Article Wood Density and Ring Width in Quercus rotundifolia Trees in Southern Portugal Vicelina Sousa 1, *, Maria Emília Silva 2 , José Luís Louzada 2 and Helena Pereira 1   Citation: Sousa, V.; Silva, M.E.; Louzada, J.L.; Pereira, H. Wood Density and Ring Width in Quercus rotundifolia Trees in Southern Portugal. Forests 2021, 12, 1499. https://doi.org/10.3390/f12111499 Academic Editors: Michele Brunetti, Alexander Petutschnigg and Michela Nocetti Received: 24 September 2021 Accepted: 27 October 2021 Published: 29 October 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 Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal; hpereira@isa.ulisboa.pt 2 Departamento Florestal/CITAB, Universidade de Trás-os-Montes e Alto Douro, Quinta de Prados Apartado 202, 5000-911 Vila Real, Portugal; emil_ms@utad.pt (M.E.S.); jlousada@utad.pt (J.L.L.) * Correspondence: vsousa@isa.ulisboa.pt Abstract: Quercus rotundifolia Lam., known as holm oak or evergreen oak, occurs naturally in the western Mediterranean region, mainly as part of the agroforestry or agrosilvopastoral systems in Portugal (“montado”) and Spain (“dehesa”), and is economically important for acorn production. Less attention has been given to Q. rotundifolia wood, and its density variability is not known, namely related to tree growth. The wood density of 20 Q. rotundifolia trees was measured along the radial direction by X-ray densitometry and the factors responsible for ring width and wood density variation within and between trees were investigated at two sites located within the main species region in southern Portugal. Ring width was significantly different between sites, with an average of 1.81 mm and 1.55 mm. Wood density was very high and averaged between 888 kg/m 3 and 914 kg/m 3 but not significantly different between sites. Ring width and wood density showed a positive and significant correlation at both sites. Cambial age was the main source of variation for ring width and wood density, while between-tree effects accounted for a considerable proportion of wood density variation. The results are an important contribution for the species valorisation aiming at high-value wood products, also adding knowledge on the species growth of interest for tree selection and sustainable management. Keywords: holm oak; Mediterranean oaks; wood density; ring width; variability; microdensitometry 1. Introduction Quercus rotundifolia Lam., known as holm oak but mostly referred to as a subspecies of Quercus ilex L. (Quercus ilex subsp. rotundifolia; Q. ilex subsp. ballota), is a Mediterranean evergreen oak mainly and naturally distributed in the western Mediterranean region: Portugal (ca. 349,400 ha), south Spain (ca. 2,794,450 ha) and Morocco (ca. 1,415,201 ha) [1–3]. Its economic, ecological, and social relevance is well acknowledged due to its presence in several ecosystems, and especially as part of the agrosilvopastoral system found mainly in Portugal (“montado”) and Spain (“dehesa”). In France, Italy, and Greece, Q. rotundifolia is replaced by Q. ilex (Q. ilex subsp. ilex, if considered a subspecies) [1,4]. The different foliar morphology and geographic distribution of these subspecies (or species) compared to Q. ilex subsp. ilex (or Q. ilex L.) are acknowledged by different authors [5–7]. Holm oak, once covering most of the Mediterranean region, is a sclerophyllous species, well adapted to both summer drought and light winter frost which are characteristic of the Mediterranean climate, and occurring from a semiarid (with markedly continental conditions) to per-humid environments [8]. However, some tree decline or dieback in montado is being reported due to climate change, land desertification, and biotic pathogens threats, with concerns related to their different impacts on this management system and species’ future distribution [9,10]. In this context, physiological and growth traits have been studied aiming at species regeneration mainly for Q. ilex (or Q. ilex subsp ilex if mentioned as subspecies) (e.g., [4,11–13]). However, it is reported that Q. rotundifolia is Forests 2021, 12, 1499. https://doi.org/10.3390/f12111499 https://www.mdpi.com/journal/forests