When the Going Gets Tough, Desert Mistletoes Cooperate

Sure, parasites can be a drain on their host, but for those parasites whose entire life depends on a living host, it doesn’t pay to kill. Such is the case for the desert mistletoe (Phoradendron californicum). These plants simply can’t live without the water and nutrients they receive from their host trees. But what happens when more than one mistletoe infects a single tree? One would think that supporting multiple mistletoes would be a dangerous drain on the host tree. However, recent research based in the Sonoran Desert suggests that desert mistletoe has a trick up its stems that involves a bit of communication with its neighbors.

Desert mistletoe isn’t completely reliant on its host for all of its nutritional needs. Though lacking leaves, the desert mistletoe is fully capable of photosynthesis via its tangled mass of green stems. Most of what desert mistletoes extract from their host consists of water and other nutrients they can’t acquire themselves. However, desert mistletoes rarely operate alone. Thanks to their nutritious berries and the territorial habits of the birds that disperse them, multiple mistletoe individuals often wind up parasitizing the same tree.

Heavy infestations may sound like a death sentence for the host tree, especially in the harsh Sonoran climate. However, by manipulating the mistletoe loads on various trees and observing how mistletoes and their hosts respond, researchers have discovered that mistletoes can apparently sense their neighbors and alter their behavior accordingly.

During dry periods, trees become stressed for both water and nutrients. For mistletoes growing on a stressed tree, it doesn’t make much sense from an evolutionary standpoint to increase their demand on the host during these times. Instead, mistletoes growing on stressed trees actually increased the amount of photosynthesis they perform without increasing the amount of water they extract from their host. By altering their metabolism in this way, the mistletoes do not add any extra burden to their already stressed host tree but nonetheless maintain their own fitness.

Amazingly, the situation got even more interesting when researchers experimentally removed some mistletoes. Somehow, depending on their position on their host tree, some remaining mistletoes can sense that their competitors had been removed. When this happens, they don’t go into overdrive and start exacting a greater share of resources from their host. Instead, the remaining mistletoe appear to sense that they no longer have to compete as much and adjust their water and nutrient uptake in such a way that actually allows their host to benefit as well.

Certainly these findings generate more questions than they answer. First, how do mistletoes sense their neighbors? Given their direct links with the host vascular tissues, they could be sensing signals from other parasites that way. There is also the potential for airborne signal detection as well. Also, do mistletoes behave differently when growing near related individuals versus strangers? What researchers have ultimately uncovered is a fascinating coevolutionary system in desperate need of more attention.

Further Reading: [1]

Meet the Ocotillo

I love the ocotillo (Fouquieria splendens) for many reasons. It is an impossible plant to miss with its spindly, spine-covered stems. It is a lovely plant that is right at home in the arid parts of southwestern North America. Beyond its unique appearance, the ocotillo is a fascinating and important component of the ecology of this region.

My first impression of ocotillo was interesting. I could not figure out where this plant belonged on the tree of life. As a temperate northeasterner, one can forgive my taxonomic ignorance of this group. The family from which it hails, Fouquieriaceae, is restricted to southwestern North America. It contains one genus (Fouquieria) and about 11 species, all of which are rather spiky in appearance.

Of course, those spines serve as protection. Resources like water are in short supply in desert ecosystems so these plants ensure that it is a real struggle for any animal looking to take a bite to get at the sap inside. Those spines are tough as well. One manged to pierce the underside of my boot during a hike and I was lucky that it just barely grazed the underside of my foot. Needless to say, the ocotillo is a plant worthy of attention and respect.

One of the most striking aspects of ocotillo life is how quickly these plants respond to water. As spring brings rain to this region of North America, ocotillo respond with wonderful sprays of bright red flowers situated atop their spindly stems. These blooms are usually timed so as to take advantage of migrating hummingbirds and emerging bees. The collective display of a landscape full of blooming ocotillo is jaw-droppingly gorgeous and a sight one doesn't soon forget. It is as if the whole landscape has suddenly caught on fire. Indeed, the word "ocotillo" is Spanish for "little torch."

Flowering isn't the only way this species responds to the sudden availability of water. A soaking rain will also bring about an eruption of leaves, turning its barren, white stems bright green. The leaves themselves are small and rather fragile. They do not have the tough, succulent texture of what one would expect out of a desert specialist. That is because they don't have to ride out the hard times. Instead, ocotillo are what we call a drought deciduous species, producing leaves when times are good and water is in high supply, and dropping them as soon as the soil dries out.

This cycle of growing and dropping leaves can and does happen multiple times per year. It is not uncommon to see ocotillo leaf out up to 4 or 5 times between spring and fall. During the rest of the year, ocotillo relies on chlorophyll in its stems for its photosynthetic needs. Interestingly enough, this poses a bit of a challenge when it comes to getting enough CO2. Whereas leaves are covered in tiny pours called stomata which help to regulate gas exchange, the stems of an ocotillo are a lot less porous, making it a challenge to get gases in and out. This is where the efficient metabolism of this plant comes in handy.

All plants undergo respiration like you and me. The carbohydrates made during photosynthesis are broken down to fuel the plant and in doing so, CO2 is produced. Amazingly, the ocotillo (as well as many other plants that undergo stem photosynthesis) are able to recycle the CO2 generated by cellular respiration back into photosynthesis within the stem. In this way, the ocotillo is fully capable of photosynthesis even without leaves.

