One of the most fertile places for plants to grow is at the edge of the forest, where it’s adjacent to an open field. Forests and fields have different advantages from the viewpoint of a plant looking for a place to grow, so if a plant can shape itself properly, it can take advantage of the good parts of both. Neither a forest nor field could provide this on their own, only the edge allows it.
The forest’s advantages include its more complex network of plants, fungi, and other organisms, many of which fill beneficial roles. Trees also shade out the ground, which keeps it cool and moist, ideal for the crucial organisms that live in the soil. The field’s advantage is that the lack of trees means it’s easier for plants to access sunlight, which is necessary for photosynthesis.
In the northern hemisphere, the trees being to the north of the clearing is ideal, because the sun sits to the south, meaning it can hit the edges where trees are to the north, but if trees are to the south, they will shade the edge. In the southern hemisphere, north and south are reversed.
Besides the comparative advantages both provide, the two ecosystems simply bring different species to the table, making the diversity of life at the edge higher than elsewhere. This is valuable for plants, because they rely on many other species, like the bacteria and fungi in the soil that gather nutrients and transport them to plant roots in exchange for sugars from photosynthesis. Above ground, birds and other animals help disperse seeds. The combination of good soil conditions, high biodiversity, and access to sunlight makes the edge between forests and clearings highly fertile.
Where there’s fertility, there will be competition. Vines can do extremely well here, as they climb the trees at the edge, getting more sunlight than they could by climbing any trees deep in the forest. Kudzu is one example, easily seen driving through the southeastern US, where forests-field edges are created for roads and other development.
Brambles/canes are kind of a cross between a shrub and a vine, and also compete well here, as they can have their roots in the moist, shaded soil, but grow out into the sunlight. Perennial herbaceous plants – ones that die back to the ground every year but come back in the spring – can thrive on the same strategy as canes, if they can grow tall enough. The race for light is eventually won by trees, which do well at the edge also, in due time pushing the forest further into the clearing if they’re not cut back.
Competition makes it harder for any individual to thrive, driving evolution faster, and eventually resulting in species highly specialized to live in their particular conditions. What traits, specifically, will help plants thrive within the tight competition of the forest-field edge? First, it’s important to understand that different plants – and organisms in general – evolve to fill different niches. Vines do one thing, brambles do another, and both can be successful at the same time at a forest edge. The next key thing to consider is that a forest abruptly turning to open field is a fairly unusual circumstance in nature.
Where there’s enough rainfall and sunlight, trees tend to be the dominant plant, as they win the vertical race for sunlight. Other plants and organisms, collectively creating a forest, then evolve to operate within the condition a canopy of trees creates.
Prairies, on the other hand, don’t have trees, and there must be a reason. If trees could grow in a prairie, they would, since they could easily outcompete the plants there for sunlight. Prairies can sometimes have a few trees here and there, but not many. Generally, this is due to lack of water and/or sunlight. The amount of precipitation and sunlight doesn’t generally change abruptly enough to have a forest-sustaining climate directly adjacent to a no-trees-at-all-sustaining climate.
Ecosystems instead blend into one another as the climate gets drier, colder, farther from the equator, etc. Trees grow sparser and sparser over many miles until there’s few, and eventually none. An abrupt forest edge is the opposite of this: the two ends of what’s normally a long transition butted up against each other. Some natural phenomena can cause this to happen, like a landslide, which exposes an area of bare earth, while the adjacent land that remains in tact is still forested.
Over the years, the clearing left by a landslide would go through a process called ecological succession. First, plants that can quickly spread sideways will have a field day since there’s no vertical competition for light, e.g. crabgrass, creeping charlie. These plants shade the soil while alive, then leave organic matter when they die, improving the soil in both steps. The improved soil allows a broader range of plants to grow, and the vertical competition for light begins. At first, it’s mainly annuals and herbaceous perennials, who shade out the early pioneer species, and add even more organic matter to the soil since they’re larger. This further improve the soil, allowing the ultimate trick in the competition for light: woody perennials. At first, only small shrubs can be sustained, but they still have a head start in spring over herbaceous perennials and annuals.
