Understanding whether and how exotic invasive plants
change ecosystems in ways fundamentally different from
native species can contribute to our understanding of plantsoil
microbe interactions, the role of secondary metabolites
in ecology in general, and the potential for regional
evolutionary trajectories in different parts of the Earth
(Callaway et al. 2005). This also is an important issue to
resolve for accurate evaluation of the biological and
economic impact of invaders. We have learned a great deal
about general ecosystems effects and some of the operating
mechanisms. However, little is known about the substantial
potential for long-term effects through exuded or litterbased
secondary metabolites on soil characteristics. Recent
work demonstrates that plants respond to environmental
conditions by releasing particular allelochemicals that can
play a role in chelation or solubilization of nutrients, but it
is not known how this mechanism or other mechanisms
differ between invaded communities and those in native
ranges of the same species. Active chelation of scarce
nutrients presumably may alter competitive relationships
among species, but the extent and importance of these
effects is not known. Little is known also about how
secondary metabolites may indirectly impact competitive
relationships through effects on soil microbial and other
processes. The literature demonstrates that invasive plant
dominance often is correlated with changes in nutrient
status and ecosystem processes in soils, but the relative
importance of secondary metabolites relative to mineral
composition in driving these effects is not well understood
and needs further investigation.
Numerous reports suggest the importance of the
indirect impacts of invasive species through the herbicides
used to control them. Herbicide applications can
alter plant secondary chemistry significantly, and impact
important symbioses with nitrogen-fixing bacteria and
mycorrhizae, but the consequences of these effects are
not yet understood. While several reports provide
evidence for differential decomposition of invasive plant
litter, the alterations in plant chemistry that have been
demonstrated in response to glyphosate applications
suggest that there could be a significant impact on how
litter from these plants is processed. The potential
interactive effects of herbicides and plant allelochemicals,
on plant-plant interactions and on soil microbiology
and chemistry, are largely unknown. The potential
impact of combinations of herbicides on soil microorganisms
and soil chemistry and interactions with
allelochemicals is another area where further study is
needed.
In summary, invasive plants have the potential to directly
affect soil chemistry and ecosystem function through the
direct effects of secondary metabolites in their tissues,
leachates, and exudates; and indirectly through chemical
attempts to control them. In this context, much is known
about highly local effects (e.g. in pots, common gardens, or
smallplots), but little is known about the scale at which
these effects function. Thus, scaling up from small scale
experiments to large scale field-based monitoring of these
sorts of effects will be a necessary step for a complete
understanding of the impact of invasive plants on soil
chemistry.