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Sylvania's Flora

Vegetation history

Vegetation now growing in areas covered by ice during the Wisconsin Glaciation “overwintered” to the south and east of the ice sheet. Species with different temperature and moisture requirements survived in different regions. When the climate improved at the end of the glaciation, species generally followed the retreating ice, but at different speeds and in slightly different directions, so that forests continually evolved.

Pollen cores taken in Sylvania tell the story of vegetation changes during the past 10,000 years, the time period since the Wisconsin Glaciation (Davis et al. 1998, Brugam et al. 1997). The analysis of variations in annual tree ring width gives a more detailed picture of changes during the past several hundred years (Frelich 2002). The interpretation of vegetation changes in terms of climate change is, however, complicated by several factors. For example, there tends to be a time lag of several hundred to a thousand years between the occurrence of optimum climate conditions for a species and its arrival in an area because migration and colonization take time, and existing vegetation may compete. Once established, trees do not immediately die when the climate worsens but simply do not reproduce. Consequently, disturbances such as fire or wind that create openings in vegetation can have an important effect on the ability of species to occupy new environments (Frelich 2002). Another complicating factor is that vegetation responds to the “effective precipitation,” which is the precipitation minus evapotranspiration. Effective precipitation can decrease, for example, by either a decrease in precipitation or an increase in temperature. Furthermore, nonclimatic factors may play a role in the composition of the vegetation, such as the progressive evolution of soils through time and the passage from pioneer to climax species.

Development of a Conifer and Hardwood Mosaic

The first tree to arrive in Sylvania (about 9700 years ago) was spruce that stayed for perhaps 2000 to 3000 years before moving northward into Canada. Spruce was largely but not completely replaced by pine during the warmest period of the Holocene in North America: Jack pine and red pine arrived around 8000 years ago; white pine joined them about 1000 years later and eventually became the dominant pine species. Several hardwoods arrived at about the same time. By about 4000 years ago, patches of conifers (white pine) and patches of hardwoods (oak, with some sugar maple and red maple) dominated Sylvania’s forests.

A relatively dry climate existed during this period of mosaic development. Forest fires were frequent, as shown by the presence of charcoal in the pollen record. A change to cooler/wetter conditions resulted in a rapid rise in the regional water table between about 4000 and 3000 years ago (and a slower rise since then). This increase in moisture was most likely the cause of a significant change in Sylvania’s forests that began at about that time.

Hemlock Invasion

Hemlock, which had overwintered in eastern North America, had gradually extended its range northward, following the ice. The cooler/wetter conditions that followed the warmest period of the Holocene seem to have favored its westward migration. Hemlock arrived in Sylvania about 3100 years ago. The species composition of the existing forest, however, played an important role in the invasion pattern: Hemlock invaded only the conifer patches where it became the dominant species as white pine declined in abundance. Since hemlock is fire resistant, the frequency of fires decreased. This fire suppression together with wetter, cooler climate conditions also favored sugar maple, which began to expand within the hardwood patches, at the expense of oak, which benefits from fire.

Sylvania’s current forest is, therefore, a mosaic of conifer and hardwood patches that has its origin in the mosaic that existed prior to the hemlock invasion although the current patches are dominated by different species. The size of the hemlock and sugar maple patches fluctuated during the past 3000 years, but the patches have persisted due to a “positive neighborhood effect” caused by some differences in environmental conditions needed to establish seedlings:

Hemlock cones are among the smallest of the pine family, about 3/4 inch (2 cm) long, and the individual seeds are tiny. The radicle of the hemlock (the first part of a seedling to emerge from the seed and to become the root of the plant) is very small and quickly dries up unless the seed falls on moist soil or a rotting log. Hemlock seedlings are unable to establish themselves if the seed falls on a thick duff layer.

Sugar maple seeds, on the other hand, develop a strong radicle that has the strength and length to penetrate heavy leaf litter and reach the soil. Furthermore, sugar maple seedlings may suffer under a hemlock canopy where the soil is acid and nitrogen poor, and where it is dark in spring when they accomplish most of their yearly growth.

Hemlock and sugar maple trees have many similar characteristics and environmental requirements. They are both tall, reaching about 100 feet (31 meters). They grow best in a cool, moist climate. Both are shade tolerant and resistant to fires and do not differ in soil preferences. Nevertheless, the hemlock and sugar maple patches in Sylvania persist because the trees exclude each other’s regeneration.

[M.D. Davies and L.E. Frelich contributed to this article]

Plant migration

The migration after glaciation of a number of plant species can be seen by going to the pollen viewer, clicking the map on the left, and selecting a species (in the box at the top). The play button > lets you change the map 1000 years at a time while the play button >| gives you the present (modern) distribution.

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Rare plants documented in Sylvania

(Scientific name/Common namerare plants
Description and habitat
_ status)

Anzia colpodes/Black foam lichen
Foliose lichen on hardwoods
_ Regional Forester: sensitive
_ (proposed Michigan: threatened)

Calypso bulbosa/Fairy slipper
Small orchid in coniferous wetlands
_ Michigan: threatened
_ Regional Forester: sensitive

Cetraria aurescens/Yellow ribbon lichen
Lichen on tree trunks in old conifer swamps
_ Regional Forester: sensitive

Gratiola aurea/Golden hedge-hyssop
Small figwort on beaches & in shallow water
_ Michigan: threatened

Littorella uniflora/American shoreweed
Small plantain in shallow water
_ Michigan: special concern
_ Regional Forester: sensitive

Phegopteris hexagonoptera/Broad beech fern
Fern in rich moist hardwoods  
_ Regional Forester: sensitive

Viola lanceolata/Lance-leaved violet
Violet on lake shores
_ Uncommon

 

Photos by USDA Forest Service & Ottawa NF

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If you think you have spotted a rare plant, please do the following:

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Visitors interested in making a plant collection (common or rare species) need to contact the Ottawa National Forest (Sue Trull) to obtain a permit.

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