Cached fungi in non-native conifer forests and their importance for red squirrels (Sciurus vulgaris L.)

1. What they did

In the commercial conifer plantations of northern England and southern Scotland, red squirrels live at low density. These forests are mostly Sitka spruce, with no shrubs or ground cover, and tree seed crops vary a great deal from year to year. Fungi are one of the few other foods. Red squirrels in Finland were known to store mushrooms on tree branches, so the authors set out to see how often squirrels cache fungi in these plantations, and how much food the caches could provide.

They chose 11 sites in Wauchope Forest (Scotland) and Spadeadam Forest (part of Kielder Forest). Earlier surveys and feeding signs (stripped cones) showed red squirrels lived at every site. In October and November 1997 they walked a transect at each site twice, scanning the branches of two rows of trees on either side. They recorded the number and species of fungi on branches and, where possible, collected the fungi and checked them for tooth marks to confirm a red squirrel had put them there. They weighed a small sample.

2. What they found

• The squirrels placed fungi on branches right next to the trunk, from 0.6 m to about 8 m above the ground. They stored single mushrooms, or three to six mushrooms placed one by one at different heights in the same and nearby trees.
• Across the 11 transects they found 58 fruiting bodies with squirrel tooth marks: 49 common yellow russula, 7 sickener, 1 bare-toothed russula and 1 spotted tough-shank. This is a minimum, since caches higher in the crowns would have been out of view.
• The two most common cached species were also the most abundant fungi in the plantations. Ten other fungi species were seen, but none of them was cached.
• More fungi were cached in trees along forest edges and rides than inside the plantations.
• Most caches were short lived: the majority had gone within 2 weeks.
• Ten fruiting bodies weighed on average 8.15 g (wet).
• Scaled up to an average home range of 9.13 ha, the authors estimated at least 42 cached fruiting bodies per squirrel, worth about 525 to 714 kilojoules (kJ). That covers a red squirrel’s energy needs (330 to 420 kJ a day) for 1 to 2 days.

3. What they concluded

• Squirrels in these plantations live with large changes in seed supply from season to season and year to year, and their flexible home ranges, habitat use and movements seem to be a response to this.
• The many Russula caches most likely reflect how abundant these fungi were at that time of year.
• More caches at the edge may come from better drying conditions there, or from squirrels spending more time at the edge, where trees are known to produce bigger cone crops.
• The caches are vulnerable to other squirrels, other species and weather (strong wind, frost), so how many are recovered may vary.
• A more detailed study is needed to learn how important fungi are in the squirrels’ diet, and whether squirrels help spread fungal spores.

4. How tracking was or was not used

The study is a sign survey. The authors walked transects looking for one kind of sign, fungi stored on branches, and confirmed that red squirrels had cached them by checking for tooth marks. Squirrel presence at each site was confirmed partly by feeding signs (stripped cones). No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the tooth marks or how. Helen Thompson is thanked for help with fieldwork, and Gordon Beaks for help identifying the fungi.