Predisposition of Willows to Elk Browsing Due to Willow Interactions with Abiotic Factors Rex G. Cates Jiping Zou Stuart C. Wooley Francis J. Singer Lauryl C. Mack Linda Zeigenfuss Abstract—Current literature indicates that the decrease in willow stature and productivity on the winter range in Yellowstone National Park is due to a complex interaction among abiotic and biotic factors. Lack of change in soluble carbohydrates, total phenolics and tannins when willows were mechanically clipped in exclosures in Yellowstone National Park show that willows were not able to respond physiologically to these treatments. These data are supportive of the premise that the willow decline is due to unfavorable growing conditions on the northern winter range. Willow physiology, growth, and recruitment are impacted adversely by ungulate browsing, but experimental manipulations within exclosures indicate that browsing may not be the only important factor. The trend toward aridity, lowered water tables, reduced stream flow, lack of flooding, and absence of beaver are suggested to be important factors in reduced willow production, reproduction, and lower levels of primary and defensive chemicals. Over the last 60 years large decreases in willow (Salix spp.) populations and stature have occurred in the northern winter range in Yellowstone National Park (YNP), Wyoming (Wagner and others 1995a; Chadde and Kay 1991; Houston 1982). Willow declines have been especially dramatic occurring primarily between 1920 and 1940 during an extended drought (Engstrom and others 1991). Reasons for these changes in willow dyamics have been heavily debated resulting in several hypotheses. The most common is that elk (Cervus elaphus) have increased to unnaturally high levels due to reduced predation including hunting, alterations of the landscape by modern man, and disruption of migration routes (Wagner and others 1995b; Boyce 1991; Pengally 1963). In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999. Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc. RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. Rex G. Cates and Jiping Zou are the Director and Associate Director, respectively, of the Chemical Ecology Laboratory, and Stuart C. Wooley, Research Assistant, Department of Botany, Brigham Young University, Provo, UT 84602. Francis J. Singer and Linda Zeigenfuss, Research Ecologist and Ecologist, respectively, Biological Division of the U.S. Geological Survey, Midcontinent Ecological Center, Natural Resources Ecology Lab, Colorado State University, Fort Collins, CO 80523. Lauryl C. Mack is a Wildlife Biologist, P.O. Box 168, Yellowstone National Park, WY 82190. USDA Forest Service Proceedings RMRS-P-11. 1999 A second set of hypotheses focus on climate and hydrological changes in conjunction with beaver declines. For the winter range in Yellowstone National Park climates are warmer and drier this century (Balling and others 1992a,b), which is suggested to have resulted in lower water tables as judged by stream downcuts, reduced stream flows, and less flooding. These changes may foster a habitat that is unfavorable for willow growth and recruitment (Singer and others 1994; Houston 1982). Compounding these adverse effects on willows is the decline in beaver, which has resulted in almost no beaver ponds, a change in stream flows, and lower water tables (Jonas 1955). Most likely, with fewer beaver ponds reestablishment sites for willow shoots, sprouts, and seedlings are greatly diminished and nutrient dynamics may have changed significantly (Naiman and others 1986; Naiman and Melillo 1984). Changes in these climatic and hydrological factors combined with the loss of beaver are purported to have resulted in willows that are unable to produce sufficient natural defenses against ungulates. As a result, willows become nutritionally more suitable to these large herbivores and are not able to grow beyond the height of browsing. Although heavy browsing exacerbates willow physiology and growth, we suggest that elk browsing may not be the only cause of willow declines. We posit that changes in water dynamics due to reduced precipitation, higher temperatures, reduced flooding, lowered water tables, and the absence of beaver may be important factors causing the demise of willows