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Invasive plants can have substantial negative impacts
on native flora and fauna. To determine the full effects of English
Ivy (Hedera helix) and ivy removal on native vegetation, we conducted
an ivy removal study in the piedmont of Georgia. Ivy was removed
by hand or by herbicide, and half of each plot was restored with
native seeds. We then counted the number of seedlings present in
each plot bimonthly. To determine mechanisms for differences in
regeneration, we determined whether ivy has allelopathic effects
on seed germination and whether ivy influences the size and diversity
of the seed bank. Ivy did not affect seed bank or have any allelopathic
effects on seed germination. Ivy removal by pulling resulted in
the greatest abundance and diversity of seedlings and responded
most favorably to restoration efforts. Spraying was effective in
removal of the ivy but significantly lowered seedling abundance
and diversity and hindered any restoration efforts. Ivy control
sites had no seedlings germinate. Therefore, the method of exotic
plant removal and if any restoration work is performed can have
profound effects on the regeneration of native vegetation and should
be taken into consideration when any exotic plant removal projects
are undertaken.
Exotic species infestation has been recognized as
the second largest threat, after habitat loss, facing native plant
and animal species across the world because some introduced exotics
can outcompete the indigenous members of the community (Westbrooks
1998). The competitive ability of exotics is enhanced by factors
such as their resistance to native pathogens and faster reproduction
rates (Sakai et al. 2001). The ecosystems of the United States are
some of the world’s most imperiled since upwards of 50,000
exotic species have been transported to the United States since
its discovery and now cause an estimated $137 billion worth of damage
a year (Westbrooks 1998).
English ivy, Hedera helix L., is one such exotic species
that is rapidly naturalizing throughout the United States and many
parts of the world. English ivy is a member of the Araliaceae family
and occurs naturally throughout Europe and the British Isles (Thomas
1998). It was brought to the United States as an ornamental introduction
to be used as a vine and ground cover due to its quick growing nature,
and its ability to tolerate sun or shade, many types of soil, and
drought (Morisawa 1999). Ivy is found in many forests in the southeastern
piedmont because ivy is more likely to invade forests that have
been disturbed by man than forests that have never been disturbed
(Thomas 1998). The invasion of ivy into forested areas also is facilitated
by its vegetative reproduction and its seed distribution and sprouting.
With its pulp removed, ivy’s seed has an almost 100% germination
rate and starts to germinate within a couple of weeks of sowing
(Cleargeau 1992). Even within its native ecosystem, once ivy appears,
it can dominate a secondary woodland within 30 years (Harmer et
al. 2001). Once ivy has invaded, it is able to achieve dominance
in a forest ecosystem with its rapid growth, excellent shade tolerance,
root sprouting ability, climbing and spreading growth ability, and
possible allelopathic ability (Slezak 1976). Ivy forms a dense ground
cover that prevents the emergence of almost all herbs (Harmer et
al. 2001) and also grows up the trunks of trees in search for the
higher light intensities required for flowering (Edlin 1970). Once
on trees, ivy will speed their death because vines, when present
in large numbers, have been shown to increase the likelihood for
damage or death of trees in forests (Penfound 1966, Sicama et al.
1976, Thomas 1980). In fact, ivy infestation over time will lead
to a vine-dominated forest (Thomas 1980). Due to the known impact
of English ivy on native vegetation, methods of ivy removal and
their effect on forest restoration must be explored.
The purpose of this study was to measure the effectiveness
of different types of English Ivy removal and their effects on regeneration
of native vegetation in a southeastern piedmont forest. In this
study, the two most common types of ivy removal, hand pulling and
spraying, were tested. Derr (1993) has shown that a 25% glyphosphate
treatment coupled with a cutting of the ivy’s leaves to facilitate
herbicide absorption will result in an almost 100% removal of ivy
from the affected area. Neal and Skroch (1985) also have shown that
timing is important with an application in March being most effective.
However, herbicide use introduces a potential environmental contaminant
and is nondiscriminatory toward native foliage. However, the use
of herbicides will leave root structure, which can prevent soil
erosion and nutrient leaching. Yet, the presence of root structure
may keep possible allelopathic chemicals present in the soil. Also,
ivy’s dense vine overlap may continue to smother seedlings
even though the plant itself is dead. In contrast to spraying, hand
pulling removes all parts of the ivy, removing any potential effect
that ivy structure has. This technique requires a large amount of
person power and time to be effective since one person can usually
weed 10 square meters of ivy an hour (Freshwater 1991). However,
this method leaves large patches of the soil barren, opens the forest
floor to the effects of erosion and nutrient leaching since ivy
prevents the substantial growth of any other understory vegetation,
and has been shown to be not as effective as spraying (Costello
1986). Also, some effects of ivy while present need to be clarified.
It is unknown whether ivy has an effect on the size or the composition
of the seed rain or seed bank, both of which will contribute to
the regeneration of the forest once ivy is pulled (Hyatt and Casper
2000). McLachlan and Bazely (2001) have shown that plants with wind
and mammal dispersed seeds and summer flowering plants are most
likely to return to habitats that are disturbed, potentially permanently
changing the dynamics of a forest once ivy has been removed. Finally,
Hines (1995) found evidence for the allelopathic effects of ivy,
but Rice (1976) in a review of the topic did not. Therefore, ivy’s
potential allelopathic effects need to be explored more to see if
ivy can hinder forest regeneration even after it is removed.
Experiment 1- To determine if differing ivy removal
treatments will have different effects on the return and the restoration
of native vegetation and on the soil chemistry of the forest floor.
