Browsing of Antelope Bitterbrush (Purshia tridentata: Rosaceae) in the South Okanagan Valley, British Columbia: Age Preferences and Seasonal Differences.
S. L. HICKS 
ABSTRACT. -- We compared browsing on twigs of small and large antelope bitterbrush (Purshia tridentata) shrubs among ten sites in the south Okanagan valley, British Columbia. We tested whether there were any age preferences by browsers and determined whether these preferences changed between seasons and mode of browsing. Two different types of browsing were observed: leaf stripping which occurred in the summer and twig clipping which occurred predominantly in the winter. We calculated age and size relationships showing that shoot volume and especially stem diameter were good predictors of shrub age. Among the ten sites, clipping removed 0.02 to 15.7% of a shrub's total twig length and stripping removed leaves from 0 to 5.2% of total twig length. Observations suggested that California bighorn sheep (Ovis canadensis california) stripped antelope bitterbrush leaves in late summer, mule deer (Odocoileus hemionus hemionus) clipped twigs in the winter and cattle clipped twigs in the summer. Browsers preferred to c lip twigs on smaller and hence younger antelope bitterbrush shrubs. In contrast, larger and older shrubs were preferred for leaf stripping. Since twig clipping was more prevalent than leaf stripping in antelope bitterbrush, overall preference for younger shrubs may lead to difficulties in seedling establishment in regions where it is heavily used as winter forage.
Severe browsing of juvenile plants can prevent regeneration of preferred browse species (Huntly, 1991). Establishment of woody plants sometimes occurs in waves of cohorts of seedlings during low browser years (Pastor et al., 1988). For example, younger balsam fir juveniles were completely uprooted by moose in Newfoundland; 2-y old trees were eliminated, but 7-y old juveniles were largely left alone (Bergerud and Manuel, 1968). Similarly, over 80% of seedlings of a tropical rainforest tree sustained some herbivory in the first year (Clark and Clark, 1985). Because of this intense selective pressure, some species have evolved specialized defenses in the juvenile stage against herbivory, including physical defenses (thorns) and chemical defenses (Bryant et aL, 1991; Bryant et al., 1992). Antelope bitterbrush (Purshia tridentata) is an important woody browse species that, instead of defending itself against herbivory, appears to be resilient to intense levels of browsing (Wandera et al., 1992). The branching arc hitecture results in the stimulation of long shoot production when more than 50% of annual production is removed (Bilbrough and Richards, 1993).
Antelope bitterbrush is considered one of the most important browse species on western ranges for a wide variety of wildlife and domestic livestock, except horses (Stubbendieck et al., 1992). The leaves and younger twigs of antelope bitterbrush provide summer forage for cattle, mule deer (Odocoileus hemionus hemionus) and domestic sheep (Wood et al., 1995; Griffith and Peek, 1989). The percent dietary contribution of this shrub increases throughout the summer as palatable forbs and other browse species become less available (Kufeld et al., 1973; Austin et al., 1984). Mule deer often use antelope bitterbrush for winter forage (Welch and Wagstaff, 1992) because it supplies more carotene, crude protein and phosphorus than dormant grass (Welch et al., 1983). Though resilient, severe browsing can result in reduced growth (Kay, 1995). Decline of antelope bitterbrush populations has been observed throughout its range (Rickard and Sauer, 1982; Updike et al., 1990). If juvenile antelope bitterbrush are browsed more s everely than established shrubs, regeneration might be limited partially because of overuse.
