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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