Arctostaphylos nipumu T. Abbo, M. A. Stickrod, A. Krohn, V. T. Parker, M. C. Vasey, W. Waycott & A. Litt., 2025

Abbo, Tito, Stickrod, Morgan A., Krohn, Alexander, Parker, V. Thomas, Vasey, Michael C., Waycott, William & Litt, Amy, 2025, Investigating a hybrid mixed population leads to recognizing a new species of Arctostaphylos (Ericaceae), PhytoKeys 251, pp. 119-142 : 119-142

Arctostaphylos nipumu T. Abbo, M. A. Stickrod, A. Krohn, V. T. Parker, M. C. Vasey, W. Waycott & A. Litt. sp. nov.

Figs 6 , 7

Type.

United States of America • California: San Luis Obispo County, Nipomo Regional Park, 200 m southwest (260 °) from parking area exit onto Pomeroy Rd. , Oceano 7.5 ’ Quad: 35°01'57.5"N, 120°30'01.8"W, ± 55 m, 114–124 m alt., 28 Dec 2023, MA Stickrod 135 (holotype: UCR!; isotype: SFSU!) . Paratypes (all from San Luis Obispo County, for coordinates and collection dates see Suppl. material 1): • Nipomo Regional Park , MA Stickrod 126 ( UCR; SFSU) • same locality, MA Stickrod 128 ( UCR; SFSU) • same locality, MA Stickrod 130 ( UCR; SFSU) • same locality, MA Stickrod 131 ( UCR; SFSU) • same locality, MA Stickrod 134 ( UCR; SFSU) • same locality, MA Stickrod 128 ( UCR; SFSU) • same locality, T Abbo 147 ( UCR; CAS; MO; NY; OBI) • same locality, T Abbo 148 ( UCR; DAV; UCSB) • Blacklake, Golf Ball Rd. , T Abbo 149 ( UCR; IRVC; SBBG; SD) • Blacklake, Black Lake Canyon Dr. , T Abbo 150 ( UCR) • Los Berros Ridge, Dale Ave. , T Abbo 153 ( UCR; OBI; RSA) • southeast of Los Berros; Summit Station Rd. , T Abbo 154 ( UCR; UCSB) .

Description.

Shrub up to 2, 3 (5) m ht. and <10 m width; often layering, rooting when branches contact soil; burl 0 but with branchlets sprouting infrequently on stems; bark (red tinged) gray and shredding, from large stems up to new growth; twigs and petioles with moderately to very dense, short, nonglandular hairs; leaves isofacial (with stomata on both surfaces), green, shiny, generally lanceolate to ovate (elliptic to rounded with mucronate tip); blade 1.4–3.2 cm length, 0.9–1.9 cm width; petiole 0.4–0.8 cm; inflorescence a raceme or few-branched panicle (generally <5 - branched), nascent and mature inflorescences of similar length and thickness, 0.5 to 1.1 cm, and ca. 1 mm; bracts ± scale-like, often grading to ± leaf-like proximally, generally green, photosynthetic in summer; flowers 5 - merous, urn-shaped, white to pinkish-white; fruit a multi-seeded drupe, generally reddish-orange, depressed axially; mesocarp mealy; endocarp generally rough, fused or separating into a variable number of nutlets.

Etymology.

Based on discussions with the yak tityu tityu yak tiłhini (Northern Chumash Tribe), we selected the name Arctostaphylos nipumu because nipumu is the ytt (Northern Chumash language) word for the Nipomo Mesa region. The word nipumu is literally translated to English as “ of the big house ”, so we treat the epithet nipumu as a noun in apposition; ergo, Arctostaphylos nipumu is translated as “ Arctostaphylos of the big house ”. We recommend that A. nipumu be referred to by the common name nipumu manzanita or Nipomo Mesa manzanita; the latter regional name, Nipomo Mesa, is more well known in current usage, but the word Nipomo is an inferior Spanish transliteration of the word nipumu .

Distribution.

