Light Pollution and Plants

I love walking around my town at night. Things really seem to slow down when the sun sets. Growing up in the country, my evening walks were lit only by the moon. Now that I live in civilization, however, street lights punctuate the darkness on every block. Walking around I can't help but wonder what all of this artificial light is doing to our photosynthetic neighbors. 

The vast majority of plants need light to make food. It doesn't matter if this light comes from the sun or a high powered electric light, as long as it is strong enough for photosynthesis. Even weaker wavelengths of light serve a purpose for our botanical friends. Plants can sense the relative length of uninterrupted darkness in their environment and they use that information for myriad internal processes. Its this dependence on light that makes many plant species vulnerable to our addiction to artificial lighting.

Just because a light isn't strong enough for photosynthesis doesn't mean it isn't affecting nearby plants. This is especially true for plants that use day length for timing events like bud break, flowering, and dormancy. The type of lighting favored by most municipalities emit wavelengths that peak especially high in the red to far-red ratio of the electromagnetic spectrum, which makes them particularly adept at disrupting plant photoperiods.

One of the most obvious effects of artificial lighting on plants can readily be seen in street trees growing in temperate regions. Though light sensitivity varies from species to species, trees growing near street lights tend to hold onto their leaves much longer in the fall than trees farther away. Because artificial lighting is enough to trick the red to far-red receptors in plants, it can "convince" trees that the days are longer than they actually are. Additional photosynthesis may not seem that bad but holding onto leaves longer makes trees more susceptible to ice damage. 

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The effects of artificial lighting continues into spring as well. Trees growing near lights tend to break buds and flower earlier in the spring. This too makes them susceptible to frost damage. Early flowering plants run the risk of losing their entire reproductive effort by blooming before the threat of frost is gone. This can really mess up their relationship with pollinators. 

The effects of artificial lighting can even influence the way in which plants grow. Research has found that plants growing near street lights had larger leaves with more stomatal pores and these pores remained open for considerably longer than plants growing under unlit night conditions. This made them more susceptible to pollution and drought, two stressors that are all too common in urban environments. This issues is made much worse if the artificial lighting never turns off throughout the night. 

Artificial lighting affects more than just plant physiology too. Scaling up, the effects of night lights can have whole ecosystem consequences. For instance, researchers found that artificial lighting was enough to change the entire composition of grassland communities. Some plants responded well to artificial lights, producing more biomass and vegetative offshoots to the point that they pushed out other species. This was compounded by the change in reproductive output, with certain species showing higher seed production than others.

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Changes in plant physiology, phenology, and composition also affect myriad other organisms in the environment. Changes in the timing of flowering or bud break can disrupt things like insects and birds that rely on these events for food and shelter. Research even suggests that forest regeneration is being altered by artificial lighting. Seed dispersers such as bats often will not fly into well-lit areas at night, therefore reducing the amount of seeds falling in those areas. Such research is still in its infancy meaning we have a lot more to learn about how artificial lighting is disrupting natural events.

Light pollution is so much more than an aesthetic issue. Artificial lighting is clearly having pronounced effects on plant life. Disrupt plants and you disrupt life as we know it. Certainly more work is needed to tease out all the ways in which lights influence plants, however, it is clear that we must work hard on reducing light pollution around the globe.

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

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

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]

Leafy Cacti?

  Pereskia aculeata

Pereskia aculeata

At first glance, there is little about a Pereskia that would suggest a relation to what we know as cacti. Even a second, third, and forth glance probably wouldn't do much to persuade the casual observer that these plants have a place on cacti family tree. All preconceptions aside, Pereskia are in fact members of the family Cactaceae and quite interesting ones at that.

Most people readily recognize the leafless, spiny green stems of a cactus. Indeed, this would appear to be a unifying character of the family. Pereskia is proof that this is not the case. Though other cacti occasionally produce either tiny, vestigial leaves or stubby succulent leaves, Pereskia really break the mold by producing broad, flattened leaves with only a hint of succulence.

 Pereskia spines are produced from areoles in typical cactus fashion.

Pereskia spines are produced from areoles in typical cactus fashion.

What's more, instead of clusters of Opuntia-like pads or large, columnar trunks, Pereskia are mainly shrubby plants with a handful of scrambling climbers mixed in. Similar to their more succulent cousins, the trunks of Pereskia are usually adorned with clusters of long spines for protection. Additionally, each species produces the large, showy, cup-like blooms we have come to expect from cacti.

They are certainly as odd as they are beautiful. As it stands right now, taxonomists recognize two clades of Pereskia - Clade A, which are native to a region comprising the Gulf of Mexico and Caribbean Sea (this group is currently listed under the name Leuenbergeria) and Clade B, which are native to regions just south of the Amazon Basin. This may seem superficial to most of us but the distinction between these groups has a lot to teach us about the evolution of what we know of as cacti. 

