Quercus tomentella 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Fagales Fagaceae

Scientific Name: Quercus tomentella Engelm.
Common Name(s):
English Island Oak
Taxonomic Source(s): Trehane, P. 2007-2015. The Oak Names Checklist. Available at: (Accessed: 2 February 2016).

Assessment Information [top]

Red List Category & Criteria: Endangered B2ab(i,ii,iv,v) ver 3.1
Year Published: 2017
Date Assessed: 2016-02-17
Assessor(s): Beckman, E. & Jerome, D.
Reviewer(s): Oldfield, S.
Contributor(s): Ashley, M.
Quercus tomentella is found only on the Channel Islands (Santa Rosa, Santa Cruz, Anacapa, Santa Catalina, and San Clemente) off the coast of California, and Guadalupe Island, Mexico. Population disturbance in the past has resulted from agriculture and ranching. In 1997, the Catalina Island Conservancy began a restoration program and successfully removed all feral goats and pigs. This species of oak has an area of occupancy (AOO) of between 200 and 250 km2 and a maximum number of locations at five. Continuing decline is projected on Guadalupe Island. Quercus tomentella has been listed as Endangered, under criterion B2ab(i,ii,iv,v).
Previously published Red List assessments:

Geographic Range [top]

Range Description:Quercus tomentella is found only on the Channel Islands (Santa Rosa, Santa Cruz, Anacapa, Santa Catalina, and San Clemente) off the coast of California, and Guadalupe Island, Mexico. The species belongs to a small, mysterious group of oaks which only exists in the western United States and northern Baja California, Mexico: the intermediate or golden oaks, section Protobalanus (Ashley et al. 2007).  Flora of North America lists the species' elevation range as 100–650 m Asl (1997).

Island Oak has an estimated extent of occurrence (EOO) of 43,500 km2 and, by generous estimates, an AOO of 230 km2. The species has been well documented, with the vast majority of stands represented in herbarium specimens. Records ranging in date from 1950 to the present were used for the AOO estimate. This is likely an overestimate considering some destruction from agriculture and livestock was still occurring on the islands after 1950. McCune's survey of Santa Catalina Island in 2005 recorded the total area occupied by stands of Q. tomentella as only 0.084 km2. This clearly does not fit the 2 km cell width for calculating the AOO. When the points from Santa Catalina alone are mapped (data for this island is currently the most extensive), the AOO is approximately 40 km2. If this is taken as the average AOO for all islands the oak occupies - since Catalina represents a mid-sized population, with some islands having much smaller areas covered by Q. tomentella and others having more area occupied – and there are six islands in the species range, then the estimated AOO would be 240 km2. So, between 200 and 250 km2 seems to be a reliable AOO, using the data currently available.
Countries occurrence:
Mexico (Guadalupe I.); United States (California)
Additional data:
Estimated area of occupancy (AOO) - km2:200-250Continuing decline in area of occupancy (AOO):Yes
Estimated extent of occurrence (EOO) - km2:43500
Continuing decline in extent of occurrence (EOO):Yes
Number of Locations:2-6Continuing decline in number of locations:Yes
Lower elevation limit (metres):100
Upper elevation limit (metres):650
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Population disturbance has been evident on the Channel Islands since the increase of agriculture and ranching, which caused dramatic vegetative changes by the late 1800s and early 1900s, as supported by records of increased non-native plant species (Halvorson 1992). Each island has a slightly different story to tell, involving varying degrees and types of habitat cover affected by agricultural use and destruction from introduced sheep, goats, and pigs. A population estimate is not available for the entire range of the species, but some data from individual islands are provided, to gauge the state of studied subpopulations.
In some cases, it is hard to know the island's complete vegetative composition before this destruction. On Guadalupe Island especially, numerous species have become extinct. Until their removal in 2006 sapling destruction by goats prevented Q. tomentella's small population on the island from reproducing sexually. Although the goats have been removed the Q. tomentella population on Guadalupe Island consists of less than 50 mature trees currently (M. Ashley pers. comm. 2015).
In 1992, 25-70%  of the vegetation cover within the Channel Islands National Park was composed of plant communities dominated by alien plants (Halvorson 1992). On Santa Catalina Island, Q. tomentella also faces damage from introduced goats. A fence was built in 1956 to separate the goat-infested from the goat-free area, but the goat-free region was still used by Mule Deer (Odocoileus hemionus), Bison (Bison bison), and feral pigs (Sus domesticus). The goat population consumed browse (mostly from sapplings of species such as Q. tomentella) during early winter, making up approximately 90% of their diet, as recorded in the 1970s (Coblentz 1977). According to a survey of Santa Catalina Island performed by McCune in 2005, the seven identified groves included 3,206 stems (mature and immature individuals) and 1,528 apparent individuals (genets). Of these stands, two are much larger than the remaining four isolated groves, and represent more than 80% of the island's total Q. tomentella population (McCune 2005). Following this survey, genetic diversity for the species on Santa Catalina has been quantified using eight microsatellite loci and 68 samples from six sites on the island. Results show that although Q. tomentella occupies a small and isolated area (island), its genetic variation is typical for oaks. Therefore, Q. tomentella does not seem to have experienced severe population bottlenecks. But the data does show an extreme amount of population structure, as compared to mainland oak populations - some stands harbour much greater genetic diversity than others (Ashley et al. 2007). But, overall, with the removal of introduced herbivores in the late 20th century, subpopulations of Q. tomentella on the United States' Channel Islands are likely increasing slowly; or, in the least, these subpopulations are stable at present (M. Ashley pers. comm. 2015). Though there is added some instability reflected in the concentration of tree density, number, and genetic diversity in a few stands. This will require monitoring and research moving forward.

