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Tufts OpenCourseware
Author: Colin Orians
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1. Discussion Questions

1.1. Chapter 4: Soils and nutrient limitation.

1.1.1. Part 1: Temperate vs. Tropical

  • Make sure that you know what is meant by CEC, AEC and PZC. What are the roles of clay minerals and organic matter for nutrient delivery and retention? What is special in the tropics in these aspects as compared to the temperate zones?
  • Which are the properties of the secondary clay minerals formed during weathering of silicate minerals and which differences in clay mineral composition can be found between temperate and tropical soils? What are the consequences for fertility?
  • Mycorrhizal fungi are often critical for the nutrient uptake by plants. Are there any special circumstances in the tropics that make mycorrhizas especially important to plants?
  • Which organism groups are involved in decomposition processes? Are there differences between tropical and temperate decomposer communities?
  • Are tropical soils more vulnerable to leakage of nutrients than soils in temperate areas? If so why?
  • What is weathering? How do organisms and climate, especially temperature, affect weathering?

1.1.2. Part 2: Intra-tropical Variation.

In answering these questions, think about how they would differ for La Selva and Sirena .


  1. Rainfall differences (see next page)
  2. La Selva is closer to recent volcanic activity and many of the rivers originate in volcanic mountains
  3. Sirena is closer to the ocean and located on an isolated peninsula farther from any recent volcanic activity.
  • Is the form in which plant nutrients exist in soil likely to differ between La Selva and Sirena?
  • Characterize oxisol, ultsol, alfisol and vertisol; their clay type content, geographical distribution and capacity for plant production. Do the sites differ?
  • In what ways have plants adapted to low soil fertility, and would you expect plants at the two sites to differ in their adaptations?
  • Which soil types in the tropics have especially high value for plant production and can support dense human populations on agriculture? Can you think of indicator species that might be useful in assessing fertility before the forest is cleared?
  • Which are the main constrains for plant growth in the wet and semiarid tropics?

1.1.3. Rainfall

Rainfall (mm)
Month La Selva La Selva Sirena
1990-2005 2000-2005 2000-2005
January 245.2 353.3 176.0
February 223.7 257.8 26.3
March 142.9 137.7 82.4
April 174.9 240.0 200.9
May 462.1 521.9 505.6
June 431.2 543.0 616.5
July 547.0 528.0 505.5
August 453.7 481.8 594.7
September 319.7 276.8 767.7
October 413.0 366.5 914.6
November 411.6 435.5 819.7
December 483.7 532.4 357.6
Total: 4308.7 4674.6 5567.4