Search

In many national guidelines and policies regarding protection of freshwater systems from stressors associated with forestry, riparian buffer width is a commonly prescribed strategy, typically with no other refinements of protection measures. In Sweden, the Strategic Management Objectives (SMOs) were developed to ensure that riparian buffers that are left after harvesting sustain important ecosystem attributes in aquatic systems, referred to as objectives, namely shading, biodiversity, reduction of sedimentation, and provision of deadwood and food. However, little specification is given on threshold targets or how to manage riparian zones to effectively provide these objectives. In this paper, we evaluated whether existing riparian buffers of different widths along small, recently harvested (

Southeast Asia is undergoing extensive conversion to oil palm (Elaeis guineensis) monocultures with fast growth expected in West Africa and South America. Despite rapid conversion rates, few tropical studies evaluate land-use change impacts on stream communities, with even fewer studies examining oil palm plantations (OP). In this pioneer study we compare macroinvertebrate community assemblages and their relationships with physico-chemical properties in OP streams bordered by different riparian buffer types to streams in a lowland dipterocarp forest in Sabah, Malaysian Borneo. Our study sites were in increasing levels of riparian disturbance (reduced vegetation density, diversity and structural complexity): (i) Native reference forested sites (NF); (ii) OP streams buffered with patches of natural forest (OPF); (iii) OP streams buffered with untreated oil palm (no chemical application at buffer sites; OPOP); and (iv) OP streams without buffer (OPNB). Macroinvertebrates were collected from leaf litter bags deployed with leaves of varying quality - native Macaranga tanarius and exotic oil palm, and retrieved after 14 days. We found that although invertebrates are typically influenced by resource quality, no differences were detected in their assemblages between the leaf species. All litter bags were dominated by collector-gatherers, with few shredding invertebrates, indicating that they were using the leaves as a habitat rather than a food source. We found no significant differences in macroinvertebrate abundance, richness and sensitive taxa richness (EPT) regardless of the rate of riparian disturbance. However, PERMANOVA showed significant differences between the macroinvertebrate community composition of heavily disturbed OPNB and pristine NF streams. Retaining high quality riparian buffers in OPF streams and lower quality understory in OPOP streams posed intermediate effects on macroinvertebrate community composition. The differences between sites were explained by the increase in nutrients, particularly potassium and phosphorus, stream water temperatures and erosion levels that negatively correlated with macroinvertebrate assemblages in oil palm streams. These findings support the use of riparian buffers in oil palm plantations to mitigate land conversion impacts on macroinvertebrate communities in tropical streams.

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.

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.