Arceuthobium littorum

Like other dwarf mistletoes, A. littorum grows on and parasitizes conifers. The principal hosts of A. littorum are Bishop pine (Pinus muricata) and Monterey pine (Pinus radiata), meaning that A. littorum shows a strong preference for infecting these hosts over other conifer species; Bolander's pine (Pinus contorta var. bolanderi) is an occasional host. Although the stems contain chlorophyll, A. littorum photosynthesizes at a low rate, relying on its host for the vast majority of its carbohydrates. Dwarf mistletoes are dioecious, meaning that A. littorum plants produce either male (staminate) or female (pistillate) flowers. The male flowers are 3.5–5.7 mm in diameter and predominantly 4-petaled, although flowers with 3 or 5 petals are also common; the presence of 5-merous flowers is a key morphological difference between A. littorum and the closely-related mistletoe species A. occidentale. == Ecology ==

Infection by A. littorum negatively impacts the health of host trees. For example, hosts display decreased longevity, lower reproductive rates, and increased susceptibility to infection by fungal pathogens compared to uninfected trees. Abnormal growth patterns by the host is a common outcome of infection by A. littorum, typically characterized by the formation of large, nonsystemic witch's brooms. While broom formation disadvantages the host tree, the brooms are ecologically significant as they provide unique nesting habitats and cover for small mammals and birds. As larvae, several species of hairstreak butterflies (Callophrys) feed only on mistletoes in the Campylopoda group to which A. littorum belongs; however, none have been documented on A. littorum. == Distribution ==

A. littorum has a coastal distribution, growing in closed-cone conifer forests at elevations from sea level to 300 m. Its distribution, consisting of scattered populations ranging from Fort Bragg in the north to Cambria in the south, is narrower than that of either of its primary hosts. Although A. littorum is considered rare throughout its range, it is likely that many undiscovered populations exist on private property. Pleistocene fossils of dwarf mistletoe fossils described by Chaney and Mason in 1933 from three coastal sites in California were later identified as A. littorum. One of these sites, near Tomales Bay, still supports a population of the species 30,000–40,000 years later; the other two sites (Carpinteria and Santa Cruz Island) lie outside the modern range of A. littorum. == Taxonomy ==

A. littorum was previously considered conspecific with A. occidentale but was described as a separate species in 1992. Morphometric analyses, comparing color, branching patterns, spike measurements, and flower structure, have been used to differentiate A. littorum from other species in Section Campylopoda, including the closely-related A. occidentale and A. siskiyouense. In 1990, shortly before A. littorum was formally described, isozyme analysis of taxa in Section Campylopoda led Nickrent and Butler to conclude that A. littorum was genetically distinct from both A. campylopodum and A. occidentale. On the other hand, phylogenetic work from 2012 comparing nuclear ribosomal ITS and chloroplast sequences of dwarf mistletoes showed little genetic differentiation between most members of Section Campylopoda, calling into question whether taxa in Section Campylopoda represent distinct species or merely subspecies of a single highly-variable species. Because there is a lack of consensus on (sub)specific boundaries in Section Campylopoda, taxonomic authorities differ in their treatment of A. littorum. Plants of the World Online considers A. littorum a distinct species, largely following a framework advocated by Mathiasen and Kenaley. The Flora of North America treats the taxon instead at the subspecific level, using the name A. campylopodum subsp. littorum, following Nickrent's interpretation of Campylopoda as comprising one variable species with several host-specific subspecies. == References ==