Natural History of James Reserve

James Reserve
Overlook of James Reserve Image by Lobsang Wangdu

The climate of the James Reserve is typical of the mountainous regions of Southern California and is a product of the highly variable daily weather we receive here (see current weather data). As a result, temperature and precipitation vary greatly over the year (Figure 1) and over years (view long-term trends here).   Detailed information on weather data at the James over the last 16 years can be found here.

Monthly Precip
Figure 1. Monthly changes in precipitation and temperature at the James Reserve. These are averages of 14 years of weather data collected on the Reserve.

We do not have temperature data at Los Osos. However, because of its relatively low elevation and proximity to Palm Springs, temperature patterns are similar. Los Osos is a hot desert site where the average maximum summer temperature is 41O C (106O F).  Precipitation data available for the site indicates annual precipitation averages around 333 mm (13.1 inches). The small amount of rain that the area receives falls primarily during the winter (Fig. 2) when temperatures are more moderate (average maximum temperature = 22O C [71O F]).
Fig. 2. Precipitation patterns for the Oasis de los Osos Reserve

The San Jacinto Mountains, in which the James and Los Osos reserves are found, is part of the Peninsular Ranges Province that extends from San Jacinto Peak down 900 miles to the tip of Baja California. The mountains in this range were formed from the collision of two tectonic plates, the Pacific Plate and the continental North American Plate. The friction of this collision formed a series of masses of molten rock that rose to the surface. Upon cooling, they formed massive solid granite rock formations called batholiths. Upon erosion and continued pressure from the San Andreas and San Jacinto Faults, the San Jacinto Mountains arose above the surrounding landscape, with predominantly steeper slopes on the northeast side.

Within the San Jacintos, location plays a major role in how an area develops. The James Reserve lays nestled within the high elevation Hall Canyon watershed on the western side of the range. Here cooler temperatures and higher precipitation produce a protective forest cover, buffering the underlying soil from weather extremes.   On the other side of the mountain, Oasis de los Osos is a steep alluvial fan on the dry northeast side of the mountain.  Dryer conditions support sparse vegetation and result in periodic torrential rains and violent floods carving deep canyons that broaden into flood plains. In both cases, the geology produces unique conditions that have resulted in diverse differences in flora and fauna of the two Reserves.

Lemon Lily
Lemon Lily. Image credit: John Laundré

James Reserve: The flora of the San Jacinto Mountains includes a community of plants characteristic of many different regions, The Sierra Nevada, Baja California, the coast, and the desert, as well as endemic plants found nowhere but here such as the lemon lily. A complete list of the plant species attests to the high floral diversity of the area. Here is the latest chapter of the Flora of the Fuller Ridge Area and and here is most up to date master list of the flora of the whole San Jacintos. The usual caveats of this being a work in progress apply.  Here is a link on how to identify the two species of Delphinium possibly found in the Reserve. Besides being able to print the species list as a handy pamphlet, you can access the list of plant specimens in the James Reserve Collection here.  If you follow this link to the search page for the Consortium of California Herbaria and put in James Reserve for Geographic Locality, you will get a list of plant specimens from the James housed at other herbaria in California. A Jepson checklist of plants found in the Hall Canyon area has been compiled by Tom Chester and can be accessed here. The designations of 4S and NT indicate whether and how many of a particular species was specifically found along the Four Saints or Nature Trails of the James Reserve.


The James is home to over 180 species of vertebrates, which include 121 birds, 27 mammals, and 23 amphibians and reptiles. At one time, California spotted owls and mountain yellow legged frogs (MYLF) inhabited the Reserve. MYLF have been successfully reintroduced to Indian Creek at the James, but the population remains small. Commonly seen reptiles include several species of small lizards, including the western fence lizard and sagebrush lizard.  A survey conducted on the Reserve in 1996-97, attests to the high diversity and abundance of reptiles found here. Abundant bird species include western bluebirds, mountain chickadees and violet-green swallows, which use the reserve nest boxes. Long term mist netting  efforts have provided insights on the changing species diversity of the avian community on the Reserve. Common mammals to be seen are Merriam chipmunks, California ground squirrels, and western gray squirrels.

Oasis de los Osos – Los Osos provides habitat for a wide variety of resident and migratory birds, mammals, reptiles and amphibians.  However, little is known of the exact species to be found there. We are currently in the process of compiling a list for this area and welcome any authenticated records that you might have.  Contact the Assistant Director ( if you would like to share your records with us.  The stream provides habitat for the freshwater snail, Laguna Mountain Springsnail (Pyrgulopsis californiensis).

On July 3-19 1908 Joseph Grinnell and Harry Swarth surveyed Strawberry Valley where Idyllwild was just starting to emerge as a resort. On July 2010 and 2011 the San Diego Natural History Museum in cooperation with Riverside County Parks were able to conduct a resurvey of the area with surprising results shown here.