Forest Health in the Apostle Islands, 2011–2021

In 2011, National Park Service scientists began long-term forest monitoring at Apostle Islands National Lakeshore. In 2021 they returned to the 48 original sites, added two more, and looked at what had changed in ten years.

A sampling quadrat around young Canada yew, a favorite food of white-tailed deer (NPS photo).

Why deer matter here

The plots span five forest types — hemlock-hardwood, aspen-birch, cedar, mountain maple-black ash and balsam fir — and three deer management zones.

Islands with no history of deer harbor plants and plant communities rarely seen elsewhere in the region, including Canada yew, a shrub that has largely vanished from the mainland because of fire, logging and heavy browsing by deer. It is most abundant on islands that have never had deer, and also common on islands where deer once lived but no longer do. The Park Service sets deer management goals for hunting on the islands, because heavy browsing can make the forest less diverse and less resilient and can wipe some species off the islands entirely.

What changed

• Tree regeneration. Many aspen-birch forests are now about 75 years old and dominated by maples. Hemlock and yellow birch are still regenerating in hemlock-hardwood forests, and yellow birch in balsam forests, but their densities have dipped since 2011.
• Fewer plants. Plant taxa richness and abundance fell across the park, especially in balsam fir forests (7.5 fewer taxa per site) and hemlock-hardwood forests (1.8 fewer). Declines were notable even in Zone 1, where deer have little effect: hemlock-hardwood sites there lost 3.6 taxa per site and mountain maple-black ash sites lost seven. There seems to be no single cause.

"Taxa" covers any level of classification — not just species but genera or families. Instead of fewer spotted coralroot orchids, for example, there might be fewer coralroots of every kind, or fewer orchids overall. The broader term is useful for plants, which have so many subspecies and varieties.

Bluebead lily, one of the study's four focal plants (NPS photo).

What might explain it

Earlier studies pointed to deer browsing, fire history, Canada yew cover, island size and location in the archipelago. The researchers add two more factors:

• Snowshoe hares. Hares were especially abundant on Devils Island, home to one of only four balsam fir sites — 0.977 hares per hectare, seven times the next-highest density, on South Twin Island. Few or none were found near the other three balsam fir sites, so the losses at Devils Island weigh heavily on the results for that forest type.
• Canopy closure. As trees grow, spreading branches gradually shade the forest floor, and sun-loving plants give way to shade-tolerant ones or bare ground. At one mountain maple-black ash site on Outer Island, next to an old landing strip, richness dropped from 63 taxa to 38 as a raspberry-and-blackberry understory gave way to open forest floor. Similar changes are under way in hemlock-hardwood and aspen-birch forests.

The park-wide decline makes deer effects hard to read. The four focal plants — wild sarsaparilla, rosy twisted-stalk, clasping twisted-stalk and bluebead lily — turned up at fewer sites and less often, but not in the pattern heavy browsing would produce, and plants also declined where there are few or no deer.

Canada yew frequency was unchanged at 25 of the 48 sites sampled both years, with mostly small ups and downs elsewhere. Larger declines appeared at all three Sand Island sites and one site each on Cat and Stockton Islands; notable increases at South Twin and one site each on Otter, Stockton and Outer Islands.

What managers can do

• Assess marten habitat — the structure of coarse woody debris in conifer forests where martens, endangered in Wisconsin, are known to live — to predict which other areas could suit them.
• Keep watching for invasive plants. None were found in the plots, but five non-native plant taxa turned up at a few sites in 2021.
• Track yellow birch regeneration. Small yellow birch saplings declined 32% park-wide, most sharply in hemlock-hardwood and balsam fir forests.

The park's forests are generally in good shape, but vigilance matters: maintaining a forest's structure and function is possible, while restoring its ecological integrity may not be.

A marten photographed by a remote camera in the Apostle Islands (NPS photo).

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network: "Forest Health in the Apostle Islands, 2011–2021," based on Sanders S, Kirschbaum J, Forest health monitoring at Apostle Islands National Lakeshore: 2021 field season, Natural Resource Report NPS/GLKN/NRR—2022/2488 (2022).