Leaf litter breakdown of native and exotic tree species in two Hawaiian streams that differ in flow.
Riparian leaf litter is a major source of allochthonous organic material to temperate and tropical streams, promoting primary and secondary productivity in lotic and nearshore habitats. In tropical island streams, where native leaf-shredding macroinvertebrates are absent, physical fragmentation from stream flow is an important factor affecting leaf litter breakdown and, thus, organic matter dynamics. Additionally, the invasion of exotic plants into riparian areas is expected to affect litter composition and, consequently, its degradation. We compared the interactions of stream flow and inputs of leaf litter from native and exotic plants on leaf litter breakdown in two streams of varying flows on Hawai'i Island. Decay rates were greater in the high flow stream than in the low flow one for exotic Spathodea campanulata (0.037 vs. 0.023 day-1), but not significantly different for exotic Psidium cattleianum (0.003 vs. 0.003 day-1), and native Metrosideros polymorpha (0.005 vs. 0.002 days-1). In contrast, the exotic Falcataria moluccana (a nitrogen fixer) decomposed more rapidly in the low flow stream (0.017 day-1) than in the high flow stream (0.010 day-1). Breakdown rates also varied among species, with S. campanulata > F. moluccana > M. polymorpha > P. cattleianum. Breakdown rates were generally positively correlated to leaf nitrogen content and negatively correlated with leaf structure characteristics (toughness, organic carbon content, percentage lignin). Our findings indicate that stream flow regimes altered by climate change are likely to influence leaf litter decomposition, and S. campanulata and F. moluccana will likely impact organic matter dynamics in Hawaiian and other Pacific Island streams. However, predicted changes depend on the species composition of riparian leaf litter.