Urbanization modifies ecosystem conditions and evolutionary processes. This includes air pollution, mostly tropospheric ozone (O3), which leads to urban and peri-urban forest decline. Such is the case of fir (Abies religiosa) forests in the peripheral mountains west of Mexico City, which have been severely affected by O3 pollution since the 1970s. Interestingly, some young individuals with low O3—related damage have been observed within a zone of great O3 incidence, which might suggest rapid tolerance/adaptation to this pollutant. We compared asymptomatic and damaged individuals of the same age (≤15 years old; n = 10) using histological, metabolomic and transcriptomic methods. Plants were sampled during days of high (170 ppb) and moderate (87 ppb) O3 concentration periods. After verifying that all individuals clustered within the same local genetic group when compared to a species-wide panel (Admixture analysis with ~1.5K SNPs), we observed thicker epidermis and more collapsed cells in the palisade parenchyma in needles from damaged than from asymptomatic individuals; such differences increased with needle age. Furthermore, damaged individuals had lower concentrations of various terpenoids (ß-Pinene, ß-Caryophylene Oxide, α-Caryophylene, ß-α-Cubebene and α-Muurolene) than asymptomatic trees, as evidenced through GC-MS. Finally, transcriptomic analyses revealed differential expression for thirteen genes related to carbohydrate metabolism, plant defense, and gene regulation. Our results indicate a rapid and contrasting phenotypic response between plants, likely modeled by standing genetic variation and/or plastic mechanisms. They open the door to future evolutionary studies for understanding how O3 tolerance develops in urban environments, and for identifying tolerant germplasm for forest restoration.

Urbanization modifies ecosystem conditions and evolutionary processes. This includes air pollution, mostly as tropospheric ozone (O3), which contributes to the decline of urban and peri-urban forests. A notable case are fir(Abies religiosa) forests in the peripheral mountains southwest of Mexico City, which have been severely affected by O3 pollution since the 1970s. Interestingly, some young individuals exhibiting minimal O3—related damage have been observed within a zone of significant O3 exposure. Using this setting as a natural experiment, we compared asymptomatic and symptomatic individuals of similar age (≤15 years old; n = 10) using histological, metabolomic and transcriptomic approaches. Plants were sampled during days of high (170 ppb) and moderate (87 ppb) O3 concentration. Given that there have been reforestation efforts in the region, with plants from different source populations, we first confirmed that all analysed individuals clustered within the local genetic group when compared to a species-wide panel (Admixture analysis with ~1.5K SNPs). We observed thicker epidermis and more collapsed cells in the palisade parenchyma of needles from symptomatic individuals than from their asymptomatic counterparts, with differences increasing with needle age. Furthermore, symptomatic individuals exhibited lower concentrations of various terpenes (ß-pinene, ß-caryophylene oxide, α-caryophylene and ß-α-cubebene) than asymptomatic trees, as evidenced through GC-MS. Finally, transcriptomic analyses revealed differential expression for thirteen genes related to carbohydrate metabolism, plant defense, and gene regulation. Our results indicate a rapid and contrasting phenotypic response among trees, likely influenced by standing genetic variation and/or plastic mechanisms. They open the door to future evolutionary studies for understanding how O3 tolerance develops in urban environments, and how this knowledge could contribute to forest restoration.

Physical disruption of gene flow among mountains is commonly viewed as an important process for the generation of hyperdiverse tropical mountain biotas. However, the role of in situ diversification within mountains has been seldom explored. Here we evaluate spatially fine-scale patterns of arthropod community assembly within a single mountain to understand the role of dispersal limitation and landscape features as drivers of tropical mountain diversity. We focus on a single tropical sky-island of the Transmexican Volcanic Belt, where we sampled whole-communities of arthropods for eight orders with a comparable design at a spatial scale ranging from 50 m to 20 km, using 840 pitfall traps and whole community metabarcoding. We explored multiple hierarchical levels, from individual haplotypes to lineages at 0.5, 1.5, 3, 5, 7.5% similarity thresholds, to evaluate patterns of richness, turnover and distance decay of similarity with isolation-by-distance and isolation-by-resistance approaches. Our results showed that distance and altitude influence distance decay of similarity at all hierarchical levels. This holds for arthropod groups of contrasting dispersal abilities, but with different strength depending on the spatial scale. Our results suggest long-term persistence of lineages within sky islands, combined with local-scale differentiation, may be an important driver of high arthropod biodiversity in tropical mountains.