Holding Tight

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American Beech trees, still holding leaves in January, are scattered throughout the understory of a local forest.

And to this very day, when all the other trees lose their leaves, the great oaks (and for the sake of this post, the American Beech) keep theirs in memory of their ancestor’s promise. Only in Spring’s time, when the other trees grow new leaves, do the oaks drop theirs. Winter cannot steal spring away and control the weather forever.”

As the days get shorter and colder, trees have an important “decision” to make. They must shed their leaves or hold tight through the winter. Evergreens hold tight to their needles or leaves, but in evolutionary terms, evergreens are old. Over time, trees diversified and accumulated many beneficial adaptations. One of those was the ability to lose leaves. It is these deciduous trees that dominate areas like Statesville, where there are patches of mixed evergreen and deciduous forests. Between evergreens and deciduous trees, there seem to be some that do not obey all the rules. Tree such as oaks and beeches belong somewhere in the middle.  To understand why these trees hold their leaves longer than others, we must first understand why deciduous trees shed in the first place.

Shorter periods of daylight coupled with colder temperatures trigger a hormone in deciduous trees that sends a chemical message to every leaf. This message is to, essentially, “get off.” Cells then appear at the exact point where the leaf and stem meet. These cells start to rapidly divide, creating what is called an abscission layer. The division of these “abscission” cells prevents nutrients from reaching the leaves and also prevents chlorophyll from being replaced. Therefore, the leaves start to die just as other pigments (yellow, red, and orange) start to be seen and enjoyed by many people. Abscission comes from the Latin word abscissio, meaning “breaking off” (think scissors).

Leaves are the main food producers for trees through the process of photosynthesis. The leaves trap energy from the sun and use it to convert water and carbon dioxide into glucose (food) and oxygen. On short, cold days, food production decreases significantly. Deciduous trees choose to push these food producers away rather than keep them around with decreased productivity. Not that there’s anything wrong with retaining what gives you food. Evergreens do it. For example, one of the luxuries that a southern magnolia, a broad-leaved evergreen, has is it is able to produce food through photosynthesis all year long. Another advantage of retaining leaves would be that the tree would not have to allocate energy and resources into growing new ones every year. For deciduous trees, though, the bad outweighs the good.

If there happened to be several warm days in the winter, the leaves would resume photosynthesis (which requires water). If this warm spell was following by a frost, the leaves would be in trouble. The water would be trapped and would freeze. Because water expands when it freezes, the ice crystals would damage internal structures beyond repair, killing cells and tissue. In areas where snow and ice accumulates, entire branches could even be pulled off of trees, putting the tree in serious danger. So, for many trees, abscission keeps trees from becoming damaged and also helps conserve water and energy.

Even with the risks, the American Beech tree, Fagus grandifolia, decides to hold most of its leaves throughout the winter. The process of retaining dead plant organs that are normally shed is called marcescence. How do beech trees get away with this strategy and is there any advantage? There are three main theories, but these theories trigger more questions than answers.

Marcescence could be a trait found in both juvenile trees and/or lower branches of older trees. Holding on to leaves throughout the winter could benefit younger trees simply because after the taller trees shed leaves, more sunlight would reach the bottom. This would allow these understory trees to take advantage of the increased sunlight and photosynthesize longer. The American Beech tree does really well in the shadows of larger maples, oaks, and birches. Could it be that retaining leaves is what allows the beech to be so shade-tolerant? It’s hard to imagine that the beech trees are able to take advantage the increased food production process since their leaves look to be out of chlorophyll in the winter (see the yellowish, brown leaves above).

Another idea is that having these leaves “stick” around plays an important role in nutrient cycling for the tree. If the leaves shed in autumn, they would join all the other leaves on the forest floor and start the decomposition process. There’s a chance that the nutrients produced from this decomposition could leach away and the trees would have none for the spring when they are growing. Instead, the beech trees wait to drop their leaves in the spring to ensure that are some nutrients in the soil. This could be important for small understory trees that have small root systems. However, the problem with this theory is that big leaves often take months to decompose. Maybe there’s another answer.

Retaining leaves may help to deter large herbivores, such as deer, from browsing. At least one researcher thinks that the leaves protect buds and twigs from being chewed off because the leaves are less nutritious and palatable. Svendsen (2001) showed that the European beech tree, Fagus sylvatica, was browsed significantly more by weight and number of branches when the leaves were removed. Chemical analyses revealed that the protein and fiber content of beech twigs was higher of higher quality when compare to marcescent leaves. Maybe these leaves do act as an herbivore deterrent.

No one seems to know for sure why these trees chose to not give in to peer pressure and drop their leaves. It could be one of the previous theories. It could, in fact, be a combination of these theories. It could be something yet to be discovered.

Ah, the beauty of nature.


Svendsen, C. R. 2001. Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests. Alces, 37: 475-482.

