Environmental Engineering Reference
In-Depth Information
6
Land Cover and Productivity Changes
For a few million years of their early evolution, hominins made a limited claim on
the biosphere, as they foraged for food and ate it raw. This began to change with
the controlled use of i re for cooking and defense against predatory animals. Inevi-
tably, this led to accidental i res, whose unchecked progress added to the phytomass
destruction that had been taking place naturally by i res ignited by lightning. Even-
tually, humans began to use i re for other than immediate food and security needs,
above all to clear forested land for shifting cultivation and later to open up new
areas for permanent cropping. Other important uses of i re were to suppress any
encroachment of shrubs and trees on grasslands used for grazing domesticated
herds, and to open up land for new settlements.
Ever since that time (this means for close to 10,000 years in some of the oldest
continuously inhabited regions with domesticated animals), the large-scale impact
of human activities on the biosphere has gone beyond the conversion of forests,
grasslands, and wetlands to crop i elds, beyond the expansion and management of
grazing lands, and beyond deforestation driven by the demand for fuelwood, metal-
lurgical charcoal, and timber. These diverse actions have caused many second-order
changes, seen not only in the directly affected ecosystems but also in distant places,
at regional and even global levels. Soil erosion (and the concomitant loss of accu-
mulated nutrients and reduction of an eroded site's potential primary productivity)
and the emission of greenhouse gases are two ubiquitous examples of such effects.
Fire and Land Conversions
Fire has always been the least labor-intensive way to clear a forest in the absence
of good tools (the axes and saws needed to cut down large trees) and strong draft
animals (to remove the logs). Preindustrial societies could not control deliberately