  Pereskia grandifolia

Pereskia grandifolia

Genetically speaking, the genus Pereskia sorts out at the base of the cactus family tree. Pereskia are in fact sister to all other cacti. This is where the distinction between the two Pereskia clades gets interesting. Clade A appears to be the older of the two and all members of this group form bark early on in their development and their stems lack a feature present in all other cacti - stomata. Stomata are microscopic pours that allow the exchange of gases like CO2 and oxygen. Clabe B, on the other hand, delay bark formation until later in life and all of them produce stomata on their stems.

The reason this distinction is important is because all other cacti produce stomata on their stems as well. As such, their base at the bottom of the cactus tree not only shows us what the ancestral from of cactus must have looked like, it also paints a relatively detailed picture of the evolutionary trajectory of subsequent cacti lineages. It would appear that the ancestor of all cacti started out as leafy shrubs that lacked the ability to perform stem photosynthesis. Subsequent evolution saw a delay in bark formation, the presence of stomata on the stem, and the start of stem photosynthesis, which is a defining feature of all other cacti.

  Pereskia aculeata

Pereskia aculeata

If you are as excited about Pereskia as I am, then you , my friend, are in luck. A handful of Pereskia species have found their way into the horticulture trade. With a little luck attention to detail, you too can share you home with one of these wonderful plants. Just be warned, they get tall and their spines, which are often hidden by the leaves, are a force to be reckoned with. Tread lightly with these wonderfully odd cacti. Celebrate their as the evolutionary wonders that they are!

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]

The Other Pawpaws

  Asimina tetramera

Asimina tetramera

The pawpaw (Asimina triloba) has been called "America's forgotten fruit." Once quite popular among Native Americans and settlers alike, it fell out of the public eye until quite recently. If one considers the pawpaw "forgotten" then its relatives have been straight up ignored. Indeed, the pawpaw shares the North American continent with 10 other Asimina species. 

  Asimina angustifolia

Asimina angustifolia

The genus Asimina belongs to a family of plants called the custard apple family - Annonaceae. It is a large family that mostly resides in the tropics. In fact, the genus Asimina is the only group to occur outside of the tropics. Though A. triloba finds itself growing as far north as Canada, the other species within this genus are confined to southeastern North America in coastal plain communities. 

  Asimina parviflora

Asimina parviflora

As I mentioned above, there are 10 other species in the genus and at least one naturally occurring hybrid. For the most part, they all prefer to grow where regular fires keep competing vegetation at bay. They are rather small in stature, usually growing as shrubs or small, spindly trees. They can be rather inconspicuous until it comes time to flower.

  Asimina obovata

Asimina obovata

The flowers of the various Asimina species are relatively large and range in color from bright white to deep red, though the most common flower color seems to be creamy white. The flowers themselves give off strange odors that have been affectionately likened to fermenting fruit and rotting meat. Of course, these odors attract pollinators. Asimina aren't much of a hit with bees or butterflies. Instead, they are mainly visited by blowflies and beetles. 

  Asimina pygmaea

Asimina pygmaea

As is typical of the family, all of the Asimina produce relatively large fruits chock full of hard seeds. Seed dispersal for the smaller species is generally accomplished through the help of mammals like foxes, coyotes, raccoons, opossums, and even reptiles such as the gopher tortoise. Because the coastal plain of North America is a fire-prone ecosystem, most of the Asimina are well adapted to cope with its presence. In fact, most require it to keep their habitat open and free of too much competition. At least one species, A. tetramera, is considered endangered in large part due to fire sequestration.

  Asimina reticulata

Asimina reticulata

All of the 11 or so Asimina species are host plants for the zebra swallowtail butterfly (Eurytides marcellus) and the pawpaw sphinx moth (Dolba hyloeus). The specialization of these two insects and few others has to do with the fact that all Asimina produce compounds called acetogenins, which act as insecticides. As such, only a small handful of insects have adapted to be able to tolerate these toxic compounds. 

  Asimina tetramera

Asimina tetramera

Sadly, like all other denizens of America's coastal plain forest, habitat destruction is taking its toll on Asimina numbers. As mentioned above, at least one species (A. tetramera) is considered endangered. We desperately need to protect these forest habitats. Please support a local land conservation organization like the Partnership For Southern Forestland Conservation today!

See a list of the Asimina of North America here: [1] 

Photo Credits: Wikimedia Commons

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

Red or White?

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Who doesn't love a nice oak tree? One cannot overstate their importance both ecologically and culturally. Although picking an oak tree out of a lineup is something many of us are capable of doing, identifying oaks to species can be a bit more challenging. This is further complicated by the fact that oaks often hybridize. Still, it is likely you have come across some useful tips and tricks for narrowing down your oak choices. One such trick is distinguishing between the red oaks and the white oaks. If you're anything like me, this is something you took for granted for a while. Is there anything biologically or ecologically meaningful to such a split?

