Effect of riparian management on stream morphometry and water quality in oil palm plantations in Borneo

Large-scale conversion of tropical forests into agricultural plantations, particularly oil palm (OP) across South East Asia exerts enormous pressure on freshwater systems. To mitigate impacts on aquatic ecosystems, the retention of riparian buffer zones along stream banks are often advocated for freshwater management. However, there is a severe lack in ecological information available on tropical stream systems advising on the efficacy of different riparian buffer types (with varying quality) to mitigate stream physico-chemical properties after conversion for agricultural use. To test the hypothesis that greater riparian disturbance will have negative effects on stream geomorphology and water quality, we assessed the impacts of riparian vegetation structure and density on stream chemical and physical properties in different riparian buffer types commonly used in OP plantations subjected to a gradient of disturbance: (i) Native forest (NF); (ii) OP - forested buffer (OPF); (iii) OP untreated palms buffer (no fertilizer and pesticide application) (OPOP); and (iv) OP - treated palms (OPNB). Across the disturbance gradient, riparian species diversity and density decreased with taller trees and high foliage cover. Foliage cover heavily influenced the amount of light received at the stream, bank and buffer zone that concur with stream water temperatures. In-stream litter substrate decreased with increased riparian disturbance. OP streams had higher phosphorus and potassium concentrations that can be attributed to the use of fertilizers while sodium concentrations were higher in NF streams. Generally, OPF was most similar to NF sites whereas OPOP and OPNB sites had similar characteristics showing that riparian vegetation type influences the physical and chemical characteristics of streams. Thus, the use of high quality riparian buffers with forested riparian vegetation in OP plantations to reduce the impacts of land conversion on streams is supported.