Handling editor: Dr. Fabian Fassnacht Received 20 January 2025; revised 16 May 2025; accepted 19 May 2025 © The Author(s) 2025. Published by Oxford University Press on behalf of the Institute of Chartered Foresters. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Forestry: An International Journal of Forest Research, 2025, 1–16 https://doi.org/10.1093/forestry/cpaf034 Original Article Costs and precisions of alternative plot configurations for estimating above-ground tree biomass in tropical forests Erkki O. Tomppo 1, *, Bryan C. Foster 2 , Laura S. Kenefic 3 , Mark J. Ducey 4 1 Department of Forest Sciences, University of Helsinki, Latokartanonkaari 7, P.O. Box 27, Helsinki FI-00014, Finland 2 Foster Forestry and Environmental Consulting LLC, 123 Chipman St., South Burlington, VT 05403, United States 3 United States Department of Agriculture, Forest Service, Northern Research Station, 54 Government Road, Bradley, Maine 04411, United States 4 Department of Natural Resources and Environment, University of New Hampshire, 114 James Hall, 56 College Road, Durham, NH 03824, United States *Corresponding author. Department of Forest Sciences, University of Helsinki, Latokartanonkaari 7, P.O. Box 27, Helsinki FI-00014, Finland. E-mail: erkki.tomppo@helsinki.fi Abstract We used field measurements and sampling simulations with existing tree-level data to assess the precision of the estimates and data measurement costs for ground-based sampling inventory for above-ground tree biomass (AGB) and basal area estimation. Data were obtained from primary wet tropical forests using field measurements in Costa Rica and, for simulations, published tree-level data on Barro Colorado Island in Panama. Plot configurations and costs were compared using criteria commonly used by forest carbon offset project developers to meet precision of +/−10% at 90% confidence interval for AGB estimation. Detached plots were tested using circular fixed-area, nested, and several types of angle-count configurations. A plot divided into a cluster of sub-plots was further tested with angle-count plots. The basal area factor (BAF) used for angle-count plots varied between 2 and 20. Key findings include (i) angle-count plots emerged as a highly efficient plot configuration for forest inventories in tropical regions with high tree density and size variability with 33%–80% cost savings without precision penalties compared to the nested and fixed-area plots respectively; (ii) angle-count plots with large BAFs (3–8) are cost-efficient and can achieve low sampling errors (∼10%) with fewer plots and trees than the fixed-area and nested plots; (iii) a cluster of angle-count subplots with a large BAF (e.g. BAF = 12, 16, 20) is an efficient configuration for further investigation; (iv) small fixed-area plots for small trees with angle-count sampling for large trees is also an efficient configuration for further investigation. Among the alternative plot configurations compared, angle-count plots with their variants emerged as a highly cost-efficient and versatile alternative in complex tropical forests with visual occlusion, and are worth studying further. Keywords: forest inventory; carbon monitoring; fixed-area plots; angle-count plots; precision; neotropical forests Introduction Global change and the possible role of forests in mitigating climate change is a pressing research topic. The International Union for Conservation of Nature’s Nature-based Climate Solutions financially incentivize source avoidance and sink enhancement of greenhouse gas emissions via forest activities that generate offset credits (Pasgaard et al. 2016). The three major intervention types for Forest Carbon Offset (FCO) projects include: conservation such as Avoided or Reduced Emissions from Deforestation and Degradation (REDD), enhancement such as Afforestation and Reforestation, and sustainable management such as Improved Forest Management (Griscom et al. 2017, Forest Carbon Offsets 2023). FCOs require frequent monitoring, reporting, and verification including on-the-ground inventories prior to verification audits every 5–6 years over 40–100-year lifespans (Foster et al. 2017, Pan et al. 2022). All major global FCO standards (American Carbon Registry, Climate Action Reserve, Gold Standard, Plan Vivo, and Verified Carbon Standard) require verifiers to check ground-based inventory plots scaled up to the project level—a “stratify and multiply” approach using expansion factors (Marvin and Asner 2016). The Intergovernmental Panel on Climate Change has articulated a simple “stock change difference” method to calculate emission factors (Achard et al. 2014) between two measurements of spatially permanent plots over time. Two key questions in any forest inventory are what type of sample plot layout (sampling design) and plot configuration (plot type and size) are most appropriate to be used (Schreuder et al. 1993, Gregoire and Valentine 2007, Kershaw Jr. et al. 2016). Many forest inventories use fixed-area plots, but fixed-area plots are known to be cost-inefficient for assessing forest volume and carbon stocks when the volume or carbon of the individual trees Downloaded from https://academic.oup.com/forestry/advance-article/doi/10.1093/forestry/cpaf034/8158007 by guest on 07 June 2025