3
Present Conditions: Vegetation

VEGETATION OF YELLOWSTONE’S northern range, a mosaic of different forest and nonforest communities, is the result of interactions among many environmental factors. To understand how management decisions may affect vegetation of the northern range, the committee reviewed the present conditions of the major vegetation types. In this chapter, the current state of a type of vegetation—for example, sagebrush or aspen communities—is described, followed by a discussion of how the driving variables may affect these conditions. In several cases, where changes in driving variables may alter ecosystem components, the nature of these changes and their consequences is discussed. Recent modification of environmental drivers is discussed to help explain the significance of recent changes of the ecosystem components. Although this chapter emphasizes the dominant plants, such as sagebrush or aspen, the concern over loss or degradation of these systems is not only for the dominant species, but also for the biodiversity—plants, animals, fungi, and microbes— that they support.

UPLAND SHRUBLANDS AND GRASSLANDS OF THE NORTHERN RANGE

Shrublands

Big sagebrush-Idaho fescue is the most abundant sagebrush-grassland type. It occurs on sites with thin cobble soils to well-developed loams, gener-



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Ecological Dynamics on Yellowstone’s Northern Range 3 Present Conditions: Vegetation VEGETATION OF YELLOWSTONE’S northern range, a mosaic of different forest and nonforest communities, is the result of interactions among many environmental factors. To understand how management decisions may affect vegetation of the northern range, the committee reviewed the present conditions of the major vegetation types. In this chapter, the current state of a type of vegetation—for example, sagebrush or aspen communities—is described, followed by a discussion of how the driving variables may affect these conditions. In several cases, where changes in driving variables may alter ecosystem components, the nature of these changes and their consequences is discussed. Recent modification of environmental drivers is discussed to help explain the significance of recent changes of the ecosystem components. Although this chapter emphasizes the dominant plants, such as sagebrush or aspen, the concern over loss or degradation of these systems is not only for the dominant species, but also for the biodiversity—plants, animals, fungi, and microbes— that they support. UPLAND SHRUBLANDS AND GRASSLANDS OF THE NORTHERN RANGE Shrublands Big sagebrush-Idaho fescue is the most abundant sagebrush-grassland type. It occurs on sites with thin cobble soils to well-developed loams, gener-

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Ecological Dynamics on Yellowstone’s Northern Range ally at elevations of 1,800 to 2,400 m within the 40- to 75-cm precipitation zone. It is distributed throughout the park but is most common in the Gardner and Lamar River drainages (Despain 1990). The habitat type is dominated by mountain big sagebrush (Artemisia tridentata ssp. vaseyana), although Wyoming big sage (A. tridentata ssp. wyomingensis) may also be present. Identification of big sagebrush subspecies is particularly important because of differences in palatability and preference to ungulates. Idaho fescue (Festuca idahoensis) dominates the understory with Agropyron spicatum and Koeleria macrantha also present. Forbs (broad-leaved herbaceous plants), such as Geum triflorum, are abundant. Primary production (the amount of carbon fixed by photosynthesis) varies widely in big sagebrush-Idaho fescue habitat depending on rainfall and temperature (Mueggler and Stewart 1980). A 50% difference in production may occur on any given site over a 3-year period. Production varied across the type from 560 kg/ha (Mueggler and Stewart 1980) to 1,610 kg/ha with grasses contributing 21% to 42% of the production, forbs 38% to 56%, and shrubs 10% to 41%. Between 88% and 98% of the shrub production is from big sagebrush. Big sagebrush-Idaho fescue habitat, which is heavily grazed in winter by ungulates, and the grassland habitat type (Idaho fescue-bearded wheatgrass) account for slightly more than half of all the nonforested vegetation in the park and on the northern range (Houston 1982). These two types probably furnish most of the forage for the large number of grazing animals in the park (Despain 1990). Wyoming big sagebrush-bluebunch wheatgrass (A. spicatum, now Pseudoroegneria spicatum) habitat type occurs in the Gardner River canyon in small areas on southern and western slopes, often between big sagebrush-Idaho fescue and other grasslands on ridgetops and upper slopes. It occurs on shallow to moderately deep soils formed over several parent materials. Mountain big sagebrush is the dominant shrub, although basin big sagebrush (Artemisia tridentata ssp. tridentata) may occur on deeper soils in drainages. Low shrubs, such as A. frigida and Gutierrezia sarothrae, are usually present. In addition to bluebunch wheatgrass, other conspicuous grasses include K. macrantha, Poa secunda, and Stipa comata. Production varies between 670 and 1,120 kg/ha with high variability between sites but not between years (Mueggler and Stewart 1980). This type is heavily grazed in winter by ungulates in the Gardiner area. Big sagebrush receives enough browsing to reduce the size of its canopies.

