Wednesday, May 28, 2008

Talk: Rod Myatt

(These talk posts will always just be my notes on the presentations; the original talks were of course much better, and all errors are my responsibility...)

Rod Myatt: An Introduction to Tropical Habitats

There are a great many habitats in the tropics, despite the appearance of uniformity from the air or the river. There are several types of climate in the tropics. Due to the tilt of the earth, the amount of incident sunlight varies by latitude and by season. The tropics receive the most heating, producing rising warm currents of air that cool as they rise, producing storms. The wet season of the tropics follows the sun, since the rain is driven by the incident sunlight, so now the southern Amazon is entering the dry season as the sun is moving northward for winter. Temperatures between the wet and the dry seasons don't actually differ much; this is one thing that is characteristic of a tropical climate. Rainfall can be very high, typically giving more water than plants are capable of using, so there is a great deal of excess precipitation (unlike many other climates).
So that's the basic picture of tropical climate. But there are many microclimates. The trade winds blow along the equator from east to west; this is the source of all incoming moisture entering the tropics. So in the Amazon region, the winds are coming from the Atlantic across the Amazon basin, slowly depositing their rainfall from the east coast all the way to the Andes. At about 5000-6000 feet elevation, the air masses cool to their point where clouds form and precipitation is highest; this is the cloud forest on the slopes of the Andes. Lowland rainforest, such as surrounds Manaus, is lower than montane rainforest (cooler and wetter), which is lower than cloud forest (coolest and wettest). So there are gradients of temperature and humidity based upon elevation. Above the cloud forest it is very dry and cold, and typically vegetation goes from shrubs up to alpine species. Similarly, the far side of the Andes lies in the rain shadow of the mountains, are are drier and colder. The more seasonality there is in the climate, the more deciduous the vegetation will be, so in the lowland rainforest vegetation is not generally deciduous, while in Peru, for example, you see tropical deciduous forest.
There is also microclimate variation due to local terrains; the land has some texture and slope, so some areas are north-facing or south facing slopes, some areas get water runoff and others don't, soils can vary quite a lot based upon such factors, some areas have volcanic influences, and so forth. The geology of the tropics is quite variable; there are many kinds of soil, and the statement that "tropical soils are poor" is an overgeneralization. But because of the long history of weathering of rock in the tropics, and the excess rainfall, many minerals have been leached out of the soils and have been lost; this is the source of the overgeneralization. Other less soluble minerals remain behind. Clay soil is composed of particles that are typically negatively charged, so minerals with positive charge are retained, while negative ions (often oxidized ions such as nitrates, sulfates, phosphates, and carbonates) are lost. The rapid buildup of organic debris in lowland rainforest also leads to faster loss of nutrients, because they are taken up by plants before they can become incorporated into the soil. This process is accelerated by the fungi that are often symbiotic with tropical plants that allow for extremely rapid and efficient absorption of minerals; in effect, plants are competing for insufficient minerals in the top layer of organic material, rather than getting minerals from the actual soil. Most of the nutrients in the tropics are in the plants, not in the soil.
This is what drives the slash-and-burn agriculture of the native peoples of the Amazon. It is not a matter of ignorance, it is actually the logical way to farm given low nutrient content in the soil and a lack of fertilizer. After a patch of forest is cleared, crops will grow well for only a few years before the soil is exhausted. The native peoples then move on to a new patch, clear it, and farm it for another few years, and so forth. Given a small population, this is actually perfectly fine; the forest recovers from clearance of small areas, and the native people have been practicing this sort of agriculture for quite a long time. The problem arises when population rises, agricultural demands rise, and the forest begins to be cleared faster than it is able to recover: deforestation. When small gaps are created, as happens naturally due to tree falls even without agriculture, it also creates microhabitats (open area, edge area, and so forth) and succession from one type of vegetation to the next. Another source of diversity in habitats involves difference in humidity, temperature, and light depending upon the height you are at, from the soil up to the canopy. Mike will be talking about this more later in detail.
So the big message is that there is a great deal more heterogeneity in tropical habitats than one might expect. The species in each habitat will differ; this is part (but not the whole) explanation for the high biodiversity of the tropics.

1 comment:

Chris H said...

I had heard and read bits and pieces of this soil chemistry information over my years, but it is revealing to have it (and more) all in one place. It even makes sense now! Thanks.