Mexican oregano (Lippia graveolens) is an important aromatic plant, mainly used as flavoring and usually harvested from non-cultivated populations. Mexican oregano essential oil showed important variation in the essential-oil yield and composition. The composition of the essential oils extracted by hydrodistillation from 14 wild populations of L. graveolens growing along an edaphoclimatic gradient was evaluated. Characterization of the oils by GC-FID and GC/MS analyses allowed the identification of 70 components, which accounted for 89-99% of the total oil composition. Principal component and hierarchical cluster analyses divided the essential oils into three distinct groups with contrasting oil compositions, viz., two phenolic chemotypes, with either carvacrol (C) or thymol (T) as dominant compounds (contents >75% of the total oil composition), and a non-phenolic chemotype (S) dominated by oxygenated sesquiterpenes. While Chemotype C was associated with semi-arid climate and shallower and rockier soils, Chemotype T was found for plants growing under less arid conditions and in deeper soils. The plants showing Chemotype S were more abundant in subhumid climate. High-oil-yield individuals (>3%) were identified, which additionally presented high percentages of either carvacrol or thymol; these individuals are of interest, as they could be used as parental material for scientific and commercial breeding programs.

The opening of a canopy gap at Los Tuxtlas rainforest has an impact on populations of the understory herb Aphelandra aurantiaca: the ratio of recruited seedlings per reproductive individual is 1:17 in mature forest vs. gaps. Predation occurring before seed dispersal seems a plausible explanation for this observed difference. In a field experiment, in which insecticide was applied to plants growing in gaps and mature forest, we evaluated the extent to which herbivore damage to flowers, fruits, and seeds reduces the number of seeds available for seedling establishment. Under natural conditions, ∼30% of the flowers and >70% of the capsules of A. aurantiaca showed herbivore damage, but its impact changed depending on the type of forest habitat. Flower and fruit herbivores caused more damage in closed forest than in gaps, and this difference was even bigger under the insecticide treatment. Insecticide effectiveness varied depending on the type of forest patch. The highest herbivore impact on seeds was found in the mature forest without insecticide treatment, where most seeds were destroyed. The percentages of seed damage reported here show that predispersal predation is limiting seedling recruitment, especially in mature forest. Other possible explanations might be differences in insect composition, densities, and behavior between gaps and mature forest.

The basis of obligate epiphytism remains poorly understood. In this study we compare mortality, growth and fecundity for two populations of the epiphytic bromeliad Tillandsia brachycaulos in Yucatán. The fate of dislodged individuals of Tillandsia brachycaulos was compared with that of specimens anchored on tree bark in a dry forest in south-east Mexico. Annual percentages of dislodgement differed among location on phorophytes and epiphyte life stage, ranging from 1% on primary branches to 32% on quaternary branches, and from 7% for adult ramets to 64% for seedlings. Fallen versus attached individuals exhibited different mean annual leaf production, 18 and 21 leaves per individual respectively. The same pattern was observed for capsule (0 and 1.04±1.09 capsules per individual) and ramet production (0.01±0.08 and 0.54±0.68 ramets per individual). Annual percentage mortality was different for fallen (92%) and attached individuals (72%). The plants on the ground and those still anchored to hosts performed differently in ways that help explain why this bromeliad is an obligate epiphyte at the study site. More fundamental causes for its obligate status are also considered.

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