Quantifying the Impact of Oil Palm Plantations on Channel Shading in Malaysian Rivers Using Drone‐Based Structure From Motion Photogrammetry

River ecosystems in tropical Asia are increasingly threatened by climate and land‐use change. The proliferation of oil palm plantations is thought to exacerbate climate change‐driven stream temperature increases through the removal of bankside tree shading. As such, there are concerns that rising water temperatures could severely affect sensitive river organisms. Despite this, research investigating how the conversion of native tropical forest to oil palm plantations impacts stream shading has been limited. River scientists and plantation managers therefore currently lack the evidence needed to develop appropriate riparian management strategies. To address this knowledge gap, we present the results of a drone‐based study quantifying differences in stream shading generated under three different tropical forest canopy treatments: native dipterocarp forest, disturbed forest and oil palm plantation. Working in the Langat River watershed, Malaysia, we generated Structure from Motion point clouds of riparian forest cover for these three treatments and used the resulting data to measure tree height and canopy density at each site. We then input these data into the shading routines of the Heat Source stream temperature model, generating values of effective shade and view‐to‐sky (VTS) along a 500 m stream reach at each site. Our results showed that both the disturbed and particularly oil palm treatments generate significantly lower effective shade (~17% and ~6%, respectively) than the native tropical forest treatment (~28%). We subsequently investigated the interaction between shade and channel width under each canopy treatment by developing an algorithm that iteratively removed pixels along the river's centreline, allowing us to calculate shading in progressively narrower channels while retaining identical riparian forest structure. While the native and disturbed forest treatments showed a strong effect of channel width on the magnitude of shade provided to the stream, the shading provided by oil palms exhibited almost no variation with width; instead, it was consistently low, with the shading offered even to very narrow channels (5 m wide) being lower than that afforded by native forests to the widest (25 m) streams. This study is the first to apply these methods to assess shading in tropical streams and highlights the significant loss of shading when native forest is replaced with oil palm. These changes may elevate stream temperatures, with cascading effects on aquatic biodiversity. Our findings emphasise the importance of retaining a buffer of natural forest along the riparian zone of streams within oil palm landscapes.