Fire-dependent forests in the Northern Rocky Mountains

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<ul><li><p>QUATERNARY RESEARCH 3, 408424 (1973) </p><p>Fire-Dependent Forests in the Northern Rocky Mountains </p><p>JAMES R. HABECK~ AND ROBERT W. MUTCH~ </p><p>Received April 15, 1973 </p><p>One objective of wilderness and parkland fire-ecology research is to describe the relation- ships between fire and unmanaged ecosystems, so that strategies can be determined that will provide a more nearly natural incidence of fire. More than 50 yr of efforts directed toward exclusion of wildland fires in the Northern Rocky Mountains (western Montana and northern Idaho) have resulted in a definite and observable impact on the forest ecosystems in this region. Fire-ecology investigations in Glacier National Park and the Selway-Bitterroot Wilderness have helped to reveal the nature of this impact and to provide a better understanding of the natural role of fire within these coniferous ecosystems. Such areas provide a unique opportunity to study and test approaches designed to perpetuate unmodified ecosystems. However, we still dont understand all of the long-term consequences of fire control in those forest communities that have evolved fire-dependent characteristics. </p><p>INTRODUCTION </p><p>Large areas have been established as na- tional parks and wildernesses in the North- ern Rocky Mountains. Their management calls for the perpetuation of all natural for- est ecosystems, including those that are fire-dependent. However, these areas have received the same degree of fire suppression that is achieved in commercial forests. There is a renewed interest in the ecology of fire-dependent species, because exclusion of wildland fires may have detrimental side effects (Heinselman, 1970; Dodge, 1972; Kilgore, 1972 ; Kilgore and Briggs, 1972). This concern is supported by an application of basic ecological theory. Many kinds of habitat alterations that cause prolonged or permanent modifications of site character- istics (e.g., changes in soil moisture or nu- trient supply) will predictably lead to sig- </p><p> Department of Botany, University of Mon- tana, Missoula, Montana 59801. </p><p>BUSDA Forest Service, Northern Forest Fire Laboratory, haissoula, Montana 59801, </p><p>nificant changes in community structure and function. The effective reduction of fire cn landscapes that historically were influ- enced by periodic fires will have a modify- ing influence that is detectable and measur- able (Fig. 1). </p><p>Fire-dependent forest ecosystems require fire treatment for their continued perpetua- tion on the landscape. The competitive suc- cess of fire-dependent species is directly re- lated to the selection of characteristics that improve the fitness of populations in the fire environment of the Northern Rocky Mountains. Many species here exhibit mor- phological and physiological adaptations that provide survival advantages on land- scapes that are subjected to fire cycles. We suspect that the diversity of community life forms engendered some sort of ecosystem equilibrium or a kind of biologic checks and balances system that governed the magnitude of the effects accompanying a given forest fire. </p><p>Personnel from the USDA Forest Service and the University of Montana are seeking </p><p>408 </p><p>Conyright c 1973 by University of Washington. All rights o P reproduction in any form remrvad. </p></li><li><p>FIRE-DEPENDEET FORESTS, SORTHERN ROCKIES 409 </p><p>FIG. 1. Tango Cr&amp; Fire, 1953, Bob Marshall Kiidwnes, blontonu </p><p>to gain understanding of the extent to which fire suppression in the Northern Rocky Mountains has modified coniferous ecosystems. Without such investigations we will not fulfill the management objectves for national parks and wildernesses. Any management prescription designed to insure a return as closely as possible to a natural incidence of fire on these landscapes must be built upon sound field data. Up to this time both research and administrative studies have been investigating fire-depen- dent ecosystems in the Selway-Bitterroot Wilderness and in Glacier National Park. </p><p>The purposes of this paper are to describe the vegetation patterns shaped by the fire environment of the Northern Rocky Moun- tains, to relate fire-dependent species to ecosystem stability, and to discuss the practical application of such knowledge to wilderness fire management. </p><p>GENERAL CHARACTERISTICS OF THE NCRTHERN ROCKY </p><p>JIGI-NTAINS </p><p>The Northern Rocky Mountains are com- posed of a series of mountain ranges that average between 2400-3100 m (7874-10,170 ft). These ranges are generally orient,ed in a northwest to southeast direction; many are separated by well-defined intermontane valleys occurring at 800-1100 m (2625- 3609 ft) above sea level. The drainage pattern for the entire region is complex; the ranges west of the Continental Divide are drained by the Columbia River system, and those on the cast side by the Missouri River system. </p><p>A large portion of the Northern Rocky Mountains was glaciated during the Pleis- tocene, either by the Cordilleran or Conti- nental