In short, yes. However, a true appreciation of these groups requires a deeper look. To start with, oaks are members of the genus Quercus, which belongs in the family Fagaceae. Globally there are approximately 400 species of oak and each falls into one of three categories - the red oaks (section Lobatae), the white oaks (section Quercus), and the so-called "intermediate" oaks (section Protoblanus). For the sake of this article, I will only be focusing on the red and white groups as that is where most of the oak species reside. The intermediate oak group is made up of 5 species, all of which are native to the southwestern United States and northwestern Mexico.

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As is common with oak identification, reliable techniques for distinguishing between the two groups can be tricky. Probably the most reliable feature is located on the inner surface of the acorn cap. In white oaks, it is hairless or nearly so, whereas in red oaks, it is covered in tiny hairs. Another useful acorn feature is the length of time it takes them to germinate. White oak acorns mature in one season and germinate in the fall. As such, they contain lower levels of tannins. Red oak acorns (as well as those of the intermediate group) generally take at least two seasons to mature and therefore germinate the following spring. Because of this, red oak acorns have a much higher tannin content. For more information on why this is the case, read this article.

Tyloses in white oak xylem.

Less apparent than acorns is the difference in the wood of red and white oaks. The wood of white oaks contains tiny structures in their xylem tissues called tyloses. These are absent from the wood of red oaks. The function of tyloses are quite interesting. During extreme drought or in the case of some sort of infection, they cut off regions of the xylem to stop the spread of an embolism or whatever may be infecting the tree. As such, white oaks tend to be more rot and drought resistant. Fun fact, tyloses are the main reason why white oak is used for making wine and bourbon barrels as it keeps them from leaking their contents.

More apparent to the casual observer, however, is leaf shape. In general, the white oaks produce leaves that have rounded lobes, whereas the red oaks generally exhibit pointed lobes with a tiny bristle on their tips. At this point you may be asking where an unlobed species like shingle oak (Quercus imbricaria) fits in. Look at the tip of its leaf and you will see a small bristle, which means its a member of the red oak group. Similarly, the buds of these two groups often differ in their overall shape. White oak buds tend to be smaller and often have blunted tips whereas the buds of red oaks are generally larger and often pointed.

 Tricky leaves of the shingle oak ( Quercus imbricaria ). Note the bristle tip!

Tricky leaves of the shingle oak (Quercus imbricaria). Note the bristle tip!

Despite this broad generalizations, exceptions abound. This is further complicated by the fact that many species will readily hybridize. Quercus is, after all, a massive genus. Regardless, oaks are wonderful species chock full of ecological and cultural value. Still, oak appreciation is something we all need more of in our lives. I encourage you to try some oak identification of your own. Get outside and see if you can use any of these tricks to help you identify some of the oaks in your neighborhood.

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

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

The White Walnut

I must admit, I am not very savvy when it comes to trees. I love and appreciate them all the same, however, my attention is often paid to the species growing beneath their canopy. last summer changed a lot of that. I was very lucky to be surrounded by people that know trees quite well. Needless to say I picked up a lot of great skills from them. Despite all of this new information knocking around in my brain, there was one tree that seemed to stand out from the rest and that species is Juglans cinerea.

Afternoons and evenings at the research station were a time for sharing. We would all come out of the field each day tired but excited. The days finds were recounted to eager ears. Often these stories segued into our goals for the coming days. That is how I first heard of the elusive "white walnut." I had to admit, it sounded made up. Its as if I was being told a folktale of a tree that lived in the imagination of anyone who spent too much time in the forest. 

Only a handful of people knew what it was. I listened intently for a bit, hoping to pick up some sort of clue as to what exactly this tree was. Finally I couldn't take it any longer so I chimed in and asked. As it turns out, the white walnut is a tree I was already familiar with, though not personally. Another common name for this mysterious tree is the butternut. Ah, common names. 

I instantly recalled a memory from a few years back. A friend of mine was quite excited about finding a handful of these trees. He was very hesitant to reveal the location but as proof of his discovery he produced a handful of nuts that sort of resembled those of a black walnut. These nuts were more egg shaped and not nearly as large. Refocusing on the conversation at hand, I now had a new set of questions. Why was this tree so special? Moreover, why was it so hard to find?

The white walnut has quite a large distribution in relation to all the excitement. Preferring to grow along stream banks in well-drained soils, this tree is native from New Brunswick to northern Arkansas. Its leaflets are downy, its bark is light gray to almost silver, and it has a band of fuzzy hairs along the upper margins of the leaf scars. Its a stunning tree to say the least. 