Arctostaphylos nipumu is one of the most narrowly distributed Arctostaphylos species, occurring exclusively on Oceano series soil (Soil Survey Staff, USDA) at 100–200 m elevation in the sandy maritime chaparral and adjacent Quercus agrifolia woodlands of the Nipomo Mesa. It is the only member of the genus found in this area, and its isolated population appears to be associated with two local waterways, the Santa Maria River to the south, and Arroyo Grande Creek to the north (Fig. 1 , inset). Its range is situated within a disjunction in the distribution of A. crustacea and just north of the northern edge of the distribution of A. purissima . Little is known about the biogeography of Arctostaphylos species, so it is unclear what has led to this distribution pattern, which is a topic we are currently investigating.

Notes.

A. nipumu is most reliably distinguished from the two populations of A. rudis using its bark characteristics. A. nipumu has gray to reddish-gray bark that shreds in short strips generally all the way to its young branches; the young branches may become progressively redder but the outer bark remains a dull gray or brown that contrasts with the inner bark. The older stem bark of A. rudis may appear similar to that of A. nipumu but often is redder and shreds in wider strips (Fig. 7 ), and younger branches tend to be smooth dark red or if shredding, then the outer bark does not contrast with the inner bark. Consistent with its high levels of introgression (Fig. 3 ), the Point Sal population of A. rudis is highly variable. Some individuals may appear very similar to A. nipumu but, in addition to having the described bark difference, will be shorter and ± mounded compared to the taller, ± erect A. nipumu . Other individuals may be strikingly distinct from A. nipumu by having burls, long hairs, and / or truncate to lobed leaves resembling A. crustacea , whereas A. nipumu lacks a burl, always has short hairs, and typically has lanceolate to ovate leaves (never truncate or lobed). In contrast, A. rudis individuals from the Lompoc area (represented in our molecular data by the Burton Mesa population) tend to be more similar to each other as well as to A. nipumu . They are, nonetheless, easily distinguishable from A. nipumu because they have the pronounced bark difference and consistently have burls. The lack of a burl in Arctostaphylos from the Nipomo Mesa was independently observed by Keil and Hoover (2022) who noted in the Vascular Plants of San Luis Obispo County the inconsistency with Jepson’s description of A. rudis and the plants on the Nipomo Mesa, writing: “ repeated unsuccessful search has been made for the basal burl which is said in the original description to be present. Apparently the presence or absence of that structure is not a distinctive feature of this species. ” (p. 448).

While the Point Sal population appears to be composed of recent first and second generation hybrid individuals, the Burton Mesa (Harris Grade south) population of A. rudis has lower percentages of admixture (Fig. 3 ). Currently, we are further investigating the Point Sal and Burton Mesa populations. The questions we are considering include: what is the extent of hybridization among individuals, are these populations genetically stable and uniform, and is monophyly present in a large sample of individuals. For now, we recommend that these populations continue to be referred to under the name A. rudis with the understanding that the name refers to hybrid individuals.

Conservation considerations.

Arctostaphylos nipumu is extremely limited and restricted to the Nipomo Mesa, and, consequently of great conservation concern, especially as the subpopulations are mostly surrounded by development. Based on our estimates from satellite images, the vast majority of wild lands on the Nipomo Mesa have been converted into dwellings and agricultural fields. Fig. 5 shows the estimated land occupied by A. nipumu based on field surveys and satellite images. Thus, the range of A. nipumu consists of only 28.63 hectares distributed across 10 fragmented multi-individual occurrences, with a total population estimated to be 300–700 individuals. Similar land use change led to the near extinction of A. franciscana , which is now represented by a lone individual in the wild and clones of just a few genetic individuals ( Gluesenkamp et al. 2010; Weiss et al. 2020). The genus Arctostaphylos reliably dates back to western Nevada in the Middle Miocene in a climate more similar to the California coast than present ( Edwards 2004; Millar and Woolfenden 2016). Conserving the integrity of the genome of A. nipumu could facilitate future research testing the hypothesis that this species represents a case of paleoendemism that may provide insight into broader patterns of diversification in the genus.