ice sheets from Canada, or by local </p></li><li><p>410 HABECK AND MUTCH </p><p>mountain glaciers. Many large freshwater lakes in western Montana and northern Idaho were formed by past glacial action. A large portion of this region was inun- dated by Glacial Lake Missoula, and the features of many of the valleys in western Montana have been modified by this past water influence. </p><p>Much of the Northern Rocky Mountains west of the Continental Divide is influenced by a Pacific coastal climatic regime. Mois- ture-laden air masses from the northern Pacific Ocean sweep along easterly moving storm tracks at this latitude. Although moisture is lost as the air masses move over the Coastal and Cascade Mountains, much moisture is retained and carried farther eastward into the Northern Rocky Moun- tains. At the higher elevations, annual pre- cipitation may be 1500 mm (59 in.) or more, much of which is in the form of win- ter and early spring snowfall. The inter- montane valleys, in contrast, typically re- ceive 380 mm (15 in.) or less annually. </p><p>Of particular importance in the annual climatic regime is the marked reduction of moisture during the midsummer months. Many weather stations in western Montana and northern Idaho report 24 mm (1 in.) or less rainfall during July and August. This reduction in moisture allows consider- able desiccation of forest fuels during the warmest months of the year. Summer light- ning storms, combined with the drying for- est fuels, set the stage for wildfire ignitions. </p><p>As a result of the Pacific coast climatic regime, the vegetation of the Northern Rocky Mounta,ins is closely related to that found along the Pacific coast. Western redcedar (Thuja plicata) , western hemlock (Tsuga heterophylla) , western yew (Taxus brevifolia) , western white pine (PiWUS monticola), and grand fir (Abies grandis) are good examples of species whose main botanical ranges lie farther to the west. Dozens of shrubs and herbaceous species also display similar eastward range exten- sions from western Oregon and Washington. In the distant past, all of these species were </p><p>more widely distributed in the Northern Rocky Mountains. However, following the upthrust of the northern Cascade Moun- tains, ranges of these species have become limited because of decreased moisture con- tent. This shrinkage of ranges has con- tributed to the complexity of vegetation, particularly in western Montana. </p><p>AN OVERVIEW OF FIRE- DEPENDENT FORESTS IN THE </p><p>ROCKY MOUNTAINS </p><p>Both authors have had over a decade of field experience with forest vegetation and fuels in the Northern Rocky Mountains. The senior author has been engaged in a continuing phytosociological analysis of grassland, forest, timberline, and alpine communities in this region; the junior au- thor has had comparable experience in the study of forest fuels in many different for- est ecosystems. During their field work, di- rect evidence could usually be found that past fires had influenced most plant com- munities. The presence of charcoal, either above ground or in the soil profile, has indi- cated that most areas have been burned. </p><p>Anyone knowing forest successional pat- terns and species relationships can readily observe that a high percentage of the vege- tation, within all forest zones, is at one stage or another of succession following past fires. Climax, or near-climax, forest stands that have escaped fire for several centuries are only rarely found in northern Idaho and western Montana. It is believed that past, uncontrolled fires did not, at any one point in time, create a completely burned over and denuded landscape, be- cause many stages of successional develop- ment can usually be found in each forest zone. </p><p>Written historical descriptions of the re- gions plant communities predate the effects of modern fire-control measures. Two of the best early descriptions of the effects of fire on Northern Rocky Mountain forest vege- tation are the reports prepared by Leiberg (1900) and Ayres (1900). Leiberg reported </p></li><li><p>FIRE-DEPENDENT FORESTS, NORTHERN ROCKIES 411 </p><p>on the forest resources within the original Bitterroot Forest Reserve. Ayres prepared a similar report for the Flathead Forest Re- serve. The boundaries of the Selway-Bitter- root Wilderness are well within the area discussed by Leiberg, and those of Glacier National Park are within the forest reserve described by Ayres. Both emphasized the nature and extent of recently burned-over forest lands. Over the past 70 yr, our under- standings of forest succession and the de- pendency of certain tree species on periodic fire treatment have expanded manyfold. </p><p>FOREST ZONES OF THE NORTHERN ROCKY MOUNTAINS </p><p>A series of vegetation zones are generally recognizable in the Northern Rocky Moun- tains (Daubenmire, 1943; Daubenmire and Daubenmire, 1968; Habeck, 1967; 1970a, 1970b, 1972). These zones are generally arranged spatially along elevational gra- dients, but often are modified locally by slope and exposure factors; thus, no perfect relation between forest zone and elevation is found. Moisture availability varies with