The Curious Case of the Yellowwood Tree

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The immense beauty and grace of the yellowwood (Cladrastis kentukea) is inversely proportional to its abundance. This unique legume is endemic to the eastern United States and enjoys a strangely patchy distribution. Its ability to perform well when planted far outside of its natural range only deepens the mystery of the yellowwood.

The natural range of the yellowwood leaves a lot of room for speculation. It hits its highest abundances in the Appalachian and Ozark highlands where it tends to grow on shaded slopes in calcareous soils. Scattered populations can be found as far west as Oklahoma and as far north as southern Indiana but nowhere is this tree considered a common component of the flora.

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Though the nature of its oddball distribution pattern is open for plenty of speculation, it is likely that its current status is the result of repeated glaciation events and a dash of stochasticity. The presence of multiple Cladrastis species in China and Japan and only one here in North America is a pattern shared by multiple taxa that once grew throughout each continent. A combination of geography, topography, and repeated glaciation events has since fragmented the ranges of many genera and perhaps Cladrastis is yet another example.

The fact that yellowwood seems to do quite well as a specimen tree well outside of its natural range says to me that this species was probably once far more wide spread in North America than it was today. It may have been pushed south by the ebb and flow of the Laurentide Ice Sheet and, due to the stochastic nuances of seed dispersal, never had a chance to recolonize the ground it had lost. Again, this is all open to speculation as this point.

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Despite being a member of the pea family, yellowwood is not a nitrogen fixer. It does not produce nodules on its roots that house rhizobium. As such, this species may be more restricted by soil type than other legumes. Perhaps its inability to fix nitrogen is part of the reason it tends to favor richer soils. It may also have played a part in its failure to recolonize land scraped clean by the glaciers.

Yellowwood's rarity in nature only makes finding this tree all the more special. It truly is a site to behold. It isn't a large tree by any standards but what it lacks in height it makes up for in looks. Its multi-branched trunk exhibits smooth, gray bark reminiscent of beech trees. Each limb is decked out in large, compound leaves that turn bright yellow in autumn.

When mature, which can take upwards of ten years, yellowwood produces copious amounts of pendulous inflorescences. Each inflorescence sports bright white flowers with a dash of yellow on the petals. It doesn't appear that any formal pollination work has been done on this tree but surely bees and butterflies alike visit the blooms. The name yellowwood comes from the yellow coloration of its heartwood, which has been used to make furniture and gunstocks in the past.

Whether growing in the forest or in your landscape, yellowwood is one of the more stunning trees you will find in eastern North America. Its peculiar natural history only lends to its allure.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]

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]

Bacterial Enduced Shield

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Legumes are famous the world over for their nitrogen fixing capabilities. These hardy plants can live in soils that would otherwise not support much of anything. As such, nitrogen fixation is one reason that the legumes have found themselves as a focus of agriculture. However, this ability is not solely the plants doing nor has it evolved to benefit humans. Legumes owe their ability to turn a gas into food to a symbiotic relationship with special soil bacteria known collectively as "rhizobia." The legumes produce special root structures called "nodules" to house these bacteria. In return for nitrogen, the bacteria receive carbohydrates and other organic compounds. The nature of this relationship may seem pretty straight forward but, as with anything in nature, the closer we look the more interesting things get. As it turns out, rhizobia also play a role in plant defense.

When a team of researchers began raising Crotalaria, a genus of legume native to Africa, they noticed something strange. Plants that were not inoculated with rhizobia didn't produce nodules nor were they producing any of the alkaloid chemicals that defend them from herbivores. Even adding artificial nitrogen to the soil didn't stimulate the plants to produce their chemical cocktails. Something was going on here and it would seem that the missing bacteria were the key to the puzzle. 

Indeed, only after the plants were inoculated with their native rhizobia did they begin producing nodules and eventually the defensive alkaloid compounds. Could it be that the bacteria produce these chemicals for the plant? 

Not quite. As it turns out, the area of biosynthesis for these defense compounds happens to be in the root nodules that house the rhizobia. The rhizobia trigger the production of the nodules, which in turn triggers the production of the alkaloids. From there, the plant can export them to above-ground structures as a means of defense. The bacteria are simply a key that unlocks a genetic pathway for defense. Seeing as the alkaloids are, in part, made from nitrogenous molecules, this is not too surprising. There is no sense in trying to make these compounds if the chemical ingredients aren't there. This research serves as further evidence of how complex the microbiome can be. 

Photo Credit: Dick Culbert - Wikimedia Commons

Further Reading:

http://www.pnas.org/content/early/2015/03/13/1423457112

The Peculiar Peanut

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Did you know that peanuts are not nuts at all? More accurately, they are a type of legume. What are you are actually eating are the cotyledons and embryo of the next generation of plants. Despite their popularity around the world, the plant itself gets very little attention outside of agricultural circles. This is a shame as the ecology of the peanut is truly fascinating.

The plant that produces the peanuts that the world either loves or fears is known scientifically as Arachis hypogaea. It was originally native to South America and it is believed to have first been domesticated in Paraguay over 7,000 years ago. Domestic peanuts are amphidiploid or allotetraploid meaning they have two sets of chromosomes from two different parent species. 

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Research points to a natural hybridization event between Arachis duranensis and Arachis ipaensis, which produced the tetraploid Arachis monticola. It is A. monticola that gave rise to the domestic peanut we know today. As a plant, it looks rather much like most pinnate legumes. Its yellow flowers are unmistakably pea-like. It is only after they have been pollinated that they become truly bizarre. 

Members of this genus exhibit what is called "geocarpy." This relatively rare form of plant reproduction involves the plant literally planting its own seeds. After fertilization, the flower stalk elongates and bends towards the ground. Once it touches soil, the stalk pushes the developing seed pod down into the dirt. Underground, the seeds mature and germinate. The embryos will only become active in the dark, subterranean environment. This is especially useful in habitats where soil disturbance is a frequent occurrence. Geocarpic plants like the peanut increase the likelihood that their offspring will survive long enough to germinate and grow by skipping over that pesky seed dispersal step. Keep that in mind the next time you tuck into a bag of peanuts.

Photo Credit: [1]

Further Reading:

http://www.biomedcentral.com/content/pdf/1471-2164-14-517.pdf

http://www.amjbot.org/content/94/12/1963.abstract