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The namesakes of several southwestern U.S. national parks are threatened by past and present human alterations to ecosystems. Giant sequoia (Sequoiadendron giganteum) trees of Sequoia National Park, California, are at risk from unnaturally severe fires related to fuels accumulating during the 1900s from exclusion of natural surface fires (Swetnam et al., 2009). Prescribed fires and thinning of woody fuels have been implemented by park managers to begin counteracting fuel accumulation to protect sequoia groves (Swetnam et al., 2009). In the desert Southwest, Joshua trees (Yucca brevifolia) in Joshua Tree National Park, California, and saguaro (Carnegiea gigantea) in Saguaro National Park, Arizona, are threatened by nonnative grasses introduced by humans, increasing prevalence of wildfire, and climate change. These factors can combine to remove mature individuals and limit recruitment (Barrows and Murphy-Mariscal, 2012; Conver et al., 2017). Managers have implemented treatments to reduce hazardous fuels produced by nonnative grasses, but the broad extent of grass invasion challenges efforts to conserve native species (Abella, 2015). These examples show the importance of assessing the status and conservation needs of park namesakes and priority natural features for effective conservation.

In this study, we focused on Arizona walnut (Juglans major), the namesake of Walnut Canyon National Monument, Arizona (Fig. 1). Little information was available on the population size of Arizona walnut trees in the monument, the health of the population, and whether a range of size classes existed indicative of recruitment potential. Our objectives were to 1) conduct a census of the population of Arizona walnut in the monument, 2) determine the size class distribution of the population, and 3) evaluate tree health.

The U.S. Government established Walnut Canyon National Monument in 1915 to conserve cliff-dwelling structures constructed between 1125 and 1250 by people of the Sinagua culture along the walls and rim of Walnut Canyon (Clark, 1968). Since 1934, the monument has been managed by the National Park Service as a national park unit. The 1, 444-ha monument is 10 km southeast of Flagstaff, in northern Arizona. The canyon is 13 km long and 0.5 km wide rim to rim, with the canyon floor and Walnut Creek about 160 m lower in elevation than the rims. Slope gradients along the canyon walls are steep, with areas of vertical rock and most gradients exceeding 25%. Major soils in the canyon are derived from Kaibab limestone, Toroweap sandstone-limestone, and Coconino sandstone geologic formations and are classified by Harrigan et al. (2015) as Ustifluvents on the canyon bottom and Ustorthents on the canyon walls. Based on a weather station along the Walnut Canyon rim (elevation 2, 040 m), annual precipitation averages 46 cm, with half falling as late summer monsoonal rain and much of the remainder as snow (157 cm/year of snow; 1951-2016 records; Western Regional Climate Center, Reno, Nevada, Daily high temperatures average 30[degrees]C in July and 7[degrees]C in January (2000-2016 available records). Daily low temperatures average 14[degrees]C in July and -7[degrees]C in January.

Forest composition changes from the canyon rims to the bottom. Pinyon pine (Pinus edulis), Utah juniper (Juniperus osteosperma), alligator juniper (Juniperus deppeana), one-seed juniper (Juniperus scopulorum), and ponderosa pine (Pinus ponderosa) predominate along the rims and upper canyon walls (Menzel and Covington, 1997). in topographically protected areas and along the canyon bottom, major tree species, along with Arizona walnut, include box elder (Acer negundo), narrow-leaf cottonwood (Populus angustifolia), willows (Salix lasiolepis and Salix laevigata), Gambel oak (Quercus gambelii), quaking aspen (Populus tremuloides), Douglas-fir (Pseudotsuga menziessii), and ponderosa pine (Joyce, 1976). Understories include diverse mixtures of shrubs, such as wild rose (Rosa woodsii) and New Mexican locust (Robinia neomexicana), forbs and perennial grasses, and obligate wetland herbaceous species near seeps and along Walnut Creek (Joyce, 1976).

During the summers of 2011 and 2012, we systematically surveyed a 130-ha study area, defined as Arizona walnut habitat along 13 km of Walnut Creek in a strip about 100 m wide including side canyons, by walking transects 5 m apart. As a complete census, we tagged every Arizona walnut tree observed and measured each tree for its location using a global positioning system (GeoXT, with submeter accuracy; Trimble, Sunnyvale, California) in Universal Transverse Mercator North American Datum 1983. We measured the height of each individual Arizona walnut using a measuring pole for seedlings (<1.4 m tall) and saplings (1.4-4.5 m tall) and a digital rangefinder (TruPulse 360B, Laser Technology, Inc., Centennial, Colorado) for trees (>4.5 m tall). We measured diameter at a height of 1.4 m with a diameter tape for each tree. For multistemmed individuals, we measured diameter on the largest stem and considered multiple stems to be a single individual. We recorded vigor of each tree as one of five categories: very low (<15% live branches; remainder dead), low (15-35% live), medium (35-65% live), high (65-85% live), and very high (85-100% live). We took two photos of each individual, with one from the upstream side and one from the downstream side. The study was designed as a complete census of Arizona walnut trees, so we tabulated population counts of the number of trees by height, diameter, and vigor classes.

