Western North American Naturalist 65(2), © 2005, pp. 269–273 REFUGIA FROM BROWSING AS REFERENCE SITES FOR RESTORATION PLANNING William J. Ripple1,2 and Robert L. Beschta1 Key words: refugia, ungulate browsing, woody browse species, predation risk, livestock grazing In the western U.S., deciduous woody species times have had significant impacts on vegeta- along riparian systems provide important eco- tion (National Research Council 2002a). Thus, logical functions. For example, they stabilize there is an increased need for restoration of stream banks and impart hydraulic resistance deciduous woody species at landscape scales. during overbank flows, enhance deposition of Such restorations would be facilitated if refer- organic matter and fine sediment on floodplains, ence sites existed that were relatively unim- support general food webs of aquatic and ripar- pacted by ungulate herbivory (i.e., refugia) since ian organisms, moderate water temperatures they (1) can provide an understanding of vege- and microclimates, and recruit large wood tation dynamics without the effects of her- (National Research Council 2002b). Measures bivory, (2) help define the degree and extent of of biodiversity, biomass, and number of rare degradation in woody plant communities for species are often much greater in riparian habi- other portions of a landscape, (3) may assist in tats than on adjacent uplands (Knopf et al. 1988). setting restoration priorities, and (4) may provide Deciduous woody species on upland sites pro- important “targets” for restoration programs. vide for watershed protection, aesthetics, wood In landscapes that have experienced the fiber, and habitats that also help support a effects of widespread and sustained herbivory wide variety of wildlife and avian species (Bar- from ungulates (either domestic or wild), refu- tos 2001, National Research Council 2002a). gia from browsing can be created with fenced Despite their significance to western ecosys- exclosures (Brookshire et al. 2002, Sarr 2002), tems, deciduous woody species have been in provided that sufficient seed or bud banks re- decline (Braatne et al. 1996, Kay 1997, Bartos main. Unfortunately, such exclosures are seldom 2001). Many western riparian systems have available. Yet, even within a heavily browsed been diminished in total area (Swift 1984) while landscape we suggest there will often exist many that remain often have been altered or scattered refugia sites with deciduous woody degraded by various human activities and land species; such sites are often small in area but uses (Wigington and Beschta 2000). may contain a relatively diverse plant commu- While the causes of loss and alteration of nity structure and composition. Where such woody species during a period of increasing refugia have persisted is notable as they typi- Euro-American influence are multiple, high cally occur in locations where there are multi- levels of herbivory from domestic ungulates ple impediments to ungulate access. The im- have often degraded ecosystem structure and portance of these sites is that they provide a function. Such degradation includes impacts glimpse of the potential structure and compo- to habitats of numerous species of vertebrates sition of plant communities where ungulate and invertebrates, various food web interac- herbivory is not of overriding significance and tions, and nutrient cycling (Fleischner 1994, may represent an initial approximation of what Braatne et al. 1996, Belsky and Blumenthal other areas in a landscape might become if 1997, Donahue 1999, Rooney and Waller 2003). herbivory levels were reduced or curtailed. Even where land has been set aside within the Because refugia are often visually different National Park system, native ungulates some- (e.g., high contrast, taller plants, higher plant 1College of Forestry, Oregon State University, Corvallis, OR 97331. 2Corresponding author. 269 270 WESTERN NORTH AMERICAN NATURALIST [Volume 65 densities) relative to the general landscape, servation because of cascading effects upon they are typically easy to locate. In the follow- lower trophic levels (Smith et al. 2003, Ripple ing discussion, we identify numerous types of and Beschta 2004). Refugia created through “impediments” to browsing that have contrib- risk-sensitive foraging involve predator/prey uted to the maintenance of refugia. interactions whereby areas of low browsing Several studies have described the role of intensity occur, either in conjunction with exist- natural physical barriers to animal movement ing physical barriers or independent of them. in creating refugia. At the microsite scale, Changes in prey behavior due to the presence Rooney (1997) described how herbaceous veg- of predators are referred to as predation-risk etation