Ecology and Diversity of the Manzanitas
Earlier this month on April 5th, CCBER's Tucker Research Botanist and Collections Manager Dr. Greg Wahlert presented about one of the chaparral's most iconic trees, the manzanita (genus Arctostaphylos), to the Channel Islands Chapter of the California Native Plant Society (CNPS). Wahlert discussed the manzanita's phylogeny, reproduction, and diversity. He also led a manzanita field trip for CNPS at La Purisima State Park the following Saturday.
The manzanita is a genus of closely related shrubs in the family Ericaceae, which includes heather and blueberry. There are 104 taxa within the genus Arcostaphylos. They are mostly found in western North America, with one species, A. uva-ursi, having a circumpolar distribution all over the world. The genus has high endemism, or species unique to an area: many species are restricted to particular soils such as serpentine, granite, sandstone, and Monterey shale. Nearly 50% of manzanitas are rare or endangered. They occupy diverse habitats, but they are most commonly found in the chaparral and are adapted to a Mediterranean climate with wet winters, dry summers, and cyclical disturbance by fire.
The characteristic red bark of the manzanita, useful for identifying the genus (Arctostaphylos) if not the species. Photo by Ellen Finch on WIkipedia.
Manzanita taxonomy, or classification, is notoriously difficult. Their evolutionary lineage and relatedness was the subject of Wahlert's master's thesis project, which was finished in 2005 and published in 2009 (Wahlert, Parker, and Vasey 2009). Wahlert came about his fascination with manzanitas when working at CNPS preserves during his high school years.
Two different herbarium specimens of reportedly the same species of manzanita, A. tomentosa. The specimen on the right has hairs on the bark, while the specimen on the left has the smooth red bark characteristic of many Arctostaphylos plants.
Wahlert discussed how important hybridization was in the diversification and formation of new species in manzanitas. Nearly half of the species in the genus are thought to be hybrid products of reproduction. Hybrids are often immediately reproductively isolated from the parent species, which causes rapid speciation (Schierenbeck et al, 1992). In reproduction, homologous chromosome sets have different methods of combination. In humans, chromosomes come in a diploid (2n) set, with reproduction creating diploid (2n) offspring. This is homoploid reproduction, where two diploid parents make a diploid. Plants have many other combinations, such allopolyploidy, where diploid parents (2n) can make tetraploid offspring (4n), which is how the manzanita species A. mewukka came about. Species with different ploidy can also reproduce with each other in plants. This hybridization also gives rise to different adaptations.
A. mewukka found in the high-elevation Sierra Nevadas, quite far from the typical coastal chaparral habitat of other manzanitas.
Manzanitas have two reproductive strategies in response to fire, resprouting or reseeding. Resprouting is when after a burn destroys the aboveground growth of a plant, the plant recovers by regrowing from the intact base. Reseeding is where the original adult plant dies and its offspring seeds, protected from the fire in the soil seed bank, germinate to take the place of the parent. Resprouting is asexual reproduction, where clones are produced, and reseeding is sexual reproduction, where genes are mixed. Resprouting does not respond to a changing environment well since the genes do not change. Seeding, on the other hand, causes higher rates of speciation, with generation time based on the frequency of fire.
Arctostaphylos are either facultative seeders (sprouters), being able to both sprout and seed, or obligate seeders, where they cannot resprout and must reproduce by seed. Most sprouters are tetraploid (4n), and most seeders are diploid (2n). 85% of Arctostaphylos are obligate seeders.
Above: Figure from Wahlert's paper showing the relationships inferred by genetic sequencing of ITS regions. Bottom right: Arctostaphylos sp. being pollinated by European Honeybee (Apis mellifera) at Arroyo Hondo Preserve. Photo by Elaine Tan.
The main body of Wahlert's research was to examine how different manzanita species were related to each other by sequencing the nuclear DNA of the ITS region, as well as the genes found in chloroplasts. His goal was to infer the phylogeny, or evolutionary history and relative relatedness of different species, through a broad sampling of different taxa. 101 taxa were sampled for ITS sequences, while nine taxa were sampled for chloroplast DNA. The research returned inconclusive results: 45% of taxa sampled shared identical ITS gene sequences in the "gray leaf" haplotypes across both seeders and sprouters. The ITS and chloroplast genes were not variable enough for clear genetic lineages to show, though two main clades within Arctostaphylos were tentatively identified.
Wahlert hopes to go further with research into manzanitas through haplotype-specific sequencing, using a direct method to sequence physically separated homologous chromosomes. This sequencing method originated from forensic science. Other goals include getting ITS DNA sequences of all species and more A. uva-ursi samples to resolve its relationship with the rest of the genus.
Wahlert also discussed the identification of manzanitas native to the Santa Barbara area. While identification is difficult, autumn is the best time for manzanita identification, as there are "nascent" petal-less inflorescence and fruit present; flowers in spring are generally not useful in identification. Identifying characteristics include the "basal burl", or thickened base of facultatively sprouting manzanitas; the "habit" of the plant, either upright or sprawling; stem characteristics such as or smooth versus shedding bark; and fruits and seeds being fused or not.
In Santa Barbara, there are 12 taxa, with endemics unique to the county such as A. refugioensis. There are also a few species found only on the Channel Islands, including A. insularis and on Santa Cruz Island and A. confertiflora on Santa Rosa Island. Some species like A. glandulosa have several subspecies, which can be wildly diverse in morphology.
There will be a manzanita identification workshop at CCBER in October 20-21, 2018 headed by Tom Parker and Mike Vasey, Wahlert's co-authors of his 2009 paper. Don't miss out!
A view of the coastal chaparral common throughout Santa Barbara. Low-growing trees and shrubs such as Arctostaphylos species make up much of the landscape.
A big thanks to Gregory Wahlert for the fascinating presentation, and for his help in editing the article! If you're interested in learning more about his research, you can contact him at email@example.com.
Additional thanks to the California Native Plant Society, Channel Islands Chapter for holding their event at CCBER. For more about our local plants, see their Calendar of Events.
Finch, E. L. Manzanita branches with red bark. Wikipedia image. <https://en.wikipedia.org/wiki/Manzanita#/media/File:ManzanitaShrubBranches_wb.jpg>
Schierenbeck, K., Stebbins, G. L., Patterson, R. W. (September 1992). Morphological and cytological evidence for polyphyletic allopolyploidy in Arctostaphylos mewukka (Ericaceae). Plant Systematics and Evolution. (179),187-205. <https://doi.org/10.1007/BF00937596>
Wahlert, G. A., Parker, V. T., Vasey, M. (November 2009). A phylogeny of Arctostaphylos (Ericaceae) inferred from nuclear ribosomal ITS sequences. Journal of the Botanical Research Institute of Texas. 3(2), 673-682. <https://www.jstor.org/stable/41971858>