Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2010
Publication Date: October 15, 2010
Citation: Uselman, S.M., Snyder, K.A., Blank, R.R. 2010. Insect biological control accelerates leaf litter decomposition and alters short-term nutrient dynamics in a Tamarix-invaded riparian ecosystem. Oikos. 120:409-417. Interpretive Summary: The northern tamarisk beetle (Diorhabda carinulata) was released as biological control to reduce the extent of saltcedar in highly-valued riparian areas in the western U.S., but very little is known about the effects of its release on ecosystem processes. This study examined the impact of beetle herbivory of saltcedar on short-term nutrient cycling in the Great Basin Desert of northern Nevada. In the first year of beetle infestation, we found that beetle-affected trees produced leaf litter that had greater concentrations of nitrogen (N) and phosphorus (P). Because N and P are the most limiting nutrients for plant growth, the availability of N and P help determine site fertility. Beetle-affected trees also produced a greater quantity of leaf litter. Together, the greater quality and quantity of leaf litter resulting from herbivory suggests potential large increases in the amount of N and P that are deposited on the soil. Furthermore, leaf litter resulting from beetle herbivory decomposed faster and released more N and P as it decomposed, because of its higher nutrient content. Results of this study show that the introduction of the beetle as a biocontrol agent in a saltcedar-invaded riparian ecosystem has lead to dramatic short-term alterations in nutrient dynamics that could lead to changes in nutrient availability. If long-term changes in site fertility ensue, then biocontrol could have implications for potential restoration of saltcedar-invaded ecosystems.
Technical Abstract: 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 6 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.