Sky Islands

From the summit of Mt. San Jacinto, looking down at the Colorado Desert.
Mt. San Jacinto from the Little San Bernadino Mountains above the Coachella Valley.

If you take the tram up to San Jacinto Mountain, you will have the unusual experience of departing from a desert landscape and arriving in a subalpine, mixed-conifer forest. The mixed-conifer forest is a rare ecosystem for California, which doesn’t generally offer enough precipitation to support big trees. Yet, 8,500′ above the desert floor, sugar, Jeffrey, ponderosa, and lodgepole pines, as well as cedars, black oaks, and white firs, grow in profusion in a landscape that looks like it was plucked directly out of the western Sierras.

This situation, wherein an ice-age remnant montane forest exists within 7 horizontal miles of one of the driest environments on the planet, is a combination of a number of geologic and ecologic forces that best describe the phenomena of sky islands. Many thousands of years ago, the world was cooler and wetter, and the deserts of California featured lusher vegetation, including mixed-conifer forests at higher altitudes similar to what occurs north of San Francisco. As the climate dried and warmed, these forests retreated upslope, where they now only remain in ecological pockets, or islands, separated by dramatically different ecosystems.

The most famous examples of sky islands are the Madrean Sky Islands in southern Arizona, New Mexico, and northern Mexico. These lofty mountain peaks tower above the Sonoran desert, which is more famous for saguaros and gila monsters than deer, oaks, and pines. While these mountains get the moniker “sky island” most often, Southern California also features a number of different sky islands throughout the region. These sky islands are surrounded by semi-arid chaparral and sage-scrub on the south/west side and desert on the east and north sides. On the top, the remnants of the ice age remain.

The separation from other similar environments also leads to biodiversity in which variations create unique species that don’t exist anywhere else. For instance, there’s a species of banana slug that is closely related to a banana slug that exists in profusion in the redwood forests of the northern coast. The climate in the southern part of the state is far dryer, and there is only one place that supports this species, which was separated thousands of years ago by climate change. Another example is the bigcone Douglas fir, which is a relative to the same Douglas fir that grows to towering heights on the coast or as a major component of forests in Colorado and Montana. The bigcone has many of the same features, only its cone is much larger, and it grows on dry slopes, usually as the first conifer to emerge with elevation gain.

This process, which is called speciation, functions in a similar way toward evolution on the Galapagos islands, where distinct species evolved due to isolation. Without the ability to share genetics with geographically separated members of the same species, the separated species gradually becomes something different, like the bigcone Douglas fir (from the Douglas fir) or the Engelmann oak (from the valley oak).

Relict species emerge this way, as well as relict ecologies. The elevation, and often the positioning of the mountains in relation to the flow of the jet stream and the presence of of impediments to moisture, create these pockets of yesterday. The forests atop the Peninsular and Transverse ranges, including Angeles, Cleveland, and San Bernadino, are relicts of an age in which glaciers grew instead of shrank and forests were the norm instead of the exception. These mountain environments are more closely related to lofty ranges like the Sierras or Rockies, or to the northern coasts, where higher average rainfall leads to an abundance of foliage.

In addition to being environmental oddities, these environments are also sensitive. For instance, the forests atop these ecological islands have been receiving less rainfall over the years. As a result, bark beetles and other pests emerge, and the trees do not have the strength to deter the pests. As the pests proliferate, the trees die. In many places in the local mountains, you will see the skeletal remains of a conifer, most likely a water-loving white fir, that has been stripped of bark and needles – a victim of the bark beetle.

The trees on the left have already been killed by bark beetles. The tree on the far right, a white fir, is exhibiting rust colored foliage. This is a symptom of a bark beetle attack. Without adequate precipitation, this tree will eventually die.

If the climate continues to warm, these forests may continue to retreat uphill. In the higher mountains, they won’t disappear entirely, but in the lower ranges, like Palomar and the Lagunas, they may disappear entirely. like the forest on Cuyamaca Peak nearly has. The effect would likely be similar to rising sea levels, except in this case the ocean would probably be chaparral. It’s hard to predict exactly how or when this could happen, so it becomes more imperative to enjoy it here and now.

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