Whether or not younger antelope bitterbrush shrubs are subjected to more browsing than older individuals is unknown. We tested whether there were any preferences for young or old antelope bitterbrush shrubs by ungulates or domestic livestock and determined whether these preferences changed between seasons and mode of browsing. There were two distinct forms of browsing, leaf stripping and twig clipping. Stripped twigs have part of their length stripped of leaves with a portion of the bark layer often removed. This form of twig use predominates in the summer in deciduous woody species when leaves provide grazers with a source of nitrogen (Danell et al., 1994; Wood et al., 1995). Clipped twigs have some portion of their length bitten off. Clipping generally occurs in the winter when grasses and other herbaceous plants are unavailable and animals are more likely to consume higher fiber diets (Willms and McLean, 1978; Danell et al., 1994). Little is known about which grazers strip leaves or clip twigs, but stripp ing and clipping can be done by the same browser; in Scandinavia moose (Alces alces) strip leaves off birch (Betula pendula) in the summer growing season and clip birch twigs in the winter (Danell et al., 1994).
STUDY SPECIES AND SITES
Antelope bitterbrush is widely distributed across western North America in sites with deep well-drained soils (Hitchcock and Cronquist, 1973; Stubbendieck et at., 1992). Not only is it a preferred shrub of browsers, but it supports more insects than other shrubs (Furniss, 1972) and small mammals cache the majority of its seed crop (Vander Wall, 1994; Shatford, 1997).
The study was conducted in the south Okanagan Valley on ten sites between Oroville, just south of the U.S. border (48[degrees]59'N, 119[degrees]25'W), and Vaseux Lake (49[degrees]16'N, 119[degrees]30'W), British Columbia. Going from south to north, the sites have the acronyms WA, ELO, BO, OS, WT, RG, BW, EL, KB, CWS. These acronyms have also been used elsewhere (Krannitz, 1997ab) to permit linkages between publications. Each site was selected on the basis of similarity other than livestock grazing history. All sites were situated between 300 to 500 m above sea level on the east side of the valley and had either loamy sand or sandy loam soils. Grazing histories ranged from little or no use to intermittent year-round grazing by horses and cattle (Krannitz, 1997a). Grazers and browsers included cattle, mule deer, California bighorn sheep (Ovis canadensis california) and horses.
In early May 1994, 12 "small" and 12 "large" antelope bitterbrush shrubs located approximately 10 m apart were selected and labeled along haphazardly placed transects at each site. Small and large shrubs differed significantly in size as measured by stem diameter (mean = 66 [+ or -] 0.4 cm (SE) and 13.3 [+ or -] 0.6 cm, respectively; F = 196.1, df = 1, 199, P = 0.0001) and by aboveground shoot volume as estimated by the shape of an inverted cone: [pi][r.sup.2] h/3 (1.0 [m.sup.3] [+ or -] 0.08 and 3.1 [m.sup.3] [+ or -] 0.1, F = 639.9, df = 1,216, P = 0.0001, ANOVA performed on log-transformed data). To test whether these measures of size reflected differences in age, antelope bitterbrush individuals were cut at ground level for accurate ring assessment in two separate experiments. In the first, 49 shrubs of varying sizes were harvested from a site not used in this study. In the second, three or four shrubs from each of the ten sites in this study were harvested and a one-factor ANGOVA was used to analyze ste m diameter or shoot volume. Ring number was the covariable and the effect of different sites was the factor. In both cases stem diameter and shoot volume correlated well with age (Fig. 1). In the ANCOVA analysis stem diameter and shoot volume did not differ among sites; nor did growth rate, as indicated by nonsignificant interactions between sites and the relationship between ring number and stem diameter or shoot volume (Table 1). Only the relationships between the covariable ring number and both stem diameter and shoot volume were significant (Table 1). Both of these destructive trials showed that non-destructive measures of antelope bitterbrush size are good predictors of shrub age and that "small" individuals were younger than "large" individuals.
To assess which animals were responsible for browsing antelope bitterbrush twigs, frequency of droppings was used as an index of animal activity (Griffith and Peek, 1989). At each of the 10 sites 40 plots (20 X 50 cm) were placed at regular 10 m intervals along the same transects as the shrubs (Daubenmire, 1959). Historically, cattle had been present on many of the sites, but in the 10 y previous to the study they were regularly only at one site, ELO, and were not there in the winter. Although horses are not known to browse antelope bitterbrush, the frequency of their droppings was recorded to test this at these sites. Droppings of California bighorn sheep and mule deer are not distinguishable in the field. Anecdotal observations and knowledge of the distribution of these browsers enabled separation. For example, mule deer were known to be present at all sites except ELO, whereas bighorn sheep were more restricted with potential access to only five sites (BW, KB, CWS, KL, RG).
