Soils in rain forests are deficient in most nutrients

In Amazon rain forests, soils are notoriously deficient in most nutrients. The frequent deluges of rain in these forests leach large amounts of calcium, phosphorus, and magne-

Selenium Plant

FIGURE 7.31

The prince's plume (Stanleya elata), shown here growing in Death Valley, California, requires selenium. Its selenium level is so high that this plant is toxic to browsing mammals.

FIGURE 7.31

The prince's plume (Stanleya elata), shown here growing in Death Valley, California, requires selenium. Its selenium level is so high that this plant is toxic to browsing mammals.

sium from the leaves of the trees. Consequently, plants in a forest depend largely on their ability to gather the relatively nutrient-rich rainwater that trickles down stems. Many plants do this by forming extensive, shallow root systems, whereas other plants intercept nutrients before they reach the soil. For example, the roots of trees such as Eperua purpurea grow upward in the fissures of the bark of nearby trees. These fissures are predictable pathways for flowing nutrients, which the roots absorb from the fissures without letting them enter the soil. Like their counterparts in the soil, these roots are extensively branched and occur throughout the canopy; they may extend more than 13 meters into the air. Interestingly, the roots of Eperua grow up along its own stems as well as those of nearby trees, but roots of nearby trees do not grow onto Eperua stems. As a result, Eperua can recycle its own nutrients as well as those of its neighbors.

Why do the roots of these tropical trees grow up, while most others do not? Do these roots respond to nutrients coming down the tree trunks, or do they fortuitously climb any vertical support that they happen to contact? To answer these questions, Robert Sanford of Stanford University sank drainpipes into the forest floor, attaching a reservoir atop each pipe. One-third of the reservoirs contained nutrient-rich cattle manure, one-third contained leaf litter, and the remaining third were empty. Within 4 months, climbing roots appeared on all of the "trees" containing manure, on 25% of those containing litter, and on none of the empty ones. Thus, the growth pattern of the roots of these plants is an adaptation to the low soil-nutrient availability of the Amazon rain forest. The roots are responding to nutrients coming down the tree trunks.

SUMMARY

Three different types of roots are found in plants: taproots, fibrous roots, and adventitious roots. These roots differ in their form and function. Taproots are single, relatively large roots, whereas fibrous root systems consist of many similarly sized roots. Although most roots absorb water and minerals from the soil, modified roots have many other functions, including asexual reproduction, storage of energy, aeration, and protection. Some roots are even photosynthetic. Roots are composed of layers of tissues including the epidermis, cortex, vascular tissue, and endodermis, which helps filter water and minerals that are entering the stele of the root. Just behind the tip of a root are the zones of cell division, cell elongation, and cell maturation.

The roots of many plants interact with organisms that dwell in the soil, including bacteria, fungi, and ne-matodes. Plants establish several kinds of associations with other organisms to obtain nutrients. The roots of almost all plants have mutually beneficial interactions with fungi. These mutualisms, which are called mycorrhizae, increase the absorption of water and nutrients. Many plants, especially legumes, establish mutually beneficial symbioses with nitrogen-fixing bacteria. The bacteria convert atmospheric N2 to ammonia, which the plant uses for growth. In return, the bacteria receive sugars from the host plant. Parasitic plants get their nutrients directly from other plants.

Soil is made up of sand, silt, and clay particles. Soil also contains air, water, organisms, and the decomposing bodies of these organisms, which makes up humus. The contents of the soil determine how well plants grow.

Plants require at least 16 essential elements that are necessary for normal growth and reproduction, cannot be substituted for, and directly or indirectly affect metabolism. Carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, and magnesium are required in relatively large amounts and are called macronutrients. Iron, chlorine, copper, manganese, zinc, boron, and molybdenum are required in relatively small amounts and are called micronutrients. Macronutrients and mi-cronutrients function as parts of structural units, parts of compounds involved in metabolism, and activators and inhibitors of enzymes. Most essential elements are absorbed by roots. This absorption is facilitated by the large surface area of roots.

Some plants are hyperaccumulators of minerals, which may provide protection from some herbivores. Humans are making use of these hyperaccumulators when cleaning up toxic materials in soil. This practice of phytore-mediation may save millions of dollars.

Rain in tropical rain forests contains many nutrients leached from the forest's dense canopy. Some trees in the rain forest have roots that grow up along the trunks of adjacent trees and absorb the nutrients from rainfall trickling down the trees.

Crop plants absorb only about half of the nitrogen supplied in fertilizer. What happens to the rest? What are the consequences of this?

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Responses

  • sanni
    What do rain forests depend on\?
    8 years ago
  • Torsten Koenig
    Why is the soil in the rain forest deficient in nutrients?
    6 years ago
  • edvige
    Do forests recycle chemical nutrients?
    6 years ago

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