Agriculture Reference
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greatly increased loading of organic carbon to the soils both in particulate
detrital matter and in dissolved organic matter. Microbial utilization of that
increased organic matter will in turn increase recycling of nutrients and
enhance the competitive advantage of the invasive species.
Many successfully invasive emergent wetland plants exhibit rapid clonal
growth through rhizomes and large structural roots that have slower turnover
rates in comparison to fine roots. Nonetheless, detailed seasonal field studies
of growth of two species of Typha demonstrated that most of its new growth
occurs in the roots and that carbon allocation to the roots increased markedly
in nutrient-insufficient hydrosoils [74-78]. Not only was biomass of roots at
any given time considerably larger than that of rhizomes, but the much higher
turnover rates of roots in comparison to rhizomes led to at least an order of
magnitude, likely two, higher carbon allocation to roots than to rhizomes.
Many of the most aggressive and successful invasive plant species are those
exhibiting clonal reproduction and rapid growth, often under nutrient sufficient
(eutrophic) conditions. Additionally most of these plants exhibit continuous
population growth with the constant growth of new cohorts or with a number of
overlapping cohorts (e.g., [79] for Typha latifolia ; [80] for Juncus effusus ).
Accompanying such productive growth strategies is constant senescence of tis-
sues with large amounts of 'standing dead' tissues and accumulated litter of tis-
sues relatively resistant to rapid degradation. For example, the leaves (culms) of
the emergent rush Juncus effusus , a plant common to many littoral areas and
wetlands of lakes and streams, senesce from the leaf tip to the base at an expo-
nential rate (over 90-225 days), the rate of which is greater with increasing
temperatures seasonally [81]. The leaves of Juncus remain standing while dead,
a feature common among emergent wetland plants. Although fungal biomass
constituted 3-8% of the total detrital mass, decomposition was slow ( k = 0.40
yr -1 ), and senescent leaves lost about half of their biomass in two years.
Availability of water was a major factor affecting rates of fungal respiration
[82-83]. CO 2 evolution from the senescent tissues increased precipitously in
the evening with increasing relative humidity (>90%) and plant water potentials
(> -1.0 Mpa). Fungal respiratory rates were manifold higher during night and
early morning hours than during daytime on clear days. Throughout this long
period of aerial degradation of senescent tissues, appreciable recycling of nutri-
ents occurs by leaching of nutrients with rainfall and transport to the hydrosoils
for utilization and recycling by microbes and living plants (e.g., [84-85]).
Similarly, with the greater intensive growth of the invasive plant species, and
the common multiple, overlapping cohort production that is found among many
of these aggressive invasive plants, accumulations of senescent litter is often
greater than is the case among native vegetation. If wet or submersed, fungal
production is often reduced and supplanted by bacterial production [86, 87].
Here again, diurnal fluctuations of environmental conditions within the litter
detrital organic matter can be very great, largely keyed to the periodicity of
insolation and photosynthesis of both the macrophytes and the algal/cyanobac-
terial photosynthesis. For example, the redox conditions often rapidly fluctuate
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