The larger a plant is, the more complex its root system needs to be, particularly in its relationships with bacteria and fungi. All the good soil where these organisms live gets lost in a landslide, and in the immediate aftermath, trees simply can’t succeed without the right soil life, particularly fungi. Fungi mostly feed on wood, while bacteria feed on the green growth that dominates the early years of ecological succession. The small shrubs that show up next play the same role as the earlier plants when they die: improving the soil. Shrubs are made of wood, which helps fungi thrive, making the soil diverse enough to sustain the eventual winners in the vertical competition for light: trees.
While forest-field edges aren’t common in nature, they do happen periodically, but will typically follow ecological progression back to a forest. What’s even rarer is (or at least once was) when a forest edge is a permanent condition. These can sometimes have geographic or geologic causes, but almost always come from a different source: humans.
This brings us back to the question in bold above: What traits, specifically, will help plants thrive within the tight competition of the forest-field edge?
Humans don’t create forest edges for the sake of the plants living there, they create them because they want to build/grow/graze something in a location that was previously forest. When they do this once, they tend to do it again later, cutting further into the forest. Thus, while humans are the main cause of the forest edge microclimate, they’re also the main threat to the plants growing there. Thus, the most important advantage the plants need to have in this location is a way to survive against humans.
Some plants that thrive are just good at quickly regrowing when humans cut them back, which are the least interesting to me. These are generally called lawn weeds. Others have defense mechanisms against humans, which fall into two categories: carrots and sticks. “Carrots” (not the literal plant/food carrot) are plants that are useful to humans, so the humans would prefer to keep them around. “Sticks” are plants that will harm humans, so they prefer to stay away, and let them be. Carrot is a reward, stick is a punishment.
Today, there’s enough personal protection, large equipment, and legal requirements like property boundaries, that the carrots and sticks don’t play as much of a role, but that’s only been true for a tiny amount of time humans have been around.
Imagine an tribe living thousands of years ago in a clearing in the woods. Their population has grown, so they want to expand. To the north, there’s blackberries growing on the forest edge, to the east there’s poison ivy, to the west there’s stinging nettle (a plant that gives humans a rash if touched), and to the south there’s a number of other plants, none particularly harmful/helpful to humans. Which way would they likely expand their settlement?
Blackberries, and their close relatives raspberries, employ a dual strategy of berries as a carrot, and thorns as a stick. Stinging nettle does the same, since it’s a highly nutritious leafy green for humans, if cooked or otherwise prepared right to destroy the tiny spines that cause the rash. Poison ivy only employs the stick. Given these facts, if they expand north or west, they’ll be cutting down a food source, and if the go east or west they’ll end up with a nasty rash.
Naturally, people without protective gear and fancy equipment to avoid contact with the plants would prefer to expand south, preserving the carrots and avoiding the sticks. As a result, the plants that are specifically good at helping or harming humans have a competitive advantage at the forest-field edge. Urushiol, the oil on poison ivy, oak, and sumac that gives humans a rash, is harmless to most animals. Many animals eat and seek protection in those three plants, they just happen to be harmful to humans. As a result, they were left alone whenever humans cut further into a forest for thousands of years, while plants not harmful to humans got cut down more often. Thus, poison ivy and the other carrot/stick plants had more chances to produce seed and spread over/under ground, and came to dominate the forest edge.
The overall effect is that the presence of humans created a previously-rare microclimate that was highly fertile, but humans were also the main threat against the plants that grew there, as those who cut into the forest one time tended to do it again. This was, and still is, the main threat the forest as a whole faced, so the forest evolved to have more of the plants that prevented humans from cutting into it. Thus, when humans create forest edges now, the first plants to pop up are specifically the ones well-equipped to appease or annoy humans. In other words, blackberries, poison ivy, and stinging nettles are the forest’s immune response against humans, a scab to help heal the wounds the humans impart on it.
This edge effect can be seen elsewhere, like at sea floor vents, which are the edge between the ocean and the Earth’s mantle. The crust is the solid outer layer of Earth, which the ocean sits on, and the mantle is the layer below the crust, made of molten rock, or magma. Sea floor vents are where there are cracks in the crust that allow magma to escape into the ocean. Basically, underwater volcanoes.
