Montana Field Guide

In northwestern Montana, wooded vernal pools occur from valley bottoms to montane elevations ranging from 866-1,585 meters (2,840-5,200 feet). Wooded vernal pools are small, shallow, circumneutral freshwater wetlands of glacial origin that partially or totally dry up as the growing season progresses. Pools are generally found on valley bottoms, lower benches, toe slopes, and flat sites. This system is well represented in the Seeley-Swan Valley in northwestern Montana. Depending on annual patterns of temperature and precipitation, the drying of the pond may be complete or partial by the fall. These sites are usually shallow and less than 1 meter in depth, but can be as much as 2 meters deep. The pool substrate is a poorly drained, often clayey layer with shallow organic sediments. Wooded vernal pools have a ring of trees surrounding the ponds that provide shade and influence their hydrology. The surrounding forest generally includes grand fir (Abies grandis), subalpine fir (Abies lasiocarpa), western larch (Larix occidentalis), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta), Douglas-fir (Pseudotsuga menziesii), black cottonwood (Populus balsamifera ssp. trichocarpa), and, to a lesser extent, quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera). In Montana, water howellia (Howellia aquatilis), a federally threatened species, is found only in wooded vernal pools. Other common species include water starwort (Callitriche heterophylla), inflated sedge (Carex vesicaria), common spikerush (Eleocharis palustris), and reed canarygrass (Phalaris arundinacea).

In northwestern Montana, these ponds and pools occur in forested environments. Occurrences are found from valley bottoms to montane elevations ranging from 866 to 1,585 m (2,840-5,200 feet) (Mincemoyer, 2005). This system is well represented in the Seeley-Swan valley in northwestern Montana. Pools are generally found on valley bottoms, lower benches, toeslopes, and flat sites, often in glaciated kettleholes that vary in size and depth.

Depending on annual patterns of temperature and precipitation, the drying of the pond may be complete or partial by the fall or during drought years. However, many of these ponds remain at fairly constant water levels throughout the growing season. These sites can be shallow and less than 1 meter (3.3 feet) in depth, but can be as much as 2 meters (6 feet) deep. The pool substrate is a poorly drained, often clayey layer with shallow organic sediments. Parent materials are typically clay alluvium or clay colluvium (Mincemoyer, 2005). These freshwater ponds have pH ranges from 6.2 to 7.8 with most measurements between 6.5 and 7.5. The size of the pools/ponds can range from .4 to 4 hectares (1-10 acres) in size.

The overstory surrounding vernal wooded pools is typically a mixed coniferous forest consisting of grand fir,, subalpine fir, western larch, Engelmannspruce,lodgepolepine, Douglas-fir, and deciduous trees like black cottonwood and, to a lesser extent, quaking aspen and paper birch. Western red cedar (Thuja plicata) often borders the ponds, especially in the northern Swan Valley. Common shrubs occurring in the forest edges surrounding the pools include thinleaf alder (Alnus incana), redoiser dogwood (Cornus sericea), buckthorn alder (Rhamnus alnifolia), and willows (Salix spp.).

The herbaceous component is dominated by graminoids such as shortawn foxtail (Alopecurus aequalis), water sedge(Carex aquatilis), beaked sedge (Carex utriculata), inflated sedge, common spikerush, and rushes (Juncus spp.). Other characteristic species include woolyfruit sedge (Carex lasiocarpa), awned sedge (Carex atherodes), and wooly sedge (Carex pellita). Reed canarygrass is invasive in this system.

Water starwort (Callitriche species), pondweeds (Potamogeton spp.), burr reed (Sparganium spp.), white water crowfoot (Ranunculus aquatilis), common mare’s tail (Hippuris vulgaris), bladderworts (Utricularia spp.), field mint (Mentha arvensis), and yellowcress (Rorippa spp.) are common herbaceous plant associates. Horsetails (Equisetum spp.) are often present. In Lake and Missoula counties, wooded vernal pools are habitat for water howellia, a federally threatened species. This annual aquatic may undergo dramatic yearly fluctuations in population size.