Precipitation, decomposition and litter decomposability of Metrosideros polymorpha in native forests on Hawai'i

1 The effects of precipitation on litter quality and surface litter decomposition were evaluated across a natural rainfall gradient, ranging from 500 to 5500 mm mean annual precipitation, on the island of Hawai'i. 2 Litter of Metrosideros polymorpha from the driest (500 mm) and wettest (5500 mm) extremes of the gradient was decomposed at five sites along the precipitation gradient. Metrosideros polymorpha litter collected from all sites was also decomposed at a single common site (2500 mm), and M. polymorpha litter collected from each site was decomposed at the site of collection (in situ). 3 Initial litter quality varied significantly with precipitation at the site. Leaf mass per area (LMA) and phosphorus content decreased, while lignin and nitrogen content both increased, with increasing site precipitation. 4 Mass loss of all common litter types increased with increasing precipitation, as did in situ decomposition, which gave k-values ranging from 0.22 to 1.06. When decomposition at the same site was evaluated, litter from the driest site decomposed up to 2.5 times more rapidly than wet-site litter. 5 Decomposition at the common site showed an inverse correlation between inherent decomposability of litter (i.e. litter quality) and precipitation at the site from which litter originated, with k-values ranging from 0.71 for litter from the driest site to 0.28 for litter from the wettest site. Decomposability correlated strongly and inversely with initial lignin concentrations. 6 Proportional nutrient release from in situ litter was slower than mass loss, but nitrogen and phosphorus mineralization generally increased with increasing precipitation. 7 At the common site, there was an inverse relationship between rate of nutrient release and precipitation at the site where litter was collected. The systematic variation in litter decomposability due to changes in annual precipitation appears to buffer the direct effects of precipitation on carbon and nutrient turnover.