Forest & Grassland Health

Porcupine Damage

A porcupine in Sitka spruce.

 

A porcupine feeding in a Sitka spruce tree.

Erethizon dorsatum L.

Primary host(s) in Alaska:

Sitka spruce (Picea sitchensis)
western hemlock (Tsuga heterophyla)
birch (Betula spp.)

Damage: feeding damage to bark and cambium wounds or girdles trees, causing forked form, topkill and tree mortality
 

Photos

Click on the image for a larger version.

Sitka spruce crop trees killed by porcupines on Wrangell Island.

Sitka spruce crop trees killed by porcupines on Wrangell Island.

A managed stand on Wrangell Island severely damaged by porcupines. Credit: Greg Roberts, USFS.

A managed stand on Wrangell Island severely damaged by porcupines. Credit: Greg Roberts, USFS.

Low-altitude imagery showing porcupine-caused mortality on Mitkof Island.

Low-altitude imagery showing porcupine-caused mortality on Mitkof Island.

Teeth marks on a fresh wound caused by porcupine feeding.

Teeth marks on a fresh wound caused by porcupine feeding.

Damage to the lower tree bole and roots of Sitka spruce from porcupine feeding.

Damage to the lower tree bole and roots of Sitka spruce from porcupine feeding.

 

Current Status in Alaska (2018 Update)

In 2018, more than 2,500 acres of porcupine damage were mapped in Southeast Alaska, similar to the acreage mapped in recent years. Despite the relatively low mapped acreage, porcupine-girdling is a significant cause of spruce and hemlock mortality in managed stands on the Tongass National Forest (see Map). Damage is typically most severe in 10- to 30-year-old stands on Wrangell, Etolin, Mitkof and Kupreanof Islands and on the coastal mainland near major river drainages (e.g., Hobart Bay/ Port Houghton). From 2013–2018, silviculture staff on the Wrangell Ranger District installed stand examination plots to assess and track porcupine impacts, with the goal of identifying tree, site and composition factors associated with elevated damage.

Historic Activity

Porcupines are absent from many islands in Southeast Alaska, including Admiralty, Baranof, Chichagof, Zarembo, Revillagigedo and Prince of Wales (although, single porcupines and damaged trees are occasionally reported on Chichagof Island).

Topkill and mortality from porcupine-feeding is often most severe in managed stands that are 10 to 30 years old, particularly on Wrangell, Etolin, Mitkof and Kupreanof Islands and on the coastal mainland near major river drainages, such as Hobart Bay/ Port Houghton. Porcupine feeding can be locally concentrated in these young-growth stands, but typically tapers off over time.

Symptoms, Biology & Impacts

Porcupine damage to tree boles can be identified by the pattern of teeth marks. See photos above for examples. Damage is most common on western hemlock, Sitka spruce and birch trees. Feeding damage to spruce, hemlock, and birch boles leads to bole scars, top-kill, or tree mortality, reducing timber values but enhancing stand structure.

Porcupine feeding damage commonly occurs during the winter, when tree branches, twigs, and inner bark become an important component of their diet. This form of tree injury can provide thinning services in forests; however, porcupines usually target the largest, fastest growing trees and feed on clustered groups of trees. Feeding is intense in managed young-growth stands that are about 10 to 30 years of age and 4 to 10 inches in diameter. As stands age, porcupine feeding typically tapers off, but top-killed trees often survive with forked tops and internal wood decay as a legacy of earlier feeding.

Management

In some places, porcupines are the leading cause of spruce and hemlock crop tree mortality. Where porcupines cause substantial damage to timber resources, managers may thin to a tighter spacing between trees to accommodate anticipated loss of crop trees and to favor tree species that porcupines avoid, such as yellow-cedar and western redcedar.

Survey Method

Freshly girdled trees can be mapped by aerial survey. GIS tools, including low-altitude imagery, may prove useful for more accurately determining the impact and extent of damage in managed stands.

Distribution & Damage Map 

Damage to young managed stands is common on Wrangell, Etolin, Mitkof and Kupreanof Islands, and parts of the coastal mainland. Porcupines occur throughout much of forested Alaska, but are absent from several large islands in the Panhandle, including Admiralty, Baranof, Chichagof, Zarembo, Revillagigedo and Prince of Wales Islands. The distribution of porcupines suggests historic points of entry and migration from the major river drainages in Interior British Columbia to mainland Alaska and nearby islands. The fragmented island landscape of Southeast Alaska affects porcupine migration and colonization; porcupines are present in some mainland locations (e.g., Cleveland Peninsula), but conspicously absent from adjacent islands (e.g., Revillagigedo).
 

Click map image for larger version.

Porcupine damage to managed young-growth stands detected through ground and aerial surveys 2016-2018

Porcupine damage to managed young-growth stands detected through ground and aerial surveys 2016-2018. 


Links to Resource & Publications

Eglitis, A. and P. Hennon. 1997. Porcupine Feeding Damage in Precommercially Thinned Conifer Stands of Central Southeast Alaska. Western Journal of Applied Forestry 12(4): 115-121. Available here.

Hennon, P. 1986. Porcupine Damage on Mitkof Island. Technical Report R10-86-1. 21pp. Available here

Control of porcupine damage- Alberta Agriculture and Forestry webpage.

Porcupine Internet Center for Wildlife Damage Management- Cornell University webpage.

 

Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection, rlmulvey@fs.fed.us

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