Sadly, it is a species in decline. As it turns out, the excitement surrounding this tree is due to the fact that finding large, robust adults has become a somewhat rare occurrence. Yet another casualty of the global movement of species from continent to continent, the white walnut is falling victim to an invasive species of fungus known scientifically as Sirococcus clavigignenti-juglandacearum

The fungus enters the tree through wounds in the bark and, through a complex life cycle, causes cankers to form. These cankers open the tree up to subsequent infections and eventually girdle it. The fungus was first discovered in Wisconsin but has now spread throughout the entire range of the tree. The losses in Wisconsin alone are staggering with an estimated 90% infection rate. Farther south in the white walnuts range, it is even worse. Some believe it is only a matter of time before white walnut becomes functionally extinct in areas such as the Carolinas. No one knows for sure where this fungus came from but Asia is a likely candidate.

A sad and all too common story to say the least. It was starting to look like I was not going to get a chance to meet this tree in person... ever. My luck changed a few weeks later. My friend Mark took us on a walk near a creek and forced us to keep our eyes on the canopy. We walked under a tree and he made sure to point out some compound leaves. With sunlight pouring through the canopy we were able to make out a set of leaves with a subtle haze around the leaf margins. We followed the leaves to the branches and down to the trunk. It was silvery. There we were standing under a large, healthy white walnut. The next day we stumbled across a few young saplings in some of our vegetation plots. All is not lost. I can't speak for the future of this species but I feel very lucky to have seen some healthy individuals. With a little bit of luck there may be hope of resistance to this deadly fungus. Only time will tell. 

Photo Credit: Dan Mullen (http://bit.ly/2br2F0Z)

Further Reading:
http://bit.ly/2b8GiMV

http://bit.ly/2aLUdMD

Meet the Redbuds

 Redbud (Cercis canadensis)

I look forward to the blooming of the redbuds (Cercis spp.) every spring. They can turn entire swaths of forest and roadside into a gentle pink haze. It's this beauty that has led to their popularity as an ornamental tree in many temperate landscapes. Aside from their appeal as a specimen tree, their evolutionary history and ecology is quite fascinating. What follows is a brief introduction to this wonderful genus.

 Redbud (Cercis canadensis)

The redbuds belong to the genus Cercis, which resides in the legume family. In total, there are about 10 species disjunctly distributed between eastern and western North America, southern Europe, and eastern Asia. All of them are relatively small trees with beautiful pink flowers. Interestingly enough, unlike the vast majority of leguminous species, redbuds are not known to form root nodules and therefore do not form symbiotic relationships with nitrogen-fixing bacteria called rhizobia. This might have something to do with their preference for rich, forest soils. Until more work is done on the subject, its hard to say for sure.

One of the most interesting aspects of the redbuds are their flowers. We have already established that they are quite beautiful but their development makes them even more interesting. You have probably noticed that they are not borne on the tips of branches as is the case in many flowering tree species. Instead, they arise directly from the trunks and branches. This is called "cauliflory," which literally translates to "stem-flower." In older specimens, the trunks and branches become riddles with bumps from years of flower and seed production.

 Redbud (Cercis canadensis)

It's difficult to make generalizations about this flowering strategy. What we do know is that it is most common in dense tropical forests. Some have suggests that producing flowers on trunks and stems makes them more available to small insects or other pollinators that are more common in forest understories. Others have suggested that it may have more to do with seed dispersal than pollination. Regardless of any potential fitness advantages cauliflory may incur, the appearance of a redbud covered in clusters of bright pink flowers is truly a sight to behold.

Further Reading: [1] [2] [3] [4] [5] [6]

The Devil's Walking Stick

The name "Devil's walking stick" just sounds cool. You can imagine my excitement then when I first laid eyes on the species it refers to. Aralia spinosa is no ordinary tree. It is a hardy species ready to take advantage of disturbance. Armed with spikes and a canopy that looks like it belongs in some far off tropical jungle, the Devil's walking stick is a tree species worth knowing. 

I used to think that spikenard (Aralia racemosa) was the most robust member of the aralia family found in North America. Not so. The Devil's walking stick is a medium sized tree capable of reaching heights of over 30 feet (10 m). Most interesting of all, its triply compound leaves are the largest leaves of any temperate tree in the continental United States.

The Devil's walking stick can be found growing in disturbed areas and along forest edges throughout a large swath of eastern North America. When young it is a rather spiny lot. These are not true spines, which are modified parts of leaves, but rather prickles, which arise from extensions of the cortex or epidermis. 

As it grows, however, it loses a lot of its prickliness. Such armaments are costly to produce after all. It is believed that younger plants develop these structures while they are still at convenient nibbling height, only to lose them once they grow big enough to avoid hungry herbivores. Research has shown that most herbivorous mammals alive today do not bother much with the Devil's walking stick, which has led some to suggest that these defenses evolved back when this side of the continent was brimming with much larger herbivores such as elk and bison. 