on the northern winter range. In support of the climate-hydrological-beaver scenario, tall productive willows growing within the upper reaches of the drainages where the habitat is more favorable for growth were browsed seven to nine times more than were suppressed willows at lower elevations (Singer and Cates 1995). Willow Chemistry and Ungulate Browsing Compounds in willows that function as defenses against herbivores and also affect nutritional quality are tannins that bind to several primary metabolites (e.g., nitrogencontaining compounds like proteins, amino acids, DNA, RNAs), and low molecular weight phenolics, which are suggested to be toxic and/or act as feeding deterrents (Robbins 1993). Of these two, low molecular weight phenolics may 191 play the major role in deterring ungulate browsing since some large animal browsers may render tannins less effective as defenses (Robbins and others 1987). For willows inhabiting sites favorable for growth, the challenge from tissue removal by herbivores and infection from pathogens normally results in an induced increase in phenolics and tannins in remaining and new leaf and stem tissues (Karban and Baldwin 1997). Where conditions for willow growth are unfavorable on the northern winter range, willows will not be able to respond by increasing their production of defensive chemicals when exposed to heavy ungulate browsing and mechanical clipping experiments. Clipping experiments using willows have produced mixed results regarding changes in nitrogen and soluble carbohydrates in new growth. Clipping Bebb willow (S. bebbiana) resulted in no significant increase in carbohydrates, total nitrogen, and phosphorus (Chapin and others 1985), but clipping S. lanata at 0, 50, and 100% for 2 years resulted in a significant increase in total nitrogen and phosphorus (Ouellet and others 1994). In this same study soluble carbohydrates decreased significantly and then returned to control levels. An important contribution of the study reported here is the determination of carbohydrate concentrations over a 3 year period in which willows located inside exclosures were clipped at 0, 50, and 100% of their current annual growth. Objectives and Hypotheses _______ Our objectives were first to test hypotheses, in the context of the climate-hydrological-beaver paradigm, regarding the physiological status of willows on the northern winter range of Yellowstone National Park. Secondly, we summarize from the literature the characteristics purported or documented for sites deemed to be unfavorable or favorable to willow growth, productivity, and recruitment, and a description of willow responses to these site characteristics. Included in this summary are projections as to how ecosystem processes may change as site conditions change, and the communities that result from alteration of these processes (Naiman and others 1988; Cummins and others 1989). The hypothesis tested is that willows from which the current annual growth (CAG) was mechanically clipped at the 50 and 100% levels inside exclosures in YNP will not be able to respond, as compared to the control (0% clip), by increasing defensive phenolic and tannin compounds. The response in carbohydrates to these treatments is mixed, but the expectation is that total soluble carbohydrate content will decrease in the presence of mechanical clipping as compared to the control. Clipping experiments inside 30-35 year old exclosures should be particularly instructive since the effect of elk browsing is eliminated, and the experiments were carried out over a 3-year period. Study Areas and Methods ________ Study Sites Study areas, and the indepth analysis of climate, growth, and physiological data, are detailed in Singer and others (1998). In summary, the northern winter range in YNP is 192 about 140,000 hectares located on the northern boundary of the park. This area is characterized by cold winters (averaging –4.90°C) and short, cool summers (15°C) with mean annual precipitation averaging about 18-40cm/year (Houston 1982). At the time of this study, elk population 2 densities on the YNP winter range were 11-16 elk/km . Beaver, however, are rare or absent on the winter range in YNP (<1% of stream length with beaver activity). Primary and Secondary Metabolites Ten to 15 shoots