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Ecological Dynamics on Yellowstone’s Northern Range In the Gardiner area, Wambolt and Sherwood (1999) also describe a Wyoming big sagebrush-bluebunch wheatgrass (A. spicatum) habitat type, as did Mueggler and Stewart (1980) and Houston (1982). Associated species include sprouting shrubs like rubber rabbitbrush (Chrysothamnus nauseosus), green rabbitbrush (Chrysothamnus viscidiflorus), and gray horsebrush (Tetradymia canescens). Prairie junegrass (K. macrantha) and Sandberg bluegrass (P. secunda) are also common. Grasslands Idaho fescue-bearded wheatgrass (Agropyron caninum) habitat type is a highly productive mesic grassland with high species diversity. It occurs on gentle slopes at elevations of 2,000 to 2,600 m, within the 46- to 76-cm rainfall zone. It is dominated by grasses but contains a higher proportion of forbs (30% to 70%) than other western Montana habitat types. It has a short growing season and is used by native ungulates in winter (Houston 1982). Idaho fescue-Richardson’s needlegrass (Stipa richardsonii) habitat type generally occurs at elevations of 1,100 to 2,100 m on gentle slopes and deep soils. It is a moderately mesic and productive grassland type dominated by Festuca idahoensis, S. richardsonii, Danthonia intermedia, Stipa occidentalis, and G. viscosissimum. This grassland is summer range for sheep or cattle, and it receives substantial winter grazing by native ungulates. Idaho fescue-bluebunch wheatgrass is the most common xeric (Houston 1982) or moderately mesic (35 to 50 cm of precipitation) grassland type in the Greater Yellowstone Ecosystem (GYE) (Mueggler and Stewart 1980). It is found on intermediate mountain slopes at elevations of 1,400 to 2,300 m and occurs on a wide variety of parent materials. Other grasses include K. macrantha, P. sanbergii, and either S. comata or S. occidentalis. Forbs cover from 10% to 60% of the area and include Achillea millefolium, Antennaria rosea, Arenaria congesta, and possibly Phlox hoodii. Medium shrubs such as A. tridentata and C. viscidiflorus are occasionally present. Annual primary production is highly variable depending on the weather. The grasses are used by elk and deer at the lower elevations for winter range and by pronghorn year-round. At the highest elevation, this type is summer range for elk and deer. At middle elevations, it is used as spring and fall range by all ungulates and as winter range by bighorn sheep and mountain goats.

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Ecological Dynamics on Yellowstone’s Northern Range Bluebunch wheatgrass-Sandberg bluegrass, or A. spicatum-P. sanbergii, is usually found between 900 and 1,800m, especially on gravelly soils on steep southern slopes. It is a moderately arid type in the 35- to 50-cm precipitation zone. Shrub and forb cover is low, and rhizomatous grasses are generally absent. Needle-and-thread-blue grama (S. comata-Bouteloua gracilis) habitat type is usually found on broad alluvial benches and valley floors. Houston found this grass type in Yellowstone National Park (YNP) in the boundary line area, upstream on the Yellowstone River to the Black Canyon. It generally occurs below 1,500m and is the driest grassland habitat type (20 to 35 cm precipitation). The type is floristically simple, containing grasses and a low cover of forbs and shrubs. Needle-and-thread grass is a bunchgrass that dominates late seral stages of the community but decreases under heavy grazing pressure. Blue grama, the other dominant sod-forming grass in the community, increases under heavy grazing. The terms decreaser, increaser, and invader refer to a plant’s response to grazing (Dyksterhuis 1949). Decreaser plants are most preferred by grazing animals and with continued heavy grazing are the first kinds of plants to decline in cover in a community. Increaser plants initially increase in cover in a community under ungulate grazing pressure because the preferred decreaser plants are declining and opening up space for increasers to grow. Eventually, as heavy grazing pressure continues, the increaser plants also decline, opening up sites for invader species of low palatability and generally low nutritional value. Common shrubs that increase with overgrazing include A. frigida, G. sarothrae, and Opuntia polycantha. Factors Influencing Present Conditions of Sagebrush and Grasslands Sagebrush: Ungulate Use Big sagebrush is a particularly important food plant for several Yellowstone ungulates, especially in winter. Consequently, lower sites where there is little snow or where snow does not deeply cover shrubs are heavily grazed. During other seasons use is less, although big sagebrush may be an important source of protein for elk during the gestation period and in summer because grasses alone do not meet their protein needs (Wambolt et al. 1997). Not all ungulates in the northern range use big sagebrush to the same

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Ecological Dynamics on Yellowstone’s Northern Range extent and not all subspecies of big sagebrush are equally used by the ungulates that feed on it. Big sagebrush is an important component of elk, mule deer, and pronghorn diets but not of bighorn sheep, bison, and mountain goats (Houston 1982). Mule deer and elk strongly prefer mountain big sagebrush to Wyoming big sagebrush and basin big sagebrush (Wambolt 1996). Wyoming and basin big sagebrush are much preferred over black sagebrush (A. nova), but during severe winters, all subspecies of sagebrush are browsed. Ungulate browsing in low-elevation sagebrush sites near the park boundary has resulted in significant negative effects on big sagebrush (Wambolt 1996). In some cases, up to 91% of the leaders were removed and unbrowsed plants had higher productivity (45 g per plant) and seed-head production (60.3 seed heads per plant) than browsed plants (10 g per plant and 0.08 seed heads per plant) (Hoffman and Wambolt 1996). Up to 35% of plants were killed between 1982 and 1992, and many plants that survived had high percentages of dead crown (Wambolt 1996). Wambolt’s exclosure work (1998) demonstrates elk-induced decreases of sagebrush even in areas where there were no other ungulates. Wambolt and Sherwood (1999) came to similar conclusions. However, according to the National Park Service (YNP 1997), on 97% of the northern Yellowstone winter range sagebrush is stable or increasing and only 3% of the land shows sagebrush decline. In general, less browsing damage is observed at higher elevations (Singer and Renkin 1995, YNP 1997). Thus, elk appear to affect sagebrush at lower elevations—including the 3% of the winter range described by NPS as having declining sagebrush—but not at higher elevations. Grasslands: Ungulate Use Grasses are important components of the diet of most YNP ungulates except pronghorns and moose (Singer and Norland 1994). The importance of grasses in the diets of most ungulates on the northern range of YNP was shown by Singer and Norland (1994) through microhistological analyses on feces, comparison with earlier published work based on rumen analyses, and, for bighorn sheep, examination of feeding sites. Mean percentage diet compositions assessed by these methods were, as follows: elk, 75% to 79%; bison, 53% to 54%; mule deer, 19% to 32%, pronghorn, 10% to 4%; and bighorn sheep, 65% to 58%. Bison also included a high proportion of sedges in their diets (32% to 56%). As in many other grasslands, ungulates in YNP move nonrandomly over