 Photo Credit: Celerylady - Wikimedia Commons

Photo Credit: Celerylady - Wikimedia Commons

As if the giant compound leaves of this tree were not stunning enough, the surprisingly large inflorescence is sure to blow you away. Typical of the family, it consists of hundreds of tiny green flowers. Despite their size, they are a boon for pollinators. A tree in full bloom comes alive with bees and butterflies alike. Flowers soon give way to clusters of berries, which are a favorite food among birds. All in all this is one cool tree.

Further Reading: [1] [2]

Why Trees?

Walking through the forest is my favorite activity in the world. It is where I feel truly myself. There is something about towering trees that calms me. The thought of why forests are even there often jumps to mind during my strolls. Plenty of plants seem to do just fine hanging out closer to the ground. Why have trees (and some forbs) taken to this vertical realm. Why do forests exist?

In essence, forests are a prime example of an evolutionary arms race. It is one that these organisms have been fighting since the Devonian, roughly 385 million years ago. As plants left the water and began covering the land, some inevitably grew taller than others. There are pros and cons to growing tall. Competition is likely the prime driver for most tree species. Getting above your neighbors means more sunlight. Not every plant is as content as an herb to live out its life in the understory.

Height also means better pollinator visibility and seed dispersal for many tree species. Out in the open, gametes and propagules can be carried great distances by the wind. Colorful blooms would prove to be more exposed and easier for pollinators to locate. Growing tall can also get you out of harms way, removing sensitive growing parts from many different kinds of hungry herbivores and all but the worst forest fires.

There are many downsides to growing tall as well. For one, trees are exposed to the elements and are often victims of strong winds or lightening strikes. What's more, all of that wood takes a lot of energy to produce and, at least for most species, gives nothing back in the way of photosynthesis. It is a rather hefty investment. However, the cost of getting shaded out by your neighbors is definitely not worth the risk of staying small for sun-loving species.

Pumping water is another serious issue. The laws of physics suggest that redwoods are pushing the limits for how tall a tree can grow and still be able to lift water to leaves way up in the canopy. Of course, humidity can assist with such issues but for a majority of the water needs of a tree, water must be able to travel against gravity via weak hydrogen bonds. Water forms an unbroken chain within the vascular tissues of plants. As it evaporates from the leaves, it pulls more water up to fill in the void. It is possible that in today's world, a tree would not physically be able to grow much over 400 feet.

Despite the seemingly lavish waste of limited resources that a forest of trees would suggest, they are nonetheless a common occurrence all over the globe and have been for millions of years. The pros must certainly outweigh the cons or else tallness in trees would never have evolved. The next time you find yourself hiking through a forest, think of how the struggle for survival has led these towering organisms from lowly green stains on rocks to hulking behemoths racing towards the sky.

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

Meet the Chinkapin Chestnut

I made a new acquaintance this week. While surveying a dry ridge top I began noticing a strange, musty odor in the air. At about the same time I began seeing what looked like spiny chestnut burs littering the ground. I looked above me and there stood the branches of a chestnut in full bloom! 

It didn't make much sense to me that I would be seeing a Chinese chestnut in such a remote high-elevation area. As it turns out, this was indeed a native species of chestnut, though one I have never encountered before. What I was looking at was a healthy stand of Allegheny chinkapin (Castanea pumila). 

The Allegheny chinkapin is a small tree compared to its cousins. It is native to the southeastern United States where it seems to prefer xeric sites. Now I am a child of the post-chestnut era and therefore I am not used to seeing a native chestnut at reproductive age. As it turns out, the Allegheny chinkapin varies in its susceptibility to the chestnut blight that devastated its relatives. 

Reports from Kentucky as well as the Ozark Mountains show that these populations have suffered severely from the blight. Here in North Carolina, however, the situation seems to be a bit better. Trees don't seem to show the signs of heavy infestation (blighted cankers and cracking of the bark), though some trees do show some scarring. Regardless of their susceptibility, it would seem that they are able to reproduce at a smaller size. On top of that, they readily sucker and it doesn't take long for the suckers to mature. 

All in all this is a lovely tree. It is refreshing to know that there is hope for our native Castanea. Its small stature makes for ample opportunity to appreciate this species when you find it. 

Further Reading:

http://bit.ly/29l3XLP

http://bit.ly/29kdSUn

In Search of Stewartia

Up until a little over a week ago, I had no idea there were native representatives of the family Theaceae other than Franklinia alatamaha in North America. Dr. Mark Whitten was looking for a tree in order to obtain some genetic samples. He showed me a picture and my jaw just about hit the ground.

Fast forward a few days. A friend sent me an email regarding a hike to see Stewartia in the wild. This was an opportunity I was not about to miss. We took the day off and headed into the mountains. We met up with a small group of people whose goal that day was to bask in the glory of the mountain camellia (Stewartia ovata). We were led by local Stewartia expert, Jack Johnston (http://bit.ly/2908lSY).

