Litter decomposition is driven by microbes and is more influenced by litter quality than environmental conditions in oil palm streams with different riparian types

Abstract

Rapid and extensive conversion of tropical forests into oil palm (Elaeis guineensis) (OP) plantations pose serious threats to tropical stream processes. To mitigate land use change impacts on stream ecosystems, retention of riparian vegetation is typically proposed. We evaluated the effectiveness of a gradient of riparian qualities in oil palm streams: (1) natural forest; (2) OP-native forested buffer; (3) OP-native understory, no chemical input (OPOP) and (4) OP-no buffer, to mitigate impacts on in-stream litter processing. Leaf bag method entry using two leaf species of contrasting litter quality (Macaranga tanarius and OP) were deployed into streams. Across all riparian types, microbes were the main drivers of decomposition with negligible macroinvertebrate shredding activities. Leaf decomposition rates were more influenced by litter quality than changes in environmental conditions in the different riparian types. Across all sites, native Macaranga litter decomposed approximately 5× faster than OP litter possibly due to high structural compounds in OP leaves. Macaranga litter was also more susceptible to changes in environmental conditions as leaf decomposition positively correlated to phosphorus and potassium content. However, OP leaves were resilient to stream environmental changes and decomposed slower only at OPOP sites. These varying responses reveal complex interactions within tropical stream ecosystems. We suggest that riparian management strategies as well as plans to restore functioning in degraded tropical streams should ensure a wide diversity of native riparian tree species in order to effectively mitigate adverse OP plantation impacts on tropical stream functioning.