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Ecological Dynamics on Yellowstone’s Northern Range the landscape, feeding preferentially on grasses that are at particular stages of development (Frank et al. 1998). Yellowstone elk migrate elevationally as grasses produce new growth in spring (Houston 1982, Frank and McNaughton 1992). Some areas are intensively grazed but they recover as animals move to other patches. Timing of feeding is critical because feeding on vegetative material can have less impact than removal of growing points or reproductive structures. Despain (1996) compared one exclosure with the surrounding area. He found that elk fed heavily on the highly palatable bluebunch wheatgrass but moved off before the grass flowered so that there was little difference between exclosures and the surrounding areas in the total amount of green biomass of all species at the end of the growing season. The grazing and migration pattern in the northern winter range results in modest spring and summer grazing on the lower ranges that receive heavy winter pressure and more intense grazing at higher elevations as snow recedes and green-up occurs (Singer and Harter 1996). There are visually apparent effects of grazing on YNP grasslands. The question is whether those effects are signs of damage induced by feeding populations that exceed the carrying capacity of those rangelands. NPS perspective at YNP (YNP 1997) is that there is a perceptional problem—observers who see YNP rangelands make comparisons with commercial livestock rangeland and interpret the differences to indicate overgrazing (Coughenour and Singer 1991, 1996a). Grasses are generally adapted to grazing and may even respond positively to appropriate grazing levels (Huff and Varley 1999). Grazing in YNP may cause enhanced plant protein (Singer 1996) and nitrogen content (Coughenour 1991, Mack and Singer 1993), and grazed plants may produce taller leaf and seed stalks (Singer and Harter 1996). Also, it is possible that animal movements, deposition of urine and feces, and the physical effects of hooves combined with plant responses to grazing could result in dense, short grass stands of enhanced above-ground growth (Frank and McNaughton 1992). Plant diversity on grazed sites is often higher than on ungrazed or heavily grazed sites (Wallace et al. 1995). Some authors have suggested that there are alarming decreases in plant diversity on Yellowstone’s northern range, implying that overgrazing is occurring on some sites (Wagner et al. 1995). Grazing in the northern range does not appear to reduce root biomass (Coughenour 1991) or soil moisture content, even though there is an increase in soil bulk density (Lane and Montagne 1996). Changes have been reported in forb biomass (Singer 1995) and soil nutrients (Lane and Montagne 1996).

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Ecological Dynamics on Yellowstone’s Northern Range Confounding Factors: Fire, Pocket Gophers, and Nonnative Plants Three other factors affect shrublands and grasslands in the northern range, but they have received little study. Usually, fire is not common in grass or big sagebrush communities in the northern range (Despain 1990), but unusual increases in sagebrush since the 1870s (Houston 1982) may have caused sufficient fuel buildup to carry fires in this fire-sensitive vegetation type. After fire, numerous seedlings may establish, although there is little evidence that these plants survive and reproduce, especially for mountain big sagebrush (Wambolt et al. 1999). Possibly, the lush, young greenery draws ungulates to the site, increasing herbivory and further decreasing sagebrush population recruitment (Wambolt et al. 1999). Fire has fewer negative effects on grasslands and may stimulate community renewal. Pocket gophers (Thomomys talpoides) are common, active, fossorial rodents that move masses of soil wherever there is sufficient below-ground consumable biomass to support them. Their digging activities, and those of bears, create patches in YNP grasslands with high densities of forbs that replace grasses and add to community diversity (Despain 1990). Pocket gophers can alter the structure of a community significantly and change the time-course of succession (Chase et al. 1982). Activities of elk and other ungulates might affect Thomomys populations by altering vegetation cover and soil compaction and consequently indirectly influence vegetation characteristics. If this had occurred, it would complicate the assessment of ungulates’ effects on vegetation, but we are not aware of relevant data for the northern range. Nonnative plants are abundant in Yellowstone’s northern range, especially in big sagebrush and grassland habitats. Three grass species, timothy (Phleum pratense), crested wheatgrass (Agropyron cristatum), and cheat grass (Bromus tectorum), occur in the area and might alter, to an unknown degree, ecosystem productivity, susceptibility to fires, and ecosystem nutrient dynamics as well as other integrated measures of ecosystem processes (Huff and Varley 1999). Conclusions Not enough data are available for the committee to evaluate the claim that abiotic factors in the northern range are currently more influential than biotic factors on vegetation (YNP 1997, Frank et al. 1998), or the reverse. Certainly, many processes, especially those in the soil, are strongly mediated by animals

