first picture is a typical xeric environment in NW Italy, at 45N. Its a river bank mostly composed of pebbles and a few sand, 10 years after the main water flow drifted away. As you may see it is fundamentally arid, unable to support much more than sparse grasses and a few weeds. However there are already some trees that begin to provide shade and litter the place with leaves. Without shade the Sun light is scorching and heats the pebbles to unbearable levels. The second picture is 50 meters far prom the first one, on the same bank, but this section of the bank has been free from heavy floods for about 20 years. As you may see, the grasses are much greener, and theres much more orgamic matter mixed in with the sand. Plus, the forest edge is expanding rapidly. There are sparse rubus fruticosus instance and the trees provide much more shade. Theres a clear difference: wherever there are trees and more shade, the soil retains more moisture and plant growth is much more vigorous. The process self-increments exponentially until theres an abrupt transition to the forest. The full forest took about 25 years to colonize a previously sterile environment. Even by human standards this is fast! The trees that come first are: Populus spp (alba and nigra) Salix alba subsequently: Robinia pseudoacacia Ailanthus altissima And then all the others, including Quercus spp.
Posted on: Wed, 25 Jun 2014 07:52:56 +0000
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