Early successional stages

[Chris J. Durden]

The tropical system I have looked at most closely (actual, not model, in 
Rondonia) is very dynamic, that is, looking past the recent non-indigenous 
human modifications.
   Look at the life cycle of a representative large tree. The surviving 
seedling grows rapidly in its light gap putting on wide growth rings for 
the first fifty years. It reaches the canopy, overtopping its neighbors, 
extends lateral branches and puts on narrow rings for the next hundred 
years. As the crown is damaged by storm events, rot and insects work on the 
light inner wood, producing a hollow. Into the hollow comes the soil 
arthropod fauna feeding on the leaf litter and detritus of decay that 
accumulates there but not on the forest floor. The tree sends roots inward 
to take advantage of its own humus resource. Millipedes feed on the bark, 
stimulating sap flow which traps insects and other organisms and 
accumulates between the buttresses forming copal. Meanwhile the lateral 
branches, grown in balance, become heavily loaded with epiphytes and 
lianas. During storm events one or more lateral branches are broken off. 
When enough are gone on one side to unbalance the tree it topples, upending 
a vast sprawl of roots and the soil attached, exposing a new opening for 
the process to begin again. This is the normal process that occurs in 
relatively protected locations, producing a forest of very mixed aged trees 
and perhaps the most diverse butterfly fauna in the world (with more than 
1,800 species in an area 10 x 20 km). Added to this are the edge effect 
slow successions occurring at the border of rock outcrops and inselbergs, 
and at the border of rivers. Prehistorically there is evidence of 
widespread savanna at times of the extended glaciations at high latitudes 
during the Pleistocene. There is prehistoric evidence of regional fire in 
postglacial time. There is indication of inundation of lowlands by a vast 
inland sea during the previous interglacial episodes when melted ice sheets 
raised global sea level.
    There is human use of the forest, gathering useful plant products from 
Brazil Nut to Rubber, meat, fur, feathers, pets for zoos, fish from the 
rivers for aquaria, butterflies and beetles for collectors and artwork, 
plants for biochemical search for drugs and useful DNA. Much of this has 
recently been prohibited.
    On top of this there is damming of rivers to create lakes for access 
and hydroelectric power generation. There is the grading of roads to 
provide access for deforestation schemes before bringing in settlers. There 
are agricultural schemes that plant crop monocultures like Cacao, which 
collapse from disease or economic planning mistakes. And then there are the 
independent loggers who help improve the value of the land by removing the 
best timber, on the fly. And then there is clearing of the land to raise 
cattle on the rather poor grasses that will grow there. And then there is 
the opening of small and large hand-worked gold mines, with the 
proliferation of malaria among people living in rather marginal conditions. 
And then there are the eco-tourists who leave their film wrappers and water 
bottles in the bushes and stimulate a patchy boom or bust economy that 
fuels political dispute.

    Early successional stages are most important in all ecosystems. Highest 
diversity is found during the intermediate stages of succession and also in 
the boundary areas where biotic communities meet.

    I see this not as a theoretical ecologist, but as one trained in 
geology and biosystematics.
.............Chris Durden

At 08:26 AM 1/17/2002 -0800, you Norbert wrote:
>Some additional natural disturbance agents in my part of the world that
>'recycle'/open up that nasty old tree canopy and create butterfly habitat
>are: pine beetles which sometimes reach epidemic proportions over large
>areas; small patch openings are regularily created by Armillaria root rot
>and this has a most salubrious effect in contributing habitat diversity to
>an otherwise monotonous coniferous forest; snow avalanching is very common
>here and maintains early succesional habitats (many of these are well known
>for their spring feeding value to Grizzly Bear but remain unstudied re.
>butterfly use. On a microscale, ground squirrel digging and bear digging
>create substrate for the Polygonum host plant of Lycaena nivalis. Lots of
>fascinating stuff going on out there. I recall somebody suggested that there
>should be more study of model tropical systems. Worthy as that is; I cannot
>see how that will be of much or any use to people that have to make land use
>and resource management decisions in other completely different ecosystems.



 
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