Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 102

from anoxic reducing conditions in darkness to highly oxidized supersaturated
conditions within the sediments and detrital masses by photosynthetic activity
of attached microalgae. These changes can occur very rapidly upon receiving
light (shifting from anoxia to dissolved oxygen supersaturation and increase
several pH units within minutes) and result in marked alterations of rates of
sorbed nutrient releases and fluxes [88-90]. Beneath the understory of emer-
gent aquatic plants, the rates of decomposition of the detrital plant are marked-
ly influenced by redox conditions and fluctuating water levels [91]. As these
recycling processes increase, the capacity of the dominating invasive species to
increase was directly correlated, and presumably these processes enhance inva-
sive expansion and ability to compete with indigenous species.
Defensive mechanisms
Selection pressure from competition among wetland plants has led to the
development of numerous competitive adaptations. Submersed, verticillate
macrophytes, such as Hydrilla, Elodea and Myriophyllum spp, are capable of
rapid shoot elongation and sloughing of shaded leaves in response to reduced
light intensity [12, 92-94], adaptations that concentrate photosynthetic tissues
within the photic zone of water bodies. High rates of aboveground production
provide a competitive advantage through shading of nearby competitor species
(e.g., Typha latifolia, Juncus effuses and Hydrilla verticillata [59, 78, 95]).
Vegetative growth strategies often rely upon vegetative clonal growth with
reduction of sexual reproduction [96]. Many of these clonal methods of prop-
agation also function in perenniation or supplemental resource exploitation
(e.g., adventitious root formation and fragmentation in Myriophyllum ; [97]).
Wetland and aquatic macrophytes not only compete with other macro-
phytes, but also face competition from attached epiphytic microbial communi-
ties for both light and nutrients. There is some indication that various allelo-
pathic interactions exist between macrophytes and epiphytic microbial com-
munities, but only in a few cases is the chemical evidence compelling (see
reviews of Gross [98] and Ervin and Wetzel [94]).
Aggressive chemical mechanisms function effectively as well. For example,
certain invasive species can release chemical compounds that have allelopath-
ic effects on selective species of the indigenous plant community [52]. The
native species may not have had previous exposure to, and sufficient time to
develop defensive mechanisms to cope with, the allelochemical compounds
and rapidly capitulate to the invasive species.
Herbivory
Various toxic metabolites (phenolic compounds, terpenoids, alkaloids) are
well known to deter herbivory upon aquatic plants [94]. Some hormonal