growing on the tops of boulders effects. These behavioral modifications include escaped deer browsing. Schreiner et al. (1996) changes in habitat use, patch selection, and discovered shrub refugia behind log barriers choices of feeding sites (Lima and Dill 1990). created by fallen conifers in Olympic National This process can produce low populations of Park. They found several species of shrubs in herbivores in a predator’s core use area, thus these refugia that successfully produced flow- creating refugia for woody browse species ers and fruit unlike the majority of the shrubs through lower herbivory. For example, in re- growing nearby in the open. They concluded sponse to the presence of predators, researchers that these refugial shrub patches may provide have documented increased concentrations of critical seed sources for recolonization of the ungulates in buffer zones away from both mam- floodplain by species that might otherwise be malian predators (Mech 1977, White et al. 1998, absent. Ripple and Larsen (2001) found that Ripple et al. 2001) and human hunters (Lalib- fallen conifers killed by the 1988 fires in Yel- erte and Ripple 2003). lowstone National Park could be dense enough Predation-risk effects on prey animals, in to provide local refugia, allowing aspen recruit- combination with varying terrain conditions, ment with high levels of ungulate browsing can also create “invisible impediments” to nearby (Fig. 1). Beschta and Ripple (2005) iden- browsing and have apparently been caused by tified increased cottonwood recruitment occur- sport hunters as well as wolves. For example, ring between highways and terrain features St. John (1995) concluded that aspen stands such as steep slopes and rivers that reduced within 500 m of roads were less impacted by the presence of animals. Larsen and Ripple wild ungulates than those farther away, suggest- (2003) discovered a lack of aspen recruitment ing that elk adjusted their foraging behavior to across the northern range in Yellowstone avoid human contact and possible predation National Park except for stands growing in the by humans. Other researchers found that aspen midst of scree deposits. They concluded that were heavily browsed on U.S. Air Force land the scree protected the aspen from ungulate that was utilized year-round by a large elk browsing. The scale of the refugia in the above population but where sport hunting was not case studies ranges from 1 to several thousand permitted. Conversely, this property is sur- square meters. Yet, physical barriers also have rounded by national forest land where hunting been described at much larger scales where is allowed and the aspen stands were mini- terrain features such as mesas and buttes im- mally browsed (McCain et al. 2003). peded ungulate access and created refugia Ripple and Beschta (2003) proposed that, (Jameson et al. 1962, Ambos et al. 2000). following the reintroduction of wolves in Yel- It is important to recognize that the wide- lowstone National Park, a “terrain fear factor” spread loss of major predators such a wolves has been playing an important role in the (Canis lupus) early in the 20th century allowed selective release of cottonwood and willow ungulates to browse with a reduced threat of from long-term browsing suppression by elk. predation. In addition to the often widespread In their predation-risk hypothesis, they sug- effects of domestic ungulates, woody plant gested that elk would increasingly forage at communities can be profoundly affected by sites that allow early detection, avoidance, and native ungulates when top predators are re- successful escape from wolves. They found moved from ecosystems (Leopold et al. 1947, cottonwood and willow to be releasing at Terborgh et al. 1999, Ripple and Larsen 2000, potentially high-risk sites with limited visibil- Beschta 2003, Soulé et al. 2003) and evidence ity of approaching wolves and/or with terrain is growing on the importance of predator con- impediments to escape from an attack, such as 2005] NOTES 271 Fig. 1. Protected aspen sprouts growing among coarse woody debris on the Blacktail Plateau in Yellowstone National Park (example of a physical barrier to browsing). See Ripple and Larsen (2001) for details on aspen recruitment in loca- tions where dead trees have created a jackstraw barrier to ungulate movement. In 2003 the aspen sapling to the left of the white pole was approximately 4 m tall (see arrow), while aspen sprouts growing nearby outside the woody debris were less than 1 m tall. high terraces, steep cutbanks, and nearby gul- represent atypical conditions for pre-European lies (Fig. 2). plant communities. There are several limitations to the use of Realizing the potential limitations of local refugia as reference sites. Because they are