from each plant were analyzed for total soluble carbohydrates, phenolics, and tannins, and were expressed on a dry-weight basis. ANOVA for unequal sample sizes on log transformed data were performed using clipping treatment as the independent variable, and total content of soluble carbohydrates, phenolics, and tannins as the dependent variables. Results and Discussion __________ Willow Physiology as an Indirect Indication of Unfavorable Habitat Quality for Willow Growth Measurements of primary and secondary metabolites over a 3 year period indicate that 50 and 100% clipped willows in exclosures in YNP were not able to respond to these treatments by increasing phenolic and tannin content (tables 1,2). Soluble carbohydrates for Bebb willow and false mountain willow (S. pseudomonticola) did not change or decrease significantly in the 50 and 100% treatments (e.g., false mountain, 1996) (tables 1, 2). Total phenolic content in Bebb (table 1) and false mountain willows (table 2) followed the pattern of not changing significantly across treatments within a year, or of decreasing significantly (e.g., Bebb willow summer 1993—100% decreased compared to 50%; for the winter 1995 collection, 50 and 100% decreased compared to 0%). Bebb willow tannin content in leaf and twig tissues for the 0 and 50% clipped plants was not significantly different, but tannin content decreased significantly in the 100% clipped plants compared to the other two treatments for 1993 (table 1). For the 1995 winter twig collection, tannin content decreased in the 100% clip compared to the 0 and 50% treatments (table 1). For false mountain willow the 1993 and 1996 tannin content in leaf and twig tissues in the summer was not different among any treatments, but did significantly increase in the 100% clip in 1995 (table 2). In the 1993 winter collections tannin content significantly decreased in the 50% treatment compared to the control and 100% clip (table 2). These physiological data are consistent with the hypothesis that willows in exclosures that have been protected from browsing 31-35 years were occupying unfavorable habitats for growth. An additional clue to the willow decline, as related to water dynamics and improved habitat quality for willow growth and physiology, is found in the phenolic data in the clipping experiments. A trend toward increased moisture and lower temperatures in YNP seems to have occurred during the late 1990’s (Stohlgren and others 1998). This USDA Forest Service Proceedings RMRS-P-11. 1999 Table 1—Effect of mechanical clipping on total phenolic, tannin, and total soluble carbohydrate content of the current year’s growth of Bebb willow from July 1993 through summer 1996 ( x , sd§), Yellowstone National Park, Wyoming. 0 Year/compound+ 50 Summer 100 Summer Winter Winter 1993 Total phenolics Tannins Sol. carbo.¤ 4380 ± 1226 76 ± 12a* 44 ± 16 5091 ± 1784 57 ± 17 23 ± 1 5191 ± 1357a* 72 ± 8a* 47 ± 12 4349 ± 427 52 ± 7 22 ± 2 3634 ± 1200b* 56 ± 13b* 47 ± 13 5200 ± 1184 55 ± 8 21 ± 8 1995 Total phenolics Tannins Sol. carbo. 3071 ± 466 66 ± 8 32 ± 5 6987 ± 1688a* 52 ± 12a* 19 ± 6 3133 ± 688 57 ± 14 29 ± 4 3287 ± 1546b* 53 ± 14a* 20 ± 5 3066 ± 749 57 ± 13 32 ± 14 3873 ± 2246b* 30 ± 10b* 17 ± 1 1996 Total phenolics Tannins Sol. carbo. 9154 ± 1595 62 ± 8 26 ± 5 — — — 9181 ± 2295 64 ± 7 32 ± 8 — — — Summer Winter 8953 ± 2950 59 ± 17 28 ± 5 — — — § Means followed by different letters significantly different at *p ≤0.05; **p ≤0.10, ANOVA. Total phenolics expressed as average peak height divided by 100. Tannins and soluble carbohydrates expressed as mg/g dry weight. ¤ From S. C. Wooley, M.S. Thesis 1998. Brigham Young University, Provo, UT + Table 2—Effect of mechanical clipping on total phenolic, tannin, and total soluble carbohydrate content of the current year’s growth of false mountain willow from July 1993 through summer 1996 ( x , sd§), Yellowstone National Park, Wyoming. 0 Year/compound+ 50 100 Summer Winter Summer Winter Summer Winter 1993 Total phenolics Tannins Sol. carbo.¤ 7141 ± 668 76 ± 10 46 ± 11 4532 ± 1423 48 ± 7a* 35 ± 7 6061 ± 1639 75 ± 11 40 ± 15 4593 ± 847 35 ± 10b 31 ± 3 6649 ± 1793 77 ± 10 46 ± 17 4847 ± 913 47 ± 10a** 23 ± 11 1995 Total phenolics Tannins Sol. carbo. 