The Desert Mistletoe: Evolution In Action

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There are a multitude of mistletoes on this planet (for example: 1, 2, 3) and all of them are parasites to one degree or another. I find parasitic plants absolutely fascinating because there are as many variations on this lifestyle as there are hosts to parasitize. On a recent botanical adventure in the Sonoran Desert, I met yet another representative of this group - the desert mistletoe (Phoradendron californicum). Once I knew what I was looking at, I could not wait to do some research. As it turns out, this species has garnered quite a bit of attention over the years and it is teaching us some interesting tidbits on how parasites may evolve.

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The desert mistletoe is not hard to spot, especially during the driest parts of the year when most of its host trees have shed their leaves. It looks like a leafless, tangled mass of pendulous stems sitting among the branches of larger shrubs and trees. It can be found growing throughout both the Mojave and Sonoran deserts and appears to prefer leguminous trees including palo verde (Parkinsonia florida), mesquite (Prosopis spp.), and Acacia.

The desert mistletoe is a type of hemiparasite, which means it is capable of performing photosynthesis but nonetheless relies on its host tree for water and other nutrients. Lacking leaves, the desert mistletoe meets all of its photosynthetic needs via its green stems. Its leafless habit also makes its flowers and fruit all the more conspicuous. Despite their small size, its flowers are really worth closer inspection. When in bloom, a desert mistletoe comes alive with the hum of various insects looking for energy-rich nectar and pollen. Even before you spot them, you can easily tell if there is a blooming mistletoe nearby as the flowers give off a wonderfully sweet aroma. It appears that the desert mistletoe takes no chances when it comes to reproduction in such an arid climate.

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As I mentioned above, the desert mistletoe has been the subject of inquiry over the last few decades. Researchers interested in how parasitic plants evolve have illuminated some intriguing aspects of the biology of this species, especially as its relates to host preference. It would appear that our interest in this species seems to be situated at an important time in its evolutionary history. Not all populations of desert mistletoe "behave" in the same way. In fact, each seems to be heading towards more intense specialization based on its preferred host.

By performing seed transplant experiments, researchers have demonstrated that various populations of desert mistletoe seem to be specializing on specific tree species. For instance, when seeds collected from mistletoe growing on Acacia were placed on paleo verde or mesquite, they experienced significantly less germination than if they were placed on another Acacia. Though the exact mechanisms aren't clear at this point in time, evidence suggests that the success of desert mistletoe may be influenced by various hormone levels within the host tree, with isolated populations becoming more specialized on the chemistry of their specific host in that region.

Speaking of isolation, there is also evidence to show that populations of desert mistletoe growing on different host trees are reproductively isolated as well. Populations growing on mesquite trees flower significantly later than populations growing on Acacia or palo verde. Essentially this means that their genes never have the chance to mix, thus increasing the differences between these populations. Again, it is not entirely certain how the host tree may be influencing mistletoe flowering time, however, hormones and water availability are thought to play a role.

Another intriguing idea, and one that has yet to be tested, are the roles that seed dispersers may play in this evolutionary drama. After pollination, the desert mistletoe produces copious amounts of bright red berries that birds find irresistible. Two birds in particular, the northern mockingbird and the Phainopepla, aggressively defend fruiting mistletoe shrubs within their territories. It could be possible that these birds may be influencing which trees the seeds of the desert mistletoe end up on. Again, this is just a hypothesis but one that certainly deserves more attention.

A Phainopepla on the lookout for mistletoe berries.

A Phainopepla on the lookout for mistletoe berries.

Love them or hate them, there is something worth admiring about mistletoes. At the very least, they are important components of their native ecology. What's more, species like the desert mistletoe have a lot to teach us about the way in which species interact and what that means for biodiversity.

Photo Credit: [1]

Further Reading: [1] [2] [3] [4]

Palo Verde

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One of the first plants I noticed upon arriving in the Sonoran Desert were these small spiny trees without any leaves. The reason they caught my eye was that every inch of them was bright green. It was impossible to miss against the rusty brown tones of the surrounding landscape. It didn’t take long to track down the identity of this tree. What I was looking at was none other than the palo verde (Parkinsonia florida).

Palo verde belong to a small genus of leguminous trees. Parkinsonia consists of roughly 12 species scattered about arid regions of Africa and the Americas. The common name of “palo verde” is Spanish for “green stick.” And green they are! Like I said, every inch of this tree gives off a pleasing green hue. Of course, this is a survival strategy to make the most of life in arid climates.

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Despite typically being found growing along creek beds, infrequent rainfall limits their access to regular water supplies. As such, these trees have adapted to preserve as much water as possible. One way they do this is via their deciduous habit. Unlike temperate deciduous trees which drop their leaves in response to the changing of the seasons, palo verde drop their leaves in response to drought. And, as one can expect from a denizen of the desert, drought is the norm. Leaves are also a conduit for moisture to move through the body of a plant. Tiny pours on the surface of the leaf called stomata allow water to evaporate out into the environment, which can be quite costly when water is in short supply.

The tiny pinnate leaves and pointy stems of the palo verde. 

The tiny pinnate leaves and pointy stems of the palo verde. 

Not having leaves for most of the year would be quite a detriment for most plant species. Leaves, after all, are where most of the photosynthesis takes place. That is, unless, you are talking about a palo verde tree. All of that green coloration in the trunk, stems, and branches is due to chlorophyll. In essence, the entire body of a palo verde is capable of performing photosynthesis. In fact, estimates show that even when the tiny pinnate leaves are produced, a majority of the photosynthetic needs of the tree are met by the green woody tissues.

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Flowering occurs whenever there is enough water to support their development, which usually means spring. They are small and bright yellow and a tree can quickly be converted into a lovely yellow puff ball seemingly overnight. Bees relish the flowers and the eventual seeds they produce are a boon for wildlife in need of an energy-rich meal.

However, it isn’t just wildlife that benefits from the presence of these trees. Other plants benefit from their presence as well. As you can probably imagine, germination and seedling survival can be a real challenge in any desert. Heat, sun, and drought exact a punishing toll. As such, any advantage, however slight, can be a boon for recruitment. Research has found that palo verde trees act as important nurse trees for plants like the saguaro cactus (Carnegiea gigantea). Seeds that germinate under the canopy of a palo verde receive just enough shade and moisture from the overstory to get them through their first few years of growth.

In total, palo verde are ecologically important trees wherever they are native. What’s more, their ability to tolerate drought coupled with their wonderful green coloration has made them into a popular tree for native landscaping. It is certainly a tree I won’t forget any time soon.

Further Reading: [1] [2]