Biomass Expansion Factors for Hedgerow-Grown Trees Derived from Terrestrial LiDAR

Abstract

Converting data from national forest inventories to carbon stocks for greenhouse gas reporting generally relies on biomass expansion factors (BEFs) that expand stem volumes to whole tree volumes. However, BEFs for trees outside forests like trees in hedgerows are not yet included in the IPCC reports. These are expected to be different from forest trees as hedgerow trees are exposed to more solar radiation and have more growing space. We present age-dependent BEF curves for hedgerow-grown pedunculate oak (Quercus robur L.), common alder (Alnus glutinosa (L.) Gaertn.) and silver birch (Betula pendula Roth). We scanned 73 trees in northern Belgium using terrestrial LiDAR (Light Detection and Ranging). Via quantitative structure models, we estimated total volume and stem volume (diameter greater than 7 cm); we then calculated BEF as the ratio of total volume to stem volume. BEFs decreased exponentially with tree age, converging at 1.18, 1.9 and 1.92 for alder, birch and oak, respectively. For alder, this value is comparable to values of forest-grown alder; for birch and oak, these values are substantially higher, indicating a bigger part of the total volume is branch wood instead of stem wood. Total wood volume in hedgerows varied from 131.2 to 751.8 m(3) per running kilometre, accounting for 30.0 to 222.8 Mg carbon stored, respectively. Only half of the produced wood in hedgerows was classified as stem wood, the other half as branch wood. Our findings show that hedgerow-specific BEFs should be used when applications for biobased economies are drafted. Also, hedgerows should be included in national carbon budgets as they represent non-negligible stocks.