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Ecological Dynamics on Yellowstone’s Northern Range in YNP (Frank and Groffman 1998). The available data indicate that over short periods (decades), browsing and, in some areas, grazing have caused declines in plant populations and productivity. Sagebrush Big sagebrush at higher elevations in the northern range appears to be at relatively high abundance and vigor. These areas are important for ungulates during the nonwinter portions of the year. Because snow is usually deep and plants are relatively protected from elk browsing, these areas do not show sagebrush decline and may even show increases. They are not obviously of immediate management concern. Lower-elevation big sagebrush stands, which have been very heavily used by elk, are decreasing in density and productivity, especially near the northern park boundary. Those sites are a critical winter range for a variety of other ungulates, especially pronghorn. It appears that, without extensive and intensive management to offset the damage done by elk browsing and grazing, the sites will continue to be degraded as resources for pronghorns and other ungulates (Wambolt and Sherwood 1999), especially near the northern boundary of the park. Grasslands Grasslands do not appear to have been altered as much by grazing as low-elevation shrublands have been by browsing. However, the few comprehensive reviews of the literature do not factor in the amount of biomass or other integrated measures of ecosystem characteristics contributed by nonnative species. The few studies available do not indicate that biodiversity is declining or that these systems are near a threshold value for some characteristic that is critical to any ecosystem process that currently appears to be within normal, long-term variations of the system. However, the committee would have more confidence if there were more data and analyses available. FOREST TYPES ASSOCIATED WITH THE NORTHERN RANGE YNP forests range from lower elevation woodlands through dense forest to timberline woodlands of whitebark pine and spruce and fir krummholz.

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Ecological Dynamics on Yellowstone’s Northern Range Knight (1994) describes seven forest and woodland types in Wyoming, six of which are found in YNP. The seven include limber-pine woodland, ponderosapine forest, Douglas-fir forest, aspen forest, lodgepole-pine forest, spruce-fir forest, and whitebark-pine woodland. There are many associations within these forest types, based on the composition of dominant and subordinate understory plants. Despain (1990) uses these associations to describe many forest habitat types. Of the seven types described by Knight, only ponderosapine forests are not found in YNP, and limber-pine woodlands are not common. The lack of ponderosa pine is considered to be caused by the predominance of rhyolitic soils on the Yellowstone Plateau, soils that create water stress conditions too extreme for ponderosa pine in the elevational zone where ponderosa pine might have established. The area may also lack the higher summer precipitation and warmer and longer growing season temperatures required by ponderosa pine. Of the six YNP forest or woodland types described by Knight (1994), aspen forests are discussed separately in this chapter because of the importance of their growth and reproductive response to changing environmental conditions in the northern range. The forests of northern YNP and adjacent areas exist in a mosaic of forests and meadows (or parks). The causes for this mosaic mostly relate to moisture availability, whether influenced by soils, topography, or other factors (Patten 1963). The forests of YNP are continuously changing as stands mature and external factors, such as long-term climatic changes (Whitlock 1993), cause decline or loss of existing stands. Additionally, because of natural or anthropogenic environmental changes, such as climate change or fire suppression, many nonforest areas have been invaded by trees (Patten 1969, Jakubas and Romme 1993). Historical photograph comparisons show that many slopes throughout the northern range have more conifer forests now than in the past (Meagher and Houston 1998). These photographic comparisons also show a decline of aspen stands throughout the area, a phenomenon also seen in remotely sensed data that show aspen changes inside and outside northern YNP (Ripple and Larsen 2000a). Invasion by conifers of sagebrush and other nonforested areas continues to occur throughout the GYE. Sagebrush areas are becoming forest (Patten 1969), and forests are invading subalpine meadows (Jakubas and Romme 1993). The most common forest type in the lower elevations of the northern range is dominated by Douglas fir (Pseudotsuga menziesii). It occurs below the lodgepole pine zone (1,800 to 2,300 m) in dense stands on cooler sites and sparse stands often mixed with Rocky Mountain juniper on drier or warmer sites. Douglas fir develops a thick bark, which makes mature trees relatively

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Ecological Dynamics on Yellowstone’s Northern Range fire tolerant. In the Yellowstone and Lamar River valleys (a major portion of the northern range), Douglas fir is the most common tree, with snowberry a common understory shrub in warmer sites and pine grass common in cooler sites. Aspen and lodgepole pineare often associated with Douglas fir in these areas. Douglas-fir forest with shiny-leaf spirea and other short woody shrubs as understory is common in the northern range on upper slopes and ridges (Despain 1990). The extensive zone above Douglas-fir forest in the northern range is dominated by lodgepole pine. Although Despain (1990) described a few pure lodgepole-pine habitat types in YNP, none were in the northern range. However, using a cover classification, he described many cover types based on different successional stages of lodgepole forests with different understory recovery phases of climax species (e.g., subalpine fir). Knight (1994), however, described the lodgepole-pine forest as the most common forest type in Wyoming, occurring in northern Wyoming from 1,800 to 3,200m. He pointed out that, although lodgepole pine is primarily a fire successional species, climax lodgepole pine can occur on cool, nutrient-poor sites where other Rocky Mountain conifers cannot survive. The forest zone above the Douglas-fir zone includes other subalpine species such as subalpine fir, Engelmann spruce, and, at higher elevations, whitebark-pine woodlands. Spruce, fir, and sometimes whitebarkpine form stunted krummholz “forest” stands on the ecotone between forest and alpine communities. In most cases, the krummholz is on exposed ridges or rocky outcrops. The elevation of these woody communities is well above the northern range especially the northern winter range. Factors Influencing Present Conditions of Northern Range Forests The present condition of forests on the northern range has been determined primarily by changes in management of fire and ungulates Fire Fire management policy has changed over the past several decades in YNP as well as in surrounding national forest wilderness areas. Before the 1970s, all fires were extinguished regardless of their location or intensity.

