The Effects of Nutrient Pulses and Community Diversity on Invasive Plant Success in Virginia
Heckman, R. W.. 2008. M.Sc. Thesis. Graduate Faculty, University of Virginia, VA
Abstract
What makes a community susceptible to invasion? Two
commonly cited mechanisms are
nutrient pulses and community diversity. In both of
these, biological differences between
native and invasive species can shift the community's competitive balance, especially
when one group more effectively utilizes a nutrient
pulse. These differences may play a
role in invasions into a variety of habitats includ
ing deciduous forests and tallgrass
prairie. As a biennial
Alliaria petiolata
(Garlic mustard, Brassicaceae) is active in
deciduous forest understories during autumn and win
ter, allowing it to utilize nutrient
pulses from leaf-litter before its competitors. I m
anipulated the timing of leaf-litter
addition (fall or spring) and simulated the nutrien
t pulse from decaying leaves with
artificial fertilizer addition. Fall nutrient-amend
ed plots had greater adult biomass than
plots receiving spring nutrient additions. In addit
ion, silique production per plant and
bolting stem was highest in fall litter-amended plo
ts. Bienniality may provide a unique opportunity for
A. petiolata
to spread through deciduous forest understories.
Tallgrass prairie is dominated by high nutrient-us
e-efficiency warm-season (C4)
grasses and is frequently invaded by less nutrient-
use-efficient cool-season (C3) forbs.
Physiological differences in phenology and nutrient
-use efficiency may explain the
invasion success of
Galium verum
(Rubiaceae)
,
a European C3 forb, in an experimental
meadow at Blandy Experimental Farm. Within each of
three warm-season grass diversity
levels (one, three or five species), I performed fo
ur nutrient manipulations: 1) depleted
nitrogen (excess carbon introduction), 2) ambient n
itrogen, 3) elevated nitrogen (fertilizer ii
addition) and 4) fire (early-season burn). I found
no effect of native grass diversity on the
abundance of exotic Galium,
nor did grass diversity significantly alter soil NO3
or NH4
.
Elevated nitrogen treatments produced significantly
higher
Galium
biomass and stem
density than depleted or ambient nitrogen treatment
s. The fire treatment significantly
reduced
Galium
biomass relative to all other treatments. As predi
cted, soil plant-available
nitrogen drove the pattern of
Galium
abundance, but early season fire inhibited
Galium
despite the nutrient release afterwards. Biological
differences between exotics and natives
in these systems may allow the invasive species to
exploit nutrient pulses more
effectively and expand their ranges.
