Citation: Antúnez, P.; Hernández- Cruz, I.; Ibrahim-Abdulsalam, F.; Clark-Tapia, R.; Ruiz-Aquino, F.; Valenzuela-Encinas, C. Effects of Distance and Neighbor Size on Abies hickelii: The Asymmetric Competition Is Aggravated in an Endangered Species. Forests 2023, 14, 1654. https://doi.org/10.3390/ f14081654 Academic Editor: Andrew M. Barton Received: 30 June 2023 Revised: 12 August 2023 Accepted: 13 August 2023 Published: 16 August 2023 Copyright: © 2023 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/). Article Effects of Distance and Neighbor Size on Abies hickelii: The Asymmetric Competition Is Aggravated in an Endangered Species Pablo Antúnez 1, * , Iván Hernández-Cruz 2 , Fatima Ibrahim-Abdulsalam 3 , Ricardo Clark-Tapia 1 , Faustino Ruiz-Aquino 1 and César Valenzuela-Encinas 1 1 División de Estudios de Posgrado, Instituto de Estudios Ambientales, Universidad de la Sierra Juárez, Avenida Universidad S/N, Ixtlán de Juárez, Oaxaca 68725, Mexico 2 Instituto de Estudios Ambientales, Universidad de la Sierra Juárez, Avenida Universidad S/N, Ixtlán de Juárez, Oaxaca 68725, Mexico; ivanhernandezcruz10@gmail.com 3 School of Public Health, Walailak University, Thasala 80160, Thailand * Correspondence: pantune@uni-goettingen.de; Tel.: +55-951-5536362 Abstract: One of the hypotheses in the field of forest ecology suggests that competition among individual plants is asymmetric or unidirectional. This notion holds particular significance for endangered species, necessitating a comprehensive assessment of the impact of their closest com- petitors to understand their spatial distribution and local-scale interactions. The mean response of Abies hickelii’s diameter at breast height and total height was determined for each unit that increased the distance from its neighbor and each unit that changed the size of its neighbor. The results obtained through the analysis of covariance suggest that, for each meter that the distance from the neighbor increases, there is a mean gain of 1.13 cm in the diameter of the individuals studied. The height results showed higher sensitivity to changes in the neighbor distance than the diameter. It was observed that the population is exposed to asymmetric competition, suggesting that larger individuals take available resources disproportionately, thereby inhibiting the growth of smaller individuals. The intraspecific competition surpassed the interspecific competition. A suggestion was made to use selective thinning during the early-growth stages of the species. The habitat of the species studied is under strong pressure from land use change due to clandestine logging, the opening of roads, and other anthropogenic effects. Thus, these results represent an important step in the implementation of action strategies to preserve the species in the wild. Keywords: Cohen’s index; effect of first five neighbors; endangered species; Mexican fir; Weibull 1. Introduction Abies hickelii (Flous et Gaussen) is a critically endangered species found in isolated populations forming strips or small patches in the mountain ranges of Mexico, mainly in Chiapas, Oaxaca, and Veracruz [1]. Currently, it is listed in the endangered category on the IUCN Red List, and its population is decreasing [2,3]. This species has also been recognized as a priority for conservation in the official Mexican standard NOM-059-SEMARNAT-2010 related to the conservation of the diversity of species in wildlife (NOM-059, 2010). Small and isolated populations like A. hickelii are particularly vulnerable to competition for lim- ited resources, which can be exacerbated by their low genetic diversity and increased inbreeding [4,5]. These factors hinder the species’ adaptive capacity and restrict its ability to expand its territory or thrive in the wild. Considering the context of climate change and land use alterations, understanding the factors influencing the growth and survival of geographically restricted populations like A. hickelii becomes vital for effective conservation efforts [6,7]. Understanding the pattern of spatial distribution of plants in wildlife in accurate and meaningful terms has not been an easy task due to various factors, including interactions such as competition and interference [8]. Conventionally, the effect of Forests 2023, 14, 1654. https://doi.org/10.3390/f14081654 https://www.mdpi.com/journal/forests