Wood decomposition following a perennial lupine die-off: A 3-year litterbag study

Woody debris is a conspicuous feature of many ecosystems and can be a large pool of stored carbon and nutrients. In the California coastal prairie, yellow bush lupines (Lupinus arboreus) experience mass die-offs, producing large quantities of woody detritus. Live lupines are fed upon by the stem-boring caterpillars of the ghost moth, Hepialus californicus, and outbreaks of ghost moths are one factor contributing to lupine die-offs. A common detritivore, the terrestrial isopod Porcellio scaber, frequently inhabits ghost moth tunnels in lupine wood. We used a litterbag experiment to test the hypothesis that H. californicus increases decomposition of woody lupine detritus by facilitating its use by P. scaber. Isopod access to wood was crossed with simulated ghost moth boring to measure the independent and interactive effects of these two arthropods on total mass loss, as well as on carbon, nitrogen, and lignin dynamics. Isopods initially colonized litterbags but were not more abundant on L. arboreus logs that had simulated ghost moth boring than on logs without boring. They were rare in litterbags collected at 12 months or later and had no effect on wood decomposition. Simulated ghost moth boring increased wood decomposition (P = 0.0021), from 50.5 to 55.1% mass loss after 3 years. This effect was likely due to increased surface area for microbial utilization of the wood. Lupine wood had an initial lignin content of 14.70 +/- 0.67%, but lignin did not appear to decompose during the 3 years of this study, and by the end of the experiment accounted for 32.6 +/- 1.12% of the remaining wood. Neither ghost moth boring nor isopod access affected lignin loss. Lupine wood from a die-off in 2002 was estimated to have contained three times more nitrogen per unit area than the yearly input of annual grass litter. The slow decomposition of lupine wood, however, restricts the rate at which nitrogen is released into the soil and results in the storage of carbon and nutrients in lupine wood for several years following such die-offs.