The effects of catchment and riparian forest quality on stream environmental conditions across a tropical rainforest and oil palm landscape in Malaysian Borneo

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Citation:
Luke, S.H., Barclay, H., Bidin, K., Chey, V.K., Ewers, R.M., Foster, W.A., Nainar, A., Pfeifer, M., Reynolds, G., Turner, E.C., Walsh, R.P.D., Aldridge, D.C., 2017. The effects of catchment and riparian forest quality on stream environmental conditions across a tropical rainforest and oil palm landscape in Malaysian Borneo. Ecohydrology e1827. doi:10.1002/eco.1827


Riparian buffer strips go some way to protecting freshwater streams but are not, by themselves, enough to fully protect freshwater ecosystems from land-use impacts. Luke and colleagues assessed forest quality in riparian zones and across whole catchments, and compared it with stream environmental conditions, such as water quality, structural complexity, and organic inputs. They found that streams with the highest riparian forest quality were cooler, deeper, had less sand and greater canopy cover, as well as more stored leaf litter and wider channels compared with oil palm streams with the lowest riparian forest quality. However, catchment‐scale forest quality strongly affected other aspects of stream conditions, with streams in the highest quality forest catchments having more bedrock and 20 times more dead wood, along with higher phosphorus, and lower nitrate‐N levels compared to streams with the lowest catchment‐scale forest quality. This study shows that, whilst riparian buffer strips appear to protect some aspects of freshwater ecosystems, consideration must also be given to the impacts of logging practices and catchment‐scale forest management on freshwaters.

Freshwaters provide valuable habitat and important ecosystem services but are threatened
worldwide by habitat loss and degradation. In Southeast Asia, rainforest streams are particularly
threatened by logging and conversion to oil palm, but we lack information on the impacts of this
on freshwater environmental conditions, and the relative importance of catchment versus
riparian‐scale disturbance. We studied 16 streams in Sabah, Borneo, including old‐growth forest,
logged forest, and oil palm sites. We assessed forest quality in riparian zones and across the
whole catchment and compared it with stream environmental conditions including water quality,
structural complexity, and organic inputs. We found that streams with the highest riparian forest
quality were nearly 4 °C cooler, over 20 cm deeper, had over 40% less sand, greater canopy
cover, more stored leaf litter, and wider channels than oil palm streams with the lowest riparian
forest quality. Other variables were significantly related to catchment‐scale forest quality, with
streams in the highest quality forest catchments having 40% more bedrock and 20 times more
dead wood, along with higher phosphorus, and lower nitrate‐N levels compared to streams with
the lowest catchment‐scale forest quality. Although riparian buffer strips went some way to
protecting waterways, they did not maintain fully forest‐like stream conditions. In addition,
logged forest streams still showed signs of disturbance 10–15 years after selective logging. Our
results suggest that maintenance and restoration of buffer strips can help to protect healthy
freshwater ecosystems but logging practices and catchment‐scale forest management also need
to be considered.