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Huggin’ Trees by Design

By Tim Ross (Own work) [Public domain], via Wikimedia Commons

By Tim Ross (Own work) [Public domain], via Wikimedia Commons

How do you survive if you’re not the most dominant canine in an area? If you have to share space and potential food with your faster and sometimes much larger cousin, how do you and your kin thrive day in and day out? These are questions the gray fox, Urocyon cinereoargenteus, must answer.

The common gray fox is a crepuscular and nocturnal canid whose home range averages around 1.5 square miles. This range contracts during the spring when they mate and have pups and then expands during the fall while they search for food. Their diet, like both red foxes and coyotes, consists of small mammals and even birds. Unlike other canids, the gray fox’s digestive system is better equipped to handle much more fruit.

In many parts of the southeast, including Statesville, gray foxes live with both red foxes and coyotes. Not much is known about how red foxes and gray foxes divide habitat in areas where they overlap. What is known is that red foxes seem to prefer more open terrain while gray foxes stick to the woods. it’s certainly possible that these two species use time-share strategies to avoid interactions.

Having to share space and resources with coyotes, however, could cause major, or fatal, problems for the foxes. Coyotes have been implicated in the decrease of red fox populations in certain areas (Cypher 1993). However, gray fox populations do not seem to be impacted as much by the larger coyote. Our small data set from greenway patches seem to support the idea that gray foxes tolerate coyotes better than red foxes. The following shows the number of pictures of each species we have collected from two camera traps from August to December during both 2015 and 2016:

                               Camera A             Camera B
Coyotes             25                         10
Gray fox            24                           2
Red fox                1                           2

Is the explanation just that the diets of gray foxes and coyotes are different or is it that gray foxes avoid coyotes spatially and/or temporally?

Both Cypher (1993) and Neale and Sacks (2001) found that there was actually a high dietary overlap among coyotes and gray foxes year-round. The implication is that this means, more than likely, that there is interspecific competition when they live in the same area. Neale and Sacks (2001) also used scat analyses to determine that gray foxes did not avoid coyotes in space. Our data (Figure 1) suggest that gray foxes don’t avoid coyotes in a temporal (time) dimension either.

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Figure 1. Gray fox versus Coyote

The answer to how gray foxes avoid coyotes seems to be found in their evolutionary past. Gray foxes were the first canid to diverge from the rest of the canid family. They are a primitive species that branched off from the rest of the canids around ten million years ago (Lindblad-Toh et al. 2005). As a matter of fact, there is scientific support that states that the gray fox should not even be considered a canine or a vulpine (red fox). The gray fox is what geneticists call a basal canid, meaning it represents an ancient lineage.

Gray fox climbing in Statesville

Gray fox climbing in Statesville

As a result of this ancient lineage, the gray fox has many unique characteristics. Some features are only shared with another ancient species, the raccoon dog (Nyctereutes procyonoides). Gray foxes have hooked claws and the ability to rotate their forearms. This, coupled with the fact that they have the shortest leg-to-body ratio of any wild canid, allows them to easily hug tree trunks or branches and climb. Ecologically, they are more like cats. When pursued, gray foxes are not really interested in running. They would rather scamper up a tree. It’s this ability that helps them minimize conflict with coyotes.

Next time you are out in the forest, take the time to look up into the trees. You may get a peek at one of these cat-like dogs.

Gray fox foraging

Gray fox foraging


Cypher, B. L. 1993. Food Item Use by Three Sympatric Canids in Southern Illinois. Transactions of the Illinois State Academy of Science, 86: 139-144.

Lindblad-Toh et al. 2005. genome sequence, comparative analysis, and haplotype structure of the domestic dog. Nature, 438: 803-819.

Neale, J. C. C. and Sacks, B. N. 2001. Food habits and space use of gray foxes in relation to sympatric coyotes and bobcats. Canadian Journal of Zoology, 79:1794-1800.

NewsBits 06- Declining cheetahs, bird migrations, and a beneficial centrifuge

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Mucus is gross, right? Have you ever wondered if this “troublesome” substance has any advantages? This article investigates the advantages of snot.

One recent paper shows that cheetahs are declining in dramatic fashion. With a global population estimated at 7100 individuals, cheetahs are thought to occupy only 9% of their historical range. Not only are they threatened by range contraction, but also range fragmentation.

If you’re interested, you can find out here why a wild monkey tried to have sex with a deer. He even acted in an aggressive manner when other monkeys got close.

Bird migrations look to be well-timed with high levels of vegetation along the way. If this is true, what will be the impact of climate change on these stops? Will there still be food where the birds expect there to be food?

Why hasn’t nectar evolved to be really sweet? The answer will drive you batty. Read here.

The rusty patched bumblebee, Bombus affinis, became the first bee of any kind in the contiguous 48 states to be declared endangered. Its population had declined 87% in the last 20 years. You can read the U.S. Fish and Wildlife announcement here.

