Tuesday, February 24, 2015

Demigod of the Desert

By: Kristen Nelson
For most, a typical desert landscape is – at best – uninspiring and – at worst – a desolate fiery hell to be avoided at all costs. Only something as shiny and gleaming as the Las Vegas strip is capable of calling a coastal Californian into the parched expanse of the desert. And even then, the pull is usually only strong enough to last a three day weekend worth of unintentionally forgotten mishaps. Inevitably, coastal folk retreat from the desert more rapidly and desperately than a prairie dog taking cover from an eagle soaring overhead.

...but I think it's time to retreat!
I don't know what happened last night...

I am intimately familiar with this desert-avoidance syndrome, as I was raised in the skin-melting heat of the Mojave desert and, as soon as the onset of adulthood freed me to discover the world beyond, I too retreated to the coastline of California… where an overnight low of 40°F is considered cold and overcast skies are considered bad weather. Nonetheless, I find myself completely captivated by the seemingly impossible adaptations to life in the desert. Perhaps my former years as a desert-dweller instilled in me a sort of empathy for those poor beings that are not capable of relocating to greener pastures.

The adaptations of plants to desert life are numerous and well-documented. Drought-deciduous, succulence, and annual life cycles are just a few of the arid climate strategies you may be familiar with. Perhaps two of the most significant adaptations to arid climates are the modified photosynthetic pathways known as C4 and CAM photosynthesis (you can read about C4 as well as “normal” C3 photosynthesis here). The definitive tradeoff in plant energetics centers on the fact that water is lost via the same pathway that CO2 is taken in – through tiny pores on the leaf surface called stomata. In C3-photosynthetic species, an average of 400 water molecules are lost for every molecule of CO2 gained! This represents a VERY substantial and costly tradeoff for plants, especially in desert habitats where water is the primary limiting resource. C4 and CAM photosynthesis substantially improve this ratio, averaging water-lost-to-CO2-gained ratios of 150:1 and 50:1, respectively (Taiz and Zeiger, 2010).

Stem-succulent barrel cactus and drought-deciduous ocotillo, doin' what they do.
While impressive, this list of desert adaptations doesn’t even scratch the surface. Creosote bush (Larrea tridentata) is the characteristically dominant plant of the Mojave, Sonoran, and Chihuahuan Deserts of the southwestern United States and northern Mexico. Interestingly, though, this wildly successful desert shrub does not possess any of the aforementioned adaptations typical of desert plants. In fact, it is quite the opposite… a perennial, evergreen, C3, non-succulent shrub that often forms vast monotypic stands in regions virtually devoid of any other plant life. What’s more… this amazing plant is capable of tolerating temperatures as high as 120°F in places like Death Valley and as low as 12°F at the northern extent of its range … without even missing a beat.  How, then, does Larrea do it???

Typical Mojave desert scene... happy, green creosote bush as far as the eye can see.

It seems that many complex, interacting factors contribute to the anomaly of Larrea’s success. One study documented that creosote bush exhibits an atypical diurnal pattern (i.e., fluctuations from day to night) of xylem water potential (Syvertsen et al., 1975). [Xylem is the botanical term for the water-conducting tissue of a plant.] Typically, you would expect to find low water potentials (water potential = water concentration) in the xylem during the hottest part of the day and high water potentials at night. This makes sense logically, if think about high midday temperatures resulting in high rates of water loss through the stomata and thus, a low concentration of water inside the plant. However, creosote bush has been shown to have an inverse pattern, with minimum xylem water potential just before dawn and a maximum xylem water potential at midday. How does that even make sense?!

Graphs depicting xylem water potential in Larrea tridentata as measured in August (left) and September (right). Dashed lines show water potential in artificially watered plants, to demonstrate that watered plants follow the expected trend. Note that unwatered plants (no treatment) have peak water potential at 12 to 3pm. 

