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Possible practices that could improve conditions in amazonia and other rainforests

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Format for printing Introduction In previous lectures we focused on general principles of ecosystems, emphasizing the processes of biogeochemical cycling and energy transfers. In this lecture we will look at one ecosystem in detail, the tropical rain forest. There are several reasons for this. First, ecosystems may all function according to the same basic principles, but they most certainly don't all look the same and it is important to have some understanding and appreciation of this diversity.

Second, it is important to learn about the "specifics" of ecosystems as well as the "generalities". Finally, we can use this examination of the tropical forest to help tie together some of the material that has been presented in the ecology and evolution sections of the course.

Today we will examine the most productive and diverse ecosystem found on land: These ecosystems are also, and perhaps more accurately, called humid tropical forests. These ecosystems are also changing rapidly, as Figure 1 illustrates in terms of the past and present distribution of tropical forests on Earth.

Present dark and original yellow extent of humid tropical forest on Earth. Climate and Geography Certain ranges of temperature and rainfall characterize the places where tropical rainforests occur, depicted in the following figures. Vegetation and latitude in Africa Temperatures generally fall between 23 - 27 deg C, with a greater daily than monthly range. In other words, there is no strong seasonality of temperature unlike what we experience in Michigan.

Rainfall tends to be highest near the equator, where the sun's evaporative power causes high evapotranspiration, and rising air cools and then sheds its moisture.

Precipitation tapers off as one moves away from the equator, and dry belts are found at 25-30 deg of latitude the great deserts. Local variation can also be great due to trade winds, ocean currents, land masses, and mountain ranges. Evergreen forests are replaced by deciduous forests as precipitation becomes seasonal. Wherever dry periods are several months or longer in duration, leaves are shed as the dry season takes hold, providing a winter-like visual appearance.

Leaves re-appear in anticipation of or with the onset of the rains. A dry month is one where evapotranspiration exceeds precipitation. The growing season is thus shortened, and so forest productivity is less than in the evergreen forests of the more humid tropics. Boundaries between ecosystems or biomes are often gradual, and they can be sensitive to changing conditions.

A prolonged period of wetness or drought, or human intervention, can cause dramatic changes because of the transitional nature of environmental conditions near the boundaries between biomes.

Humid tropical forests appear superficially similar everywhere, but in fact they differ widely in species composition and ecosystem attributes. In the coterminous United States, botanists recognize 135 natural plant formations based on dominant species, and driven by latitude and climate. This system is called the Holdridge system Figure 3 and it is widely used for conservation purposes. However, its use is far from feasible in tropics. Instead, the future in classifying tropical rainforest plant formations may lie in remote sensing, where large areas can be mapped relatively quickly.

Tropical tree with thin topsoil and deep, unweathered rocks The first Europeans to view humid tropical forests were stunned by the luxuriant growth, giant trees with huge buttresses, thick vines, plants growing on plants epiphytesand so forth. Such luxurious growth signals high productivity, and modern scientific measurements confirm this. If one measures the photosynthesis, or captures the litterfall of leaves, branches, fruit and other plant parts to the forest floor, one finds the production of tropical evergreen forests to be roughly double that of temperate forests.

  1. Learn about alternatives such as reclaimed or recycled lumber, composite lumber, and independently certified wood. In fact, we know more about the stars in the sky than we do about the species on Earth.
  2. Such projects can act as alternatives to the expansion of agricultural areas.
  3. This was recently experienced in 2001 when severe droughts lowered reservoirs causing an energy crisis.
  4. As a result, Project Amazonia has developed possible solution to this cycle that involves the leasing of lands by the government to landless farmers. However, much of the deforestation is being used for large-scale agriculture and ranching e.

It would be natural to infer that tropical soils are very fertile in order to support this high productivity. But, as we have seen in other instances, or general inferences about what makes sense are often incorrect -- we must look more closely at the system and analyze how it functions. In fact, tropical soils tend to be very thin and the rock below them is highly weathered with few nutrients remaining Figure 4.

