Restoring logged tropical forests: the removal of woody, climbing plants

In a world facing twin climate and biodiversity crises, the protection and restoration of logged tropical forests is pivotal. Woody, climbing plants (lianas) restrict the recovery of logged tropical forests, but their removal can restore timber and carbon value. While liana removal is employed to restore logged forests in several tropical countries, the efficacy, application, and monitoring of this technique to track the outcome for timber and carbon require further consideration before it can be rolled out widely. In this thesis I, firstly, use meta-analytic techniques to quantify the overall effect of liana removal on the growth of trees and aboveground biomass, and to explore the drivers of variation in efficacy. I find that liana removal significantly enhances tree growth and nearly doubles aboveground biomass accumulation, but the Neotropical bias in liana removal studies prevents me from drawing meaningful conclusions about the causes of variation in liana removal efficacy. Secondly, I conduct a novel liana removal experiment across 320 ha of logged forest in Sabah, Malaysian Borneo, in which I remove varying proportions of lianas. I acquire satellite data across this experiment and find that liana removal can be detected using Normalized Burn Ratio – a vegetation index based on spectral reflectance that differentiates leaf from non-photosynthesising material. In this chapter I also provide the first experimental evidence that partial removal has a smaller impact on the canopy, potentially minimising negative impacts on biodiversity. Finally, I find that satellite data can also detect commercial-scale liana removal (applied across 17,000 ha in Sabah). Overall, my thesis demonstrates the significant potential of liana removal as a restoration action to enhance timber and carbon in logged tropical forests and develops a simple remote sensing method to validate the application and monitor the influence of large-scale liana removal on the canopy. However, much is yet unknown about liana ecology and the myriad impacts of liana removal on biodiversity and forest function, so I urge further research into these questions and strongly recommend that at least 20% of lianas are retained if liana removal is rolled out to restore logged forests across the tropics. Further work should also focus on operationalizing the use of remote sensing for monitoring.