Direct and indirect effects of insect herbivores on terrestrial ecosystem processes
Frost, Christopher John
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Herbivores can influence terrestrial ecosystem functioning through a number ofmechanisms. Deposition of waste products (‘frass’) introduces labile nitrogen (N) andother nutrients directly to the soil, potentially altering soil N availability. Herbivoresstimulate changes in the chemistry and nutrient allocation patterns of plant tissues, whichmay indirectly affect N uptake dynamics and ecosystem N cycling. These processesoccur rapidly and are “fast” cycle components of the terrestrial N cycle. Herbivore-mediated changes to the chemistry of foliage may alter rates of leaf litter decomposition,which is a “slow” cycle component of the terrestrial N cycle. This dissertation exploresthe direct and indirect effects of insect herbivores on “fast” and “slow” cycle N dynamicsin forest ecosystems. We first present evidence from a field mesocosm manipulation experimentdemonstrating that insect herbivores can facilitate the export of N from terrestrial toaquatic components of a watershed via feces (frass) deposition. This is the first study toprovide mechanistic evidence that aqueous N export from terrestrial systems can resultfrom frass deposition. Using the same experimental design, we examined the influenceof herbivore damage and frass deposition on foliar quality and litter decomposition. Despite expected herbivore-mediated changes to leaf chemistry, leaf litter decompositionrates were not affected by herbivore activity. Collectively, our results indicate thatherbivore damage and frass deposition affected “fast” but not “slow” cycle N dynamics.15We then discuss an experiment in which we generated and applied N-labeledfrass to oak mesocosms to explore the distribution and plant recovery of frass N. Surprisingly, oaks accumulated new N in their foliage throughout the growing seasondespite declines in total N as a result of senescence. Herbivore damage reduced therecovery and apparently the allocation of frass N to oak foliage, which affected the Nsupply to the next cohort of spring-feeding insect herbivores. In addition, only a smallpercentage of N in frass was exported via leachate, with rainfall as a likely factorinfluencing the loss. The majority of N in frass was retained in the surface soil,suggesting that surface soils are strong sinks for exogenous, labile N.1315Finally, we report the results of a dual-isotope (C and N) microcosmexperiment designed to examine the effects of foliar herbivore damage on belowgroundcarbon allocation (BCA) and N uptake. In the presence of herbivores, BCA in oakseedlings was lower and stem storage of new N was higher. The data are the first toshow that BCA in oaks is altered by foliar herbivores and have implications for C cyclingand sequestration in forest ecosystems.