We counted 2, 065 Arizona walnut trees within Walnut Canyon National Monument. Except for small gaps, Arizona walnut was distributed throughout the canyon bottom along Walnut Creek (Fig. 2). Trees were distributed across all height and diameter classes (Fig. 3). At the extremes, 53% of trees were in seedling-sapling height classes ([less than or equal to] 4.5 m tall), while 3% of trees exceeded 15 m in height to the maximum recorded height in the monument of 23 m. For diameter, 78% of trees were smaller than 20 cm, while 3% of trees exceeded 46 cm in diameter up to the maximum recorded diameter of 103 cm. Trees generally exhibited healthy characteristics. Over 80% of trees were in at least the medium vigor category, and only 3% of trees had very low vigor.

While Arizona walnut trees were well distributed spatially along Walnut Creek in the canyon bottom, some gaps as well as dense clusters were evident. Gaps were associated with the likely driest sections where the canyon was wide with minimal shade from the walls (Joyce, 1976). illustrating potential correlates with tree distribution in a microclimate analysis of Walnut Canyon, Joyce (1976) found that June-August relative humidity averaged 38% in protected canyon bottoms, compared to 31% on exposed south-facing slopes. Average maximum daily air temperature was 32[degrees]C on south-facing slopes, compared to only 27[degrees]C on protected canyon bottoms (Joyce, 1976). Seed size, germination proportion, and seedling survival increase in Arizona walnut in moist sites and years (Stromberg and Patten, 1990). Seeds mature in fall and usually germinate the following year during late summer (August-September) monsoonal rains (Stromberg and Patten, 1990). Seedling mortality is a major limiting factor in Arizona walnut recruitment, with most seedlings dying in early summer, in May-June, often the driest months (Stromberg and Patten, 1990). In Walnut Canyon, the maximum air temperature in June on canyon bottoms was only 26[degrees]C, compared to 39[degrees]C on a south-facing slope, including a 7% higher relative humidity on the canyon bottom (Joyce, 1976). This indicated sharp differences in moisture stress during this critical month.

The greatest concentration of Arizona walnut trees was around the Island Trail in the center of the monument. This area contains among the highest concentration of cliff dwellings and evidence of ancient human use, though it is difficult to attribute Arizona walnut distribution to human use. In comparing flora of three ancient habitation sites with one control site within the monument, Clark (1968) hypothesized that certain plants were associated with ancient human habitation sites but that this affinity was likely related to favorable environmental site conditions or dispersal by nonhuman means. Arizona walnuts could have been a food source utilized and promoted by prehistoric humans, but other species also utilize and disperse Arizona walnuts (Stromberg and Patten, 1990).

The size class distribution, together with the proportionally high vigor ratings of trees, suggested that the monument's current Arizona walnut population is generally healthy. All height and diameter classes were represented, and there was no evidence of particularly poor health in either the smallest or largest trees. For example, none of the 70 largest trees ([greater than or equal to] 46 cm in diameter) had a very low vigor rating, and 86% of the largest trees had at least medium vigor. Similarly, of the 1, 211 trees of seedling size (<8 cm in diameter), 77% exhibited at least moderate vigor, and only 5% had very low vigor.

Placing the current population status of Arizona walnut into a longer-term context of possible change is challenging without additional data on age structure. Humans have altered the hydrology of Walnut Creek, by building two dams (in 1904 and 1941) 10 km upstream of the monument to create Lower and Upper Lake Mary for Flagstaff's water supply, and one downstream (Santa Fe Dam), in 1885-1886, for the railroad. Brian (1992) noted that before 1904, Walnut Creek likely had a reliable seasonal flow in spring fed by snowmelt, with some flow in warmer months depending on rainfall, and pools of water and wetlands likely more persistent. However, since 1941, sustained flows (weeks) in Walnut Creek have mainly only occurred after both lakes filled during wet winters (Brian, 1992). Potential effects of these hydrological alterations to riparian biota are poorly understood, but might have increased abundance of Arizona walnut. Repeat photography in the monument indicates that riparian vegetation has increased since the early 1900s, possibly because the dams tempered the flooding and scouring of the creek channel that had kept the canyon bottom more open (Brian, 1992). Riparian vegetation also increased in many locations of Grand Canyon National Park after dam building on the Colorado River (Sankey et al., 2015). Further data collection to identify the age structure of Arizona walnut might improve understanding long-term trends in population change.

With a population of over 2, 000 Arizona walnut trees and most exhibiting good health, Walnut Canyon National Monument's namesake species does not appear currently threatened. However, continued monitoring is warranted, especially given emerging forest health problems in other parks, including those with other Juglans species. For example, in eastern forests, butternut (Juglans cinerea) has experienced over 90% mortality from butternut canker caused by the nonnative fungal pathogen Ophiognomonia clavigignenti-juglandacearum (Parks et al., 2013). In Great Smoky Mountains National Park, butternut recruited continuously during the 1900s, until abruptly declining in the 1980s (Parks et al., 2013). This situation with butternut is representative of forest health challenges in many other parks. At least 80, and likely several hundred, species of nonnative forest pests (pathogens and insects) inhabit U.S. national parks (Fisichelli et al., 2014). Continued assessment of forest conditions is prudent for assisting with ongoing conservation of Arizona walnut and associated species.

