Monday, June 16, 2008

Thinking About Tropical Diversity (in the redwood forest!)

On my daily walk, the forest of redwoods (Sequoia sempervirens) the S. gigantea is pictured here), oaks, Douglas firs, and many understory plants seems rather diverse. I wondered if this diversity had any implications to gardens. Maybe yes, maybe no. But today, for some reason I'm thinking about the tropics as well as the temperate climates.

In my opinion, using tropical diversity as a model in the USA is childlike. While the tropics often have lots of vertically-integrated plants, the temperate American landscape, with its hardwood forests, meadows, and prairies, is less vertically complex.

The nutrient mass is primarily above the ground in the tropics, the soil is rather “thin”. By comparison, the deeper soil is the reservoir of nutrients in a temperate ecosystem. Some plants in the tropics actually fix nitrogen on their leaves – “Experiments in a Costa Rican rainforest revealed that fixed nitrogen [by blue-green algae] is directly transferred into the leaf “ (Many tropical plants and trees do fix nitrogen in the soil, but it is “recycled” much more quickly.) From MONGABAY.COM: “The colonial settlers did not realize that they were dealing with an entirely different ecosystem from their temperate forests where most of the nutrients exist in the soil. In the rainforest, most of the carbon and essential nutrients are locked up in the living vegetation, dead wood, and decaying leaves. As organic material decays, it is recycled so quickly that few nutrients ever reach the soil, leaving it nearly sterile”. While in the temperate climates, nitrogen-fixing bacteria grow only the roots beneath the soil’s surface—primarily Rhizobium spp. bacteria found on the roots of the bean family (Fabaceae or Leguminosae) gather the nutrients, especailly nitrogen.

The difference is considerable when applied to the practical. Gardeners in temperate climate work to enhance the fixation and storage of nitrogen the soil mass. While tropical gardeners can rely on foliar feeding of nitrogen and the rapid recycling of nitrogen in a thin soil. (Sugarcane production requires no additional nitrogen due to the independent fixation of this element.)

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