You already know that ecosystems are open with respect to nutrient cycling, meaning that inputs and outputs are significant. Remember that the entire globe is closed with respect to nutrient cycling, because the inputs and outputs to Earth as a whole are extremely small. What makes humid tropical forests so productive is the combination of high temperatures, light, and rainfall year-round good growing conditionscoupled with especially efficient nutrient recycling.

What is the evidence for this claim? First, analysis of soils of tropical regions shows them to be virtually devoid of soluble, mineral forms of nutrients. Rocks weather rapidly due to high temperatures and abundant moisture, and millennia of rapid weathering and torrential rains to wash away nutrients from the soils have left the soils very low in nutrient stocks. This is supported by the analysis of stream water draining tropical regions, which likewise reveals a scarcity of dissolved nutrients.

Most tropical soils are clays with little soluble mineral content, and moderate to strong acidity which interferes with the ability of roots to take up nutrients.

6) Improvement of agricultural methods and productivity

If the nutrients aren't in the soils, where do they come from? Nutrient budget in a tropical forest. Note specifically that phosphorus P is stored more in the vegetation than in the mineral soils. Figure 5 shows a budget accounting that indicates nutrients are found mainly in living plant biomass and in the layer of decomposing litter; there is little nutrient content of the deeper soil, as there is in temperate-zone ecosystems.

This suggests that plants are intercepting and taking up nutrients the moment they are released by decomposition.

There are many organisms that are players in this decomposition process: Of particular importance are micorrhizal fungi that invade the roots of trees to obtain nourishment. As we learned in the lecture on Microbesthese fungi gain carbon nourishment from the tree and they benefit the tree by providing a vastly expanded nutrient gathering network in the soils. In some circumstances tree roots even grow upward toward the soil surface, permeating the litter layer.

Isotope experiments have shown the importance of roots in nutrient uptake. What happens when the forest is harvested for timber or other plant products, or the forest is burned?

  1. These methods could reverse the nutrient depletion characteristic of so many cultivated soils in tropical areas.
  2. However, this source of energy is frequently at the mercy of nature and if an especially dry year ravages the Amazon, power shortages are experienced. Precipitation tapers off as one moves away from the equator, and dry belts are found at 25-30 deg of latitude the great deserts.
  3. Finally, we can use this examination of the tropical forest to help tie together some of the material that has been presented in the ecology and evolution sections of the course. This would allow reduction in artificial inputs, so that fertilizer and pesticide use could be considerably reduced.
  4. Remember that the entire globe is closed with respect to nutrient cycling, because the inputs and outputs to Earth as a whole are extremely small.

In all these cases nutrients will be lost from the ecosystem, but the outputs cannot exceed inputs for very long because the stock of nutrient capital in the system will be depleted. When forests are burned, or the cut timber is removed as in logging, the nutrients that were in the tree biomass are either washed out in the case of burning or simply removed from the system.

Because there was only a small stock of nutrients in the soil and most of the nutrients were in the biomass, there is little nutrient stock remaining to support regrowth of the forest.

This is why slash and burn agriculture does not work for more than a few years after burning, and why the land is made very infertile and growing new vegetation is difficult.

We can't simply "regrow" tropical rainforests once they are burned -- once they are lost they are gone forever or at least for 1000s of years, and even then the species that regrow will be different from the original forest species.

Table 1 above shows 7 tropical possible practices that could improve conditions in amazonia and other rainforests, arranged roughly from least to most fertile soils. Note the range of above-ground biomass is about twofold. Yet the nutrient stocks vary by several orders of magnitude an "order of magnitude" is equal to one power of ten, so that 100 is one order of magnitude greater than 10, and 1000 is two orders of magnitude greater than 10. The greatest above-ground productivity is in the Ivory Coast forest, on soils of intermediate fertility.

