Hydrocotyle americana L.
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Basis for Listing
The circumstances of Hydrocotyle americana in Minnesota illustrate several features that are characteristic of special concern plant species. For one thing, it has been found only a few times even after highly directed and intensive searches. Furthermore, it is restricted to a rare habitat type that is found in only a small portion of the state. These habitats are small, sensitive wetlands that are typically imbedded in upland forests along streams or rivers, and they are often dependent on the local discharge of groundwater. Such habitats are known to harbor an unusually high percentage of local biodiversity, particularly locally or regionally rare species (Flinn et al. 2008). Hydrocotyle americana was listed as a special concern species in Minnesota in 1984.
Hydrocotyle americana is a small, inconspicuous, perennial forb. The stems are slender and tend to creep over moist or wet ground, rooting at the nodes. The leaf blade is more or less circular in outline and the petiole is attached at the base (not peltate). The blade is 1-5 cm (0.4-2.0 in.) across and has 6-10 shallow lobes on the margin. The flowers are whitish or greenish but they are so small they are rarely noticed. There are 2-7 flowers arranged in small, sessile umbels in the axils of the leaves. The fruits are basically circular in outline, strongly flattened, and about 1.5 mm (0.06 in.) across.
In Minnesota, H. americana is a wetland species restricted to a rather narrow range of habitat types in the east central, and possibly the southeast, parts of the state. Most occurrences are at the wet margins of small, cold, groundwater streams that emerge from small ravines within larger river valleys. These streams may broaden into open meadows or sedgy seeps with shallow pools. Such pools may have patches of bare muck as well as scattered rocks and logs laden with wet moss; all are excellent micro-habitats for H. americana. These wetlands are often inclusions in Fraxinus nigra (black ash) seepage swamps, and because they are fed by groundwater they tend to remain wet all summer, even in years of below-average rainfall when other small wetlands might become dry. These are very rare and localized habitats that are known to harbor a number of other rare or specialized plant species such as Carex bromoides ssp. bromoides (brome-like sedge), Chrysosplenium americanum (American golden saxifrage), Symplocarpus foetidus (skunk cabbage), and Poa paludigena (bog bluegrass).
Biology / Life History
The stems of H. americana are slender and tend to creep along the ground, rooting at the nodes. At the end of the growing season the stems die, but the roots are perennial and survive the winter. In this way H. americana can, under favorable conditions, create small but dense colonies, particularly in exposed habitats where there is little competition from other plants. In many cases, each colony is likely a single clone, and it is possible that all the individuals and colonies within a particular habitat may be fragments of a single clone. Hydrocotyle americana also reproduces by seeds, which are presumably dispersed on water currents. The seeds have been reported in soil seed banks (Hanlon et al. 1998), although the role of a seed bank in population dynamics is unknown.
Conservation / Management
The key to maintaining a population of H. americana is maintaining the hydrologic regime that supports its habitat. This includes the depth, duration, and timing of water fluctuations as well as the physical and chemical characteristics of the substrate. In most cases, all or a portion of the water will originate from a nearby groundwater seep. There may also be surface water run-off reaching the habitat, especially in the spring as the snow is melting and in the summer after a rain storm. The extent of the groundwater source (the recharge area) and the extent of the surface water source (the watershed) may not be readily apparent, but such knowledge may be critical when attempting to gauge the impacts of off-site projects.
Conservation Efforts in Minnesota
At least three populations of H. americana are known to occur in wetland habitats that are contained within State Parks. These are probably the most secure sites in Minnesota, and yet in each case the groundwater systems that support the habitats extend far outside the park boundaries. As a result, the populations could be vulnerable to a variety of activities such as road building and mining.
Flinn, K. M., M. J. Lechowicz, and M. J. Waterway. 2008. Plant species diversity and composition of wetlands within an upland forest. American Journal of Botany 95(10):1216-1224.
Hanlon, T. J., C. E. Williams, and W. J. Moriarity. 1998. Species composition of soil seed banks of Allegheny Plateau riparian forests. Journal of the Torrey Botanical Society 125(3):199-215.