The Pioneer Cabin tree, a giant sequoia in California, fell as a result of heavy rains. It was carved out in 1880’s and instantly became a tourist attraction. See here.

And, finally, you can find out here how a researcher constructed a DIY centrifuge that will help developing countries.

Cautious Around the Trails

Leo Tolstoy once stated that the most important advice he could give would be to “stop a moment, cease your work, look around you.” It’s doubtful Tolstoy had white-tailed deer in mind when he said this, but surely this advice applies to cervids, especially those living near areas that attract walkers, joggers, and cyclists.

Over the past year, a team of biology students from Mitchell Community College have used trail cameras to monitor deer activity throughout greenway patches in Statesville, NC. These cameras have been used to estimate deer population, but researchers have also used them to compare deer activity in different areas. Two groups of cameras were used. Cameras in group A were placed deep in the woods away from human-made recreational trails and group B cameras were placed less than 25 m from the actual greenway trails. The researchers were interested in knowing if deer activity differs as the deer get closer to human used trails. During a 3 month period in the fall 2016, the cameras collected 139 pictures of deer on trails or less than 25 m from the trails and 522 pictures of deer in the deep forest. The pictures were then analyzed. Figure 1 shows the results.

Figure 1. Trail Activity vs. Forest Activity

Figure 1. Trail Activity vs. Forest Activity

 

Deer activity around trails peaks around 8:00 a.m., then drops off significantly. Activity is pretty much nonexistent around trails from 10:00 a.m. to 5:00 p.m. The deer don’t start using the trails again until after 6:00 p.m. On the other hand, deer activity in the deep woods peaks close to 9:00 a.m. and does not hit a low point until 11:00 a.m. During the day, deer activity in the deep woods stays consistently higher than activity along trails. White-tails tend to rest during the day in areas of highly dense vegetation. However, they are not resting for long periods of time. They are alternating between moving, eating, and resting. Our data suggests that this cycle occurs in the deep woods during the daytime hours. Is it simply because that’s where the vegetation is or do human-made trails have something to do with it?

Notice that right around 8:00 p.m., deer activity along the trails becomes higher than activity in the deep woods. It stays like this the entire night. There may be some simple explanations. It could be that the trails provide easier travel for the deer during the night. Maybe the deer are avoiding coyotes, especially when they have fawns to protect. The trails could also provide the deer easier access to the corn and bean fields nearby. Finally, maybe the deer know when people will not be out on the trails, and they purposely chose those times. Is it possible that the presence of people is what actually impacts the deer behavior on a daily basis?

While there is really no way to answer these questions from our small data set, there have been other studies over longer periods of time with more cameras that have looked at some of the same questions regarding this behavior.

Lashley et al. (2014) showed that female white-tailed deer, at baited sites, were 46% more vigilant when fawns were present. They also showed that does and bucks spent more time feeding as the size of the group increased, implying that there is safety in numbers for deer.

Parsons et al. (2016) found that prey species like deer avoided potential predators like dogs, humans, and coyotes in time, but not in space. The deer in this study did not greatly increase vigilance. The researchers concluded that dogs that are on the recreational trails with their human owners have a lesser impact on prey vigilance than free-ranging dogs.

More related to our study, Schuttler et al. (2016) analyzed the head posture of deer in over 3400 pictures to determine if deer are more vigilant in areas with both high human hunting and high coyote activity. What they found was not what they expected. They found that deer vigilance was actually lower in areas with high human recreation. This result is interesting because it may indicate that deer become habituated to human presence in areas such as our greenway spaces.

More than likely, there are a combination of factors that cause deer at this greenway patch to use the trails more during the overnight hours. In the future, more cameras will be used over a longer period of time to attempt to understand how these urban deer stay safe.

 

Not Playing by the “Rules”

Red fox (Vulpes vulpes)

Red fox (Vulpes vulpes)

Legend tells of a fox as black as night, so that it can never be seen in a man’s shadow. Although sometimes seen in juveniles, this black-phase phenomenon is quite rare. Foxes love urban green patches where they can hunt and also enjoy a life of secrecy. Remarkably, in ecosystems with larger predators, they continue to have success.

Regarding this color morph, we must ask several questions:

1) Physiologically, how does this happen?

2) Is this the product of natural selection?

3) If so, what is the evolutionary advantage?

There does not seem to be any specific studies that have looked at this condition in red foxes. However, there are two that look at the genetics of melanism in cats and wolves.

This article looks into the melanistic variants of the Felidae family. The researchers studied the genetic basis, evolutionary history, and adaptive significance of this dark color.

Interestingly enough, the researchers in this article show that traits that are selected in domestic dogs can influence fur color in wild wolves and coyotes.

In the meantime, be on the lookout for this guy in Statesville.