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Premise: In plant groups with limited intrinsic barriers to gene flow, it is thought that environmental conditions can modulate interspecific genetic exchange. Oaks are known for limited barriers to gene flow among closely related species. Here, we use Quercus as a living laboratory in which to pursue a fundamental question in plant evolution: Do environmental gradients restrict or promote genetic exchange between species? Methods: We focused on two North American oaks, the rare Quercus dumosa and the widespread Q. berberidifolia. We sampled intensively along a contact zone in California, USA. We sequenced restriction site‐associated DNA markers and measured vegetative phenotype. We tested for genetic exchange, the association with climate, and the effect on phenotype. Results: There is evidence for genetic exchange between the species. Admixed plants are found in areas of intermediate climate, while less admixed plants are found at the extremes of the climatic gradient. Genetic and phenotypic patterns are out of phase in the contact zone; some plants display the phenotype of one species but are genetically associated with another. Conclusions: Our results support the hypothesis that a strong climatic gradient can promote genetic exchange between species. The overall weak correlation between genotype and phenotype in the contact zone between the species suggests that genetic exchange can lead to the breakdown of trait combinations used to define species. This incongruency predicts ongoing problems for conservation of Q. dumosa, with implications for conservation of other oaks. [ABSTRACT FROM AUTHOR]

North America is home to approximately 55 species of Ceanothus (Rhamnaceae), more than 30 of them endemic to the biodiversity hotspot known as the California Floristic Province. Although several species of Ceanothus are widespread in North America, most of the species are micro-endemics, known from a small number of occurrences and generally associated with special ecological conditions. As botanical exploration of North America continues, it is expected that more such micro-endemics will be discovered. However, field work is not the only way in which additional Ceanothus diversity will come to light; the strongly variable morphology of Ceanothus means that specimens are difficult to identify, and many biological entities that probably deserve taxonomic recognition are already represented in herbaria but have not yet been recognized as new. We here describe three new taxa of Ceanothus, two initially noted among existing collections, and another discovered during botanical exploration in San Diego County. All three are edaphic micro-endemics found only in the southern California Floristic Province, California. These include 1) Ceanothus foliosus var. viejasensis D. O. Burge&Rebman, restricted to gabbro-derived soils on ViejasMountain in SanDiego County, 2) Ceanothus pendletonensis D. O. Burge, Rebman, &M. R. Mulligan, found on granodiorite-derived soils of the central Santa Margarita Mountains in San Diego County, and 3) Ceanothus thyrsiflorus var. obispoensis D. O. Burge, endemic to rhyolite-derived soils near Hollister Peak in San Luis Obispo County. [ABSTRACT FROM AUTHOR]