Summer Bed Sites of Elk (Cervus elaphus) in the Black Hills, South Dakota: Considerations for Thermal Cover Management

1. What they did

Shade and shelter that help animals keep cool or warm are called thermal cover, and managers of elk range have long debated whether elk need it in summer. Some elk herds do well in open country with no shade, by being more active at night and less active by day. In Custer State Park, South Dakota, about 40% of the forest had burned in two wildfires (1988 and 1990), and around 1.5 million visitors arrive each year, mostly on summer days. The researchers expected that elk there would choose daytime bed sites mainly for hiding cover (cover that hides them from people) rather than thermal cover.

• Collaring elk. Elk were caught in salt-baited Clover traps in 1993 and 1995, and with a net gun from a helicopter in 1993, and fitted with radio collars.
• Finding bed sites (5 June to 30 August, 1994 to 1996). Each day one or two collared elk were chosen at random. A researcher homed in on the radio signal until the elk was seen, trying not to disturb it, then made the elk move off so the bed site could be measured.
• Measuring each site. They recorded air temperature at the bed, wind, slope and aspect (the direction the slope faces); ground cover (grass, forbs, shrubs, rock, litter, bare ground, wood) in small plots along four 5 m lines; the number, size and basal area of trees in a 0.01-ha plot; canopy closure overhead; and hiding cover with a checkered board at 25 to 100 m. Distances to roads, trails and water came from a Geographic Information System (GIS).
• Comparing with random sites. They measured the same things at 131 randomly chosen sites within the elk’s summer range, and compared bed sites with random sites.

2. What they found

The sample

• 131 bed sites of 26 elk (11 bulls and 15 cows): 44 in 1994, 48 in 1995 and 39 in 1996. Bed sites were found between 09:13 and 18:49.
• There were no differences between bulls and cows or between years, so all data were pooled.

Cooler, shadier places

• Elk bedded at temperatures from 15 to 36 °C. At the bed sites it averaged 24.8 °C, compared with 30.3 °C measured before homing in on the elk.
• Canopy closure averaged 54% at bed sites and 29% at random sites.
• Trees were present at 128 of 131 bed sites (97.7%) but at only 54 of 131 random sites (41.2%). Bed sites had more trees and more basal area (12.4 versus 4.9 m² per hectare).
• Elk chose north-facing slopes more than expected: 26% of bed sites faced north.
• Bed sites had less grass (15% versus 36%), more litter (57% versus 35%) and slightly more bare ground (3.9% versus 2.6%).

What made no difference

Hiding cover, wind speed, forbs, shrubs, rocks, wood, slope, average tree size, elevation, and distance to roads, trails and water were similar at bed sites and random sites.

Best predictors

Canopy closure, microsite air temperature and tree basal area together correctly sorted 86.2% of sites into bed sites or random sites.

3. What they concluded

• Contrary to their expectation, the elk appeared to choose daytime summer bed sites for thermal relief rather than hiding cover, picking the most thermally neutral sites available.
• Bedding itself may be part of how elk keep cool: by lying on bare ground and needle-covered surfaces, they may lose more heat by contact with the ground.
• Distance to roads and trails did not affect where elk bedded. The authors suggest the elk had become used to predictable traffic and that there was enough cover near roads and trails, although the elk were very wary of people.
• Elevation and slope, important for elk summer habitat in other studies, did not influence bed sites here. The authors think elevation differences in the park’s vegetation are too small to matter.
• The study did not look at topography, position on the slope, or which way elk faced relative to the wind, all of which may relate to security, so the hiding versus thermal cover question is not fully settled.
• Elk can thrive without thermal cover, but where it exists they appear to use it to cope with heat.

6. Other technologies, skills or innovations

• Mortality-sensing radio collars (Lotek Engineering), with elk located by homing in on the signal and confirmed by sight.
• SPANS GIS for elevation and distances to roads, trails and water, with roads and trails digitized from 1:24,000 United States Geological Survey (USGS) maps.
• Spherical densiometer for canopy closure; checkered density board for hiding cover; Daubenmire plots for ground cover.

7. Implications for conservation (from the paper)

• Thermal cover should be kept where it exists.
• Stands with basal area over 12.4 m² per hectare, over 110 trees per hectare and canopy closure over 54%, on northerly slopes, appear important for helping elk in the park cope with summer heat.