Insect herbivory accelerates leaf-litter decomposition in Patagonian deciduous forests

CHANETON, E.C.; MAZÍA, C.N.; KITZBERGER, T.

Montreal, Canada

Congreso; 90th Annual Meeting of the Ecological Society of America; 2005

Ecologial Society of America

ABSTRACT- Forest insects may stimulate nutrient cycling through various mechanisms including frass deposition, enriched canopy throughfall and improved litter quality. These effects become apparent during insect outbreaks but might be unimportant at endemic herbivory levels. Since herbivory often varies along environmental gradients, effects on carbon and nutrient dynamics may also be overriden by local site factors. Nothofagus pumilio forests in northern Patagonia, Argentina, are exploited by different guilds of foliar insects along a west-east precipitation gradient. Higher damage by leaf miners and leaf tiers occur in dry forests (800 mm/yr), yet the quantity and quality of litterfall is greater in wet forests (3000 mm/yr), where dominant leaf chewers cause little damage. We hypothesized that litter decomposition is faster in wet forests, because abiotic conditions and leaf chemistry would enhance soil microbial activity, regardless of low herbivory. We established a reciprocal litter-transplant experiment between dry and wet forests (n=2 stands/forest). In each stand we placed mesh bags containing either leaf litter produced in situ or litter brought from the other end of the gradient. This allowed us to separate effects of site conditions and litter identity on decay rates (% mass loss). As expected, decomposition was higher in the wet forest. However, in both habitats, the dry forest litter decomposed at a faster rate than the wet forest litter (35% vs 15%). Leaf litter quality as measured by nutrient and lignin contents, and secondary chemistry, was greater for the wet forest, suggesting that other foliar attributes may be controlling decomposition. When freshly collected litter showing different natural levels of insect herbivory were incubated in the laboratory, we found that % mass loss increased with prior insect damage, especially in the dry forest litter. Insect damaged litter had lower toughness, higher N content and lower C:N ratios than the intact litter. We conclude that canopy insects dominating in these dry Nothofagus forests may facilitate litter processing by soil biota and thus contribute significantly to nutrient cycling.