Consumption of seeds of southwestern white pine (Pinus strobiformis) by black bear (Ursus americanus).
Farther south, black bears consume seeds of limber pine (P. flexilis, of the genus subsection Strobi) in Colorado (McCutchen, 1996), pmon pine (P. edulis, of the genus subsection Cembroides) in Utah and New Mexico (Bunnell, 2000; C. M. Costello et al., in litt.), and Jeffrey pine (P. jeffreyi, of the genus subsection Ponderosae) on the east slopes of the Sierra Nevada in California (Kuhn and Vander Wall, 2007). Bears apparently obtain limber pine seeds primarily by raiding caches made by red squirrels and obtain seeds of Jeffrey and piflon pines primarily by licking up seeds on the ground or foraging on cones in tree canopies. Consumption of seeds of southwestern white pine (P. strobiformis, genus subsection Strobi) by black bears has not been previously documented anywhere in the Arizona, New Mexico, Colorado, or Mexico range of this species of trees, although a definitive conclusion regarding use of this species by bears is complicated by the know hybridization of southwestern white pine with limber pine (Anderson et al., 1971; Perry, 1991).
We document, for the first time, consumption of seeds of southwestern white pine seeds in its southwestern United States range by black bears, apparently from cones excavated from larder hordes made by red squirrels. We discovered this activity in an area of Arizona that is considered to be unproductive habitat for black bears (Arizona Game and Fish Department, http://www.azgfd. gov/h_f/hunting_units_7.shtml#bear) as well as former range of grizzly bears prior to extirpations in the late 1800s and early 1900s (Brown, 1996).
Our study area in the uplands surrounding Flagstaff, Arizona, was 34.70 to 35.80[degrees]N latitude, 111.13 to 111.77[degrees]W longitude, and 1,330 to 3,850 m above sea level. Ponderosa pine (P. ponderosa) forests and pinon-juniper (Juniperus sp.) woodlands dominated most of this area. Mixedspecies forests of Douglas-fir (Psuedotsuga menziesii), aspen (Populus tremuloides), southwestern white pine, corkbark fir (Abies lasiocarpa var. arizonica), Engelmann spruce (Picea engelmannii), and bristlecone pine (Pinus aristata) occurred at elevations >2,440 m on the San Francisco Peaks.
We discovered evidence of black bears consuming seeds of southwestern white pine on three occasions, during mid-September 2005 and during mid-May and mid-July 2011. We made these discoveries on north slopes of the San Francisco Peaks in Arizona, all within roughly 1 km of 35.378[degrees]N latitude and 111.678[degrees]W longitude at nearly identical sites. These sites were on lower-slope positions oriented 340[degrees], on 10-12[degrees] slopes, and at elevations of 2,660-2,740 m above sea level. For these sites, we estimated 65-78% forest canopy cover and 1820% coarse woody debris (deadfall) cover, which we classified as dense forest with heavy deadfall. The overstory was dominated by aspen, corkbark fir, and southwestern white pine. The sparse understory consisted mostly of Oregon-grape (Berberis repens) and Arizona peavine (Lathyrus arizonica). We found bear feces (scats) comprised wholly of the finely masticated coats of seeds from southwestern white pine only at two of the sites (three at one site and one at the other). At the third site, we found one scat containing pine seeds plus sign that a bear had broken apart southwestern white pine cones excavated from a larder horde in a midden made by red squirrels. The midden also contained corkbark fir cones, but there was no sign that bears had fed on them. The midden was roughly 27 [m.sup.2] in size and centered on two fallen logs 24 and 13 cm in diameter at midpoint. Approximately 15% of the midden had been excavated. We estimated forest basal area of a plot centered on the midden to be 64.3 [m.sup.2]/ha, of which 71% was comprised of five southwestern white pines averaging 63.1 cm in diameter (range of 40.5-83.0 cm) at 1.4 m above ground level (dbh).
This is an important discovery for three reasons. First, it offers insight into foods and foraging behaviors of black bears in comparatively unproductive habitats of Arizona that, to date, have not been scientifically investigated. Newly identified foods, such as southwestern white pine seeds, could lead to revision of how we currently assess habitat of black bears and its productivity in this region. Second, it offers insight into potential foods and behaviors of grizzly bears in a range unlike any they currently occupy. This potential is highlighted by currently unique behaviors of grizzly bears in the Yellowstone ecosystem, such as excavation of food caches made by pocket gophers (Thomomys talpoides); these behaviors are unlike any exhibited by grizzly bears farther north but are representative of behaviors that were probably much more common farther south prior to historic extirpations (Mattson and Merrill, 2002; Mattson, 2004). Third, this discovery expands the documented extent of consumption of pine seeds by bears, in terms of species of pine and habitats.
Retrospectively, we did not find it surprising that black bears consumed seeds (southwestern white pine) in cones excavated from middens made by red squirrels at the sites. Southwestern white pine seeds are comparable in size to other pine seeds eaten by bears (average 6.0 seeds/g compared to 4.0, 5.7, 8.2, and 10.8 seeds/g for Siberian, whitebark, Jeffrey, and limber pines, respectively; Bonner and Karrfalt, 2008), which would engender comparable energetic rewards. Red squirrels also are known to harvest a large percentage of ripe cones from southwestern white pine (as much as 93%; Samano and Tomback, 2003), making their middens a potentially rich source of food. Southwestern red squirrels also are densest in older mixed-species forests, where they tend to make middens around deadfall and at the base of large trees (Vahle and Patton, 1983; Patton and Vahle, 1986). This description fits not only the site where we found southwestern white pine cones excavated by bears but also sites where most excavations by grizzly bears for whitebark pine cones occurred in the Yellowstone ecosystem. Yellowstone grizzly bears were more likely to consume whitebark pine seeds in areas with high densities of large middens made by red squirrels and, upon encountering a midden, were more likely to excavate those surrounded by greater amounts of mature whitebark pine (Mattson and Reinhart, 1997). using a model developed for the Yellowstone ecosystem based largely on size of middens and basal area of food-source pines (Mattson and Reinhart, 1997), the predicted point probability that bears would have excavated pine seeds at our third site was 0.82.
We appreciate expert advice from T. Ayers, Curator of the Deavor Herbarium at Northern Arizona University, on the taxonomy and presence of southwestern white pine on the San Francisco Peaks, comments by D. E. Brown on an earlier draft of this paper, and suggested revisions by two anonymous reviewers.
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Submitted 7 August 2011. Accepted 30 April 2013. Associate Editor was Jennifer K. Frey.
David J. Mattson * and Terence A. Arundel
United States Geological Survey, Southwest Biological Science Center, Flagstaff, AZ 86002
* Correspondent: David_Mattson@usgs.gov
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|Author:||Mattson, David J.; Arundel, Terence A.|
|Date:||Jun 1, 2013|
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