Biodiversity and Change

 responsibility/how-brazil-cut-deforestation- rates-to-record-lows.html responsibility/how-brazil-cut-deforestation- rates-to-record-lows.html

 Life, and, thus, biodiversity, requires two things to exist: a) Energy and b) Nutrients;  The ultimate source for the mineral nutrients that plants in rainforests require are weathered rocks;  But, in a mature rainforest, the nutrient pool is contained largely in the living biomass and decaying organic matter (humus) of the forest floor

 2 of the most important nutrients to plants, Phosphorus (P) and Potassium (K) are cycled almost entirely from biomass to litter (plant debris) and back again (the decayed matter turns to soil and is absorbed by plants).  That’s one of the reasons why most of the roots of the trees in rainforests lie at a depth of less than half a meter from ground level, and indeed many roots are found on the ground surface

 Some tropical rainforests have developed on rich, recent or alluvial material (material deposited by rivers  this can be sediments such as rocks, tree branches, plants and, mostly, sand).  That is why in some tropical rainforests the soil is indeed very sandy;

 However, in many rainforests the mechanisms for recycling nutrients are largely independent of the nutrient supply from the soil.  THIS MEANS THAT THE TRANSFORMATION OF DECAYED MATTER IN BIOMASS AND FURTHERMORE ON NUTRIENTS DOESN’T HAVE, NECESSARILY, AN ASSOCIATION WITH THE SUPPLY OF NUTRIENTS TO THE SOIL.  Because of this, such evergreen forests are able to prosper even on poor soils.

IMPORTANT QUESTION  But then, how do rainforests’ soils get nutrients?

 A: Rainforests have adapted to the low nutrient levels of the soils under conditions of high rainfall (that leach and erode soils) and high temperatures. A dense root-mat forms over the soil in contact with the litter layer above it. Root tips grow upward and become attached to the fallen litter. The root-mat also acts to absorb nutrients from sources such as rainfall. The leaves of many tropical rainforests have long active lives and some transfer of nutrients back into plants takes place before leaf shedding occurs. Some plants also have leathery leaves which contains chemicals that are repellant to insects, to discourage herbivores eating the leaves while they’re still active.

Text Reading: from §III, p.134 to p.136 (prior to QUESTION BLOCK)  TEXT GUIDELINE:  How nutrients are filtered from rainwater?  What is the meaning of slash-and-burn agriculture?  Outline, with your own words, the pros and cons of shifting cultivation.  When does shifting cultivation is harmful? When it is harmless(or even beneficial)?  Associate a) Tropical deforestation with b) Destruction of the ozone layer.  What are some of the economic implications of tropical rainforests’ deforestation explicited in the text?