2011 International Year of Forests – Micro-climates in a tree

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Micro-climates provide a variety of life. The underside of this leaf has favorable conditions for aphids that feast on the plant juices. The aphids in turn will attract ants which will transform the aphid population into a kind of livestock farm. They 'milk' the aphids regularly. The aphids secrete sugary juices which act as food for the ants. An entire natural history story on one small leaf. Photo: Leon Pors (Curaçao)

Micro-climate – the definition

Search for the word climate in the dictionary and you’ll get the following description: “Average atmospheric and meteorological conditions in a given area over a long period of at least 30 years”. Enter the word micro-climate and you’ll get a lot of different definitions ranging from “climate in the bottommost layer of air (up to 2 m above the ground), dominated by the contact with the earth’s surface,” to “a micro-climate occurs when the conditions on a very small scale are different than what can be expected based on the climate, for example in case of a fallen tree with exposed root ball on which very small scale, diverse micro-climates will develop as a result of varying humidity and light intensity.” The latter definition is the one that this article is about, because if you take a close up look at a tree, a whole new world will reveal itself to you, a world of micro-climates.

Micro-climates create unique living conditions

Forests are richer than you think. The diversity of forests all over the world is largely determined by the variety of conditions prevailing therein where wildlife can adapt itself to. Tropical forests are often the most species rich, due to the different micro-climates such as the ‘canopy’ and ‘undergrowth’ that were mentioned in the previous article. But the story goes much further than that. Even within the “undergrowth” or the “canopy” sub-worlds can be recognized. Sub-worlds that can be as small as the area under a fern that grows on one specific tree in the forest. The conditions at this location can be so special that a certain kind of moss or some special kind of animal occurs only there and nowhere else in the forest.
Sometimes it is the location where a certain tree grows that is responsible for the specific micro-climate. The side of the trunk that is illuminated by the bright morning sun, for example on Curacao, is drier than the other side of the trunk. Ditto with the side on which the prevailing wind blows. In a certain place where branches intersect a ‘bed’ of dead leaves might have accumulated that were converted into soil. Some plant roots excrete substances that other plants do not. All circumstances that will make a plant or a tree in a certain spot suddenly very attractive for other species of plants and animals to settle upon.

The snack-behavior of the Trupial

On the moist part of a trunk, out of direct sunlight, swarms of small insects and spiders might accumulate, which in turn will attract birds which will not pass up a good meal. Ever wondered why Trupials are constantly picking at, and digging in the trunk of the Wayaca? At some point the bark of this tree will flake off, and beneath the flakes an ideal micro-climate for caterpillars, spiders, beetles and other critters exists, all organisms on the snack menu of the Trupial. With its sharp beak the bird chops and digs holes in the bark in order to snap off the flakes to gain access to the food underneath. The birds can spend hours doing this. It is thought that the Wayaca also benefits from this behavior to get rid of the old bark, to prevent the bark from suffocating.

A small spot in between two rocks with sufficient nutrients and the right humidity is the ideal place for moss to grow abundantly. Ten centimeters away it is already too dry and too hot to survive. Photo: Michelle da Costa Gomez (Curaçao)

Moss and micro-climate

Mosses grow abundantly on trunks and branches in places where it is humid, preferably with little sunlight and little wind. Look closely at a wind distorted tree in the style of the Dividivi. On the leeward side, chances are you will not only encounter mosses, but also insects and other small animals.

What exactly determines a micro-climate?

Factors responsible for the development of micro-climates are differences in light intensity, wind, ventilation and chemical composition of the environment, in addition to the differences in temperature and humidity already mentioned. The micro-climate under every rock might be different, as well as the micro-climate at different spots in a tree and even from branch to branch and leaf to leaf. A good example of the influence of light intensity, which directly translates into temperature differences on Curacao (with its overall hot, semi-arid climate) is the behavior of snails. If you pay attention during a walk in the ‘mondi’ in the dry season, you may encounter some trees completely covered with snail shells on only one side. In their enthusiasm some walkers will even collect such presumed “empty” shells, not knowing that the occupant of the shell is still alive.

Shells on the shadow side of a tree. The picture clearly shows how 'unfriendly' the other side of the tree would be for the animals. Photo: Michelle da Costa Gomez (Curaçao)

These snails have a special survival strategy. When it gets too hot and dry for them they move, often en masse, to a tree or plant of their preference and move up to a spot where the light intensity is the least and therefore the temperature lower. In the right spot they will create a vacuum in between themselves and the plant and attach themselves and seal the opening to the shell with a layer of mucus which dries to a membrane. This film is so well designed that the moist body of the snail is perfectly shielded against the hot and dry conditions. The animal enters what can be thought of as a “drought sleep ‘, which is equivalent to a hibernation. Only when the climatological circumstances have changed in favor of the animals, or if the side of the tree where the animal attached itself becomes too inhospitable, the animal gets the signal that it is okay to get moving again, or to escape from the spot to look for a better place to resume its ‘nap’.

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