Agriculture Reference
In-Depth Information
species may not have had previous exposure to, and sufficient time to develop
defensive mechanisms to cope with, the allelochemicals.
One observes at any given time a balance of multiple processes among both
positive and negative non-exploitative mechanisms that often can enhance plant
growth and competition. In contrast, exploitative interactions are by definition
negative. Some parameters, such as increased concentrations of atmospheric
CO
2
, can impose both exploitative and non-exploitative interactive mechanisms
simultaneously, and certainly these interactions vary physiologically among
species within the same communities. The balance between these competing
processes results in the net direction of effects of invasive species in the receiv-
ing ecosystem. The relative magnitudes of these processes are dynamic and
constantly changing - as a result, the regulating mechanisms controlling the
dynamics and net balance are difficult to quantify [44]. That complexity of
shifting dominance of interactive mechanisms in regulating competitive success
makes it difficult to develop predictive models to account for variations in the
strength of impacts of invasive species on the indigenous communities.
Invasive species clearly impact processes at the ecosystem level. Alterations
are induced in nutrient cycling [53-56], hydrology [57], and other processes.
Competitive exploitation for light is often a dominant mechanism of interac-
tion, particularly at high fertility and high plant densities. Among aggressive
invasive wetland plants (e.g.,
Typha, Juncus, Phragmites
, and similar emergent
plants), severe shading of incident light (70-90%) is a most effective competi-
tive strategy against native species [58-61]. However, in the early stages of col-
onization by such invasive plants, shading is not the dominant contributor to the
negative effects on the indigenous community. Other factors, such as exploita-
tion of limiting nutrients, can dominate competition [61]. Hence, dominance of
exploitative competitive interactions can shift very rapidly spatially and tempo-
rally as the dynamics of the species invasion proceeds in a new environment.
A relatively large area of ignorance centers on competitive belowground
interactions of invasive plant species with indigenous species. Complex inter-
actions occur among rooting tissues and rhizospheric microflora. The relative-
ly rapid rates of fine root turnover (hours to days) results in both active release
of root exudates as well as release of rooting tissue by fragmentation in vari-
ous stages of degradation with the release of inorganic and largely dissolved
organic carbon and nutrients. These nutrients and organic substrates are rapid-
ly assimilated and recycled by soil microflora to inorganic forms more readi-
ly assimilable by mycorrhizae and active roots. These two sources of poten-
tially important nutrient regeneration and repeated recycling are very poorly
understood and rarely quantified, particularly among wetland plants [7, 49]. It
is probable that accelerated aggressive growth of invasive plants owes part of
their competitive success to simultaneous active growth of roots with high fine
root turnover, root expansions, and efficient nutrient recycling.
A number of studies indicate marked variability in the types and periodici-
ties of organic exudates from fine roots. In the immediate rhizosphere of the
soil, plants perceive and respond to the chemical environment of the soil par-
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