Well, Syvertsen and his team hypothesized that the depth of Larrea’s fibrous root system - 20 to 40 cm below the soil surface – is the key to explaining this unusual phenomenon. This root depth is shallow enough to take advantage of atmospheric sources of water vapor diffusing in and out of the soil and deep enough that the soil temperature is relatively stable. However, the soil shallower than 20 cm experiences dramatic diurnal temperature swings, with daily changes ranging from about 105°F (midday) down to about 85°F (pre-dawn) during the spring and summer. This group hypothesized that the temperature gradient in the soil between the root zone and the soil surface results in a downward movement of water vapor during the day – concentrating at the root zone - and an upward diffusion of water vapor at night – away from the roots (see diagram below for a visual). This movement of soil water would explain the unusual pattern exhibited by creosote bush. While difficult to prove, measurements of soil temperature and water potential in May, July, August, and September support this hypothesis.

Illustration of the hypothesis tested and supported in the Syvertsen study.

More recent work has turned toward molecular and genetic analyses to explain the impossible resiliency of creosote bush to such harsh environments. One group (Hunter et al., 2001) conducted an extensive genetic analysis over the entire range of creosote bush (range map below) and discovered a revealing pattern. It turns out that the Chihuahuan Desert population of creosote bush is diploid (contains 2 copies of genetic material, the normal number), the Sonoran Desert population is tetraploid (twice the normal amount of genetic material), and the Mojave Desert population is hexaploid (three times the normal amount of genetic material). 

Range of creosote bush; points designate sites where genetic material was collected for analysis.

Now, genome duplication (and even triplication) is not unusual in plants, but these guys hypothesized that the distinctness of the pattern in creosote bush hinted at something more. Follow-up work revealed that genome size is linked to the size of guard cells (specialized cells that regulate the opening and closing of stomata – i.e., the gatekeepers for prevention of water loss). Specifically, smaller genome equals smaller guard cells (see data below).

Data correlating genome size to guard cell size.

The current assumption is that hexaploidy allowed creosote bush to expand north into the dry-summer climate of the Mojave desert, whereas tetraploid and diploid populations are restricted to the more southern Sonoran and Chihuahuan deserts, characterized by summer rain.

Creosote bush accomplishes so many astonishing feats that I don’t have the space or time to indulge in even half of them. But I would like to share one last thing, in case you are still not impressed. Here is a picture of the King Clone ring of Larrea tridentata, shown to be nearly 12,000 years old, ranking it among the oldest living organisms on Earth…

This ancient clonal organism is about 45 feet across and can easily been seen in GoogleEarth!

So the next time you are speeding across the desert through creosote country toward the glittery oasis of the Las Vegas strip (or on your return to the unsurpassed beauty of the California coast), keep in mind the challenges Larrea (and friends) must overcome to survive such an extreme environment...

…and with such grace and beauty. 

Photo Credits (in order of appearance):

References and Literature Cited:

Ashby, E. 1932. Transpiratory organs of Larrea Tridentata and their ecological significance. Ecology 13(2): 182-188.

Gowik, G., and P. Westhoff. 2011. The path from C3 to C4 photosynthesis. Plant Physiology 155:56-63.

Hunter, K.L., J. L. Betancourt, B.P. Riddle, T.R. Van Devender, K.L. Cole, and W.G. Spaulding. 2001. Ploidy race distributions since the last glacial maximum in the North American desert shrub, Larrea tridentata. Ecology and Biogeography 10:521-533.

Syvertsen, J.P., G.L. Cunningham, and T.V. Feather. 1975. Anomalous diurnal patterns of stem xylem water potentials in Larrea tridentata. Ecology 56(6):1423-1428.

United States Forest Service. Larrea tridentata (Sesse' and Moc. ex DC.) Coville, Zygophyllaceae. Retrieved February 19, 2015 from: http://www.fs.fed.us/global/iitf/pdf/shrubs/Larrea%20tridentata.

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