Decomposition of leaf litter in karri ( Eucalyptus diversicolor) forest of varying age

Abstract

Rates of weight loss and release of N, P, K, S, Ca, Mg, Na and Cl from decomposing leaf litter were measured in regrowth karri ( Eucalyptus diversicolor F. Muell.) forest aged 6, 9 and 40 years and in mature karri forest growing in south-western Australia. Decomposing leaf litter from the four sites lost 47–55% of original dry weight after exposure for 82 weeks on the forest floor. Rates of weight loss from leaf litter at the four sites varied in the order: mature<40-year-old=9-year-old=6-year-old-forest. A composite exponential function, with separate decay functions for labile and resistant litter components, explained 98–99% of the variation in mean dry weight loss in relation to time of exposure. Loss of labile litter components was rapid (half-lives 5–14 weeks). Loss of the resistant components was faster for litter from regrowth stands (half-lives 116–119 weeks) than for litter from mature forest (half-life 189 weeks). Slower rates of leaf litter decomposition together with increasing amounts of leaf and non-leaf (twigs, bark and fruit) litterfall as the stands mature indicate that the rate of accumulation of forest litter will be greater in mature forest than in regrowth stands. The order of element mobility in decomposing karri leaf litter (Na>Cl>K>S>Ca>N>P) was similar to that reported for other eucalyp litters. Nutrient content of fresh leaf litter, and rates of nutrient release during leaf litter decomposition, tended to be greater in the younger regrowth stands than in mature forest. Therefore, although the amounts of leaf litterfall were smaller in the younger stands, the amounts of nutrients released during the initial stages of decomposition were similar to, or greater than, the amounts released from decomposing leaf litter at the mature forest. However, since the proportion of non-leaf components in annual litterfall increases with age of the forest, total release of K, S, Ca, Mg and Na from all litter fractions is likely to be greater on older stands. Nitrogen and P occur at low concentrations in the major non-leaf litters and are imported into these tissues during decomposition. Therefore, in the initial stages of decay, immobilization of N and P will be greater in the litter layer of mature forest than in young regrowth stands.