Shoreline habitats around water bodies with fluctuating water levels present challenging conditions for most plants. Such habitats occur along the shores and in the shallows of man-made reservoirs and natural depressions containing seasonal lakes. These habitats may be either dry and exposed for several years in succession during periods of drought, or continuously inundated during periods of above average precipitation.
Some plants readily tolerate annual cycles of inundation and exposure, but fewer can thrive where there is both multi-year dewatering and multi-year inundation. Examples of such habitats in northern Mono County include the natural shallow seasonal lakes on Dry Lakes Plateau, and the shallows of Bridgeport Reservoir and Grant Lake Reservoir.
Surprisingly, some plants actually thrive in receding shoreline habitats, both natural and artificial. Some are annuals, some are perennials, some are native, some are introduced. They are most prevalent where the ground is nearly level or sloping only gradually; steeper slopes tend to be eroded, with thinner, less favorable substrates. The following highlights several species I’ve seen flourishing in these habitats during the recent series of drought years.
Taraxia tanacetifolia (Tansy-leaf evening primrose) can be the most abundant and colorful plant in receding shoreline habitats. In low-water years it creates a spectacular carpet of yellow across the exposed eastern shallows of Bridgeport Reservoir. Similar displays can occur across the beds of both seasonal lakes on Dry Lakes Plateau. These carpets of yellow can be seen from miles away.
Tansy-leaf evening primrose is a stemless, taprooted perennial with leaves that are deeply and irregularly pinnately lobed, 4 petals up to an inch long that open bright yellow and fade to orange, and a capitate stigma that extends beyond the 8 anthers. It looks a lot like Oenothera flava, but Oenothera flowers have a prominently 4-branched stigma.
Potentilla rivalis (Brook cinquefoil) can be abundant too, but not necessarily together with the Taraxia, and its yellow flowers are much smaller and less showy. It can, nevertheless, form a patchy to dense groundcover in strands along the east shore of Bridgeport Reservoir and on shallow flats near the south end of Grant Lake Reservoir. This is a widespread species, found in many ecoregions and in a variety of habitats across western and central North America. It isn’t restricted to receding shoreline habitats, but it’s well adapted to be very successful in some such areas.
Brook cinquefoil is an annual or biennial with ascending stems. Its leaves are palmate with 3–5 leaflets; it has open, branching (cymose) inflorescence with many flowers, but tiny petals barely 2 mm long. It is a prolific seed producer, and you are likely to see uncountable thousands of these tiny achenes scattered on the ground among the plants.
Potentilla newberryi (Newberry’s cinquefoil) is known mostly from the shallows and shorelines of natural seasonal lakes in northeastern California (Modoc Plateau), south-central Oregon, and northwestern Nevada. It’s been collected in west-central Nevada from Silver Lake in Washoe County south to a pond in the Pine Nut Mountains in Douglas County. In 2021 it was discovered along the sandy/gravelly eastern shore of Bridgeport Reservoir, with additional observations there in 2022. Also in 2022 it was found on the south shore of Lake Tahoe.
When flowering, this plant is clearly Potentilla-like (though it was originally described as an Ivesia), with pinnately compound leaves and prostrate to decumbent stems, and is unusual among cinquefoils of our region in having white petals.
Verbena bracteata (Bigbract verbena) looks unlike anything else in this area, with its many narrow-triangular, hairy bracts along many-flowered spikes on long, lax stems. This is another widespread (mostly western) North American native, usually found at pond or lake margins or other open, disturbed places. These photos show it at the south end of Grant Lake Reservoir.
Persicaria amphibia (Water smartweed) flourishes during periods of shallow inundation rather than drought, but clearly it survives the dry periods easily. These photos show it rooted in a sandy beach at Bridgeport Reservoir. When the water is high, this site is a few feet under water. When inundated, the plants have several ovate-elliptic leaves floating on the surface, and a terminal spike of bright pink flowers.
Artemisia biennis (Biennial wormwood) occurs in moist, disturbed sites across much of North America. It’s locally common in sandy soils at Bridgeport Reservoir, especially north of the boat launch at Ramp Road. The Flora of North America considers Artemisia biennis to be native in the northwestern United States and possibly introduced in other parts of its range; it is introduced in Europe.
The above is far from being an all-inclusive list of plants that can be found in these habitats. The list will vary from one lake or reservoir to another, depending on a variety of factors. The take-away for me, after looking at Bridgeport and Grant Lake Reservoirs particularly, is that the vegetation of receding shoreline habitats is not all weedy—some interesting and unexpected native plants are likely to be found there too.
Copyright © Tim Messick 2023. All rights reserved.