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Ecological Dynamics on Yellowstone’s Northern Range During the 1970s, the benefits of fire were recognized, and fires that were unlikely to damage human activities or structures were left to burn. This was very successful as most fires in the park during the decades leading up to 1988 burned a few to several hundreds of hectares in Yellowstone’s conifer forests, and the forest mosaic normally formed by disturbance processes was gradually returning to a more “natural” landscape. Fires did not occur in the sagebrush-grasslands during this time. (The fires of 1988 would have occurred even if decades of controlled burns had preceded them.) Forests of the northern range are a mosaic of burned and unburned stands, most of which burned in 1988 (Despain et al. 1989). Before 1988 the forests were pure lodgepole-pine or mixed lodgepole-Douglas-fir and lodgepole-spruce-fir forests (Keigley 1997a). Most of the burned forests are recovering as nearly pure stands of young lodgepole pine. Ungulate Use Many forest stands in the northern winter range and in the upper Gallatin River drainage are heavily browsed and highlined (i.e., a browsing pattern on trees caused by ungulates removing foliage and live twigs as high as they can reach, thus creating a high line usually a few meters above the ground) (Kay 1990). Stunted conifers may not be browsed during mild winters and thus grow into branched trees (see following paragraph for analysis of tree architecture). All conifer species are browsed. Spruce, fir, and Douglas-fir trees are highlined, and adventitious branches, which grow on tree stems, are also browsed. In the upper Gallatin River drainage near the YNP boundary, trees are highlined throughout most of the area, but highlining decreases or disappears several miles south of the park boundary at higher elevations or several miles north of the boundary, where ungulates migrate but few overwinter (committee observations). The forest-ungulate interaction found on the Gallatin may represent a microcosm of the northern winter range. This interaction demonstrates the influence of ungulates on the forest under winter conditions where ungulates now stay at higher elevations in areas that once were primarily summer range (Patten 1963). Branch architecture and growth form have been used for determining the intensity of browsing of shrubs and trees (Keigley 1997a). Young trees branch after the terminal shoots are browsed and then may grow into branched rather than single-stem tall trees if browsing pressure is reduced. Dating of the origin of branching and other tree-architectural anomalies have been tied to periods

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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 3–1 Annual peak stream flow in cubic meters per second (cms) at Corwin Springs on the Yellowstone River a few miles downstream from Gardiner, MT. USGS gage 06191500. willows declined (i.e., reduced stature and loss) during the earlier decades of the twentieth century (Smith et al. 1915, Warren 1926). Therefore, it is necessary to use hydrological data from most of the century to detect whether hydrological changes have caused willow decline. Increased precipitation at high elevations has been used to explain the vigor of higher-elevation willow communities (YNP1997); however, willows occur along stream courses where they have access to groundwater and soils wetted from the stream or from capillary rise of water from the water table (Dawson and Ehleringer 1991, Busch et al. 1992, Flanagan et al. 1992). Consequently, the amount of local precipitation has very little influence on willow growth and survival, except when it is in the form of a snow cover (see below). Snow Accumulation Annual snow accumulation during the twentieth century was within normal variability for the period of record based on spring runoff data (Farnes 1998).

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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 3–2 Annual discharge volumes (thousands of acre-feet per year) at Corwin Springs on the Yellowstone River a few miles downstream from Gardiner, MT. USGS gage 06191500. There has been considerable variation in the past few years, with 1996 and 1997 having high accumulations, and 1998, 2000, and 2001 having accumulations well below the mean. Some evidence shows that runoff from snow melt peaked earlier, by three days on average, after the 1988 fires (Farnes 1998) and that peak flows may have been higher, but peak discharge data do not support the latter conclusion. Also, runoff appears to have returned to normal for the snowfall amounts within a decade after the fires. Snow accumulation may play a more important role in riparian vegetation structure through protection of riparian shrubs as pointed out by Singer (1996). Not only does deep snow accumulation around riparian woody plants reduce browsing use, it also may prevent use of higher-elevation riparian vegetation by ungulates. Conversely deep snow can cause greater use of riparian vegetation by browsers if herbaceous plants are buried and woody stems are the only forage exposed. This is especially true when a thaw-freeze cycle creates an impenetrable ice layer over herbaceous vegetation. The present condition of riparian vegetation in the northern range may be caused by heavy utilization when it is one of the few sources of forage in deep snow years or years with significant thaw-freeze events.

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Ecological Dynamics on Yellowstone’s Northern Range Stream Discharge (Hydrographs) Streams often recharge adjacent groundwater, although in some cases groundwater movement may be toward the stream rather than away from it. Consequently, reduction in stream flows combined with reduced groundwater movement from uplands when the precipitation is below normal may reduce groundwater availability to riparian vegetation. However, the USGS daily historical hydrological data for rivers flowing from the northern range (e.g., Yellowstone at Corwin Springs) do not indicate any unusual reduction or increase in stream flows over the past century beyond normal variation. Consequently, surface hydrological changes do not appear to be sufficient to have directly affected riparian availability of groundwater, although if some streams incised their channels during the past several decades, that could have lowered the surface flow enough to lower the alluvial water table to levels that could stress riparian vegetation. Spring floods can scour channel margins and, through overbank flows, scour and flood the adjacent floodplain to produce suitable sites for recruitment. High spring flows that could enhance recruitment of riparian plants may occur only every 5 or 10 years, resulting in spaced age-classes of the woody riparian species, which is especially evident in large woody species such as cottonwood. The lack of evidence of regular recruitment events in the northern range indicates that perhaps there have been fewer spring flood events during the past few decades. However, USGS peak flow data for rivers coming from the northern range watershed show periodic high or flood flows, sufficiently high to produce a recruitment event and probably high enough to have seedling establishment above either ice scour or smaller scouring flood events. Therefore, the lack of periodic recruitment by riparian plants in the northern range cannot be attributed to absence of flood events. Groundwater (Alluvial Water Table): Streams, Seeps, and Beavers Groundwater monitoring data obtained by the committee show that groundwater in the floodplains, where most of the riparian vegetation in the northern range occurs, has not changed sufficiently in the past several decades to cause stress on riparian vegetation. Data from one well north of Gardiner showed water table decline in the 1980s (YNP1997), but the 1980s were a dry decade and domestic use of groundwater in the area of the well probably increased as well. Groundwater monitoring wells within the northern range are needed if

