The tree genus Juglans (nogal, walnut) is generally thought of as a north temperate group, but most of the 16 species of black walnuts (Juglans sect. Rhysocaryon) occur in subtropical areas of the New World, spanning Central America, Andean South America, and the Greater Antilles. The two species native to Central America (J. olanchana var. olanchana and J. steyermarkii) and two closely related Mexican taxa (J. olanchana var. standleyi and J. pyriformis) have not been studied thoroughly from a comparative systematic perspective, so here we provide a detailed investigation of their morphology and natural history as a first step towards a broad-based synthesis of J. sect. Rhysocaryon. Fieldwork in Guatemala and Mexico provided the material for germination studies, morphological analyses, and phylogenetic reconstruction using DNA sequences. Juglans steyermarkii was rediscovered in its type locality in Guatemala, and new populations were located during the course of fieldwork. SEM analysis of trichome diversity and density, along with leaflet shape, provided characters to differentiate among five taxa of Juglans in the accompanying key. Staminate flowers and pollen examined for the first time in J. steyermarkii showed close morphological similarities to those of other species of black walnuts. Variation in the pattern of nut-wall sculpture appears to be a promising source of variation; however, adequate capture and quantification of this variation will likely require three-dimensional scanning techniques. There was high variability in percent seed germination of the two Central American species and no observed advantage to leaching, scarification, or a prolonged stratification period. DNA studies included in the exploration of three promising sources of nuclear sequence variation: the ETS and ITS (external and internal transcribed spacer of ribosomal DNA), and the second intron of the LEAFY gene. Combined phylogenetic analysis of these sequences provided a moderately resolved topology containing two major groups of black walnuts and a general correlation between phylogeny and geography. The analyses supported a division between taxa from northern Mexico and the USA, and those from southern Mexico, Guatemala, the West Indies (Greater Antilles), and South America. Observations on the geographic range of Juglans in Mexico and Guatemala indicated a frequent association with coffee plantations, suggesting similar climatic requirements. Our studies of the dispersed and most likely fragmented populations indicate that Juglans may still be found in native plant communities as a canopy tree, although it is much more common to observe populations in cleared areas where the trees were retained as shade for coffee. We are not aware of any major efforts to conserve the native nogal in Central America, though they are sometimes left and/or planted as shade trees in coffee fincas. A study conducted in Mexico suggests that Juglans might be useful in agroforestry efforts to rehabilitate degraded sites. Â© Torrey Botanical Club.
A comprehensive systematic investigation was conducted on the extant Juglandaceae based on 25 species representing a broad sample of generic and infrageneric diversity. A total of 206 phylogenetically informative characters derived from morphological, chemical, chromosomal, and sequence-based studies formed the basis for comparative studies. Phylogenetic analysis was used to infer relationships and examine patterns of convergence in key biochemical and morphological traits associated with dispersal biology. Separate and combined parsimony analyses of three previously unpublished data sets (ITS, chloroplast DNA, morphology/chemistry) supported two major clades, Juglandoideae and Engelhardioideae, in agreement with a recent subfamilial classification. Within Engelhardioideae, the genus Engelhardia was found to be paraphyletic, as E. roxburghiana of the monotypic section Psilocarpeae was resolved as sister taxon to a New World subclade composed of Oreomunnea + Alfaroa. Within Juglandoideae, two tribes are recognized: Platycaryeae and Juglandeae. The monotypic genus Platycarya formed the sister group to Juglandeae, which was resolved fully (Carya-(Juglans-(Cyclocarya + Pterocarya))). Two new subtribes, Juglandinae and Caryinae, are described based on the cladistic pattern. Unique morphological apomorphies were detected for all genera, including the previously little-studied Cyclocarya, which was also determined to possess a novel base chromosome number for the family (N = 28). The nested position of Annamocarya sinensis within Old World Carya, combined with its lack of unique apomorphies suggested sectional recognition within Carya might be more appropriate for this taxon. Phylogenetic context was used to interpret patterns of morphological and chemical variation associated with the evolution of seed dispersal and the tropical versus temperate habitat. Although the syndrome of wind dispersal appears to be ancestral within the family, four novel origins of wing tissue are represented by Engelhardia/Oreomunnea, Platycarya, Pterocarya, and Cyclocarya. The convergence on animal dispersal has been achieved through three different developmental pathways in the production of a husk in Alfaroa, Carya, and Juglans. In general, wind-dispersed seeds have epigeal germination and those that are animal-dispersed are hypogeous, but Oreomunnea and Cyclocarya are exceptions in their respective clades by having wind-dispersed seeds with hypogeal germination. The seed-energy reserves are also revealing. With the exception of Oreommunea, wind-dispersed seeds have relatively high concentrations of the unsaturated linolenic (C) and linoleic (B) fatty acids (CB pattern), whereas all animal-dispersed fruits (viz., Alfaroa, Carya, and Juglans), and Oreomunnea, have relatively high concentrations of the unsaturated oleic (A) and linoleic (B) fatty acids (BA or AB pattern). Tropical genera, whether wind- or animal-dispersed (viz., Oreomunnea, Alfaroa, Annamocarya), have relatively high concentrations of the saturated palmitic fatty acid. Conversely, wind- and animal-dispersed fruits of temperate genera (viz., Carya, Juglans, Cyclocarya, Pterocarya, and Platycarya) have relatively low percentages of palmitic acid. The explanation here is based on the fact that seed fats must be fluid at the temperature of the living plant, thus selecting for saturated fats in warm tropical climates and unsaturated lipids in cool temperate climates.