Saltcedar (Tamarix ramosissima) invasion and subsequent dominance in biologically and functionally diverse riparian ecosystems across the western U.S. has lead to release of the leaf beetle (Diorhabda elongata) as a biological control agent, and has resulted in large-scale rapid defoliation of saltcedar across several western states. Little is known about the effects of using this insect as biocontrol on ecosystem nutrient dynamics and the effect of multiple early defoliation events. However, changes in nutrient cycling, due to multiple defoliation events could impact the states and transitions of plant succession in these sites. In this study, we examined the effect of insect herbivory on quantity and quality of saltcedar litterfall and litter decomposition along the lower Truckee River, 35 miles NE of Reno, Nevada. Green leaf and freshly senesced litter was collected from beetle-affected trees and trees without beetles, leaf tissue was analyzed for differences in chemistry, and leaf litter was allowed to decompose in 1 mm mesh litterbags in the field for one year.
Results/Conclusions
Beetle-affected trees had reduced nitrogen (N) and phosphorus (P) leaf resorption efficiency before leaf drop, thus beetle-affected leaf litter quality differed significantly in several chemical characteristics correlated with rates of decomposition, including %N, %P, % lignin and C:N, C:P, and lignin:N, suggesting that beetle-affected litter should decompose faster than control litter. Quantitative collections of litterfall indicated that beetle-affected trees produced 82% to 71% greater quantities of total aboveground and leaf litterfall under trees (g m-2 y-1), respectively (P=0.001 and P=0.001). Due to changes in litter quantity and quality, estimates of annual inputs of N and P from leaf litterfall (g m-2 y-1) were 3 to 5 times greater under beetle-affected trees than control trees. Beetle-affected litter decomposed faster than control litter over a 6-month period, with 64 ± 1% mass remaining of beetle-affected litter versus 78 ± 1% of control litter (P<0.001).Our results show that biocontrol herbivory leads to short-term stimulation of nutrient cycling, but it is unclear whether these changes in nutrient dynamics will lead to long-term changes in site fertility. It is important to understand changes in nutrient dynamics due to biocontrol, due to the ramifications for potential secondary invasion versus restoration of these degraded systems.