Through the good times and the bad, the ocotillo and its relatives are important components of desert ecology. They are as hardy as they are beautiful and getting to see them in person has been a remarkable experience. They ad a flare of surreality to the landscape that must be seen in person to believe.

Further Reading: [1] [2] [3] [4] [5]

The Wild World of the Creosote Bush

Apart from the cacti, the real rockstar of my Sonoran experience was the creosote bush (Larrea tridentata). Despite having been quite familiar with creosote as an ingredient, I admit to complete ignorance of the plant from which it originates. Having no familiarity with the Sonoran Desert ecosystem, I was walking into completely new territory in regard to the flora. It didn’t take long to notice creosote though. Once we hit the outskirts of town, it seemed to be everywhere.

If you are in the Mojave, Sonoran, and Chihuahuan Deserts of western North America, you are never far from a creosote bush. They are medium sized, slow growing shrubs with sprays of compact green leaves, tiny yellow flowers, and fuzzy seeds. Apparently what is thought of as one single species is actually made up of three different genetic populations. The differences between these has everything to do with chromosome counts. Populations in the Mojave Desert have 78 chromosomes, Sonoran populations have 52 chromosomes, and Chihuahuan have 26. This may have to do with the way in which these populations have adapted to the relative amounts of rainfall each of these deserts receive throughout the year, however, it is hard to say for sure.

Regardless, creosote is supremely adapted to these xeric ecosystems. For starters, their branching architecture coupled with their tiny leaves are arranged so as to make the most out of favorable conditions. If you stare at these shrubs long enough, you may notice that their branches largely orient towards the southeast. Also, their leaves tend to be highly clustered along the branches. It is thought that this branching architecture allows the creosote to minimize water loss while maximizing photosynthesis.

Deserts aren’t hot 24 hours per day. Night and mornings are actually quite cool. By taking advantage of the morning sun as it rises in the east, creosote are able to open their stomata and commence photosynthesis during those few hours when evapotranspiration would be at its lowest. In doing so, they are able to minimize water loss to a large degree. Although their southeast orientation causes them to miss out on afternoon and evening sun to a large degree, the benefits of saving precious water far outweigh the loss to photosynthesis. The clustering of the leaves along the branches may also reduce overheating by providing their own shade. Coupled with their small size, this further reduces heat stress and water loss during the hottest parts of the day.

Creosote also secrets lots of waxy, resinous compounds. These coat the leaves and to some extent the stems, making them appear lacquered. It is thought that this also helps save water by reducing water loss through the leaf cuticle. However, the sheer diversity of compounds extracted from these shrubs suggests other functions as well. It is likely that at least some of these compounds are used in defense. One study showed that when desert woodrats eat creosote leaves, the compounds within caused the rats to lose more water through their urine and feces. They also caused a reduction in the amount of energy the rats were able to absorb from food. In other words, any mammal that regularly feeds on creosote runs the risk of both dehydration and starvation. This isn’t the only interesting interaction that creosote as with rodents either. Before we get to that, however, we first need to discuss roots.

Creosote shrubs have deep root systems that are capable of accessing soil water that more shallowly rooted plants cannot. This brings them into competition with neighboring plants in intriguing ways. When rainfall is limited, shallowly rooted species like Opuntia gain access to water before it has a chance to reach deeper creosote roots. Surprisingly this happens more often than you would think. The branching architecture of creosote is such that it tends to accumulate debris as winds blow dust around the desert landscape. As a result, the soils directly beneath creosote often contain elevated nutrients. This coupled with the added shade of the creosote canopy means that seedlings that find themselves under creosote bushes tend to do better than seedlings that germinated elsewhere. As such, creosote are considered nurse plants that actually facilitate the growth and survival of surrounding vegetation. So, if recruitment and resulting competition from vegetation can become such an issue for long term creosote survival, why then do we still so much creosote on the landscape?

The answer may lie in rodents and other burrowing animals in these desert ecosystems. Take a look at the base of a large creosote and you will often find the ground littered with burrows. Indeed, many a mammal finds the rooting zone of the creosote shrub to be a good place to dig a den. When these animals burrow under shallowly rooted desert plants, many of them nibble on and disturb the rooting zones. Over the long-term, this can be extremely detrimental for the survival of shallow rooted species. This is not the case for creosote. Its roots run so deep that most burrowing animals cannot reach them. As such, they avoid most of the damage that other plants experience. This lends to a slight survival advantage for creosote at the expense of neighboring vegetation. In this way, rodents and other burrowing animals may actually help reduce competition for the creosote.

Barring major disturbances, creosote can live a long, long time. In fact, one particular patch of creosote growing in the Mojave Desert is thought to be one of the oldest living organisms on Earth. As creosote shrubs grow, they eventually get to a point in which their main stems break and split. From there, they begin producing new stems that radiate out in a circle from the original plant. These clones can go on growing for centuries. By calculating the average growth rate of these shrubs, experts have estimated that the Mojave specimen, affectionately referred to as the “King Clone,” is somewhere around 11,700 years old!

The ring of creosote that is King Clone.

For creosote, its slow and steady wins the race. They are a backbone of North American desert ecosystems. Their structure offers shelter, their seeds offer food, and their flowers support myriad pollinators. Creosote is one shrub worthy of our respect and admiration.

Photo Credit: [1] [2]

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

Palo Verde

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.

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. 

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.

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]