Isle Royale’s forest under browsing pressure: what monitoring from 2010 to 2019 found

The National Park Service calls Isle Royale's forests currently healthy, but it blames growing moose and snowshoe hare numbers for marked changes in the forest plant community. Resampling forest plots in 2019 that were first measured in 2010 showed fewer seedlings, fewer maple saplings and fewer of the herbs moose like, especially on the island's west end.

Balsam fir heavily browsed by moose. NPS photo.

How the forest is watched

Monitoring began in 2010, and in 2019 the team returned to resample the 52 original sites and eight additional plots. Each site belongs to one of five forest types: Sugar Maple-Yellow Birch, Cedar-mixed, Paper Birch-mixed, Mixed Conifer-Hardwood and Balsam Fir. A plot has three parallel 50-meter (164-foot) transects along which the team measures live trees, seedlings, snags, shrubs and herbs and records signs of browsing.

Measuring the diameter at breast height of a paper birch. NPS photo.

What changed

• Trees. Regeneration of trembling aspen, balsam fir and yellow birch remains impaired, especially on the west end, where heavy moose browsing means neither balsam fir nor aspen is common. Sugar maple is still abundant there, but sapling density fell 33% between 2010 and 2019. Paper birch forests are dominated by old trees with little hardwood regeneration, and in places dense bracken fern in the understory. In the central and eastern Mixed Conifer-Hardwood and Balsam Fir sites, the smallest aspen increased, probably because moose were fewer in 2006 to 2011.
• Seedlings. From 2010 to 2019 seedling density for all tree species fell 26.2% in Sugar Maple-Yellow Birch sites and 25.8% in Cedar-Mixed sites, and browsing on woody species was markedly higher in 2019, greatest on red maple and sugar maple.
• Herbs. Site-level herb richness fell 20% in Sugar Maple-Yellow Birch sites and 6.6% in Paper Birch-mixed sites. Herbs that moose prefer declined in both richness and abundance; on the west side their abundance fell 38% in Sugar Maple-Yellow Birch sites and 22% in Cedar-Mixed sites. The park's list of herbs moose prefer includes bluebead lily, starflower, rosy twisted-stalk, bunchberry, fireweed and wild sarsaparilla.

Bluebead lily, rosy twisted-stalk and starflower are three herbs moose prefer, beside sugar maple seedlings, another favorite. NPS photo.

Why

The monitoring team attributes the changes to growing herbivore populations. Moose numbered slightly more than 500 when sampling began in 2010, after about four years of low numbers, but as wolf numbers fell the population rose to an estimated 2,000 by 2019. Snowshoe hares also peaked in 2012 and 2013, which is thought to be an indirect result of few wolves. Hares eat many of the same plants over the same period, and the data cannot show which animal has the larger effect.

Snowshoe hare and moose browse many of the same plants. NPS photo.

Climate and invasive species

As air and water temperatures rise, Lake Superior will moderate them less, and boreal habitat will be lost, leaving the overstory uncertain especially in the east side's boreal forest and a band around the island's edge. The greatest invasive plant threat may be bird-dispersed buckthorns and honeysuckles, which could establish themselves far from trails before managers notice. The emerald ash borer, found in Thunder Bay, Ontario, and Houghton, Michigan, could kill the black ash that sometimes forms the only overstory in very wet places and so remove it completely.

What can be done

The team recommends keeping herbivore density low, studying the four moose exclosures to compare moose and hare browsing, and continuing to monitor for invasive plants, especially around the island's edge. Maintaining ecological diversity and function should help the island's resilience and reduce the chance of costly restoration.

Sources

• National Park Service, Great Lakes Network, "Forest Health on Isle Royale, 2010–2019" (https://www.nps.gov/articles/000/forest-health-on-isle-royale-2010-2019.htm), a summary of a park monitoring report; rewritten in hubnx's own words.