SCIENCE LITERACY WORKING WITH DATA

The following table shows six tropical forests, arranged from least to most fertile soils, estimated from total soil nitrogen:

Interpretation

Question 8.16

Which two areas have the poorest soils, in terms of nitrogen availability?

Lower Montane rainforest, Puerto Rico and Amazon Caatinga, Venezuela

Question 8.17

Look at the root biomass in the two areas from Question 1. What is the relationship between root biomass and poor soils? Propose an explanation.

When there are few nutrients in the soil, more roots are produced; the tree must invest much more energy sending out roots to obtain enough nutrients.

Question 8.18

Can you determine soil nutrient levels by looking at the above-ground biomass? Explain.

No, you cannot determine the nutrient levels. The greatest above-ground productivity is in the Ivory Coast forest, on soils of intermediate fertility.

Advance Your Thinking

Question 8.19

Notice how long it takes leaves to break down (“Turnover time of leaves”). Speculate as to why trees in poorer soils produce leaves that last longer.

When soils are poor trees need to hold their leaves longer (turnover time) since they cannot afford to make new ones frequently. This means the leaves need to be sturdy, and they need to be able to resist insects. The leaves are tough and contain chemicals to repel insects. When they finally do fall they take a very long time to decompose. This means the nutrients in them do not recycle quickly, and are not available for new growth.

Question 8.20

If you removed the trees through logging or burning, would these areas be good for agriculture? (Hint: Cleared areas are usually used for small farms that do not have funds to purchase farm equipment, fertilizers, or other chemicals.)

Possibly the Lowland Rainforest in Costa Rica would be a good area for agriculture. The others are unlikely because of the poor soils: since the soils are lacking in nutrients, any farm would need fertilizers within a few years if not immediately. When you remove the trees, you are removing most of the nutrients that were contained within the biome.