Rhus ovata, commonly known as sugar bush or sugar sumac, is a shrub or small tree found growing in the canyons and slopes of the chaparral and related ecosystems in Southern California, Arizona, Baja California and Baja California Sur. It is a long lived-plant, up to 100 years, and has dense evergreen foliage that make it conspicuous. It is closely related to and hybridizes with the lemonade sumac.
Description
Morphology This plant is a tall, wide woody shrub to small tree that ranges in height from 2–10 m (6.6–32.8 ft) with a rounded appearance. The stout twigs are thick and reddish when young. The foliage is suspended on a petiole 10–30 mm long. The leaves are 3 to 8 cm long, and roughly the same size in width, shaped broadly ovate to broadly elliptic, folded at the midrib, and have a leathery, glabrous texture. The leaf tips range from acute to acuminate, and the leaf margins are entire.The flowers are small, less than 1 cm wide, composed of 5 white to pink petals and 5 reddish sepals with ciliate margins. They are gynodioecious and self-incompatible. Some plants may only have female flowers, others may be hermaphroditic with bisexual flowers, and some with a combination of both male-sterile female flowers and bisexual flowers. The flowers occur clustered on branched inflorescences at the end of current seasonal branches. The branches of the inflorescence are stout, with the bractlets being less than 2 mm large. The fruit is a reddish, hairy and sticky drupe, 3–5 mm long and 6–8 mm in diameter, with a flattened appearance, producing a single seed surrounded by a stony endocarp. Male-sterile plants tend to set the most fruits.
Taxonomy
Phylogeny and hybridization Over 20 species in the genus Rhus occur in North America, Europe, and Asia. Phylogenetic analysis based on numerous traits placed Rhus ovata in the section Styphonia, alongside Rhus integrifolia, the lemonade sumac. Both plants are their closest relatives, and hybridize where their range overlaps, this overlap typically being in coastal mountains. Lemonade sumac tends to occur with a more western distribution along the coast and on islands, while sugar bush is typically found more inland and to the east, with both intergrading where they meet. The two species diverged roughly 3 million years ago, and had several period of expanding and contracting distributions, with fossils being found as far north as Nevada. Species distribution modeling implies a future northward shift for the populations in Arizona, but ecosystems in California will likely remain stable into the future. However, human impact will likely continue to fracture suitable habitats as climate change affects human development, with some implications for gene flow and adaptation. A worst-case scenario of global climate change may even see R. ovata being pushed towards the coast by 2070. The hybrids produced are a result due to the fact that both species are visited by the same pollinators. Hybrids possess morphological traits intermediate between both species. Research has shown that only 19% of interspecific crosses produced seeds, a much lower fertility rate compared to the 61% and 73% exhibited by crosses within species. The low fertility rate of hybrids indicates that they have a loss of fitness from their parents, suggesting that there is some incompatibility between the two species. Although hybrids may be sterile, populations of sugar bush have high levels of genetic diversity.
Etymology The epithet ovata refers to the egg-shaped leaves present on the plant.
Distribution and habitat The sugar bush is primarily located in inland localities, as it grades into its relative, the lemonade sumac, near the coast. However, it does occur on Santa Cruz and Catalina Island. It may be found throughout the inland mountains and foothills of Southern California, continuing southward through the Peninsular Ranges of San Diego County into northern Baja California, in the foothills and mountains of the Sierra de Juarez and the Sierra de San Pedro Martir. Further south, more disjunct distributions are found in the sky islands of the Central Desert of Baja California, primarily in the mountains of the Sierra La Asamblea and the Sierra San Borja. The southern limit occurs in Baja California Sur, on the Tres Virgenes volcano. The species is also found distantly in Arizona, on the Mogollon Rim. The plant may be found along the slopes of canyons in the foothills of mountains, mostly in the chaparral and associated ecosystems. They are drought-tolerant plants, and even occur along the edges of the Colorado Desert in the eastern foothills of the Peninsular Ranges. Their preferred soil types include well-drained mediums derived from both granitic and sedimentary materials, but not alkaline soils.
Ecology The flowers bloom from March to May, and a variety of bee species may be seen visiting the flowers, including the Western honey bee and the black-tailed bumblebee, along with smaller bee genera such as Andrena, Perdita, Nomada, Evylaeus. After pollination, fruits mature over the summer and may be collected from July to August. They generally will fall onto the ground, forming a seed bank in the soil, but many will also remain on the plant into fall. Seeds may be consumed by the larvae of eurytomid wasps.The fruits and the seeds of Rhus species are generally dispersed by birds and mammals, and in the related Rhus integrifolia, many animals even dispersed fruits before they fell off the shrub. The larvae of the eurytomid wasps may predate up to 50% of the fallen seeds in the wild, with a singular larvae consuming the seed by entering, eating the entire interior, leaving an exit hole upon departure, which may be noticeable. Other interactors with the fruits include rodents and birds, who will also eat or disperse the seeds. Some rodents, like the dusky-footed woodrat, will strip and consume the bark of the plant, leaving entire branches bare. Pieces of the sugarbush form a minor portion of the food supply within wood-rat nests.
Uses
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