6745 ± 1721 58 ± 12a 32 ± 6 5069 ± 1534 42 ± 12a 26 ± 9 6888 ± 1474 59 ± 21a 30 ± 8 4771 ± 539 46 ± 7a 25 ± 7 7264 ± 1125 79 ± 13b* 31 ± 6 5141 ± 597 62 ± 10b* 25 ± 6 1996 Total phenolics Tannins Sol. carbo. 17,166 ± 3343 65 ± 9 47 ± 9a § — — — 15,265 ± 4864 62 ± 14 25 ± 4b* — — — 16,283 ± 3034 68 ± 12 24 ± 4b* — — Means followed by different letters significantly different at p ≤0.05; p ≤0.10, ANOVA. Total phenolics expressed as average peak height divided by 100; tannins and soluble carbohydrates expressed as mg/g dry weight. From S.C. Wooley, M.S. Thesis 1998. Brigham Young University, Provo, UT. + ¤ trend is associated with a two to three times increase in phenolic content in the samples taken from Bebb and false mountain willows (tables 1, 2). Climate Change, Beaver Abundance, Ecosystem Processes, and Riparian Communities Several factors noted in recent publications indicate a trend toward aridity and change in habitat quality on the winter range toward one less suitable for willow growth and physiological response (table 3) (and perhaps aspen, Romme et al. 1995). The summer temperature increase has been 0.87°C, while the average January-June precipitation decreased 61 mm. Beaver and their associated activities are an additional factor affecting water dynamics and ecosystem processes (table 3). Beaver were found to be almost USDA Forest Service Proceedings RMRS-P-11. 1999 nonexistent on the winter range in YNP. Their activity results in local water tables being higher near beaver dams and surrounding areas, in the formation of a mosaic of braided, meandering, slow-moving water in streams, and ponds that are rich and efficient in processing nutrients, carbon, and litter (table 3) (Naiman and others 1988). Reduced production, leader length and weight, plant height, and recruitment of unbrowsed and browsed willows also suggest poor growing conditions for willows (table 3). Clipping willows over a 4 year period in YNP exclosures showed a cumulative negative change in production and height for the 0, 50, and 100% clipped plants (Singer and others 1998). Noted also were decreases in catkin production. These, combined with the lack of physiological response in soluble carbohydrates, low molecular weight phenolics, and tannins in mechanically clipped willows in exclosures, suggest that browsing may not be the only factor in willow declines. 193 Table 3—Verified and predicted parameters describing sites favorable and unfavorable for willow growth, and willow and habitat responses on these sites, in the northern winter range of Yellowstone National Park, Wyoming. Site Parameter/response Unfavorable Parameters Precipitation1-4, 8 Temperature1-4, 8 Water Tables1-3 Streambanks1-3 Flooding1-3 Streams6, 7 Beaver3, 5, 6 Willow Response Production1-3 Recruitment1-3 Leader Length and Weight1-3 Plant Height1-3 Browsing Effect on Growth Form2 Defense Response Habitat Nutrients6, 9 Aquatic Invertebrates6, 7 Carbon6, 7 Litter Plant Community2, 7, 8 Less, drier winters Warmer Higher, wetter Cooler Lower Prevalent downcutting Common, extensive Single or few streams, narrow streams; increased flow Few to none; loss of food supply Perennially higher Less downcutting, vegetated Infrequent, reduced intensity Network of braided, expounded streams, ponds; impounded water, high sediment organic matter Common; food supply abundant (aspen, willow) Reduced; negative gain in CAG Reduced to nonexistent Short, lighter Reduced in stature Morphological juvenility response; reduced physiological response Reduced or none Net increase, vigorous gain in CAG Seed productivity high, asexual Longer, heavier Able to grow beyond reach of elk Morphological and physiological juvenility Induced response vigorous Reduced input of N Shredders, scrapers; biomass less Low carbon input More recalcitrant; slower processing Grass, sedges, conifer encroachment, dryland shrubs Enhanced N availability Collectors, predators; greater biomass High carbon input and processing More rapidly processed Mosaic of riparian vegetation; willow aspen maintained 1 4 7 2 5 8 Singer and others 1998 Singer and others 1994 3 Singer and Cates 1995 Balling and others 1992a,b Consolo-Murphy and Hanson 1993 6 Naiman and others 1988 9 Naiman and Melillo 1984 Houston 1982 Jonas 1955 References _____________________ Balling, R. 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