Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 100

ticles and associated microbes. Although a large range of organic and inor-
ganic substances are exchanged between the roots and soil, plants can quickly
modify their rhizosphere in response to environmental signals and stresses [62,
63]. For example, certain plants are capable of rapidly synthesizing, in
response to organic elicitors from pathogens, increasing concentrations of sol-
uble and wall-bound phenolic polymer organic acids and esters that function
as anti-microbial defenses in the root tissues [64]. Variance in defensive capa-
bilities certainly exists among species, and it is suggested that highly success-
ful invasive plant species potentially have superior capabilities in relation to
those of indigenous species.
It should also be noted that anthropogenically altered environmental factors
can induce changes in rates of the release and chemical composition of
leachates and root exudates from living and senescent tissues. Differences
among plant species in growth and biochemical responses to these climatic
changes likely confer advantages to many invasive plants that are highly suc-
cessful in competitive interactions with other species. For example, under ele-
vated atmospheric CO 2 (e.g., double ambient, 720 ppm) growth of many plants
can be accelerated, often leading to nitrogen limitation [65]. Increased carbon
uptake is utilized in secondary compound synthesis, and lignin and related
phenolic compounds often increase appreciably (double). C:N ratios and per-
centage lignin and total phenolic compounds were 15-100% higher in live and
senescent plant tissues in some wetland plants grown on elevated than in ambi-
ent CO 2 conditions [66-68]. As these plants senesce and slowly degrade,
leachates from those tissues grown on elevated CO 2 concentrations with more
recalcitrant organic compounds are degraded more slowly than those grown at
ambient concentrations. Similarly, there are indications that the stimulation of
growth by elevated CO 2 alters and enhances releases of root exudates and, as
a result, the metabolic activities of rhizospheric microbes and coupled nutrient
recycling rates. For example, elevated CO 2 -grown grasses effected a shift from
metabolism of older soluble carbon compounds to more easily degraded exu-
date compounds [69] or reduced utilization rates of carbohydrates, amides,
amines, carboxylic acids, and phenolic compounds [70, 71]. Similarly reduced
oxidation of polymers and more rapid utilization of carbohydrates, amino
acids and carboxylic acids occurred in soils of the rhizosphere of a shrub
grown under elevated CO 2 [72]. The findings are inconsistent, and suggest
considerable variability, even if one assumes the methodology employed
allows direct comparisons. In general, however, one would anticipate that
highly productive species, particularly growing in nutrient-rich habitats, would
allocate less carbon to rooting tissues. These plants, which include many suc-
cessful invasive species, can exhibit a greater intensity of CO 2 assimilation and
higher efficiency of conversion of CO 2 to organic carbon, with smaller carbon
losses to root respiration and exudates (see [73]). It is important to note, how-
ever, that although the direct root inputs to the hydrosoils may be reduced with
enhanced aboveground growth, the total organic matter of aboveground tissues
of the invasive species is often much increased which in turn will result in