It wasn't long before we had our first sighting. Just off the trail leading to a campsite was a spindly looking tree that stood roughly 15 feet in height. Without flowers I don't know if I could pick it out of a lineup. Lucky for us, this small tree was covered in large white blossoms. For the second time that week my jaw had to be pulled up off the ground.

The blossoms were absolutely stunning. About the diameter of a softball and with bright white petals, they are impossible to miss. At the center of each flower is a dense cluster of filaments supporting bright yellow anthers. The filaments themselves are quite attractive. They range in color from pure white to deep purple. What's more, any given tree can sport multiple flowers of with different filament colors.

The color did not seem to influence pollination whatsoever. Each flower we saw was crawling with solitary bees. To be fair though, very little research has been done on this species. Aside from some genetic work, the ecology of the mountain camellia remains a bit of a mystery. What we do know about this tree is that it has its roots in Asia. North America is lucky to have two of the 18 - 20 species of Stewartia. The rest are spread around the Asian continent. North America's Stewartia serve as a reminder of an ancient geologic connection North America and Asia once shared.

By the end of our hike we had lost count of the amount of trees we encountered. Despite their abundance, they are by no means common. Though not technically endangered, their limited distribution and low germination rates make it a sensitive component of the Appalachian flora. With tentative introductions into the horticultural trade, the best way to see this species is in the wild. Look for it growing in cool, shaded edge habitats, most often near mountain streams and rivers. It is a sight you will never forget.

 

Further Reading:

http://bit.ly/28ZrctJ

http://bit.ly/28ZtA4g

Why Trees Have Rings (and why they are so useful)

Dendrochronology is a field of study that focuses on tree rings. Though it may not be obvious, the amount of information we gain from looking at these rings is astounding. This research goes far deeper than simply finding out how old a tree was when it died. Dendrochronological data can be used to investigate paleoclimates, paleoecologies, and the archaeological dating of buildings and artwork. It is amazing how a practiced eye can look back in time. To date, we have an unbroken dendrochronological record for the northern hemisphere dating back some 13,900 years!

All of this would not be possible if it were not for tree rings. But what exactly are they and how do they form? The answer is physiological. Essentially tree rings result from patterns in vascular tissues. Early in the spring, before the leaves start to grow, a layer of tissue just under the bark called the cambium begins to divide. In this cool, water-laden time of the growing season the vessels that are produced are large and less dense. This is the beginning of the spring or early wood. Although they are not as strong as vessels that are produced later in the season, they sure can move a lot of water. Things are a bit different for conifers. Because they do not produce vessel elements in their wood, this large cell growth is initiated instead by large amounts of a growth hormone called auxin that is produced by the new buds. This causes the cells of the early wood in conifers to grow large in a similar way to that of the hardwoods. 

As summer heats up, things start to change. The cambium starts producing smaller, thicker cells. The vessels that result from this are much stronger than those of the early wood. This late wood as it is called gives trees much of their rigidity and strength. Late wood is also resistant to what is called cavitation, a process in which water within the tree can literally vaporize, causing a damaging embolism during the hottest months of summer. In conifers, bud growth stops by mid to late summer and with it much of the production of auxin. This results in smaller vessels as well. 

In temperate regions, this cycle of growth occurs over the course of a growing season. As such, each ring demarcates a year in that trees life. Because so much of a trees growth is determined by environmental conditions, the size and shape of the rings can tell a lot about the conditions in which that tree was growing. That is why dendrochronology is such a useful tool. By looking at tree rings from all over the world, researchers can tell what was going on at that point in time. And, though it was long thought that this was a phenomenon restricted to seasonal forests, we are finding that even some tropical trees produce annual growth rings. This is especially true in regions that have a measurable dry season. It just goes to show you that data comes in many shapes, sizes, and forms.

Further Reading:
https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/16947

http://bit.ly/1m9cwvR

http://bit.ly/20COCbI

My oh my, What a Beautiful Magnolia

Magnolia fraseri is, in my opinion, one of the most beautiful trees in our eastern forests. To see this species, one must travel to the Southern Appalachian Mountains where it is endemic. With its whorls of massive leaves, large, cream colored flowers, and smooth gray bark, it is an unmistakeable component of the Appalachian cove forests.

M. fraseri needs canopy gaps to persist. Anywhere that disturbance opens up the canopy and allows light in, M. fraseri is soon to follow. This tree has surely benefitted from the mass die off of eastern hemlock due to the invasive hemlock wooly adelgid. This species flowers in the spring. Magnolias as a whole are an ancient lineage of flowering plants, arising before bees evolved. For that reason, their flowers are pollinated by beetles instead of bees. The large, showy flowers soon give way to your typical magnolia seed pod. As the seeds mature, they are pushed out of their pod and their bright red coloration helps to attract their main seed disperser, birds.

