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CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration Acer
platanoides, a superior competitor or a
novel intruder? Fang Wei (Department
of Ecology and Evolution, State University of New York, Stony Brook,
NY 11794-5245,USA) Abstract: Unprecedented high rates of biological invasion are
increasingly homogenizing the earth's biota, with serious consequences
both for humans and for natural communities and ecosystems. Invasions vary in the extent to which they alter the
structure, function and dynamics of natural ecosystems.
There are very few accepted generalizations about the
mechanisms with which invasive species cause the greatest ecological
effects. One of
them is the "superior competitor hypothesis" (SCH).
It predicts that invader that has a superior competitive
ability can cause a significant effect by drawing down resources to
very low levels, and may lead to competitive displacement and
numerical reductions in native species. An
alternative hypothesis is the "novel function hypothesis" (NFH).
It predicts that invaders with novel functions, such as
altering nutrient cycles within an ecosystem or altering the
disturbance regime would have significant effects. In this study, I used an invasive tree species, Acer
platanoides (Norway maple), to test the mechanisms of the impact
of invasion. I addressed
the difficulty of measuring interspecific competition in a forest
community by using an experimental design that assessed asymmetric
competition between existing adult trees and transplanted seedlings.
I then measured light availability, soil drying curve, N
mineralization rates over a growing season, and soil nutrient contents
in one-time soil samples from all experimental plots to examine the
potential ecosystem effects of Norway maple invasion.
I addressed two
groups interrelated questions in this study.
First, is competitive suppression happening under the adults of
the invasive species? In
another word, is SCH
supported by this case study? If
yes, is it through aboveground or belowground competition for
resources? If not, what
novel ecosystem effects are associated with the invasive species?
Second, what will happen to the recruitment (potential
succession) of the existing species assemblages in the invaded natural
community and the adjacent mono-specific stand of the invasive
species? Do the
functional groups and the indigenousness of species matter to their
response to the species invasion? By addressing these questions
simultaneously in the experiment, one can link the presence and
intensity of competition and other ecosystem functions to its
structuring power more thoroughly. The experiment was conducted in Muttontown nature preserve in Nassau
County, New York (40.79°N, 106.47°W), a mesic hardwood forest with sandy loam soil derived
from glacial moraine. The
experiment had a split plot design with the individual trees as
blocks. There were three
dominant-tree types (NM in the monospecific stand: NMmo; NM in a mixed
stand: NMmix, and RM in the native mixed forest: RMmix).
Eight adults, with comparable sizes and distances from each
other, were chosen under each dominant tree type, giving a total of 16
NM adults and 8 RM adults. Four
of the eight adults were randomly chosen for removal of the southern
half of the canopy. Under
each tree, three 1.5m ´ 1.5m plots were set up and assigned randomly with either
control, trenching, or litter-removal treatment. Within each plot, 40 seedlings of 8 species were randomly
transplanted with maximum distances from each other, giving a total of
2,880 transplanted seedlings. The
seedling species, i.e. the phytometer species, included 4 native
species dominant in the native mixed forest: Acer
rubrum (tree), Betula lenta (tree),
Parthenocissus. quinquefolia (vine),
and Maianthemum canadense (herb),
and 4 exotic species commonly found under the NM monoculture: A. platanoides (tree), L.
japonica (vine), Celastrus orbiculata (vine), and Alliaria petiolata (herb). All
seedlings were transplanted to the experimental plots in Muttontown
during May 2000. Survival
and aboveground heights of transplanted seedlings were measured after
2 months and 4 months following transplanting and measured again in
late June 2001 and late Aug. 2001 in the second growing season.
All living seedlings were harvested during late Aug.-early
Sep., 2001 before senescence. Stem
length, root vertical length and root horizontal length of each
seedling were measured. Leaves,
stems and roots were dried and weighted to the nearest milligram. Seedlings of six woody species altogether survived better, grew faster,
had higher biomass accumulation and higher Root/Shoot ratio under
RMmix than under NMmo and NMmix.
This illustrated that NM adults had strong competitive effects
on the performance of understory seedlings than RM adults did.
There was no significant interaction between canopy treatment
and dominant-tree type on seedling survival.
The confidence intervals of effect sizes on seedling growth and
biomass accumulation suggested that seedling growth under NM trees
responded to canopy removal more positively than that under RM trees.
This matched the pattern of canopy structure and light
availability in the experimental plots.
This suggested that aboveground part of NM adults had stronger
competitive effect for light over understory seedlings than that of
RM. Seedlings in trenched plots survived the best and grew the fastest, and
those in litter removed plots survived the worst and grew the slowest.
However, there was no significant interaction between
plot-level treatment and dominant-tree type on seedling survival.
Soil drying curves did not differ among dominant-tree types,
which suggested that soil moisture was not a limiting factor in this
experiment. Soil N
mineralization rates were different under NM trees from under RM
trees, but trenching or litter removal did not alter the pattern.
All these together suggested that NM adults did not obtain
their competitive superiority through belowground competition for soil
water or nitrogen. There was no detectable negative effect of NM litter over the
transplanted seedlings. Like
N mineralization and nitrification rates, most macro- and
micro-nutrient concentrations in the soil samples were different
between under NM trees and under RM trees, but not altered by either
canopy or plot-level treatments.
This suggested that the invasion of NM may have extensive
ecosystem effects on the recipient system other than through direct
competition for resources. Seedling survival, growth and allocation patterns of different species,
of different functional groups, or between native versus exotic
species, differed among different dominant-tree types and among
different combinations of experimental treatments.
The invasion of NM is likely to alter the community composition
and ecosystem dynamics of natural woodland profoundly.
Key
words: Acer platanoides;
invasive species; mechanisms of impact of invasion; asymmetric
competition; community composition; ecosystem functions
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