Round-leaved sundew is most often found in bogs, but also grows in
swamps, rotting logs, mossy crevices in rocks, or damp sand along
stream, lake, or pond margins [31,37,39,52,59]. It is generally
associated with sphagnum mosses and grows on floating sphagnum mats or
sphagnum hummocks [8,29,32,37,50]. It may also grow on peat soils of
other bryophyte or of graminoid origins . In the northern part
of its range the sphagnum bogs in which round-leaved sundew grows are
generally found surrounding glacial lakes. In the Appalachians from
Pennsylvania to Alabama, the bogs are most often at confluences of
springheads, around seeps, or along streams rather than lake margins.
The same is true for sphagnum bogs of the southeastern coastal plain,
but there round-leaved sundew may also grow in grass-sedge bogs. In the
Pacific Northwest, sphagnum bogs are typically found along streams and
occasionally develop around high elevation seeps and shallow lake
margins in the northern Rocky Mountains [3,45].
Round-leaved sundew is usually confined to sites with a high water table
or high precipitation and humidity . It requires continually moist
or wet situations . Round-leaved sundew grows in organic acid soils
that are low in available nutrients (nitrogen and phosphorous, and
calcium [3,24,49,54]. According to Crowder , the normal range of the
water table on sites where round-leaved sundew grows is from 1 inch (2
cm) above to 16 inches (40 cm) below the soil surface. Flooding can be
tolerated for several weeks, but dry periods can only be tolerated for a
very short time. Lloyd  reported that it is not found on limestone
soils; high calcium concentrations may be toxic to the plant.
Round-leaved sundew grows in sedge meadow communities of the Huntingdon
Marsh in Quebec on peat underlain by clay at 24 inches (60 cm) or more.
The soil surface is slightly above or up to 10 inches (25 cm) below the
water table . Round-leaved sundew has been reported as growing on
sites ranging from neutral pH (7.3) to very acidic (3.2) [18,38].
Acidic soils with low nutrient concentrations (nitrogen, phosphorous, or
calcium) seem to be the most common substrate [2,11,38,49,61].
In British Columbia, round-leaved sundew is an indicator of wet to very
wet, nitrogen-poor soils in boreal, cool temperate, and cool mesothermal
climates. It is associated with sphagnum moss in nonforested,
semiterrestrial communities .
An atypical site was found on Ile Perrot, Quebec, where round-leaved
sundew was growing on moderately dry, abandoned pastureland that
originally had been a swamp. The soil was well-drained loamy sand with
an average pH of 6.1. The site was "basically infertile" with extremely
low calcium and nitrogen concentrations .
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