Quercus engelmannii 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Fagales Fagaceae

Scientific Name: Quercus engelmannii Greene
Common Name(s):
English Engelmann Oak
Taxonomic Source(s): Trehane, P. 2007-2018. The Oak Names Checklist. Available at: (Accessed: 2 February 2016).

Assessment Information [top]

Red List Category & Criteria: Endangered A3c ver 3.1
Year Published: 2017
Date Assessed: 2016-10-19
Assessor(s): Beckman, E.
Reviewer(s): Oldfield, S.
The Engelmann Oak (Quercus engelmannii) is a rare oak of California and Baja California with particular habitat requirements. With an area of occupancy (AOO) of approximately 1,200 km2, continuing decline based on various ways of measuring, and only two locations, Q. engelmannii can be classified as Vulnerable under B2ab(ii, iii, iv, v). The species can also be assessed as Vulnerable A2c, which is slightly different from the 1998 Red List publication which listed A1c, because it is now believed that some of the causes of past reduction such as land use and development have not ceased. But, recent studies have projected a greater population decline in the future due to continuously shifting and unpredictable climates. Conlisk et al. found in 2014 that when a dynamic species distribution model (suitable habitat prediction) was combined with a stochastic, stage-based metapopulation model (prediction of population trajectories), the possible effects of shifting dispersal, fire, and masting values caused a 33–61% decline in total population by the year 2100. Taking a slightly generous approach, a 50% decline falls roughly between these two predicted extremes. This places Q. engelmannii in the Endangered category under A3c.
Previously published Red List assessments:

Geographic Range [top]

Range Description:Quercus engelmannii is native to southern California and northwestern Baja California, Mexico. It occurs in four of California's Floristic Provinces: South Coast, San Gabriel Mountains, Peninsular Ranges, and San Jacinto Mountains (The Jepson Herbarium).

At one time Engelmann Oak populations reached east to Arizona, but now exist in a restricted western range, attributed to the ancient climatic changes that formed the Sonoran and Mojave deserts. This also explains why Quercus engelmannii is somewhat unique within its range, and boasts the title "northernmost subtropical oak." It most closely resembles Arizona White Oak (Q. arizonica) and Blue Oak of Mexico (Q. oblongifolia). This oak's already small range faces pressure from suburban sprawl in the San Gabriel Valley, especially thinning subpopulations located in its northern range (Henrich 2012). Many know this species by phrases such as, “the rarest tree-oak species in the United States” or “the most sensitive type of oak woodland in California” (Merenlender and Heaton). 

Using 662 points from the GBIF database, it is found that Engelmann Oak has an AOO of approximately 1,200 km2, and an extent of occurrence (EOO) of about 19,000 km2. These values exclude the Santa Catalina island, because the single occurrence of Q. engelmannii recorded by Thorne in 1967 has never been relocated (Knapp 2010); the EOO is about 25,500 km2 with the island included.
Countries occurrence:
Mexico (Baja California); United States (California)
Additional data:
Estimated area of occupancy (AOO) - km2:1220Continuing decline in area of occupancy (AOO):Yes
Estimated extent of occurrence (EOO) - km2:18800
Continuing decline in extent of occurrence (EOO):Yes
Number of Locations:2
Upper elevation limit (metres):1300
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:In 2012, four subpopulations of Q. engelmannii were described based on percent of total population in California: 93% on the Black Mountains of central San Diego County; 6% on the Santa Rosa Plateau of Riverside County; 0.5% in Orange County, and <0.1% in Los Angeles County. At the same time these percentages were calculated, the Los Angeles County Arboretum was recorded to have a population of nearly 250 Engelmann Oak trees, which is the largest extant population in the county (Henrich 2012).

The largest remaining stands of Engelmann Oaks are located in San Diego County, on the Santa Rosa Plateau and Black Mountain, with smaller, fragmented subpopulations existing in the surrounding counties (California Native Plant Society). There is no current estimate of the total population of Q. engelmannii, but it can be reasoned that if the Los Angeles County subpopulation contained approximately 200 mature individuals and they represent 0.1% of the total, there was likely a fairly large total population in 2012.

In the 1990's, there was indication through aerial photography that after the severe land degradation experienced in the late 1800's and early 1900's, better land management practices had allowed enough oak regeneration to increase the overall population of Engelmann Oaks, as well as produce more vigorous trees (Merenlender and Heaton). But, recent information likely points to a currently decreasing population size overall, or at least a stagnant and more unpredictable population, affected heavily by fire and climate change trends.
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Population severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:The current natural population of Engelmann Oak is scattered among restricted habitats which maintain adequate rainfall (at least 15 inches per year), rare instances of frost, and moderate summer temperatures (Henrich 2012). This species can be found in valley grassland, foothill woodlands above the dry coastal plain, and margins of chaparral (Calflora 2016). Gentle, southern facing slops are a favourite, with soil type ranging from deep loamy-clay soils to shallow rocky soils. Healthy stands are usually found near a source of summer water, including springs, streams, or summer monsoonal rains (Henrich 2012).

This tree reveals an open structure reaching about 12 m, with blue-green foliage and long, rounded, evergreen leaves (Tree of Life Nursery). Surprisingly, seedlings are quite tolerant of fire compared to mature trees, which may be killed in prolonged flame. Within a stand, canopy coverage ranges from 10 to 50%, and its dominate companion is commonly Coast Live Oak (Henrich 2012).
Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):30

Use and Trade [top]

Use and Trade: Engelmann Oaks held an important position within the lives of native peoples. Acorns were shelled, ground, and leached for use as flour by the Gabrielino-Tongva tribes. These ground acorns were also made into porridge by the Luiseño and Diegueño tribes living in Riverside and San Diego counties, or whole acorns were stored in granaries for winter use (Henrich 2012, Tree of Life Nursery).

