Opposing effects of substrate quality and site factors on forest floor turnover rates: an example from the tropics

In boreal-to-cool temperate regions, there is a consistent relationship between litter quality, litter decay rates in litterbags and forest floor turnover rates across different tree functional types such as conifers and deciduous trees. This is partly due to the synergistic relationship between substrate quality and site conditions (understorey temperatures)incontrollingforestfloordynamics.Thatis,specieswithhigherlightpenetrationhavehigherunderstorey temperatureand litterof higher qualitycomparedwith species withlower light penetration. Thefindings of our studyin Sarawak werepartlyconsistent withthistrend,with substrate quality, microbial respiration and mass loss rates of the litter in litterbags being higher for pioneer species from a young secondary forest (YSF) than for climax species from a primary forest (PF). However, the microbial respiration in the forest floor and its turnover ratewerelowerintheYSFwithlowerforestfloorwatercontentthanthoseinthePF.Thisdiscrepancywasexplained bytheopposingrelationshipbetweensubstratequalityandsiteconditions(forestfloormoisture).Thatis,intheYSF, thefavourableeffectsofsubstratequalityondecompositionwereoverriddenbytheunfavourableeffectsofforest floor moisture and the reverse was the case in the PF. It was concluded that the relationship between substrate quality and site factors (synergistic vs. opposing) in controlling decomposition is macroclimate dependent and that the power of substrate quality for predicting forest floor dynamics is lower in the tropics than that in boreal-to-cool temperate regions.