weightings were subjectively assignedbased on observed textural differences in adjacent vegetation types.

Adjacent Patches Edge Weighting

YNP Burned-Unburned 0.25

Burned/Unburned – Non-forest 1.00

Map Boundary – All patches 0.25

TNF Clearcut – Seedling 0.10

Clearcut – Sapling 0.50

Cle arcut – Mature forest 1.00

Clearcut – Non-forest 0.10

Seedling – Sapling 0.40

Seedling – Mature forest 0.90

Seedling – Non-forest 0.10

Sapling – Mature forest 0.25

Sapling – Non-forest 0.90

Mature forest – Non-forest 1.00

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this category did not change in any map. While thepercentage of unburned forest decreased after the1988 fires, the number of unburned patches increasedby nearly an order of magnitude over the previouslyhighest number, from 41 to 230 patches. Similarly,the number of burned patches across the landscapevaried from 1-48 during the 283 years prior to 1988,but increased to 137 patches in 1988. The mean patchsize of unburned forest patches diminished followingthe 1988 fires from a previous minimum of 2,276 hato only 354 ha. Patch size standard deviation (for un-burned patches) was also reduced compared to theprevious 283 years, suggesting patches had becomemore similar in size. Interestingly, edge density inunburned forests was only slightly higher after the1988 fires than during the previous 283 years, from aprevious maximum of 11.9 m ha−1 to 15.3 m ha−1 in1988. Total core area of unburned forest fluctuatedbetween 87,040-ha and 110,550-ha prior to 1988, butwas reduced to 73,709-ha after 1988. The meanamount of core area per patch of unburned forest wasconsiderably smaller after 1988, falling to only320 ha, compared with a previous minimum of2,123 ha (Table 2).

Targhee National Forest landscape

In the TNF, all four landscape metrics were greatlyaltered by 30 years of clearcut timber harvesting (Ta-ble 3). The number of patches increased more thanthree-fold from 437 before logging to 1,481 after log-ging, while the mean patch size decreased from over309 ha to only 91.3 ha. Patch size standard deviationalso decreased by a factor of three and edge densityincreased from 9.5 m ha−1 to 25.1 m ha−1 (Table 3).

The percentage of the TNF landscape composed ofunharvested mature forest decreased by almost 27%as a result of clearcutting, and the number of patchesof mature forest increased by an order of magnitude,from 12 to 123 (Table 3). Mean patch size of unhar-vested forest and standard deviation both decreasedsignificantly, suggesting smaller, more similar patchesfollowing 30 years of clearcutting. Edge density in-creased from 9.3 to 19.6 m ha−1, total core area ofunharvested, mature forest decreased by 37%(111,741 to 70,615 ha), and the mean core area perpatch was reduced nearly 20-fold from 9,312 to574-ha (Table 3). Notably, non-regenerating clearcutsand regenerating seedling patches comprised almost63% of the total number of patches, but only 27% ofthe total landscape area (Table 3).

Table 2. Comparison of Pre-1988 YNP landscape (1705-1985) to post-1988 YNP landscape (1995).

Pre-1988 landscape Post-1988 landscape

Minimum Maximum Value

Landscape Metrics Number of Patches 340 404 694

Mean Patch Size (ha) 318.9 378.9 186

Patch Size SD (ha) 3,917 6,181 2,121

Edge Density (m/ha) 9.747 14.16 17.3

Class-level Metrics Unburned Forest Percent of Landscape 72.4 90.1 63.2

Number of Patches 12 41 230

Mean Patch Size (ha) 2,276 9,672 354

Patch Size SD (ha) 12,035 31,500 3,585

Edge Density (m/ha) 9.7 11.9 15.3

Total Core Area (ha) 87,040 110,550 73,709

Mean Core Area/Patch (ha) 2,123 9,212 320

Burned Forest Percent of Landscape 0.01 17.7 27

Number of Patches 1 48 137

Mean Patch Size (ha) 12.25 632.5 253

Patch Size SD (ha) 0 1,324 1,028

Edge Density (m/ha) 0.02 6.59 10.4

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None of the four landscape-level metrics calcu-lated for the post-harvest landscape of the TNF fellwithin the historic range of variability for subalpineforest landscape structure as defined by nearly 300years of fires and post-fire succession in YNP (Fig-ure 4). The total number of patches in the harvestedTNF landscape (1,481) was more than double that ofthe highest number of patches identified for any YNP(Figure 4a). Mean patch size in the post-harvest TNFlandscape was 91 ha, a reduction of over 50% fromthe smallest mean patch size in YNP (Figure 4b). Thestandard deviation of patch sizes in the post-harvestTNF landscape also was less than all YNP values, al-though the difference was smaller than other metrics(Figure 4c). The clearcut TNF landscape also showedhigher edge density (25.1 m ha−1) than any YNP land-scape (Figure 4d).