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Ecological Dynamics on Yellowstone’s Northern Range seasonal and annual fluctuations in groundwater levels are to be determined. The existence of riparian vegetation, especially willow, along most of the rivers and on the floodplains in the northern range indicates that the water table is still high enough to maintain these communities. The occurrence of seeps and springs is additional evidence that the water table is near enough to the surface to maintain riparian vegetation. Groundwater change appears to have been an important factor in those few areas where riparian vegetation once occurred but is no longer present because the beaver ponds were abandoned. Stem xylem water potential can indicate water stress in woody plants. Measurements of xylem water potential of short (browsing suppressed) and tall stature willows near exclosures showed water potential averages of -1.17±0.68 and -2.4±0.85 bars, respectively. Comparisons between short and intermediate stature willows were -1.71±0.68 and -2.86±1.35 bars, respectively (Singer et al. 1994). Thus, tall and intermediate willows with greater canopies had greater water stress than short willows, but these values are much less than wilting-point values (i.e., -15 bars) for mesic plants. Loss of willows during the 1988 drought period may be partly explained by water stress (Singer et al. 1994) but may be better explained by possible water table declines and warm, dry winds that occurred during that period. Beavers, whose activity currently helps maintain riparian vegetation along many streams in the GYE, are currently rare or absent in most of Yellowstone’s northern range. The reasons for beaver decline and the consequences for willows have recently been controversial (YNP 1997; Singer et al. 1998, 2000; Keigley 2000). An early theory attributed beaver decline to reductions due to ungulate browsing of woody plants (willow and aspen), which are beaver food and building material (Bailey 1930, Wright and Thompson 1935, Jonas 1955). In the absence of beavers, water tables in areas elevated by beaver dams may decline so that riparian vegetation cannot survive. Singer et al. (1998, 2000) argued that this process is exemplified by the healthy stands of willow at Willow Meadows (a location in the transition zone from winter to summer northern range use) that occur on a broad floodplain where the water table is less than a meter deep are maintained by beavers. Geomorphology: Stream Banks and Channels Many of the streams in the northern range on the broader floodplains now produce braided channels where formerly they produced single or multiple meandering channels (Meyer et al. 1995). The braided channels are dynamic

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Ecological Dynamics on Yellowstone’s Northern Range and have moved considerably over the past several decades (Mowry 1998). Changes in the width of Soda Butte Creek measured when it was full to the banks near the upper edge of the winter range were related to hydrological history and perhaps vegetation cover (Mowry 1998). Locations with tall willows and little winter ungulate use remained stable (i.e., no statistically significant changes) during a relatively constant hydrological period from 1954 to 1987, whereas channels with grass-covered banks and reaches with little willow bank stability narrowed over that time. That demonstrates, for these sections of Soda Butte Creek upstream of most wintering ungulates, that vigorous woody riparian vegetation tends to maintain bank stability under most hydrological conditions whereas non-woody banks tend to aggrade, a process explained by Lyons et al. (2000) from research on Great Plains rivers. However, the floods of spring 1996 (one of two back-to-back hundred-year floods on the Yellowstone River and its tributaries) altered the willow-stabilized banks more than banks stabilized by grass or low willows. That happened because woody riparian species (e.g., willows) stabilize banks only to a flood-discharge threshold, beyond which shear stress scours and uproots the plants, so that banks collapse (e.g., Stromberg et al. 1997). Stream banks that are stabilized only by vegetation that offer low resistance to flowing water (e.g., grasses and low willows), may withstand high-velocity floods better than banks with woody plants. Also, if one high-flood year is followed by another, as was the case in 1996 and 1997 in Yellowstone, the instability created by the first year may allow the subsequent flood to alter the channel morphology more than it would if it had occurred alone. For example, the 1997 hundred-year flood on the main stem of the upper Yellowstone River downstream from YNP removed many hectares of mature cottonwood trees that had been made unstable by the 1996 flood (committee observation). The sandy deposits in several of the rivers of the northern range, primarily the Lamar River, are thought by some observers to be a product of increased erosion along the river channels (R.Beschta, Oregon State University, comments to committee, 1999) and high sediment supply from steep erodible terrain and tributary streams (Rosgen 1993). Rosgen suggested that poor riparian conditions due to excess browsing cause unstable channel conditions and increased sediments in the Lamar River valley. In addition, Chadde and Kay (1991) claimed that the Lamar River valley channels have incised, perhaps as much as several meters, because of heavy ungulate use. If incision has occurred, the shallow riparian water table needed to sustain riparian communities and vigorous growth response to browsing will not be maintained. For example, willows growing along an apparent water table gradient away from the