Ivy Plots Study site: Mature hardwood forest that is dominated by
Liriodendron tulipifera, Liquidambar styraciflua L., and Quercus
sp. Research area: Completely covered by ivy, with the presence
of other vegetation and trees avoided. Plots: 15 5m x 5m plots-
5 replicates/treatment. Treatments: (1) a control infestation where
ivy was not removed (2) a spray treatment where the ivy was cut
and treated twice with the herbicide Roundup (30% glyphosphate)
until dripping (3) a manual pull where ivy roots and shoots were
removed from the test plot by hand. Restoration Each plot was split
and one half has been restored with native seeds. Species of seeds
used to restore the forest were all native seeds that are able to
tolerate moderate shade and are found somewhere in the piedmont
forests of north Georgia. Seeds were spread evenly throughout the
experimental restoration area, at a density of 515 seeds/m2. Plant
Sampling Plot areas were monitored for seedling germination and
success bimonthly. All seedlings were classified to seedling species
and whether they were exotic or native. Soil Analysis Baseline soil
samples were collected immediately after the ivy treatments were
completed and every two months thereafter. Analyzed for pH, phosphorus,
potassium, nitrates, and total organic matter. Depth of topsoil
determined as the depth at which the first specks of clay appear.
Experiment 2- To determine if ivy releases any residual
allelopathic chemicals in the soil that may hinder seed bank germination.
Allelopathy Top 5 cm of soil was collected from areas where ivy
was present and not present and each type of soil was placed in
ten flats Seeds of Coreopsis lancelota were sown in each flat at
a rate of 64 seeds/flat Number of Coreopsis seedlings germinated
was recorded.
Experiment 3- To determine if ivy cover affects the
abundance and the diversity of the seed bank in the forest. Seed
bank 10 samples each of the seed bank of areas infested with ivy
and not infested with ivy Soil from each point was the spread to
a depth of less than 3mm on flats containing seedless, soilless
medium (MetroMix360) and allowed to germinate. Species and number
of seedlings were recorded.
Experiment 1- Ivy Removal Effects Seedling Germination
(Figure 1) No seeds germinated on sites where ivy was left. Seedling
germination increased dramatically on sites where ivy was pulled
and increased on sites that were sprayed. Restoration significantly
increased the number of seedlings in pulled sites but was not effective
in sites that were sprayed Seedling Diversity (Figure 2) Pulled,
restored sites were significantly more diverse than pulled, non-restored
sites, but restoration had no effect on diversity in spray or ivy
areas. Diversity has significantly increased in pulled, restored
and pulled, non-restored sites over the summer while fluctuating
in the other treatments Tulip poplar and Virginia creeper most abundant
non-restored species Soil Analysis Potassium levels significantly
increased for spray while decreasing for Pull and Ivy (Figure 3a)
Phosphate level, nitrate level, and percent organic matter did not
vary among treatment over time. (Figures 3b, 4a, 4b) pH decreased
for all treatments (Figure 5a). Sample Depth significantly increased
between April and June for Ivy and Spray while decreasing for Pull
(Figure 5b).
Experiment 2- Ivy and Allelopathy Allelopathy No allelopathic
effects of ivy were apparent (Figure 6 )
Experiment 3- Ivy Effects on Seed Bank Seed Bank No
difference in abundance or diversity of seed bank for ivy and non-ivy
areas (Figure 7).
The restoration of native vegetation after ivy removal
is dependent on the method of removal and if any restoration occurred.
Hand pulling, though much slower and more time intensive than spraying,
results in a much more diverse and abundant seedling crop, which
will lay the foundation for future forest vegetation. Seedling crop
in area, though not directly affected by ivy infestation, is heavily
populated by trees, vines, and annuals. Restoration of native perennials
provides an opportunity for the understory level, which is missing
from the seed crop, to develop. Native vines, like Virginia creeper,
poison ivy, and muscadine, return quickly to ivy-free areas and
may hinder recovery (Penfound 1966, Sicama et al. 1976). Tulip poplar,
the most dominant species found in seedlings, has high mortality,
especially in shade (Skeen 1976). There are changes in soil properties
due to ivy removal and the method of removal makes a difference.
Ivy has very nutrient rich leaf litter and spraying allows the nutrients
to remain in area, which may explain higher potassium levels found
in study (Badre et al. 1998). This could be potentially important
in areas where soil fertility is low since the ivy could act as
a natural fertilizer. Pulling removes most of vegetation and disturbs
soil, which promotes erosion and compaction of the soil and may
explain the decrease in sample depths found in pulled areas. Large
scale ivy eradication efforts could magnify this effect and create
large scale erosion in the cleared area. All plots becoming more
basic may be part of seasonal cycle and does not appear to be linked
to ivy in any way. Ivy does not appear to have any residual allelopathic
effects on seed crop so growth in sprayed areas is probably hindered
by the thick carpet of dead ivy vines. Seed bank is also not directly
affected by ivy.
This material is based upon work supported by the
Howard Hughes Medical Institute under Grant No. 52003071 and a Faculty
Science Council nanogrant for undergraduate research.
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Invasive species, species that don't naturally belong
in an area but grow very well there, are a large threat to native
species. English Ivy is an example of an ivasive species. In my
study, I looked at different ways of removing ivy, pulling by hand
and by spraying with Roundup, an herbicide to see how ivy's removal
would effect the return of seedlings to a forest. I also looked
at the effect of restoration, or the seeding of an area with native
species, to see if it was worth the time and expense with both removal
treatments.
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