Resting structures and resting habitat of fishers in the southern Sierra Nevada, California

1. What they did

Fishers are forest members of the weasel family. In California they now live in less than half of their historic range, and the southern Sierra Nevada population is isolated, small and low in genetic diversity. Earlier work suggested that places to rest are the habitat fishers are most selective about. Fishers rest in trees, snags and logs for shelter from predators and weather, and they rarely use the same spot twice, so they need many good resting places across their home ranges.

From 1999 to 2001 the researchers live-trapped fishers in the Sierra National Forest, California, and fitted adults with radio collars. They used radio telemetry to find animals that were not moving, then walked in on foot to find the exact tree or other structure the fisher was resting in. A location counted when the fisher stayed while the observer pinpointed it, or was seen using the structure.

At each resting structure they measured the structure itself (size, height, tree species, decay of snags, mistletoe). They then measured the surrounding forest in a 15 m circle and a 1 ha circle: tree and snag sizes, logs, shrubs and canopy cover (with a moosehorn, a sighting tube pointed straight up). They did the same at 154 random sites in similar forest types and compared the two with statistical models to find which features best predict where fishers rest.

2. What they found

Resting structures

• They tracked 14 fishers and located 78 different resting structures used by 11 of them (57 by females, 21 by males), from 1 to 26 per animal.
• 76% were in live trees and 15% in snags (standing dead trees); 3 were in logs, 2 in stumps and 2 in rock crevices. All fishers resting close to the ground were found during rain or snow, or just after snow fell.
• Rest trees and snags were large, averaging over 95 cm in diameter. For 58 structures in trees or snags, 55% were the largest in the surrounding 15 m circle and 71% were the largest or second largest. The smallest trees used were 43 cm and the smallest snags 34 cm across.
• Live rest trees were 37% ponderosa pine, 32% white fir, 14% California black oak, 12% sugar pine and 5% incense cedar. Compared to the large trees available nearby, ponderosa pines were used more than expected and incense cedars less; there was some evidence that black oaks were favoured.
• 84% of the rest trees checked for mistletoe (oaks and cedars excluded) had obvious mistletoe infestations. Half of the snags used were in the most advanced stage of decay.
• Fishers rested in cavities, old squirrel nests or platforms, witches brooms and on large branches. All resting spots in black oaks were cavities.
• The same fisher reused a resting structure only 4 times (5%), all in live trees.

Resting habitat

• Compared to random sites, resting sites had higher canopy cover, more snags and hardwoods (by basal area), more variable tree sizes, smaller trees on average around the rest structure, steeper slopes, and were closer to streams.
• Canopy cover was the most important feature. Resting sites averaged 74% cover against 55% at random sites, and 97% of resting sites had more than 53% cover. The chance a site was a resting site rose with canopy cover up to about 50 to 55% and peaked at 72% (95% confidence interval 61 to 82%).
• Cutting canopy cover from 72% to 50% reduced the predicted chance of use by 16 to 46%; cutting it to 61% reduced it by only 3 to 12%.
• Males rested closer to ephemeral streams than females (29 m against 46 m on average); no other differences between the sexes were found.
• Different ways of measuring canopy give very different numbers: the same forest that reads 82 to 95% with a spherical densiometer reads about 65 to 80% with a moosehorn and 59 to 69% in the Forest Vegetation Simulator.

3. What they concluded

• Structurally complex forests with dense canopies, large trees, snags and hardwoods are key resting habitat for fishers in the southern Sierra Nevada.
• Unlike earlier studies, they found large rest trees surrounded by smaller trees. They link this to logging and fire suppression, which have turned forests of large, widely spaced trees into dense stands of smaller trees. The smaller trees still seemed to give enough canopy cover, as long as a suitable rest structure was there.
• Large trees and snags are probably limiting for fishers. Snags may matter for reasons beyond resting, such as prey.
• Nearness to streams and steep slopes may reflect a search for cool microclimates in a hot, dry climate; this needs more study, as do the direct effects of forest management on fishers.
• The authors caution that their results show which habitat features are associated with use, not which habitats fishers would choose if available or which are most productive.

6. Other technologies, skills or innovations

• Radio collars (Telonics) with a 16-hours-on, 8-hours-off cycle and a mortality sensor; handheld receivers and triangulation, then walk-ins on foot.
• Passive integrated transponder (PIT) tags, colour-coded ear tags and reflective tape for identifying individual fishers.
• Global Positioning System (GPS) for structure locations and Geographic Information System (GIS) layers for slope, aspect and distances to streams and roads.
• A moosehorn for canopy cover, and regression equations that convert canopy figures between moosehorn, spherical densiometer and Forest Vegetation Simulator methods so studies can be compared.

7. Implications for conservation (from the paper)

• Retain large trees, snags and logs, and support the growth of more large trees, while keeping dense cover and a complex vertical and horizontal forest structure.
• Consider keeping trees with defects and decay (multiple tops, rot, cavities) during thinning and fuel reduction. If large trees or snags must be removed for safety, leaving them on the ground may help. Large snags may need protection during prescribed burns.
• Keep large, decaying black oaks and thin around the best acorn producers to help oaks regenerate.
• Some loss of canopy cover is probably acceptable because canopy recovers faster than large trees and snags, but to keep suitable resting habitat, canopy cover should stay at no less than 61% by moosehorn (56% by the Forest Vegetation Simulator).