aspect, as well as elevation, but it also varies because of the gradual disintegration of the Pacific coastal influence. Thus, the vegetation changes continually in time and space. </p><p>Originally, the lower vegetation zones within the intermontane valleys were cov- ered with grassland, sagebrush, juniper, pine savanna, wet-bottomland, streamside, and marsh communities. The lowest contin- uous forest zone, which marks a transi- tional change between grassland com- munities and the more mesic forest zone dominated by Douglass fir (Pseudotsuga menziesii) , was dominated by ponderosa pine (Pinus ponderosa) . </p><p>Douglas fir occupies a wide elevational range in the Northern Rocky Mountains, occurring at elevations ranging between 600 and 2800 m (1968 and 9186 ft) . The forest zone dominated by Douglas fir is generally restricted to elevations ranging between 900 </p><p>and 1500 m (2953 and 4921 ft). The west- ern redcedar/western hemlock forest zone is found just above the Douglas fir zone in much of northern Idaho and in portions of northwestern Montana, where annual mois- ture reaches 760 mm (30 in.) or more. However, as one moves south toward Mis- soula, only scattered, isolated pockets of this western redcedar/western hemlock zone are found in moist canyons. Those relict, communities of this zone that are found in the vicinity of Missoula are actually depau- perate of many of the floristic elements dis- played in northern Idaho and in Glacier Park. </p><p>Where the cedar/hemlock zone is not found in the Northern Rocky Mountains, the upper Douglas fir zone comes in direct contact with the spruce/fir zone dominated by subalpine fir (Abies Zasiocarpa), Engel- mann spruce (Picea engelmannii), lodge- pole pine (Pinus contorta), and whitebark pine (Plnus albicaulis) . Communities dom- inated and/or codominated by mountain hemlock (Tsuga vnertensiana) and sub- alpine fir are found in high mountain areas along the Idaho-Montana State border. </p><p>Many of the same species found in the spruce/fir zone are also found in the tim- berline forest zone, because the spruce/fir zone extends to elevations over 2800 m (9186 ft). On some high, rocky sites at tim- berline, alpine larch (La&amp; lay&amp;ii) also occurs. This species is common in the Bit- terroot Mountains at elevations above 2100 m (6890 ft) (Arno and Habeck, 1972). These timberline species are joined on t#he Continental Divide by limber pine (Pinxs flex&amp;s) and by species that surprisingly enough are also found in certain portions of the grassland zone on the lower east slopes of the Divide. </p><p>Other tree species also occur within these various zones therefore the total tree flora found in these zones numbers about t,wo dozen species. As a result, compositional variation of these species within and he- tween these forest zones is complex. A por- tion of this complexity is attribut,ed to as- </p></li><li><p>412 HABECK AND MUTCH </p><p>pect and climatic factors, but much of it has been influenced significantly by fires. </p><p>Low-Elevation Vegetation Types </p><p>The drier plant-community types in the intermontane valleys and on the adjacent lower mountain slopes have not remained intact. They have been physically altered by mans activities, although relict exam- ples of each still exist. Such relict communi- ties, however, often occur on sites that were not easily disrupted by agricultural activi- ties or stock raising and consequently may not be typical in their botanical features. The valley grasslands and associated pine savannas burned periodically during settle- ment by Indians and early white people. Evidence of grassland fires does not persist long, but the occasional occurrence of indi- vidual trees or small groups of old-aged, open-grown, fire-scarred ponderosa pines on the margins of the valleys provides some testimony today of the influences of previ- ous fires. </p><p>Ponderosa pine savannas have been al- tered considerably during the past century. The scattered pines mentioned in early fed- eral land surveys no longer exist in some of the valleys. These were removed for croplands or for fuel and building along with the native grasses. Thus, fires that might have earlier originated in the valley grasslands and extended into the mountain forests were modified. Such cultural changes probably had an effect on the pine savanna communities, particularly those dependent upon fire for their continuance. However, a portion of the open ponderosa pine stands in western Montana appear to perpetuate themselves without routine fire treatments because of the severely dry sites upon which they occur. One still can find pine openings that have been maintained by site aridity and high temperatures on south and southwest slopes or on rocky ridges and talus slopes. </p><p>Some of the original pine savanna com- munities occurred within the lower Douglas fir zone as major seral stages; these </p><p>eventually were invaded by Douglas fir. Periodic fire in such areas favored the con- tinuance of the pine and delayed succes- sional replacement by Douglas fir. When...</p></li></ul>


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