If the subpopulation on Guadalupe is continuing to decline, while the other subpopulations are held constant, the holistic population trend would be one of decline. This is a worst-case estimate, since if the Channel Island subpopulations are increasing then the overall trend would be considered stable, or perhaps even increasing if the gain outweighs the loss on Guadalupe.
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Population severely fragmented:No
Continuing decline in subpopulations:Yes

Habitat and Ecology [top]

Habitat and Ecology:While there is fossil evidence that Quercus tomentella may have once occupied the southern California mainland, it has been pushed to islands off the coast where maritime Mediterranean climates remain as the closest ecology to those ancient times. The islands now housing the species give protection from frost and drought as well as provide ample amounts of rain and fog, but strong winds keep the tree from thriving too close to the coast. Q. tomentella prefers deep, moist soils within sheltered locations, but can survive in almost any soil type. In the best conditions, a height of 7 to 12 m can be reached, but many individuals facing the harshest winds are shrunken and bent, pruned by salt spray. Reproduction through acorns is difficult because of winds and rocky soil, so many inland groves are held constant by sprouting from adult trees. The species forms a woodland community inland, mainly accompanied by Canyon Oak (Q. chrysolepis) and Coast Live Oak (Q. agrifolia), where a closed canopy is created with very a sparse vegetative understory (Pavlik et al. 1991). On Santa Catalina Island, Island Scrub Oak (Q. pacifica) is the most common species within the island's oak woodlands, while Q. tomentella and Q. chrysolepis are less frequent (Stratton 2002).
Continuing decline in area, extent and/or quality of habitat:Unknown

Threats [top]

Major Threat(s): The main threats to Island Oak are past overgrazing from introduced herbivores and competition from invasive plants (M. Ashley pers. comm. 2015). Feral goats (Capra hircus), sheep (Ovis aries), and pigs (Sus scrofa) were imported to the islands, where they began to multiply outside of domestic cultivation due to the lack of predators (Pavlik et al. 1991). Dramatic vegetative changes were evident by the late 1800's and early 1900's, as shown by records of increased non-native plant species (Halvorson 1992). Livestock consumed much of the ground cover, moving on to the leaves, acorns, and even roots of Q. tomentella, which suffered declines (Pavlik et al. 1991). 

This has certainly been supported within more detailed studies on Santa Catalina, where the introduction of feral pigs occurred in the mid-1930's, from Santa Rosa Island. In the 1980's, the pigs were still abundant and altering native plant communities. Although the pigs prefer riparian and grassland cover, oak woodland made up about 20% of their annual diet (Baber and Coblentz 1986). Once the non-native livestock was finally removed in the late 20th century, Stratton (2002) studied the restoration potential for Q. tomentella on Catalina Island; she lists the main challenges as "bison disturbance, deer browsing [Odocoileus hemionus], long dry seasons and disturbed, weed saturated soils," including such invasives as Avena fatua, Cynodon dactylon, Phalaris aquatica, Foeniculum vulgare, and Nicotiana glauca. Evidence of hybridisation with another oak species including Q. chrysolepis has also been recorded, although it is unclear yet whether this hybridisation is negatively impacting Q. tomentella subpopulations (Ashley et al. 2007). 

Guadalupe Island has faced additional threats. In 1953, Howell and Cade describe what remained of the "once abundant" native vegetation on the Island. Introduced domestic livestock had destroyed most of the native shrubs, and remaining plants are located only on vertical cliffs too steep for goats. Several species of non-native grasses cover most of the island. A few large, old Q. tomentella specimens exist of the northern ridge and its west slope, but "no seedlings of any of the three species of trees are to be found as those that come up are eaten by goats" (Howell and Cade 1953).

Conservation Actions [top]

Conservation Actions: There have been large scale removal of introduced herbivore on all of the islands where Q. tomentella occurs. In 1997, the Catalina Island Conservancy began a restoration program, following 50 years of intensive ranching and farming. The program set out to remove all feral goats and pigs and convert 80 acres of hay fields in the centre of the island to native plant communities. All non-native livestock have now successfully been removed. Trials were performed regarding the potential for regeneration by acorn, for various native oaks species on the island. It was determined that recruitment is feasible, even without providing additional water to the acorn or sapling. The first objective in restoration of oak populations on the Channel Islands is to establish trees that stand above the browse line (Stratton 2002). In Santa Rosa all feral pigs were eradicated by 1993, by 1998 all cattle were removed, although free-ranging populations of elk, deer, and horses remain. In Santa Cruz the majority of sheep, which reached peak densities and subsequent ecosystem degradation in the late 1880s, had been removed by 1987. Other remaining non-native animals were removed in 2000. In San Clemente goats were all removed by 1991 and pigs and deer were eradicated in the 1990's (Knowlton et al. 2007). On the Island of Guadalupe native vegetation restoration began with the eradication of goats by 2006. Seedlings of Q. tomenella which hadn't been seen since 2003 were found in 2009 (Aguirre-Muñoz et al. 2011). 

According to BGCI, there are 14 ex situ live specimens currently existing worldwide.

Citation: Beckman, E. & Jerome, D. 2017. Quercus tomentella. The IUCN Red List of Threatened Species 2017: e.T30959A2799049. . Downloaded on 23 October 2017.
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