Biology Reference
In-Depth Information
shown by the relatively low water tension levels recorded (
= −0.062 mPa at mid-
day and −0.079 at predawn) (DiTommaso et al. 2005b). This may be due, in part,
to its extensive root system and waxy leaf cuticle, suggesting that this species can
effectively tolerate environmental stresses that may reduce the vigor and perform-
ance of associated plant species (DiTommaso et al. 2005b).
Soils at sites invaded by PSW have been found to have greater AMF inoculum
potentials than adjacent, uninvaded sites (DiTommaso et al. 2005b; Greipsson
and DiTommaso 2006; Smith 2006; Smith et al. 2008). Swallow-wort plants also
showed significantly greater growth in the presence of locally associated micro-
bial communities than nonlocal communities. The authors proposed that by alter-
ing the species of mycorrhizal fungi at sites, swallow-wort could facilitate its
establishment and expansion by displacing resident flora dependent on native
fungal species.
Ψ
13.7.3
Habitat and Environmental Variability
In Central New York, PSW plants are normally found on shallower soils over lime-
stone bedrock or deep, well-drained silt-loam soils in wooded ravines, calcareous
cliffs, talus slopes, alluvial woods, pastures, and grasslands (DiTommaso et al.
2005b; Weston et al. 2005). PSW exhibits a wide tolerance to light and moisture
conditions, but appears to be particularly successful and aggressive on shallow,
droughty soils or deeper silt loams with partial to full sun (Lawlor 2002; Lawlor
and Raynal 2002; DiTommaso et al. 2005a; Smith 2006). BSW seems to share
comparable habitat preferences, but is often limited to sunny, open field sites rather
than shaded forest sites (Lumer and Yost 1995).
Shaded sites are characterized by greater densities of PSW, and taller plants with
longer internodes (Lawlor 2000; Sheeley 1992; Smith 2006). Smith et al. (2006)
reported a seasonal variation in seedling stem densities of PSW in a northern New
York State site, with an almost fivefold decrease between late July and August.
Moreover, growth and fecundity of both swallow-wort species is substantially
greater in open, sunny sites or gaps in the forest understory (DiTommaso et al.
2005a; Hotchkiss et al. 2008; Sheeley 1992). Seeds produced by plants at shaded
sites are significantly more likely to posses dormant or nonviable embryos (Smith
2006; Smith et al. 2006).
Our recent findings with regard to impact of soil type upon establishment of both
swallow-wort species suggest that soil pH and precipitation levels play significant
roles in influencing their success of establishment (Douglass 2008). Soil pH levels
in particular were significantly negatively correlated (
P
< 0.05) with overall plant
height of both species. Also, it appears that sites invaded by PSW were character-
ized by significantly (
P
< 0.05) high pH, calcium and magnesium levels than those
invaded by the black species. Further studies are needed to assess these factors and
their relationship to successful invasion with respect to both the swallow-worts and
other nonnatives in New York.