Aside from seed production, the most common form of reproduction for M. fraseri is via stump sprouts. In fact, it is believed that many of the oldest M. fraseri in the Appalachian forest region are stump sprouts that harken back to a time in which forest clearing was more rampant.

The overall appearance of this tree feels tropical. The large leaves are are arranged like an umbrella and these whorls stack themselves all the way up the trunk. Why this species is not cultivated as a native landscape tree is beyond me and I think the following excerpt by Richard E. Weaver Jr. sums it up quite nicely:

 

 

"Many of our fine native plants remain rare in cultivation in our own
country for a variety of reasons. Over-familiarity with them as wild
plants; lack of commercial availability; ignorance as to culture and
propagation; or plain snobbishness. Many are far better appreciated abroad."

Magnolia fraseri was one of the first plants that greeted me upon entering North Carolina. It was growing alongside a pawpaw at a scenic overlook that showcased the hardwood forests that coat these mountains. I never pass up an opportunity to appreciate this tree and indeed I will carry the image of it in my mind wherever I go.

Photo Credit: Jim Dollar (http://bit.ly/1R2Qpjy)

Further Reading:
http://link.springer.com/article/10.1007/BF00346412

http://arnoldia.arboretum.harvard.edu/…/1981-41-2-magnolia-…

http://www.na.fs.fed.us/…/si…/volume_2/magnolia/fraseri.html

Cashews

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I love cashews. I can't seem to get enough of them. Did you know that when you eat a cashew, you are only experiencing part of the fruit? Indeed, cashews are kind of weird and many of us in temperate climates never get a chance to fully appreciate what the cashew has to offer. You may also be surprised to learn that cashews and poison ivy are cousins.

Cashews or Anacardium occidentale as they are known scientifically are large trees belonging to the family Anacardiaceae. This makes them cousins of plants like poison ivy, sumac, pistachio, and mango (just to name a few). Like other members of this family, cashews produce chemicals that can cause severe skin allergies in humans. For cashews, this chemical is known as anacardic acid and is similar in its chemical makeup to urushiol. Because of this, cashews must be roasted before they can be sold. 

As I stated above, the cashew "nut" is only part of the reproductive effort of this species. They are not nuts in the true sense but rather a drupe similar to the pit of a cherry or peach. The drupes themselves hang from the bottom of a much larger accessory fruit called a cashew apple. This pear-shaped pseudocarp is quite juicy and does not ship well. Though it is a delicacy in tropical climates where these trees are cultivated, it rarely makes it to more temperate climates.

Cashews are currently native only to Brazil but fossils found in Eocene deposits from Germany hint at a much wider distribution. It is now believed that the group that gave rise to cashews originated in Africa and subsequently migrated outwards while South America was still attached. Today, the cashew is regaining some of its lost ground thanks to its agricultural importance. 

Speaking of agriculture, cashews are offering an interesting model for more sustainable farming practices. Cashews, like most other crops, are grown in large-scale monocultures. Thousands of gallons of pesticides are used on these crops to stave off pests. However, the pesticides kill more than just unwanted insects. What is interesting about cashews is that they naturally produce extrafloral nectaries (glands that secrete nectar) on their leaves. In the wild these glands attract ants looking for a high energy meal. The ants in turn guard these nectar sources from anything that may interfere with their feeding. As such, many potential pests are driven off by the ants. Research is being done to compare the rates of insect pests between cashew plantations that use pesticides and those that don't. It could be possible that by allowing ants to guard these nectar sources, farmers could avoid the use of pesticides to control insect damage. More work is needed but cashews are certainly a great model for developing such a system. 

www.indefenseofplants.com

Photo Credit: Peter Nijenhuis (http://bit.ly/1A0MmLI)

Further Reading:

http://www.jstor.org/stable/10.1086/520728

http://www.amjbot.org/content/85/6/835.full.pdf

Mangroves

 

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These strangely beautiful flowers are that of a mangrove! A red mangrove in the genus Rhizophora to be more specific. These peculiar trees are known the world over by their stilted appearance and tolerance of salt water. With roughly 110 species worldwide and a lot of erroneous labeling of what it means to be a "mangrove," the taxonomy of this group is a bit messy. 

Mangroves are a tropical species. They form the backbone of saline coastal habitats all around the world. Their real claim to fame is their ability to deal with salt water on a level that would kill pretty much every other plant out there. They do this in a very interesting way. Upwards of 97% of the salt is excluded from the roots by spongy material that acts as a filter. What little salt does make it in ends up in the leaves. In some species, salt gets hyper accumulated in the leaves and then disposed of when those leaves are shed. Some have taken to calling these "sacrificial leaves" but recent evidence suggests that there may be no difference in salt concentrations between leaves on any given tree. Other species excrete salt through special glands, which can be readily seen by turning over the leaves. 