Threats [top]

Major Threat(s): There are many current threats to Q. engelmannii, and experienced in singularity, each threat may not produce a detrimental outcome. But, in the last few decades threats have layered upon each other, creating a type of compounding pressure on this species.

Perhaps the most obvious obstruction is suburban sprawl, especially in the San Gabriel Valley, causing natural populations to become fragmented to the point of falling rates of pollination and acorn production. Previous studies showing the long travel distances of oak pollen have recently been questioned and this distance estimate lowered, explaining the greater impact of fragmentation (Henrich 2012).

Another direct effect of development is the increasing risk of human induced wildfire. The entire range of the Engelmann Oak exists within these higher-risk areas. Principe found in 2015 that of the three matrix vegetation communities occupied by Q. engelmannii (grassland, sage scrub, and chaparral), fire damage to the trees went from low damage in grasslands, to high damage in chaparral. This is not good news, since continued human development will likely spread over grassland before chaparral, leaving the remaining Engelmann Oak population in greater risk. Principe also found that although Q. engelmannii trees "appear resistant to grass-layer fires, there appears to be a trend toward increased mortality for trees with basal scars when they are exposed to subsequent fires." Two of the largest wildfires in California burned large portions of this species’ range in the 2000's, with the 2003 Cedar Fire burning approximately 53% of monitored trees within the Santa Ysabel Open Space Preserve (Principe 2015). This preserve is within the Black Mountain region, where the vast majority of the total population is located.

Known past land-use damages and current stress from pests have and are causing considerable damage, especially regarding the tree's regeneration. On the Santa Rosa Plateau, Lathrop et al. found in 1991 that "reproduction is insufficient to maintain the present occurrence and abundance of older age classes in the future." This was attributed to past, nearly continuous grazing of the area for the last 75 years, causing soil compaction and damage to existing trees (Henrich 2012). Later studies determined that within southern oak woodland on the Santa Rosa Plateau, Pocket Gophers (Thomomys bottae) were causing extensive damage to roots of Engelmann Oak seedlings and saplings, up to several centimetres basal diameter. The difference between the overall rate of resprouting from damage due to Pocket Gophers (7%) compared to other factors such as dehydration, browsing by other deer, etc. (32%) was found to be statistically significant (Harper et al.). There is also recent concern surrounding Polyphagous and Kuroshio Shot Hole Borers (PSHB/KSHB) in Southern California. These beetles are a host for the pathogenic fungus Fusarium euwallacea, which they bring with them as they drill into the trunks and branches of trees to reproduce (University of California). The fungus has spread throughout Los Angeles and San Diego Counties, and is harmful to Q. engelmannii, especially considering it is very difficult to detect before it is too late (T. Thibault pers. comm. 2016).

In 2014, Conlisk et al. combined dynamic species distribution models (suitable habitat prediction) with stochastic, stage-based metapopulation models (prediction of population trajectories) to evaluate the effects of climate change, land use change, and altered fire frequency to Q. engelmannii. Special attention was placed on the roles of seed dispersal and predation. They concluded that although climate change will be the largest threat to Q. engelmannii, their scenarios of different dispersal, fire, and masting combinations found that the species' abundance in 2100 ranges from 39-67% of the current population; this leads to the idea that these compounding factors play an important role in addition to the concept of climate change shifting habitat ranges. It was also noted that the projected population declines (metapopulation model used) are proportionally greater than habitat losses (species distribution model used). This suggests that climate change predictions based solely on habitat suitability may under-estimate the total impact of climate change.

Finally, putative hybrids between Q. engelmannii and Q. cornelius-mulleri are common in areas of contact within Riverside and San Diego counties (eFloras 1997). This could cause concerns if the total Engelmann population begins decreasing due to encroaching hybrids.

Conservation Actions [top]

Conservation Actions: The Los Angeles County Arboretum has a population of nearly 250 Engelmann Oak trees, which is the largest remaining extant population in Los Angeles County. There is a neighbouring, smaller subpopulation at Santa Anita Racetrack, as well as another small stand within The Huntington Botanical Garden in San Marino. At the easternmost edge of the species' range is Rancho Santa Ana Botanic Garden in Claremont, where since the late 2000's they have been establishing a grove from acorns collected from isolated, wild individuals ranging from Pasedena to Monroviagrove (Eubank 2008).

The Santa Rosa Plateau is the only preservation area established specifically for Engelmann Oaks and is managed by The Nature Conservancy (Henrich 2012). At their Ecological Reserve (located at the southern end of the Santa Ana Mountains), management fires of the grass-layer were initiated in a study plot in 1988. This data is being compared to a plot in Riverside County, which is now the northernmost ecologically-intact range of Q. engelmannii, and a plot in Santa Ysabel Open Space Preserve within San Diego County, at the centre of the current natural range (Principe 2015).

Outside of managed wild populations, the California Native Plant Society provides information to homeowners regarding the tree's use in landscape, including ecological requirements, and locations for purchase. Other organizations such as the Arroyo Seco Foundation have sold thousands of trees to parks and natural areas by collecting acorn donations and then propagating into trees each year.

Citation: Beckman, E. 2017. Quercus engelmannii. The IUCN Red List of Threatened Species 2017: e.T34020A2840625. . Downloaded on 25 September 2018.
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