Biology Reference
In-Depth Information
southern Canada (DiTommaso et al. 2005b; Lawlor 2000). BSW flowering tends to
peak in mid to late June, though in shadier sites it can be delayed by up to a month
(DiTommaso et al. 2005b). The swallow-wort species are self-compatible, but are
also insect pollinated by a variety of fly, ant, bee, wasp, and beetle species (DiTommaso
et al. 2005b; Lumer and Yost 1995).
Fruit development for PSW typically begins in early June in Central New York,
with maturity occurring 4-5 weeks after flowering, and finally dehiscence peaking
at the end of July; development in BSW is normally 2-4 weeks slower (DiTommaso
et al. 2005b; Lawlor 2000). There appears to be a physiological dormancy require-
ment for PSW seeds, and this likely applies as well to the black species. Though
minimal germination is often found experimentally with PSW without subjecting
seeds to a cold stratification period, greater germination occurs in seeds subjected
to a stratification treatment (Cappuccino et al. 2002; DiTommaso et al. 2005a).
While some studies have found a significant positive correlation between seed
size and the probability of germination in PSW (DiTommaso et al. 2005a), other
studies have reported no correlation between seed size and germinability (Cappuccino
et al. 2002; C.H. Douglass, unpublished data). Cappuccino et al. (2002), however,
did find that seed size in PSW was positively correlated with final dispersal dis-
tance, especially for seeds that had been subjected to a stratification period of
3 months. This effect was weaker in a later study (Ladd and Cappuccino 2005)
though the positive trend did generally hold true. While these authors also found
that larger seeds tended to produce taller seedlings during the first growing sea-
son, they concluded that there was a nominal advantage in survivorship associated
with a greater initial seed weight during three growing seasons.
Swallow-wort species produce polyembryonic seed, and estimates suggest that
45-75% of PSW seeds are polyembryonic (Sheeley 1992; St. Denis and Cappuccino
2004). Our own work indicates that the occurrence of polyembrony is much lower
in BSW in comparison to PSW, with the probability of a PSW seed being polyem-
bryonic roughly ten times greater than that for a BSW seed (C.H. Douglass, unpub-
lished data). Research suggests that polyembryonic seeds are more successful than
monoembryonic seeds in undisturbed habitats and in the absence of strong compet-
itors (which often occurs in disturbed areas) (Cappuccino et al. 2002; Ladd and
Cappuccino 2005). However, in a recent 3-year field study in central New York
State, Hotchkiss et al. (2008) reported that polyembryonically derived PSW plants
were not afforded a survival or growth advantage over plants derived from single
embryo seeds under both high and low light environments within a forest site.
PSW has a stout and often large root crown that produces perennating buds and
extensive, fleshly, fibrous roots (DiTommaso et al. 2005b). Many plants also pos-
sess a horizontal, woody rhizome, though this structure does not appear to substan-
tially facilitate dispersion of the plants (Cappuccino 2004; Weston et al. 2005). The
root-to-shoot biomass ratio of PSW can be substantial, up to 6.7 in New York soils
that contained beneficial arbuscular mycorrhizal fungal (AMF) species (Smith
2006). Root structures in BSW are similar but tend to be thicker and more fibrous,
and rhizomes in this species are reported to contribute more significantly to popula-
tion expansion (DiTommaso et al. 2005b; Lumer and Yost 1995). For example,
