Insect biological control accelerates leaf litter decomposition and alters short-term nutrient dynamics in a Tamarix-invaded riparian ecosystem

Insect herbivory can strongly influence ecosystem nutrient dynamics, yet the indirect effects of herbivore-altered litter quality on subsequent decomposition remain poorly understood. The northern tamarisk beetle Diorhabda carinulata was released across several western states as a biological control agent to reduce the extent of the invasive tree Tamarix spp. in highly-valued riparian ecosystems; however, very little is currently known about the effects of this biocontrol effort on ecosystem nutrient cycling. In this study, we examined alterations to nutrient dynamics resulting from beetle herbivory in a Tamarix-invaded riparian ecosystem in the Great Basin Desert in northern Nevada, USA, by measuring changes in litter quality and decomposition, as well as changes in litter quantity. Generally, herbivory resulted in improved leaf litter chemical quality, including significantly increased nitrogen (N) and phosphorus (P) concentrations and decreased carbon (C) to nitrogen (C:N), C:P, N:P, and lignin:N ratios. Beetle-affected litter decomposed 23% faster than control litter, and released 16% more N and 60% more P during six months of decomposition, as compared to control litter. Both litter types showed a net release of N and P during decomposition. In addition, herbivory resulted in significant increases in annual rates of total aboveground litter and leaf litter production of 82% and 71%, respectively, under the Tamarix canopy. Our finding that increased rates of N and P release linked with an increased rate of mass loss during decomposition resulting from herbivore-induced increases in litter quality provides new support to the nutrient acceleration hypothesis. Moreover, results of this study demonstrate that the introduction of the northern tamarisk beetle as biological control to a Tamarix-invaded riparian ecosystem has lead to short-term stimulation of nutrient cycling. Alterations to nutrient dynamics could have implications for future plant community composition, and thus the potential for restoration of Tamarix-invaded ecosystems.