Article Developmental Dynamics of Gilbertiodendron dewevrei (Fabaceae) Drive Forest Structure and Biomass in the Eastern Congo Basin Henry B. Glick 1,† , Peter M. Umunay 1,2, * ,† , Jean-Remy Makana 3 , Sean C. Thomas 4 , Jonathan D. Reuning-Scherer 1 and Timothy G. Gregoire 1   Citation: Glick, H.B.; Umunay,P.M.; Makana, J.-R.; Thomas, S.C.; Reuning-Scherer, J.D.; Gregoire, T.G. Developmental Dynamics of Gilbertiodendron dewevrei (Fabaceae) Drive Forest Structure and Biomass in the Eastern Congo Basin. Forests 2021, 12, 738. https://doi.org/10.3390/ f12060738 Received: 16 May 2021 Accepted: 29 May 2021 Published: 4 June 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 School of the Environment, Yale University, New Haven, CT 06511, USA; henry.glick@yale.edu (H.B.G.); jonathan.reuning-scherer@yale.edu (J.D.R.-S.); timothy.gregoire@yale.edu (T.G.G.) 2 United Nations Environment Programme, P.O. Box 30552-00100, Nairobi, Kenya 3 Faculty of Science, University of Kisangani, Kisangani, Democratic Republic of the Congo; jeanremymakana@gmail.com 4 Institute of Forestry and Conservation, University of Toronto, Toronto, ON M5S 3B3, Canada; sc.thomas@utoronto.ca * Correspondence: peter.umunay@yale.edu † These authors contributed equally to this work. Abstract: Patterns of structural change associated with monodominant tropical forest complexes have remained enigmatic for decades. Here, we extend previous efforts in presenting a longitudinal, local- scale analysis of forest dynamics in central Africa. Using four 10-ha census plots measured across three time periods (959,312 stems ≥1 cm DBH), we analyzed changes in a number of biometrical attributes for four distinct forest types capturing the developmental gradient from mixed species forest to Gilbertiodendron dewevrei-dominated forest. We modeled above-ground biomass (AGB), basal area (BA), and stem density across all species, and diameter at breast height (DBH), recruitment, and mortality for Gilbertiodendron dewevrei. We hypothesized that trends in these attributes are consistent with a slow spread of Gilbertiodendron dewevrei into adjacent mixed species forest. We identified statistically significant increases in AGB and BA across sites and positive, though nonsignificant, increases in AGB and BA for most forest types. DBH and relative recruitment increased significantly for Gilbertiodendron dewevrei stems, while relative mortality did not. When looking from mixed species to transitional to monodominant forest types, we found a statistically significant pattern of developmental aggradation and net expansion of monodominant forest. We do not attribute this to atmospheric forcing but to a combination of (a) landscape-scale recovery or response to widespread disturbance (primarily historical fires), (b) Gilbertiodendron dewevrei’s ectomycorrhizal association, and (c) Gilbertiodendron dewevrei’s exceptional stress tolerance traits. Keywords: Gilbertiodendron dewevrei; longitudinal; modeling; forest dynamics; Congo; Ituri 1. Introduction The structural dynamics of tropical forests and their relation to species diversity have been a topic of interest for most of a century [3–8]. Patterns of forest structure vary biogeographically [1,2,7,9–17], and an exceptional pattern is the dominance of a particular tree species or cohort of species in lowland tropical rain forests [15,17–19]. Tropical forest monodominance was once viewed as anomalous [6,17,18,20], though we now know that it occurs to varying degrees in all of the major tropical forest regions [16,18,21]. Still, we lack a unified understanding of how these monodominant forests grow and change through time, and on the ways in which structural changes lead to, and promote, monodominance, or vice versa [9,16,21–23]. Central Africa’s Congo River basin contains 30% of the world’s tropical forests [1,8], which themselves possess unique structural characteristics among global rain Forests 2021, 12, 738. https://doi.org/10.3390/f12060738 https://www.mdpi.com/journal/forests