In contrast to the clearcut landscape, the pre-har-vest TNF landscape was within, or nearly within theHRV as identified in YNP for every landscape metricmeasured (Figure 4). Only edge density was slightlyoutside the HRV; the value for the unharvested TNFstudy area was less than one meter of edge per hect-

are lower than the lowest value calculated for theYNP landscape (Figure 4).

Discussion

Large wilderness areas like YNP provide unique op-portunities to understand ecosystem dynamics withina setting that has been minimally altered by 20th cen-tury technological capabilities and objectives. Yel-lowstone’s subalpine landscape has been shaped pri-marily by climate, geological processes, bioticinteractions among native species, and natural dis-turbances, especially fire, throughout the last severalcenturies. As such, it provides one of the best avail-able reference areas against which we can comparethe ecological effects of our past and anticipated fu-ture forest management activities elsewhere theRocky Mountains. In this study, we have recon-structed and quantitatively measured landscape struc-ture during the last 300 years, describing how thatstructure has fluctuated in response to a largely natu-ral fire regime. These results provide an empirically-based historic range of variability that includes the

Table 3. Comparison of landscape metrics in TNF to historic range of variability for YNP from 1705-1995 as defined by this study. Textshown in italic type indicates metrics for which TNF is outside the historic range of variability.

HRV for YNP TNF Before Logging TNF After Logging

Landscape Metrics Number of Patches 340-694 437 1,481

Mean Patch Size (ha) 186-379 309.3 91.3

Patch Size SD (ha) 2,121-6,181 5,584 1,736

Edge Density (m/ha) 9.8-17.3 9.5 25.1

Class-level Metrics Unharvested Mature

Forest

Percent of Landscape 63-90 86.6 59.7

Number of Patches 12-230 12 123

Mean Patch Size (ha) 354-9,672 9,751.5 656.3

Patch Size SD (ha) 3,585-31,500 32,289 5,954

Edge Density (m/ha) 9.7-15.3 9.3 19.6

Total Core Area (ha) 73,709-110,550 111,741 70,615

Mean Core Area/

Patch (ha)

320-9,212 9,311.7 574.1

Recently Burned

(YNP) or Clearcut

(TNF) Forest

Percent of Landscape 0.01-26.9 N/A 6.1

Number of Patches 1-137 N/A 390

Mean Patch Size (ha) 12.2-632.5 N/A 21.1

Patch Size SD (ha) 0-1,324 N/A 18.9

Edge Density (m/ha) 0.02-10.37 N/A 6.6

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effects of large infrequent disturbances (the 1988 Yel-lowstone fires) as well as more frequent smaller dis-turbances (Table 2 and Figure 4).

Both landscape and class-level metrics in the Yel-lowstone study area varied greatly during the period1705-1995, as would be expected in a non-equilib-rium, crown-fire ecosystem (Turner et al. 1993;Turner and Romme 1994). The greatest magnitude offluctuation was introduced by the large 1988 fires –which created a greater number of patches, a smallermean patch size, and a greater edge density, than hadbeen measured on any of our reconstructed landscapemaps for the previous 280-year period. This resultsupports the idea that large infrequent disturbancesdominate landscape structure and dynamics in subal-pine and boreal coniferous forest landscapes, and arefar more important determinants of ecosystem struc-ture and function than the more frequent but smalldisturbances that occur between major disturbanceevents (e.g., Johnson (1992) and Moritz (1997), Fos-ter et al. (1998)). Although the post-1988 landscape

forms one of the outer boundaries of the YellowstoneHRV for all four landscape-level metrics that wetreated in this study (Figure 4), the actual landscapestructure of our study area in 1705 and 1725 prob-ably was more similar to the 1995 landscape thanFigure 4 implies. This is because Romme and De-spain (1989) only detected the major patches of dif-ferent stand ages in 1985, and undoubtedly missedmany of the small patches created by extensive, 1988-like fires that occurred around 1700. However, the1985 base map does very accurately depict the nu-merous small fires that occurred from 1885-1985; andthe landscape-levels metrics in Figure 4 show that in-deed the effects of these small fires on landscapestructure were minimal, compared with the great ef-fects of the large 1988 fires.

Prior to the extensive clearcutting program initi-ated around 1960, the parameters of landscape struc-ture that we measured for the Targhee National Foreststudy area was within HRV for the Yellowstone studyarea. This was not surprising, given the relative simi-

Figure 4. Landscape metrics for the YNP landscape from 1705-1995, and for TNF in 1991 and prior to commercial timber harvesting.Dashed horizontal lines enclose our estimated historic range of variability for each metric, as defined by YNP landscape structure.