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My favorite aspect of mangrove ecology is their reproduction. Mangroves come about as close as a plant can get to live birth. Their long, pod-like seeds will actually germinate while still attached to the parent tree. Because they are so long and slender, the pods will often spear themselves into the sand when they fall where they will continue to grow. In other cases, the seed can change buoyancy over time. This allows them to float on the surface and travel great distances. Upon germination, the buoyancy of the seed changes, causing it to suspend itself vertically, thus increasing its chances of lodging itself into the mud or sand. In this way, mangroves are excellent long distance colonizers.

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Mangroves are so much more than simple trees. They are ecosystem engineers supremely adapted to harsh environmental conditions. They create habitats that provide breeding grounds for myriad other organisms. Entire economies rely on the bounty these trees provide and yet they are all too often leveled under the guise of economic gain. What's more, their coastal habit provides storm protection that can be counted in the billions of dollars. Sadly, the loss of mangroves often translates into not only a loss of capital for coastal communities but also a collapse in the ecosystems they depend on. Worldwide, mangrove forests have seen a 35% decline over the last few decades. Countries are starting to wake up and realize what they have lost but we still have a ways to go. We ignore species like mangroves at our own peril. 

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Photo Credit: Peripitus, Phil's 1stPix (http://bit.ly/1GTMK41), Holly (http://bit.ly/1wIrC0q)

Further Reading:

http://www.glomis.com/ej/pdf/EJ_8-4.pdf

http://muse.jhu.edu/journals/pacific_science/toc/psc60.3.html

http://www.therakyatpost.com/news/2014/09/15/mangroves-protect-malaysias-coast-also-shield-illegals/

http://www.nhmi.org/mangroves/phy.htm

http://www.botgard.ucla.edu/html/botanytextbooks/worldvegetation/marinewetlands/mangal/index.html

I've Got the Colorado Blues

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You would be hard pressed to find a resident of temperate North America who has never seen a Colorado blue spruce. These iconic trees are a staple of every sapling give-away and can be found in countless landscape plans all over the continent. There is no denying the fact that the blue hues of Picea pungens have managed to tap into the human psyche and in doing so has managed to spread far beyond its relatively limited range. However, despite its popularity, few people ever really get to know this species. Even fewer will ever encounter it in the wild. Today I would like to introduce you to a brief natural history of Picea pungens

Despite its common name, P. pungens is not solely a denizen of Colorado. It can be found in narrow swaths of the Rocky Mountains of Wyoming, Idaho, south to Utah, northern and eastern Arizona, southern New Mexico, and of course, central Colorado. There are also some rumored populations in Montana as well. It has a very narrow range compared to its more common relative the Engelmann spruce (Picea engelmannii). Whereas some authors consider the Colorado blue spruce to be a subspecies of the Engelmann spruce, the paucity of natural hybrids where these two species overlap suggests otherwise. It is likely that Colorado blue spruce split off from this lineage and has since followed its own evolutionary trajectory.

 Male cones are short-lived but quite attractive.

Male cones are short-lived but quite attractive.

One of the reasons P. pungens has become such a popular landscape tree is due to its extreme hardiness. Indeed, this is one sturdy tree species. Not only can it handle drought, P. pungens is also capable of surviving temperatures as low as -40 degrees Celsius with minimal foliar damage. Little stands in the way of a well established Colorado blue. In the wild it can be found growing on gentle mountain slopes at elevations of 6,000 to 10,000 feet. It is also a long lived and highly fecund tree. The most highly productive seed years for P. pungens begin at age 50 and last until it reaches roughly 150 years of age. Seeds germinate best on bare soils, which probably keeps this species limited to these mountainous areas in the wild.

 The typical female cone of the Colorado blue spruce.

The typical female cone of the Colorado blue spruce.

Another component of its landscape popularity is its characteristic blue color. In reality, not all trees exhibit this coloration. Its blue hue is the result of epicuticular wax deposits on the leaves as they are produced in the spring. Not all trees produce the same amount or consistency of wax and therefore not all look blue. Wax production seems to be controlled by a genetic factor and therefore is often a shared trait among isolated populations. The wax functions as sun screen, reflecting harmful UV rays away from sensitive developing foliage. This is why it is most prominent in new growth. The wax can and often does degrade over the span of a growing season, resulting in duller trees come fall. 

Despite how interesting this spruce is, Picea pungens, in my opinion, represents the epitome of lazy landscaping. Like Norway spruce and Norway maples, P. pungens seems to be an all too easy choice for those looking to save a quick buck. As a result, countless numbers of these trees line streets and demarcate property boundaries. Though P. pungens is native to North America, its narrow home range makes its ecological function elsewhere quite minimal. Though one could certainly do worse than planting this conifer, it nonetheless overshadows more ecologically friendly tree choices. 

Photo Credit: [1] [2] [3]

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