Exotic herbivores indirectly decelerate litter decomposition via increased resistance to herbivory in exotic plants

The chemical components of exotic plants can change after invasion as they
adapt to local conditions. Studying these changes is important because
they can have a marked effect on ecosystem processes and dynamics. We
examined the leaf and litter chemistry of exotic goldenrods (Solidago
altissima) that invaded Japan from the USA approximately 100 years ago. We
investigated how changes in leaf chemistry caused by herbivory by the
exotic lace bug (Corythucha marmorata) affected litter decomposition rates
in three native (USA) and three exotic (Japanese) populations under
semi-natural experimental conditions. In both native and invasive
goldenrods, populations in areas where lace bugs were absent or present at
low densities had lower foliar phenolic concentrations (defensive
compounds) than populations in areas where lace bugs were abundant. The
observed pattern of reduced herbivory (i.e., stronger resistance) in lace
bug-abundant areas suggests that an increase in defensive compounds may be
involved, although the causal relationship was not directly examined in
this study. Except for one population, goldenrod litter was separated into
two groups: one produced nitrogen-rich but phenol-poor litter, and the
other produced nitrogen-poor but phenol-rich litter. Litter of the former
group decomposed more rapidly than that of the latter group, probably due
to higher nitrogen concentrations (mg/g), which had a significant positive
effect on decomposition, whereas phenolics showed no direct effect. High
phenolic concentrations indirectly affected decomposition rates by
decreasing litter nitrogen. Interestingly, the grouping of litter traits
was independent of the source region (i.e., native [USA] vs. introduced
[Japan]) and instead reflected variation in lace bug abundance across
goldenrod habitats. Resistance to herbivory may decrease in exotic
goldenrods after invasion, but when exposed again to herbivores from their
native range, resistance was restored. These findings suggest that exotic
plants subjected to different herbivory pressures can alter litter
decomposition rates through invasion-driven changes in litter chemistry.