We thank B. Hetzler and B. Rasmussen for help with fieldwork; M.P. Whitefield, M. Jones, K. Gaiz, and B. Hansen of Flagstaff Area Monuments for GIS support and assistance with the project; and the Associate Editor and an anonymous reviewer for reviewing the manuscript.

Literature Cited

Abella, S. R. 2015. Conserving America's national parks. CreateSpace, Charleston, South Carolina.

Barrows, C. W., and M. L. Murphy-Mariscal. 2012. Modeling impacts of climate change on Joshua trees at their southern boundary: how scale impacts predictions. Biological Conservation 152:29-36.

Brian, N. H. 1992. Historical review of water flow and riparian vegetation at Walnut Canyon National Monument, Arizona. Cooperative National Park Resources Studies Unit Technical Report NPS/WRUA/NRTR-92/44.

Clark, A. B. 1968. Vegetation on archaeological sites compared with non-site locations at Walnut Canyon, Flagstaff, Arizona. Plateau 40:77-90.

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Fisichelli, N. A., S. R. Abella, M. Peters, and F. J. Krist. 2014. Climate, trees, pests, and weeds: change, uncertainty, and biotic stressors in eastern U.S. national park forests. Forest Ecology and Management 327:31-39.

Harrigan, J. M., M. W. Burney, H. A. Hosler, and J. M. Puttere. 2015. Soil survey of Walnut Canyon National Monument, Arizona. U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, D.C.

Joyce, J. F. 1976. Vegetation analysis of Walnut Canyon, Arizona. Journal of the Arizona Academy of Science 11:127-133.

Menzel, J. P., and W. W. Covington. 1997. Changes from 1876 to 1994 in a forest ecosystem near Walnut Canyon, northern Arizona. Pages 151-172 in Proceedings of the Third Biennial Conference of Research on the Colorado Plateau (C. van Riper and E. Deshler, editors). U.S. Department of the Interior, National Park Service, Washington, D.C.

Parks, A. M., M. A. Jenkins, K. E. Woeste, and M. E. Ostry. 2013. Conservation status of a threatened tree species: establishing a baseline for restoration of Juglans cinerea L. in the southern Appalachian Mountains, USA. Natural Areas Journal 33:413426.

Sankey, J. B., B. E. Ralston, P. E. Grams, J. C. Schmidt, and L. E. Cagney. 2015. Riparian vegetation, Colorado River, and climate: five decades of spatiotemporal dynamics in the Grand Canyon with river regulation. Journal of Geophysical Research: Biogeosciences 120:1532-1547.

Stromberg, J. C., and D. T. Patten. 1990. Seed production and seedling establishment of a Southwest riparian tree, Arizona walnut (Juglans major). Great Basin Naturalist 50:47-56.

Swetnam, T. W., C. H. Baisan, A. C. Caprio, P. M. Brown, R. Touchan, R. S. Anderson, and D. J. Hallett. 2009. Multi-millennial fire history of the giant forest, Sequoia National Park, California, USA. Fire Ecology 5:120-150.

Submitted 3 March 2017. Accepted 5 May 2017.

Associate Editor was James Moore.

Charles D. Schelz, Douglas A. Scher, Tibor Vegh, and Scott R. Abella *

National Park Service, Flagstaff Area National Monuments, 6400 North Highway 89, Flagstaff, AZ 86004 (CDS, DS, TV) University of Nevada Las Vegas, School of Life Sciences, Las Vegas, NV 89154-4004 (SRA)

Present address of CDS: Bureau of Land Management, Cascade-Siskiyou National Monument, 3040 Biddle Road, Medford, OR 97504

Present address of TV: Duke University, Nicholas Institute for Environmental Policy Solutions, Durham, NC 27708

* Correspondent:

Caption: Fig. 1-Views of Walnut Canyon National Monument, northern Arizona. Top: looking into Walnut Canyon; bottom: Arizona walnut tree (Juglans major; bright green foliage) along the bottom of Walnut Canyon. Top photo by S. R. Abella, 5 April 2016. Bottom photo by C. D. Schelz, 21 September 2011. (Color version is available online.)

Caption: Fig. 2-Distribution of Arizona walnut (Juglans major) trees in Walnut Canyon National Monument, northern Arizona.

Caption: Fig. 3-Arizona walnut (Juglans major) population including 2, 065 trees by size and vigor classes in Walnut Canyon National Monument, Arizona. Vigor ranges from very low (10% live stems) to very high (>90% live stems and <10% dead).
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Title Annotation:Notes
Author:Schelz, Charles D.; Scher, Douglas A.; Vegh, Tibor; Abella, Scott R.
Publication:Southwestern Naturalist
Geographic Code:1U8AZ
Date:Jun 1, 2017

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