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Ecological Dynamics on Yellowstone’s Northern Range Gallatin river have decreased growth and recovery to browsing with distance from the river (Patten 1968). Floodplains with shallower alluvial deposits, or steeper low-elevation valleys, tend to have bedrock conditions nearer the surface that prevent decline of the water table even if stream flows are reduced. In such places, willow vigor is maintained even when browsing occurs. Ungulate Use Several studies have assessed the response of willows to protection from browsing (Kay 1990, 1994; Chadde and Kay 1991; Singer et al. 1994, 1998). Some of these studies have used measurements that show that frequency and occurrence of willows inside and outside exclosures do not differ significantly (YNP1997). These measurements count the number of individuals present, regardless of their size or vigor. Therefore they cannot detect changes in size, cover, or productivity—key vegetation components. Other studies have measured willow cover (Kay 1990, 1994; Singer et al. 1994; Singer 1996), which is a better metric for determining shrub vigor because it measures actual areal cover of the canopy. Singer (1996) showed that, during the three decades after construction of upland exclosures, total canopy areas for several species of willow inside the exclosure were 200% to 600% greater than those outside. Also, annual biomass production was 5 to 10 times greater inside the exclosure than outside (Kay 1994, Kay and Chadde 1992). During the period between measurements of willow canopy cover in the 1950s and 1960s and measurements in the 1980s, cover increased many fold inside and hardly changed outside exclosures (Kay 1994). Data from exclosures showed that only willows protected from browsing reproduced (Kay 1994). No seed-producing catkins were found outside, but they averaged over 300,000 per m2 inside. Ungulate browsing also removes pollen-producing catkins. Near Geode Creek, no catkins were found on willows below the browse height, but many were found above the browse height (Kay 1994). This reduction in reproduction of willows in the northern range may account for the reduction in willow pollen in recent lake sediments (Barnosky et al. 1988). Although exclosure studies show that browsing has a dramatic effect on willows, exclosures represent the extreme of no browsing—rare in ecosystems that have browsers, as YNP does. Thus, the best way to determine how ungulate populations within and outside YNP cause changes in willow communities is to compare natural willow communities inside and outside YNP but still

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Ecological Dynamics on Yellowstone’s Northern Range within the northern range. Unfortunately, this type of study has only been done for aspen (Kay 1990), although several recent publications comparing early and recent park photos show that tall-willow communities have declined in the northern range (Kay 1990, YNP 1997, Keigley and Wagner 1998, Meagher and Houston 1998). Explanations for the differences between early and recent photos include excessive browsing by ungulates (Kay 1990, Keigley and Wagner 1998) and climate change, especially during the 1930s drought (YNP 1997, Meagher and Houston 1998). Groundwater conditions, which have not been measured in any of the studies of ungulate utilization of riparian shrubs, should be measured at all sites. The lack of cottonwood recruitment may be due to ungulate browsing. When ungulate numbers were reduced in the northern range in the 1950s and 1960s, cottonwoods were temporarily “released,” put on new shoots, and grew taller than during the preceding or following decades (Keigley 1998). Concentrations of defensive or secondary chemicals in riparian vegetation may influence the magnitude of ungulate use. Several studies were designed to determine whether low-stature willows have lower concentrations of defensive chemicals and thus are more palatable and more heavily used by herbivores than taller willows (Singer et al. 1994). Extremely short stature is not a natural condition for most willows. Consequently, very short shrubs must be a result of browsing. A controversy has developed about whether the level and composition of certain secondary chemicals is a result of short stature or a response to browsing. This controversy has resulted in a series of commentary papers following Singer et al. (1994) (Singer and Cates 1995, Wagner et al. 1995). Studies on secondary chemicals in willows generally show that low-elevation, short willows tend not to produce the concentrations of defensive secondary chemicals (e.g., tannins) found in higher-elevation and tall willows. This supports the argument that the tall willows have greater defenses against ungulate browsing and thus remain tall because ungulates avoid them. However, many of the tall willow communities are at higher elevations where snow cover or other conditions prevent heavy winter ungulate use. Thus, tall willows occur in areas with few wintering elk. Also short willows might not produce as high a concentration of secondary chemicals as tall willows because growth after browsing, the only type of shoot these short willows produce, tends not to produce as many of these chemicals. One question that is not answered in the many studies dealing with secondary chemical defense is whether the concentrations of secondary chemicals found in tall-stature willows are sufficient to prevent browsing even by ungulates that are starving. Starving ungu-

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Ecological Dynamics on Yellowstone’s Northern Range lates will eat almost any woody plant that is available if other food such as grass and forbs are buried and unavailable. Browsed highlines on lodgepole pine, a low-quality food, are an example of this type of ungulate use. The evidence that lower levels of defensive chemicals increases utilization of willows or other riparian shrubs by browsers is insufficient to explain the decline in stature or loss of willows and riparian vegetation in the northern range during this century. Conclusions During the first six decades of the twentieth century the NPS was concerned that ungulates were reducing the riparian communities; however, since then, factors other than ungulate browsing, such as climate change, have been hypothesized to explain the loss or reduction in stature of the riparian communities of the northern range (Houston 1982, YNP 1997). The committee concludes that some riparian losses may be due to changed hydrological conditions in addition to responses of vegetation to ungulate use. Flooding events and water tables are still suitable for recruitment of the dominant riparian vegetation. However, channel incision might have lowered the water table, reducing the ability of willows to recover from browsing. The increase in ungulate browsing over the past century, as evidenced by hedged willows and lost willow stands throughout lower elevations of the northern range, has caused most of the reduced willow cover and lowered willow reproduction this area. Ungulate use also appears to be the primary factor preventing cottonwoods from recruiting, because seedlings do appear on good seedbed sites along the rivers of the northern range after appropriate hydrological events, but they fail to survive. WETLAND VEGETATION OF THE NORTHERN RANGE Wetlands are a prime source of biodiversity in YNP (Elliot and Hektner 2000). Several types of wetlands, such as natural depressions, beaver dam wetlands, thermal wetlands, and wetlands along rivers and creeks (i.e., riparian wetlands), are found on the northern range. Chadde et al. (1988) defined 62 wetland communities in the northern range. These included wetlands dominated by trees (e.g., spruce [Picea] and aspen), willows, shrubs (e.g., cinquefoil [Potentilla fruticosa] and silver sagebrush [Artemisia cana]), and gram-