Like forest-field edges, ocean-mantle edges are pretty rare. In most locations, the ocean is comfortably separated from the mantle by the crust, but the few places where it’s not have a higher diversity and density of life.
The advantages of the two sides of this edge are strikingly similar to the forest-field edge. For any life form to survive, it needs, at a base, the right nutrients and an energy source. At the sea floor, there’s no sunlight to do photosynthesis, so organisms do chemosynthesis instead, which basically means a controlled burning of molecules like methane, which the magma provides. The magma provides chemical fuel for energy, like the field provides sunlight.
The ocean is made of water, which is exceptional in its ability to suspend a large variety of elements and molecules, meaning it provides a diversity of nutrients, like the forest does through soil microbial life. Completing the analogy, both sides bring different specific elements and molecules to the table, increasing the general chemical diversity in those locations.
Additionally, there’s the fact that magma’s too hot and the sea floor’s too cold for most organisms, but where they meet they can cause small pockets that even out to milder temperatures.
The result is sea floor vents are like an oasis of biological density in the sparsely populated deep sea. However, they’re more interesting and important than that. The prevailing theory for the origin of life on Earth was that it began with a self-replicating molecule (or group of molecules that collectively replicated themselves) in one of these sea floor vents. Once a self-replicator was created, and it had access to the right minerals around it, it would create more and more of itself, but sometimes mess up slightly, launching the process of biological evolution.
Most molecules/groups of molecules do not replicate themselves, but there are a rare few complex ones that do, collectively studied as “biology.” A self-replicator coming into existence means a highly complex molecule – the right highly complex molecule – has to be found through random chemical reactions. It only makes sense that the most chemically diverse location on Earth, where more types of atoms and molecules can interact than anywhere else, would be the most likely place for that to happen. Most likely, we owe all of life on Earth to one of the few edges between mantle and ocean.
The final example is social instead of biological: borders. Similarly to how ecosystems usually blend into one-another, human cultures did the same until relatively recently. While there were different, distinct cultures, that would often come to understandings about who had claim to which land, there was nothing like a modern border, with customs agents ready to greet anyone that crossed. Different tribes would govern themselves, even within a single culture, rather than the large states that dominate the globe today. Different tribes would communicate with those around them and naturally learn and borrow ideas from them, since things that help with survival in one location generally help a few miles away.
Today, the US-Mexico border is a well-defined, abrupt edge between two countries whose capital cities are almost 2000 miles apart, and both far from the border itself. As such, the lives of people mere miles apart, but on opposite sides of that border, vary much more than the lives of people in the exact same locations 1000 years ago would have. National and state borders are an abrupt transition imposed by humans, where the transition previously happened far more gradually, just like forest edges. As such, national borders provide a similar opportunity for humans to take advantage of the different conditions on the two sides.
A familiar example of this is fireworks stores right next to the border with a state/country where they’re illegal to sell. The legal side of course provides security in the state allowing them to do business, and the illegal side provides a large population of customers whose demands can’t legally be met anywhere closer to home. Thus, fireworks and other items legal in some states but not others tend to line the borders where legal states meet illegal.
The US-Mexico border is a much more drastic difference than state borders within the US, with the two countries having more varied political and economic situations. The weaker democratic institutions of Mexico allowed drug cartels to gain power within the government, giving them legal cover to operate within the country. The US has a mass of citizens who do drugs at a rate among the highest in the world and have much more money to spend on them – the perfect customers for the cartel. Thus, the unusual conditions created by the national border provided a fertile spot to make illicit money, just like the unusual forest-field edges provided a fertile spot for plants to grow.
In all these cases, the key is diversity – when you put a lot of variety of any sort of thing in close proximity to one another, more possibilities of different interactions are explored as a result. This can be good or bad, and which it is often depends on who you ask. To the cartel member, the US-Mexico edge is an opportunity, while to many Americans it’s a hazard. To the forest, poison ivy is a key piece of its immune system against its greatest threat, while to humans it’s an annoying, even dangerous plant. Hopefully we can all agree the diversity around sea floor vents is cool.
Write a comment