Global application of an unoccupied aerial vehicle photogrammetry protocol for predicting aboveground biomass in non‐forest ecosystems
Authors: Cunliffe Andrew M. , Anderson Karen , Boschetti Fabio , Brazier Richard E. , Graham Hugh A. , Myers‐Smith Isla H. , Astor Thomas , Boer Matthias M. , Calvo Leonor G. , Clark Patrick E. , Cramer Michael D. , Encinas‐Lara Miguel S. , Escarzaga Stephen M. , Fernández‐Guisuraga José M. , Fisher Adrian G. , Gdulová Kateřina , Gillespie Breahna M. , Griebel Anne , Hanan Niall P. , Hanggito Muhammad S. , Haselberger Stefan , Havrilla Caroline A. , Heilman Phil , Ji Wenjie , Karl Jason W. , Kirchhoff Mario , Kraushaar Sabine , Lyons Mitchell B. , Marzolff Irene , Mauritz Marguerite E. , McIntire Cameron D. , Metzen Daniel , Méndez‐Barroso Luis A. , Power Simon C. , Prošek Jiří , Sanz‐Ablanedo Enoc , Sauer Katherine J. , Schulze‐Brüninghoff Damian , Šímová Petra , Sitch Stephen , Smit Julian L. , Steele Caiti M. , Suárez‐Seoane Susana , Vargas Sergio A. , Villarreal Miguel , Visser Fleur , Wachendorf Michael , Wirnsberger Hannes , Wojcikiewicz Robert 
Published: July 2021

Non‐forest ecosystems, dominated by shrubs, grasses and herbaceous plants, provide ecosystem services including carbon sequestration and forage for grazing, and are highly sensitive to climatic changes. Yet these ecosystems are poorly represented in remotely sensed biomass products and are undersampled by in situ monitoring. Current global change threats emphasize the need for new tools to capture biomass change in non‐forest ecosystems at appropriate scales. Here we developed and deployed a new protocol for photogrammetric height using unoccupied aerial vehicle (UAV) images to test its capability for delivering standardized measurements of biomass across a globally distributed field experiment. We assessed whether canopy height inferred from UAV photogrammetry allows the prediction of aboveground biomass (AGB) across low‐stature plant species by conducting 38 photogrammetric surveys over 741 harvested plots to sample 50 species. We found mean canopy height was strongly predictive of AGB across species, with a median adjusted R2 of 0.87 (ranging from 0.46 to 0.99) and median prediction error from leave‐one‐out cross‐validation of 3.9%. Biomass per‐unit‐of‐height was similar within but different among, plant functional types. We found that photogrammetric reconstructions of canopy height were sensitive to wind speed but not sun elevation during surveys. We demonstrated that our photogrammetric approach produced generalizable measurements across growth forms and environmental settings and yielded accuracies as good as those obtained from in situ approaches. We demonstrate that using a standardized approach for UAV photogrammetry can deliver accurate AGB estimates across a wide range of dynamic and heterogeneous ecosystems. Many academic and land management institutions have the technical capacity to deploy these approaches over extents of 1–10 ha−1. Photogrammetric approaches could provide much‐needed information required to calibrate and validate the vegetation models and satellite‐derived biomass products that are essential to understand vulnerable and understudied non‐forested ecosystems around the globe.