Farewell Spring, Hello Summer!
By Staff Biologist Mitchell Coleman, M.S.
We are still drying off from what was a wild, wet winter and spring. Rains persisted in a series of atmospheric rivers straight through mid-May, making much of Tejon Ranch both ideal for water-stressed organisms and difficult to access via dirt road.
We are just now getting back into the high country, after months of muddy roads, tree falls, and impassable walls of snow. Now the seasons shift, even though, due to the later-than-normal rains, many wildflowers persist. As the Ranch dries off and spring annuals senesce, the Tejon Ranch Conservancy shifts gears into the melodic season of summer. It’s a perfect time to discuss how the richly biodiverse organisms on Tejon are adapted to these hot, dry conditions.
Farewell to Spring blooms from late April to early July and often occurs in dense patches that can turn early summer hillsides a vibrant pink. Photo by Laura Pavliscak.
As is often discussed, Tejon Ranch sits at the confluence of four ecoregions: the coastal range comes in from the west, the southern spine of the Sierra Nevada from the north, the Mojave Desert from the east, and the San Joaquin Desert from the northwest. Each region has its own suite of native species, which thus co-occur on Tejon in ways that cannot be seen elsewhere. This is the overarching cause of Tejon’s amazing biodiversity.
A black-tailed jackrabbit cautiously posed in front of Joshua trees in the Antelope Valley. Both jackrabbits and Joshua trees are well-adapted to the hot and dry conditions of the Mojave Desert. Jackrabbit ears are highly vascularized (i.e., inundated with blood vessels), an adaptation which helps “dump” excess body heat. Joshua trees have a special photosynthetic pathway wherein their leaf stomata (small pores for gas exchange) remain firmly shut during the day to reduce water loss.
Despite a wide variance in temperature, precipitation, and geomorphic patterns, the confluence of the four ecoregions on Tejon all share a Mediterranean-type climate, broadly characterized by hot, dry summers and cool, moist winters. While this climate type occurs in approximately 5% of the earth’s surface, it harbors almost 20% of known vascular plant taxa, many of which occur nowhere else. Long, drought-prone summers are a huge driver of this diversity and endemism. The organisms that have evolved in these areas have many behavioral and physiological adaptations to cope with the extremes of heat, light, and water stress.
A Mojave rattlesnake shaded under a California buckwheat in the Antelope Valley. Photo by Todd Battey.
Desert animals, for example, employ numerous methods to maintain sublethal body temperatures. This can be achieved through a combination of activity and metabolic regulation, body size, microhabitat selection, and hyperthermia (storing body heat to be released later). Many animals (including mammals, reptiles, and invertebrates) burrow into the ground during the day to avoid the extremes of heat, preferring to venture out into the relative cool of night.
Such creatures must also work to conserve water during the summer, not merely because of the hot, dry conditions, but also because it is the time of year when water is most limited. Desert vertebrates, for instance, greatly limit water loss through reducing water in feces and body surfaces, using nasal passages to reclaim water and using salt glands to excrete salts. Desert-adapted vertebrates, such as black-tailed jackrabbits (Lepus californicus) and coyotes (Canis latrans), obtain almost all their water through their food, be it plant or prey. They do not drink water at all!
A buckwheat blue butterfly alights on an inflorescence of California buckwheat in the Canyon Del Gato Montes. This butterfly is so named because of its preference for buckwheat flowers, a large genus that blooms primarily in the summer.
Summer is a great time to observe invertebrates on the Ranch.
Many pollinators, for instance, busy themselves with the pollen and nectar of flowers that bloom in the summer. Desert invertebrates have their own suite of adaptations for survival in the harsh conditions, such as larger body sizes, higher abundances of predatory insects, and increased sociality in groups with predictable food sources compared to related non-desert taxa.
Larger body sizes help reduce the rate of heat intake, and thus, how quickly the insects can remain active without overheating. Waxy coats called cuticles also help prevent water loss. On the extreme end, some desert invertebrates can undergo anhydrobiosis, the ability to suspend all activity until winter rains arrive. Conversely, some species can replenish water by absorbing it from unsaturated air in their burrows, thus maintaining their activity year-round.
A tarantula hawk feeding on woolypod milkweed, another summer bloomer in the San Joaquin Desert.
A longhorn beetle feeding on Mojave ceanothus flowers on Bronco Ridge. Mojave ceanothus is one of the most drought-tolerant plant species on record, in part due to strict stomatal regulation, a thick leaf cuticle (waxy layer covering leaf epidermis, restricting water loss), deep roots, and resistance to cavitation (drought-induced bubbles which form in the water-conducting vessels of plants, restricting water flow).
A California sister butterfly alights on a canyon live oak in the Mojave Desert. Eggs of this species are generally laid on the upper surface of oak leaves, which hatch in late spring and early summer. Broods remain active into late October each year. Photo by A. Schmeirer.
Much, much more can be said of the amazing adaptations of desert organisms. Indeed, in the context of climate change, fresh questions and perspectives are being used to understand the ecological importance of these adaptations, and what they might mean going forward. This is one reason desert-focused research is so vital, and much of that research is taking place right here at Tejon Ranch.