This reinforces the point that you cannot guess the fertility of the soils of a tropical forest by looking at the productivity of the trees. Look next at the root biomass, which correlates pretty well with the soil fertility. Root biomass is highest where soil quality is poorest, and vice-versa. There is also a "causal" or mechanistic explanation for this correlation; when soil nutrients are high, the tree does not need to spend as much energy in building roots to forage in the soil for new nutrient sources.

Competition for nutrients presumably is very strong at the Venezuelan sites with low soil fertility and high root biomass, whereas competition for light presumably is most important in the Ivory Coast forest.

What Can I Do?

At the Venezuelan sites we have higher root biomass and there is well-developed surface root mat infiltrating the litter. We also see tougher, more slowly decaying leaves at the poor fertility sites.

To see this in the data, note how the turnover time leaves, or how long a plant "holds on" to a leaf it has made, is around two years at the low nitrogen, high root mass sites left side of the Tableand the turnover time of leaves is shorter and only around 1 year in the higher fertility soils right side of the Table. This is because leaves are held longer under poor growth conditions their turnover time is longerand because they are exposed to herbivores longer they must be better protected meaning they need to be tough and unpalatable.

This causes slow decomposition of the leaves once they are dropped to the forest floor, and further retards the forest's productivity. What causes this prodigious concentration of biological diversity? Until ten years ago, if you had asked any biologist how many species of plants and animals lived on earth, the answer probably would have been "about 2 million.

Breeding bird species of North and Central America. Note that there is a general trend of increasing species diversity moving from higher latitudes toward to the tropics. Then, in 1982, Smithsonian biologist Terry Erwin developed a technique to fumigate the crowns of individually selected forest trees with biodegradable pyrethrin the same chemical in mosquito coils. A huge number of previously unknown spiders, insects, and other invertebrates came tumbling down onto tarps spread on the forest floor.

Based on the high proportion of undescribed species, and the expectation that each tree species contains many host-specific species species that live only in that host treeour estimates of biological diversity have been drastically altered.

One commonly hears the figures of 5 to 30 million species, but the truth is we don't know. In fact, we know more about the stars in the sky than we do about the species on Earth. We are certain that tropical forests contain a great many species, and most of these are unknown to science. We also know that the tropics are highly diverse based on evidence from well-studied groups, including the vertebrates and flowering plants. Whenever one compares the number of species along a latitudinal gradient Figure 6one observes a trend of increasing numbers of species towards the equator.

Why are tropical forests so diverse? We only have partial answers, but they revolve around 1 the high ecological "specialization" found in the tropics, and 2 around the extent of geographic isolation over time in the tropics, including repeated environmental shifts due to climate change. Let's look first at the spectacular specialization we observe in tropical forests. Tropical species exhibit highly specialized ecological roles.

However, this begs the question of how these species became so specialized and diverse, and here we turn to an evolutionary perspective that is related to the many niches associated with the high degree of diversity of habitats in the tropical forests.

Let's examine two examples of the high degree of specialization found in tropical forests: The tropical forests themselves are multi-layered. A temperate forest usually has two or three main layers. In an undisturbed forest the trees are fairly similar in height, there usually is some ground vegetation, and in between there may or may not be a shade-adapted middle layer. But tropical forests are vertically more complex Figure 7and as many as five distinct layers exist, although the strict distinctness of layers is arguable that is, one layer "grades" into another layer.

Arboreal versus ground-dwelling mammals, day and night Figure 9: Bird guilds and ecological specialization light bars for the tropics, dark bars for the temperate zone We also see evidence of great ecological specialization, as illustrated for mammals living in trees and on the ground that are active at day versus at night Figure 8.

The term "guild" is used in ecology much as it is used in human society, to describe a collection of individuals who make a living in the same way. Human examples include cobblers, brewers, and professors. In the animal kingdom we use possible practices that could improve conditions in amazonia and other rainforests word guild to indicate a finer sub-division than trophic level.