Quercus engelmannii

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.

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.