Twig use.--We measured twig use (TU) as the percent removal of the total twig growth of the previous year (Ferguson and Marsden, 1977). Regression equations were developed from the relationship between twig length and base diameter of unbrowsed twigs. The equations were then used to predict the unbrowsed length of randomly selected browsed twigs. As all shrubs could not be visited simultaneously, new growth from 1994 was not included in twig length measurements; only twig growth and browsing before 1994 was assessed. New growth could be differentiated from 1993 growth by twig color and texture. New twigs were a light brown or green color and were more pliable than older woody growth. If the season of growth of a twig was questionable, we did not include it in our sample.
To determine the relationship between twig length and twig base diameter for each shrub, we sampled terminal and lateral twigs that were unbranched, unbrowsed and at least 3 cm long. Each shrub was divided into four quadrants using two meter sticks placed in the North-South and East-West planes. In each quadrant we selected six twigs with a range of lengths (3 to 60 cm) to facilitate regression analysis. Twig diameters were measured to the nearest 0.001 mm using dial calipers and lengths to the nearest 0.5 cm using a tape measure. Diameter measurements were taken directly above the nodal swelling of the twig base and lengths were measured from the point of diameter measurement to the end of last season's growth. For each shrub simple linear regressions were calculated with total twig length as the dependent variable and base diameter as the independent variable. If a regression equation was nonsignificant (P [greater than] 0.05, or [R.sup.2] [less than] 0.20) due to highly variable diameter-length relationsh ips, that shrub was omitted from TU analysis. This reduced the sample size by 1 for the WI, BW, BO and OS sites; by 2 for KB and WA; by 3 for ELO; and by 4 for OWS and KL sites.
To assess the use of antelope bitterbrush twigs at a particular site, previously tagged shrubs were revisited and each was divided into four quadrants as before. We haphazardly sampled five twigs per quadrant by standing in front of the quadrant with eyes closed and extending one arm into the shrub approximately 30 cm above, below and to the left, right and center of the shrub's center. With each extension the first twig touched was assessed for evidence of browsing. If a twig was clipped its remaining length ([L.sub.R]) and base diameter were recorded. Twig length before browsing ([L.sub.T]) was estimated using the regression equation calculated for that shrub. The length of twig removed by browsers was the difference between [L.sub.T] and [L.sub.R]. If a twig was stripped the amount of twig length affected could be measured directly.
For a single twig TU was calculated using TU = 100[1 - ([L.sub.R]/[L.sub.T])] (Basile and Hutchings, 1966; Ferguson and Marsden, 1977). For each shrub the average percent twig use ([TU.sub.shurb]) was calculated by summing all percent TU values for that shrub and dividing the total by the number of twigs sampled per shrub (n = 20). Unbrowsed twigs were considered 0% used and included in the calculation of the mean value (Ferguson and Marsden, 1977). Site averages for twig use ([TU.sub.Site])were calculated using shrub averages.
Data analysis.--A two way ANOVA was used to test for the effect of site and shrub size on TU by clipping or stripping (PROC GLM: SAS, 1990). To normalize percent data, TU values were arcsine square-root transformed before analysis. Spearman rank correlations were used to measure the degree of association between animal activity and mean number of twigs browsed. Number of twigs was considered to be a more accurate portrayal of animal number than twig use as it would better reflect the number of mouthfuls. In addition, twigs that were browsed in any year before 1994 were included because the age of droppings was difficult to determine.