We would like to emphasize that the characters used to separate A. nipumu from A. rudis are readily apparent. We were able to immediately distinguish the ca. 53 relictual A. nipumu individuals from the ca. 168 A. rudis individuals which we believe were planted as a mitigation measure to develop the Blacklake Golf Resort, as well as the restoration planting along Osage Street on the Nipomo Mesa. The ecological impact report in the Black Lake Specific Plan recommended that Arctostaphylos rudis be planted in parts of the Blacklake Golf Resort with low vegetation cover, not used as turf ( San Luis Obispo County 1982: IV- 33). Based on their burls and smooth red young bark, we propose that the majority of plants at these sites were planted cultivars originally sourced from the Burton Mesa or Point Sal; however, the origin of the nursery stock used for restoration at this site remains uncertain. Even when sound ecological considerations are implemented, mitigation and revegetation efforts such as these have often historically neglected to consider potential genetic impacts on native plants and population structure through processes such as genetic swamping, demographic swamping and outbreeding depression, among others ( Hufford and Mazer 2003, Prentis et al. 2007, Byrne et al. 2011). The integrity of the genome of A. nipumu appears to depend on its isolation from other Arctostaphylos species, and these introduced plants have the potential to hybridize with A. nipumu . Because of their potential to hybridize, allopatry is hypothesized as an important factor maintaining the separation of Arctostaphylos species within the same clade ( Parker et al. 2020). This stresses the importance of utilizing plants with local genotypes for propagation in restoration and mitigation projects ( Maschinski and Albrecht 2017). Below is a key of Arctostaphylos species from San Luis Obispo and Santa Barbara Counties to aid in the identification of A. nipumu .

Figures

Figure 1. Distribution and sampling map generated in QGIS version 3.224. The background map of the central California coastline is from OpenStreetMap using the EPSG: 3857 - WGS 84 / Pseudo-Mercator projection. The distributions of Arctostaphylos purissima (orange polygon), A. crustacea (purple polygon), and A. rudis (green polygon) were inferred using collection data from the Consortium of California Herbaria (CCH 2 Portal 2024). The locations where we obtained samples for this study are marked with solid triangles, and site names, taxa sampled, and number of samples obtained for each site are annotated on the figure. The inset shows a closer view of the Nipomo Mesa and northern Santa Barbara County, and the key waterways of the Arroyo Grande Creek, and the Santa Maria River are highlighted in blue. Note that the northern green polygon was formerly considered a northern extension of A. rudis but herein is described as the new species A. nipumu.

Figure 3. Structure plot / stacked bar graph of the K = 2 using uncorrelated SNPs from ddRADseq data. Structure was run using STRUCTURE _ THREADER version 1.3. 10 for 50,000 MCMC generations with the first 10,000 discarded as burn-in. The figure was generated using LibreOffice Calc version 7.3.7.2 and edited in Inkscape version 1.1. 2. Each individual sample is represented by a unique 1-2 letter and single number code given at the bottom of the x axis. Above the code, we indicated the individual’s source population, or its taxonomic identity for control / nonfocal populations of Arctostaphylos purissima, and A. crustacea (see Suppl. material 1 for more information about samples). The y axis indicates the genotype percentage of the two ancestral groups (colored as green and orange) inferred for each sample.

Figure 5. Known occurrences and distribution of Arctostaphylos nipumu. Nipomo Mesa, San Luis Obispo County, California. Map figure was produced using ArcGIS Pro 3.3. 2 (Esri Inc 2024). Coordinate data for individual occurrences of A. nipumu were converted to point data shapefiles and mapped using the NAD 1983 (2011) StatePlane California V FIPS 0405 (US Feet) coordinate system.

Figure 6. Line drawing of Arctostaphylos nipumu. Top right (mature main trunk): bark shredding; base lacking burl. Top left (fruiting branch): twigs and petioles with moderately to very dense, short, nonglandular hairs; leaves generally lanceolate to ovate; fruit depressed. Bottom Left (nascent inflorescences): nascent inflorescences short / compressed; bracts overlapping, ± scale-like. Bottom center (flowering branch): inflorescences short, ± same length as nascent inflorescences; flowers urn-shaped.

Figure 7. Diagnostic photographs of Arctostaphylos nipumu (Nipomo Mesa, San Luis Obispo County, CA) and A. rudis (Burton Mesa, Santa Barbara County, CA) A A. nipumu in situ, maritime chaparral habitat B depressed fruit of A. nipumu C nascent inflorescence, with leaf-like bracts proximally grading to scale-like bracts distally D A. nipumu, lacking a burl at the base of the stem E A. rudis with a pronounced burl at the base of the stem F – H gray to reddish-gray, thinly shredding bark of A. nipumu I – K pronouncedly red to reddish-gray, coarsely and variably shredding bark of A. rudis.