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larity in geology, climate, vegetation, and previousdisturbance history of the two nearly contiguous ar-eas. Edge density actually was slightly lower in theTNF study area before clearcutting than at any timemeasured in the YNP study area; however, number ofpatches, mean patch size, and patch size standard de-viation, all were within the HRV measured for YNPfrom 1705-1995 (Figure 4). However, three decadesof intensive clearcutting dramatically changed thelandscape-level metrics of the TNF study area. In1991, patch number was far higher and mean patchsize was far smaller in the TNF than we measured forany time in the YNP study area from 1705-1995 (Fig-ure 4). Even after the large 1988 fires, the burnedYellowstone landscape was not nearly so fragmentedas the clearcut Targhee National Forest. The TNFlandscape also was more homogeneous after clearcut-ting, as indicated by a patch size standard deviationlower than any measured in the YNP study area (Fig-ure 4). In a similar study that compared landscapestructure in burned and clearcut stands in YNP andthe TNF, Wilmer (2000) also found that uncut forestpatches were less variable in size within the clearcutlandscape of the TNF than in the unburned portion ofthe landscape in YNP. His study also identified lesstotal core area in the undisturbed patches of a clearcutforest than unburned patches. However, in contrast toour findings, he did not detect any differences in edgedensity between the burned and clearcut landscapes.This difference is likely due to the specific regions ofthe TNF and YNP that were analyzed. Of the fourlandscape-level metrics that we measured in the TNF,only edge density after clearcutting was not far out ofthe HRV defined for YNP (Figure 4).

Some class-level metrics for the TNF were stillwithin the HRV defined for YNP even after clearcut-ting (e.g., number of patches, mean patch size, andpatch size standard deviation for unharvested forest),but others were outside the HRV (e.g., edge densityand total core area in unharvested forests – Table 2).One reason why mean patch size of unharvested for-est remained within the HRV is that this metric in theTarghee study area (before clearcutting) actually wasgreater than for any of the temporal sequence of land-scapes that we measured in the Yellowstone studyarea. Thus, the mature forest in the TNF was excep-tionally un-fragmented at the outset of the clearcut-ting program, but was barely within the HRV after 30years of clearcutting.

We did not include roads in any of our landscapemaps or in any FRAGSTATS analyses. Other studies

in the Rocky Mountain region have shown clearlythat roads have an even greater fragmenting effectthan clearcuts (Miller et al. (1996) and Reed et al.(1996a),b; Tinker et al. (1998) and Baker (2000)). TheYellowstone study area has never contained manyroads, but the Targhee National Forest area has adense network of roads built after 1960 to access theclearcuts. Had we included the effects of roads in ourFRAGSTATS analyses, the TNF study area probablywould have been far outside the HRV defined forYNP in almost every landscape-level and class-levelmetric.

This study has produced an empirically-based,temporally dynamic, and spatially quantitative esti-mate of the historic range of variation in landscapestructure that has resulted from a largely natural dis-turbance regime in a subalpine coniferous forest land-scape of the Rocky Mountains. An HRV of this kindprovides an appropriate reference condition that man-agers can use to evaluate the legacies of past man-agement policies and programs, and to assess poten-tial effects of future activities, on landscape structureand pattern in other ecologically similar settings(Wallin et al. 1996; Cissel et al. 1999; Swetnam et al.1999; Romme et al. 2000). Although we believe theseresults are useful, we close with two important cave-ats. First, caution must be used in extrapolating thequantitative values of HRV measured in YNP to otherportions of the Rocky Mountains, even if the areas arerelatively close together. This is because of inherentdifferences in climate, soils, disturbance history, andbiogeography, and also because the numbers we re-port here are strongly influenced by the scale-relateddetails of our analysis (e.g., our 4-ha minimum map-ping unit). Also, slight differences do exist betweenthe two study areas considered in this study, and someof these differences may not be adequately repre-sented in the spatial data derived from the USFS RISdata. Consequently, we need more local case studiesof this kind from throughout the Rocky Mountain re-gion, to better evaluate the local effects of historicdisturbance regimes and forest management activitieson landscape structure. Second, although we nowhave excellent tools for quantitatively measuring spa-tial patterns in landscapes, we still can say relativelylittle about the consequences of these patterns for ec-osystem, community, or population-level function –e.g., biogeochemical cycling, successional trajecto-ries, or wildlife population dynamics (Knight et al.2000). The results of this study, and others like it,suggest that important functional changes may be tak-

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ing place in our fragmented Rocky Mountain land-scapes, but specific, focused research is urgentlyneeded to evaluate the magnitude and significance ofthese suspected functional changes.

Acknowledgements

The University of Wyoming – National Park ServiceResearch Center on Jackson Lake and YellowstoneNational Park Ranger Division provided logistic sup-port for the field work associated with this study. Wewish to thank Shannon Savage of the YellowstoneNational Park Spatial Analysis Center and Judy War-rick of the U.S. Forest Service for providing much ofthe spatial data used in this study. We also thank themany field assistants from Fort Lewis College, Du-rango, CO and the University of Wisconsin – Madi-son for their help with data collection. Monica G.Turner generously provided a careful review of anearlier draft of the manuscript. Financial support forthe research was provided by the National ScienceFoundation (BSR-8408181 and DEB-9806440).

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