To assess whether winter browsing by clipping altered shrub shape, a ratio was calculated using stem diameter as the numerator and shoot volume as the denominator. If browsing stimulated twig production, shoot volume would be high relative to stem diameter and the ratio would be lower compared to unbrowsed shrubs of the same age. Alternatively, if browsing of antelope bitterbrush was intense enough to decrease shoot volume (as in Yellowstone National Park: Kay, 1995), the ratio would be higher compared to unbrowsed shrubs of the same age.
Using this ratio as the dependent variable, we performed two ANOVAs to test for the effect of browsing intensity, first measured as the number of twigs clipped out of 20 (four classes): 0 and 1, 2 to 5, 6 to 9 and 10 to 16 twigs and, secondly, as TU (three classes): 0%, [less than]10% and [greater than]10%. The classes were chosen to most evenly distribute the sample population. ANOVA with browsing classes was used instead of correlation analysis so that the effect of age on shrub shape could be factored out with a second factor of stem diameter class (four classes): 3 to 5.9, 6 to 9.9, 10 to 14.9 and 15 to 26.9 cm. We did not include site as a factor because the association between age and shrub size did not differ among sites (Table 1).
Twig clipping: summer use.--Cattle clipped antelope bitterbrush twigs extensively during the summer at site ELO. Cattle clipped smaller shrubs (18.5 [+ or -] 4.7%) more frequently than larger shrubs (8.6 [+ or -] 2.4%; Table 2), and some shrubs at the site (not used in the study) were so heavily browsed that they looked like trimmed hedges.
Winter use: clopping.--During the winter of 1993-1994 the average percentage of twig length clipped per shrub per site ranged from 0.02% to 15.7% (Fig. 2b). The sites differed significantly in the mean percentage of twig length that was removed from shrubs (Table 2). On two sites the amount of browsing by clipping was not significantly different from zero (LSMEANS test, SAS 1990) whereas on five sites shrubs had more than 8% of their total twig length clipped (Fig. 2b). At the nine sites where clipping occurred only in the winter, browsers preferred small shrubs (11.5 [+ or -] 3.0%) over large shrubs (4.7 [+ or -] 1.6%) (Table 2), and this effect did not differ across sites (nonsignificant interaction term). In general, TU in the winter was less than that by cattle in the summer at site ELO.
Preference for clipping smaller antelope bitterbrush shrubs might be attributable to finer twigs. Shrubs in the large size category generally had broader tips (1.6 [+ or -] 0.04 mm, n = 92) compared to the small size category (1.4 [+ or -] 0.02 mm, n = 96) (F = 5.2, df = 1,143, P = 0.02). Tip diameter did not differ among sites (F 0.8, df = 8, 143, P = 0.6), and differences in tip diameter between size categories did not change from site to site (nonsignificant interaction term: F = 0.4, df = 8,143, P = 0.9).
Leaf stripping: summer use.--Leaf stripping by browsers in the summer of 1993 affected a smaller percentage of twig length than clipping, ranging from 0 to 5.2%. Similar to clipping, the sites differed significantly in the percentage of twig length stripped per shrub (Fig. 2a and Table 2). Twig use by stripping was significantly different from 0% on all but four sites (Fig. 2a). Site RG experienced significantly more leaf stripping than all other sites. This was also the only site at which stripping by Californian bighorn sheep was observed. In contrast to clipping, browsers preferred large shrubs (1.9 [+ or -] 0.5%) over small shrubs (1.3 [+ or -] 0.5%), which was consistent across all sites (Table 2).
Browsers and effects on antelope bitterbrush morphology.--Winter browsing did not affect the ratio of stem diameter to shoot volume of antelope bitterbrush of any stem diameter class (Fig. 3, Table 3). Only age affected the ratio, with stem diameter of older shrubs increasing disproportionately over shoot volume.
Winter browsers were likely deer or sheep, or both, as the frequency of droppings was associated with the average number of twigs clipped at a site ([r.sub.s] = 0.74, P = 0.01, n = 10; Fig. 4). Excluding site ELO (where clipping was done in the summer by cattle) increases the strength of the association ([r.sub.s] 0.85, P = 0.004, n = 9). Site WA was an outlier, with few clipped antelope bitterbrush twigs and yet a relatively high proportion of deer droppings. Deer may have used adjacent orchards for browse and the native shrub-steppe habitat for bedding purposes only. Removing that site from the correlation further strengthened the association between ungulate droppings and winter clipping ([r.sub.s] 0.93, P = 0.0009, n = 8).
