Mexico is a major center of evolutionary radiation for the genus Quercus, with oak species occurring across different habitat types and showing a wide variation in morphology and growth form. Despite representing about 20% of Mexican species, scrub oaks have received little attention and even basic aspects of their taxonomy and geographic distribution remain unresolved. In this study, we analyzed the morphological and climatic niche differentiation of scrub oak populations forming a complex constituted by six named species, Quercus cordifolia, Quercus frutex, Quercus intricata, Quercus microphylla, Quercus repanda, Quercus striatula and a distinct morphotype of Q. striatula identified during field and herbarium work (hereafter named Q. striatula II). Samples were obtained from 35 sites covering the geographic distribution of the complex in northern and central Mexico. Morphological differentiation was analyzed through geometric morphometrics of leaf shape and quantification of trichome traits. Our results indicated the presence of two main morphological groups with geographic concordance. The first was formed by Q. frutex, Q. microphylla, Q. repanda and Q. striatula, distributed in the Trans-Mexican Volcanic Belt, the Sierra Madre Occidental and a little portion of the south of the Mexican Altiplano (MA). The second group consists of Q. cordifola, Q. intricata and Q. striatula II, found in the Sierra Madre Oriental and the MA. Therefore, our evidence supports the distinctness of the Q. striatula II morphotype, indicating the need for a taxonomic revision. Within the two groups, morphological differentiation among taxa varied from very clear to low or inexistent (i.e. Q. microphylla-Q. striatula and Q. cordifolia-Q. striatula II) but niche comparisons revealed significant niche differentiation in all pairwise comparisons, highlighting the relevance of integrative approaches for the taxonomic resolution of complicated groups such as the one studied here.

Background: The genus Quercus has a keystone role in the temperate forests in the northern hemisphere; thus this offers an interesting opportunity to use it as a model to know altitudinal species richness patterns which could be used in further studies and projects in biodiversity conservation. Questions: It is possible to detect an altitudinal gradient based on the genus Quercus distribution? What climatic variables are most important in the altitudinal distribution of the genus Quercus ? Study site: The physiographic province of Sierra Madre del Sur (SMS) located southwest of Mexico. Methods: Based on 3,267 herbarium registers of 61 species, a data matrix was created with the presence/absence of each species in altitudinal intervals of 100 m. Then a similarity matrix was obtained using the Sorensen-Dice index in the R software. Through a discriminant analysis, we evaluated for environmental differences among the altitudinal zones previously obtained through a cluster analysis. Results: We found three altitudinal zones, each one defined by exclusive species, and two important species turnover points. The species richness distribution showed a hump-shaped pattern along the altitudinal gradient. The overall model was highly significant, evidencing the existence of different temperature and precipitation regimes throughout the altitudinal distribution of oaks species in SMS. Conclusions: The altitudinal distribution of oak species in the SMS is not homogeneous and is restricted mainly due to differences in the precipitation and temperature regimes. The altitudinal distribution pattern found in this study could be explained considering ecological and historical factors.

Within the genus Quercus there are species with a shrubby habit that have been little studied worldwide. In Mexico, several shrub species within section Quercus are taxonomically highly problematic. These include eight named taxa that share traits such as abaxially tomentose leaves and growth of stems from one or multiple rhizomes, and there is evidence of additional undescribed species. Here, Quercus purhepecha is described and illustrated as a new tomentose shrubby white oak species, with a very restricted distribution within the Cuitzeo basin in the Trans-Mexican Volcanic Belt (TMVB), Michoacán state, Mexico. This new species is morphologically compared with Q. frutex, Q. microphylla and Q. repanda, which are similar taxa also distributed in the TMVB. In addition, a potential distribution model of Q. purhepecha for the Cuitzeo basin is presented.

Hybridization is frequent in oaks and may drive various evolutionary and ecological effects on involved plant populations and their associated species. Quercus castanea is a species of Mexican red oak that has served as a valuable model for examining the effects of hybridization events. We used a multimarker approach to characterize the morphological expression patterns and chemical production in parental and hybrid genotypes of the Q. castanea × Q. crassipes complex in a hybrid zone in central Mexico. Leaf macro- and micro-morphological (stomata) characters were measured in 27 trees previously recognized as Q. castanea, Q. crassipes, and hybrids (nine trees/taxon). The expression of foliar shape and the production of secondary metabolites and nutritional chemicals between hybrids and parental species were also evaluated. We found that hybrids exhibit a mosaic of macro- and micro-morphological characters, including the expression of intermediate, parental, and transgressive attributes. Parental species were well differentiated based on foliar shapes, with hybrids overlapping in both species, having a greater affinity with Q. crassipes. Chemical analysis also supports three chemical groups, with hybrids producing new metabolites. This multimarker approach evidences the formation of ecological islands in this hybrid oak complex in which Q. castanea is involved, a fact that has consequences at higher levels of organization in ecology.