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Ecological Dynamics on Yellowstone’s Northern Range inoids (e.g., grasses and sedges). Most wetlands in YNP have been mapped under the U.S. Fish and Wildlife Service’s National Wetlands Inventory mapping program. Although wetlands are important across YNP, they are hardly mentioned in the park’s documents on the northern range (YNP 1997) or in a vegetation description of YNP (Despain 1990). There have been limited studies of wetlands in the northern range. Houston (1982) sampled five highly productive wetland meadow sites with humic soil types dominated by sedges. Relatively few species were sampled (1 to 13 species on the five sampled sites). The sites were dominated by sedges and rushes and graded to species characteristic of mesic grasslands. One wetland type was identified as tufted hairgrass-sedge type, described by Mueggler and Stewart (1980). This habitat type dominated by Deschampsia caespitosa, is found on high-elevation valley bottoms between 2,000 and 3,000m (Mueggler and Stewart 1980). The soils are deep and poorly drained, with water standing on the soil surface at least part of the growing season. D. caespitosa is the dominant grass but sedges (Carex spp.) are always present. Other grasses include Danthonia intermedia, Phleum alpinum, and Agrostis and Juncus spp. Forb species present include Potentilla gracilis, Polygonum bistortoides, and Antennaria corymbosa. This is a highly productive habitat type with annual production reaching 2,900 kg/ha. Other wetlands were on alkaline soils formerly dominated by alkali grass and meadows cut for hay, now dominated by introduced timothy grass. All wetland communities receive substantial winter grazing by elk and especially bison, a species that may consume large quantities of sedges. Brichta (1987) studied 21 of the 62 northern-range wetland community types defined by Chadde et al. (1988). He also measured soil types, surface-soil saturation status, and groundwater depth and chemistry. Of the 180 plots studied, 24 mostly were aspen or willow stands in exclosures that excluded herbivores. Depth of soil organic horizon was highly variable, ranging from less than 5 cm for about half of the study plots to more than 60 cm for many sedge-dominated sites. Community types with abundant sedges had saturated soils all summer. Types with aspen, as well as some graminoid communities, were saturated for part of the summer; community types with spruce, and cinquefoil, as well as other graminoid-dominated communities, were dry most of the summer. Water tables were variable among study sites. For example, mean depth of water below soil surface was more than 100 cm for spruce wetlands around 100 cm for aspen wetlands, about 20 to 40 cm for willow wetlands and as low as 100 cm, from 40 to 100 cm for cinquefoil wetlands, and for graminoid wet-

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Ecological Dynamics on Yellowstone’s Northern Range lands from near surface to below 100 cm. Many water tables remained steady throughout the summer; others dropped from near surface to below 100 cm. Several sites with declining water tables were near glacial ponds. Water chemistry was also variable with most pHs near or slightly below neutral. Water chemistry differences did not appear to influence plant diversity, but availability of shallow water through the growing season did. Wetland community types typically changed across moisture gradients. All of the aspen wetlands and some willow sites studied by Brichta (1987) were within exclosures. At the Junction Butte exclosure, a Salix geyeriana/Deschampsia cespitosa community was inside the exclosure, but a Potentilla fruticosa/Deschampsia cespitosa community was immediately outside it. Brichta concluded that “the influences of grazing and succession upon wetland community type distribution should be further studied. Water regimes and soils were similar for paired plots on either side of the exclosures, therefore differences in soils and water levels could not account for the marked difference in vegetation that existed inside and adjacent to exclosures.” Brichta’s study was during a dry period that eventually resulted in the 1988 fires. Perhaps water tables would have been higher and more sites would have had saturated soils throughout the summer had the study been done during wet years. Only four species of amphibians are known with certainty to occur in YNP now: boreal toad (Bufo boreas), tiger salamander (Ambystoma tigrinum), boreal chorus frog (Pseudacris maculatd), and Columbia spotted frog (Rana luteiventris). Two other species have been reported but not verified in recent times. The population sizes of the boreal toad have declined significantly, those of the spotted frog less so, and the chorus frog and tiger salamander appear to vary within normal limits. Wetlands are critical habitat for amphibians, which are often considered indicator species of environmental health (e.g., EPA1998 Star Grant: Environmental Factors That Influence Amphibian Community Structure and Health as Indicators of Ecosystems). Habitat changes resulting from the interplay between vegetation, hydrology, elk, and beaver could influence available wetland habitat for amphibians, but none of the decreases is clearly related to the known direct or indirect effects of elk population size or feeding. Conclusions Hydrological changes in the northern range are the most likely cause of changes in wetland communities, although evidence from exclosures indicates

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Ecological Dynamics on Yellowstone’s Northern Range that hydrological factors do not account for the differences between plant communities inside and outside of exclosures. Lowering water tables from stream incision and loss of beaver ponds may reduce wetland habitat. There may be drying of wetland depressions but there are no long-term data on shallow groundwater levels in northern range locations where drying may be occurring to explain changes in these depressions. Some of the depressions may also be filling in, reducing the amount of area available for wetland species (committee observation, Yellowstone National Park northern range, June 1999). Wetlands in the northern range that support herbaceous vegetation may be grazed, but use of these areas probably is not as detrimental to their long-term sustainability as potential changes in groundwater availability. However, wetlands dominated by woody plants appear to be significantly degraded by browsing.