As expected, horses were not observed browsing antelope bitterbrush and frequency of their droppings was not significantly associated with average number of twigs clipped ([r.sub.s] = -0.038, P = 0.9, n = 10) or stripped ([r.sub.s] = 0.37, P = 0.3, n 10). Horses were especially abundant during the winter at site WT where bunchgrasses were severely grazed. Even though we observed bighorn sheep stripping antelope bitterbrush leaves, frequency of ungulate droppings was also not significantly correlated with the average number of twigs stripped ([r.sub.s] = 0.29, P = 0.4, n 10).
Browsing on antelope bitterbrush varied with both shrub age and season. As size correlates with age, our results showed that twigs of young bitterbrush shrubs were significantly preferred in the winter by wild ungulates and in the summer by domestic cattle, whereas leaves of older bitterbrush shrubs were preferred in the late summer and early fall. These results were consistent among the 10 study sites, so that differences in twig use were not resource induced. Age related differences in preference have been found for other browse species but age related preference according to season has not. For example, larger saplings of pin cherry (Prunus pennsylvanica) were preferred by moose and white-tailed deer (Odocoileus virginianus) throughout the year in New Hampshire (Shabel and Peart, 1994). Similarly, larger and older individuals of big sagebrush (Artemisia tridentata) were preferred by domestic sheep during summer and fall grazing trials (Yabann et al., 1987),. In this case, the older shrubs and the preferre d previous year's leaves contained fewer secondary chemicals than the younger leaves and shrubs. In contrast, during the winter red deer (Capreolus capreolus) and roe deer (Cervus elaphus) preferentially browsed smaller saplings of both silver birch (Betula pendula) and pedunculate oak (Quercus robur) (Van Hees et al., 1996).
Browsing by other vertebrates has also indicated differential selection based on age, but with no clear seasonal effects. Red grouse (Lagopus lagopus scoticus) preferred older heather during both the winter and summer (Moss et al., 1972). The relationship between age, palatability and secondary chemicals is especially well known for plants consumed by Arctic hares (see review by Bryant et at., 1992). In general, younger shrubs are better protected by secondary chemicals and are consumed less by Arctic hares in the winter. This includes 14 different species of woody plants from five different genera (Bryant et al., 1992).
Little is known about secondary metabolites in antelope bitterbrush or possible differences in metabolite content between age classes or leaves and twigs. However, concentrations of crude protein, which is closely linked to palatability (McKell, 1989), have been extensively studied. For example, crude protein of leaves was almost double that of new twigs in both August and February (Wamboldt et al., 1996). In general, crude protein content of antelope bitterbrush twigs was barely enough to meet ungulate nutritional requirements (Welch et al., 1983) so that unless leaves remain attached to the stem, consuming twigs in the winter seems of limited benefit. In bog birch (Betula glandulosa Michx.), crude protein is higher in the smallest twigs with diameters less than one mm (Sinclair et at., 1988). In the present study, antelope bitterbrush shrubs from the smaller size category had twigs with narrower tips and, if they also had more crude protein, this would help explain winter browsing preferences for smaller a nd younger shrubs.
