Abies jaliscana (Jalisco fir) is an endemic fir species limited to six populations in a transition zone between the Trans-Mexican Volcanic Belt (TMVB) and Sierra Madre del Sur (SMS) mountain range, in Western Jalisco, Mexico. Despite its relictual distribution, these populations and forests are continually threatened, and studies focusing on microenvironmental and climatic factors affecting forest structure are much needed for their effective management and conservation. Here we assess vegetation stand characteristics, current distribution, and the relationships among climate and microenvironmental factors with vegetation structure in six localities that contain all the known relict populations of A . jaliscana . We sampled 38 study sites (plots) of 0.1 ha across a latitudinal geographic range of ca. 100 km, and woody species were identified and measured at each site. Sixteen environmental variables were recorded at each site, in four subsets: physiographical, disturbance, biotic and climatic.To explore relationships amongclimatic variables, microenvironmental factors and vegetation structure (fir trees and co-occurring species), we used non-metric multidimensional scaling (NMDS). Woody species ranged from three to 25 species per site in the canopy and the understorey. Basal area of fir trees varied between 21.65 and 68.89 m 2 /ha, and density from 130 to 495 stems/ha at the six localities. A . jaliscana comprised 18.2–98.7% of total basal area, and 2.11–76.32% of total density at each site, forming stands that ranged from nearly monospecific to codominant with Quercus martinezii , Quercus obtusata , Pinus montezumae , Pinus pseudostrobus and Ternstroemia lineata . Diameter distribution of Jalisco fir populations showed an inverted J-shape, a bell-shape and irregular patterns; the first one suggests a multiaged population, whereas the other two, indicate disturbance. NMDS results identified elevation and slope as the main variables associated with fir basal area. Basal area of fir trees is positively associated with gentle slopes at high elevation sites. The annual humidity index also contributed to explain differences in vegetation structure. Based on estimates of extent of occurrence and area of occupancy, and applying IUCN-Red List criteria we propose that A . jaliscana should be classified as an Endangered species. Our study provides evidence that climatic and microenvironmental conditions associated with elevation play an important role in determining the distribution limits and vegetation structure of relict temperate-like fir forests.
Guerrero-Hernández et al. (Fri,) studied this question.