agriculture Article Assessing Baseline Carbon Stocks for Forest Transitions: A Case Study of Agroforestry Restoration from Hawai‘i Angelica Melone 1,2,3 , Leah L. Bremer 3,4, *, Susan E. Crow 1 , Zoe Hastings 5 , Kawika B. Winter 1,2,6 , Tamara Ticktin 5 , Yoshimi M. Rii 2,6 , Maile Wong 3,4,5 ,K¯ anekoa Kukea-Shultz 7 , Sheree J. Watson 8 and Clay Trauernicht 1   Citation: Melone, A.; Bremer, L.L.; Crow, S.E.; Hastings, Z.; Winter, K.B.; Ticktin,T.; Rii, Y.M.; Wong, M.; Kukea-Shultz, K.; Watson, S.J.; et al. Assessing Baseline Carbon Stocks for Forest Transitions: A Case Study of Agroforestry Restoration from Hawai‘i. Agriculture 2021, 11, 189. https://doi.org/10.3390/ agriculture11030189 Academic Editors: Marco Lauteri, Tommaso La Mantia and Anastasia Pantera Received: 1 December 2020 Accepted: 19 February 2021 Published: 25 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Natural Resources and Environmental Management, University of Hawai‘i at M¯ anoa, Honolulu, HI 96822, USA; ajmelone@hawaii.edu (A.M.); crows@hawaii.edu (S.E.C.); kawikaw@hawaii.edu (K.B.W.); trauerni@hawaii.edu (C.T.) 2 He‘eia National Estuarine Research Reserve, K¯ ane‘ohe, HI 96744, USA; shimi@hawaii.edu 3 University of Hawai‘i Economic Research Organization, University of Hawai‘i at M¯ anoa, Honolulu, HI 96822, USA; mailekw@hawaii.edu 4 Water Resources Research Center, University of Hawai‘i at M¯ anoa, Honolulu, HI 96822, USA 5 School of Life Sciences, University of Hawai‘i at M ¯ anoa, Honolulu, HI 96822, USA; zchastin@hawaii.edu (Z.H.); ticktin@hawaii.edu (T.T.) 6 Hawai‘i Institute of Marine Biology, University of Hawai‘i at M¯ anoa, Honolulu, HI 96822, USA 7 K¯ ako‘o ‘ ¯ Oiwi, He‘eia, HI 96744, USA; admin@kakoooiwi.org 8 Pacific Bioscience Research Center, University of Hawai‘i at M¯ anoa, Honolulu, HI 96822, USA; sheree.jwatson@gmail.com * Correspondence: lbremer@hawaii.edu Abstract: As the extent of secondary forests continues to expand throughout the tropics, there is a growing need to better understand the ecosystem services, including carbon (C) storage provided by these ecosystems. Despite their spatial extent, there are limited data on how the ecosystem services provided by secondary forest may be enhanced through the restoration of both ecological and agroecological functions in these systems. This study quantifies the above- and below-ground C stocks in a non-native secondary forest in Hawai‘i where a community-based non-profit seeks to restore a multi-strata agroforestry system for cultural and ecological benefits. For soil C, we use the equivalent soil mass method both to estimate stocks and examine spatial heterogeneity at high reso- lution (eg. sub 5 m) to define a method and sampling design that can be replicated to track changes in C stocks on-site and elsewhere. The assessed total ecosystem C was ~388.5 Mg C/ha. Carbon stock was highest in trees (~192.4 Mg C/ha; ~50% of total C); followed by soil (~136.4 Mg C/ha; ~35% of total C); roots (~52.7 Mg C/ha; ~14% of total C); and was lowest in coarse woody debris (~4.7 Mg C/ha; ~1% of total C) and litter (~2.3 Mg C/ha; <1% of total C). This work provides a baseline carbon assessment prior to agroforest restoration that will help to better quantify the contri- butions of secondary forest transitions and restoration efforts to state climate policy. In addition to the role of C sequestration in climate mitigation, we also highlight soil C as a critical metric of hybrid, people-centered restoration success given the role of soil organic matter in the production of a suite of on- and off-site ecosystem services closely linked to local sustainable development goals. Keywords: agroecology; biocultural restoration; soil carbon; ecosystem services; land-use change; equivalent soil mass method; sustainable development 1. Introduction Secondary forests account for over 40% of existing tropical forest cover [1] and they are projected to dominate tropical landscapes into the future [2,3]. Secondary forests can support high biodiversity and provide other societal benefits including carbon storage, nutrient cycling, timber and non-timber forest products, cultural services, and wildlife habitat [4–6]. However, these benefits depend on the kind of forest transitions which Agriculture 2021, 11, 189. https://doi.org/10.3390/agriculture11030189 https://www.mdpi.com/journal/agriculture