Twig use by clipping did not affect the ratio between stem diameter and shoot volume, possibly because of low levels of twig use. This might also explain similar relationships between age and shrub size among sites despite differences in twig use. The maximum amount of twig use was substantially lower than that found in the more southern parts of the antelope bitterbrush range where shrub size is affected by browsing. For example, in Utah, Bilbrough and Richards (1993) showed that removal of 60% of annual twig production in antelope bitterbrush stimulated twig production relative to controls. In southern Idaho, browsed antelope bitterbrush were larger than those protected from browsing by an exclosure (Peek et al., 1978; Ferguson and Medin, 1983) and winter twig use by mule deer ranged from 21% to 35% (Ferguson and Marsden, 1977). Browsing can often exceed 20% of annual twig production. In Lakeview Oregon, fall grazing by cattle removed approximately 40% (McConnell and Smith, 1977), whereas in Utah, the perc entage of current year's growth removed by wintering mule deer was 52.3% (Welch and Wagstaff, 1992). In Yellowstone National Park, ungulate populations were so elevated that severe overbrowsing resulted in much smaller bitterbrush shrubs outside of exclosures (Kay, 1995). In our study only a few individuals at site ELO were browsed so heavily by cattle that they were very short (approximately 0.5 m in height) and hedgelike. Generally at our sites age affected the relationship between stem diameter and crown size more than browsing. As a long lived woody shrub, older individuals favor seed over twig production, unless intensely browsed (Bilbrough and Richards, 1993).
Winter browsing was positively associated with wild ungulate droppings. We could not differentiate between deer and sheep droppings, though bighorn sheep are more limited in distribution than mule deer. Normally, antelope bitterbrush is a very important part of the mule deer diet (Kufeld et at., 1973). In Utah, antelope bitterbrush made up 91.7% of the diet of mule deer by late summer and early fall (Austin and Urness, 1983). Winter use of antelope bitterbrush by mule deer is consistently high; in Washington, just south of our study sites, it made up 70% of the diet, dropping to 17% in the spring when forbs became available (Burrell, 1982). It is likely that the same pattern also occurs at our study sites in the south Okanagan in British Columbia and that mule deer are the primary winter browsers of antelope bitterbrush.
In late summer and early fall California bighorn sheep were observed browsing on antelope bitterbrush. Published observations of browsing of antelope bitterbrush by bighorn sheep are few, but domestic sheep also show preference for antelope bitterbrush in late summer (Jensen et al., 1972), eating more herbaceous plant material in the spring (Smith et at., 1979). In diet trials with captive bighorn sheep, percent browse of shrubs other than bitterbrush declined from spring to late summer which suggests that the response of sheep to bitterbrush is different from that to other shrubs (Wikeem and Pitt, 1992). It is not known whether the dietary interest in antelope bitterbrush by bighorn sheep in late summer continues through the fall and into the winter.
In our study we did not observe horses browsing on antelope bitterbrush which is consistent with previous observations of horse diet preference (Reiner and Urness, 1982). Cattle browsed bitterbrush in the summer in our and previous studies (Smith and Doell, 1968, as cited in Reiner and Urness, 1982; Monsen and Shaw, 1983), but were not responsible for winter browsing, as they were kept off site in recent winters.
Though browsers that stripped antelope bitterbrush leaves preferred larger shrubs, the percentage of twig use was so much lower than that for clipping that, on average, selection of younger antelope bitterbrush shrubs for browsing would predominate. In some cases this can be quite severe (Kay, 1995) and may affect establishment of antelope bitterbrush, especially in degraded sites that do not offer much other forage.
Acknowtedgments.--We are grateful to landowners for access to the sites: Osoyoos Indian Band from the Okanagan Nation (ELO, WT, OS, RG, BW), Blake Kennedy and George Kennedy (KB, KL), BC Environment (BO), Puget Properties Inc. (WA) and Canadian Wildlife Service (CWS). For assistance in the field, we thank Jolene Kruger, Jennifer Grant, Phyllis Gabriel, Remco Tikkemeijer, Jeffrey Shatford and Catrin Westphal. An earlier version of the manuscript was improved by Robert Elner and two anonymous reviewers. Funding was provided by Environment Canada, Habitat Conservation Trust Fund, and Human Resources Development Canada.
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|Author:||KRANNITZ, P. G.; HICKS, S. L.|
|Publication:||The American Midland Naturalist|
|Article Type:||Statistical Data Included|
|Date:||Jul 1, 2000|
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