Arceuthobium

Abstract: Ribosomal DNA (rDNA) internal transcribed spacer (ITS) and 5.8S rDNA sequences were obtained from 22 species of dwarf mistletoes (Arceuthobium-Viscaceae) to test phylogenetic relationships. Interspecific distances ranged from 0 to 21.4% between New World species, values two to five times higher than those measures for the ITS region in other plants. One Old World species (A. oxycedri) and one New World species (A. abietis-religiosae) were remarkably similar to each other but exhibited up to 41% sequence divergence from the remaining species. Minimum length trees support the concept of a verticillately branched subgenus Arceuthobium; however, interspecific distances indicate this group is extremely heterogeneous. Subgenus Vaginata, Section Vaginata, is centered in Mexico and encompasses all the taxa previously placed in this group but is expanded to include several species previously classified in Section Campylopoda (e.g., A. divaricatum, A. rubrum, and A. strictum). The sister group relationship between A. divaricatum and A. douglasii, first seen following isozyme analysis, is supported by ITS sequence data. Section Campylopoda s. s. is now composed of 13 mainly U.S. species that show a high degree of morphological and genetic similarity. The eastern dwarf mistletoe, A. pusillum, is not closely related to A. douglashi but rather with A. bicarinatum from Hispaniola, which suggests that these taxa represent highly modified relicts that shared an ancestor in the early Tertiary. Two endemic species from Mexico and Central America (A. guatemalense and A. pendens) formed a sister group and have been placed in a new Section (Penda). Rapid molecular evolution in Arceuthobium may be associated with the adaptive radiation of this genus on numerous conifer hosts.

Abstract: Dwarf mistletoe ( Arceuthobium spp.) is a hemiparasite that is said to be the single-most destructive pathogen of commercially valuable coniferous trees in many regions of the world. Although its destructive nature is well documented in many respects, its effects on the physiology of its host are poorly understood. In the present study, water and carbon relations were characterized over a range of scale from leaf to whole tree in large (40- to 50-m-tall) individuals of western hemlock ( Tsuga heterophylla (Raf.) Sarg.) that were either heavily infected, or uninfected with hemlock dwarf mistletoe ( Arceuthobium tsugense ). Specific hydraulic conductivity ( k s ) of infected branches was approximately half that of uninfected branches, yet leafspecific conductivity ( k L ) was similar because leaf area : sapwood area ratios ( A L : A S ) of infected branches were lower. Pre-dawn and minimum leaf water potential and stomatal conductance ( g s ) were similar among infected and uninfected trees because adjustments in hydraulic architecture of infected trees maintained k L despite reduced k s . Maximum whole-tree water use was substantially lower in infected trees (approximately 55 kg d - 1 ) than in uninfected trees (approximately 90 kg d - 1 ) because reduced numbers of live branches in infected trees reduced whole-tree A L : A S in a manner consistent with that observed in infected branches. Maximum photosynthetic rates of heavily infected trees were approximately half those of uninfected trees. Correspondingly, leaf nitrogen content was 35% lower in infected trees. Foliar d 13 C values were 2.8‰ more negative in infected than in uninfected individuals, consistent with the absence of stomatal adjustment to diminished photosynthetic capacity. Adjustments in hydraulic architecture of infected trees thus contributed to homeostasis of water transport efficiency and transpiration on a leaf area basis, whereas both carbon accumulation and photosynthetic water use efficiency were sharply reduced at both the leaf and whole-tree scale.

Abstract: Whereas the biology, physiology and systematics of mistletoes have been explored in considerable detail, their ecology has received less attention and our understanding is highly fragmentary. A conspicuous exception is the dwarf mistletoes (Arceuthobium spp.)—a genus that exclusively parasitises coniferous trees, including many commercially valuable species in the forests of the western United States. Accordingly, these plants have been the subjects of intensive cross-disciplinary research for the past five decades, initially from a control and management perspective but extending into most aspects of their ecology and life history. This review summarises our understanding of dwarf mistletoes, focusing on recent developments in the areas of mistletoe–wildlife interactions, fire, ecosystem ecology and conservation biology. We also compare dwarf mistletoes with Australian mistletoes in the genus Amyema, a diverse suite of species found throughout the continent. Despite fundamental differences in their evolutionary origin and most aspects of their autecology and life history, the genera exhibit many similarities in terms of their ecological role in forests and woodlands, and their influence on stand- and forest-scale dynamics. In particular, both groups provide nesting resources for a range of birds and mammals, and nutritional resources for a diverse assemblage of species. Both also interact with fire, potentially leading to changes in successional dynamics at the stand scale. At an applied level, both groups are widely considered as pests but, as our understanding of these keystone species improves, they have the potential to serve as sensitive ecological indicators for their respective ecosystems. Key research priorities are identified for further research on both groups of mistletoes and more explicit comparative research, with Arceuthobium serving as a valuable template for future work on Amyema and Australian mistletoes in general.