Combining long-term field data and remote sensing to test how tree diversity influences aboveground biomass recovery in logged tropical forests

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The over-arching goal of this project is to test the unanswered question of whether the functioning of tropical forest ecosystems-such as how much biomass they produce-depends on how diverse they are and therefore whether replanting degraded forests with diverse mixtures of species can accelerate their recovery and help fund replanting costs through carbon credits.Tropical forests are being lost all over the world and of the remaining area more is degraded secondary forest than undisturbed old growth. This degradation occurs largely through selective logging which has negative effects of the functioning of the forest (e.g. how productive it is and how much carbon it stores) as well as on its biodiversity. This is important because work outside of the tropics in ecosystems like temperate grasslands has demonstrated a positive relationship between biodiversity and a range of ecosystem functions, including primary productivity. However, the challenges of scientific research in the tropics mean we have much less evidence of whether there is a positive relationship between levels of tree diversity and aboveground biomass production in tropical forest ecosystems. Indeed, some ecological theory suggests that species in these ecosystems are so similar ecologically that many species could be lost with little or no impact on how these ecosystems function. Twenty years ago, we started a collaboration with local foresters, conservationists and scientists in Sabah (Malaysian Borneo) to set up a long-term experiment to test whether there is a relationship between biodiversity and ecosystem functioning in these tropical forest ecosystems. The project, the Sabah Biodiversity Experiment, is one of the world's largest ecological experiments, making it of particular relevance to real-world efforts to restore and sustainably manage tropical forests. The experiment has adopted the existing forest restoration activity of 'enrichment planting' to plant more than 100,000 seedlings of native tree species across an area of 500 hectares of logged forest, allowing the recovery rates of naturally regenerating control plots to be robustly compared with those planted with different diversities (1, 4 or 16 species) and mixtures of tree species. We will assess restoration progress relative to nearby old growth forest, especially the neighbouring undisturbed area where we have repeatedly mapped and measured every tree within a 50-hectare area (the Danum Valley ForestGeo Plot) as part of a global network to monitor forest health.The large size of our research plots means they are difficult to repeatedly monitor in fine detail using traditional field methods. We will therefore combine targeted field sampling with cutting-edge remote sensing technologies to explore how tree enrichment planting has shaped the recovery of canopy structure, aboveground carbon stocks and plant diversity across hundreds of hectares of degraded forest. Our project will take advantage of existing data acquired for our study sites using two complementary technologies: LiDAR and satellite remote sensing. Airborne LiDAR uses a laser mounted on an airplane or helicopter to precisely measure the height of the vegetation and reconstruct the 3D structure of the forest canopy and underlying terrain in exquisite detail, allowing forest carbon stocks to be accurately mapped across entire landscapes. To get a more detailed and longer-term picture, the project will also use freely available satellite timeseries data (Sentinel 1-2, Landsat 8 and PlanetScope imagery) to map annual changes in forest aboveground carbon stocks (using Google Earth Engine).In the final phase we will work with local partners including the Sabah Forestry Department on a cost-benefit analysis of forest restoration and communicate these results so that they can inform their local and regional management policies for the conservation and restoration of forests and the carbon that they store.