Principles of terrestrial ecosystem ecology.pdf

48 3. Geology and Soils
and locations of uplift and the kinds of rock
uplifted ultimately determine the distribution
of different kinds of bedrock across Earth’s
surface.
Plate tectonics are the driving forces behind
rock formation. The lithosphere, or crust—the
strong outermost shell of Earth that rides on
partially molten material beneath—is broken
into large rigid plates, each of which moves
independently. Where the plates converge and
collide, portions of the lithosphere buckle
downward and are subducted, leading to the
formation of ocean trenches, and the overriding
plate is uplifted, causing the formation of
mountain ranges and volcanoes (Fig. 3.2).
Regions of plate collision and active mountain
building coincide with Earth’s major earthquake
belts. The Himalayan Mountains, for
example, are still rising due to the collision
of the Indian subcontinent with Asia. If plates
converge in one place, they must diverge or
separate elsewhere. Eurasia, Africa, and the
Americas were once the single supercontinent
of Pangaea, 200 million years ago. The mid-
Atlantic and mid-Pacific ridges are zones of
active divergence of today’s ocean plates.
Climate
Mountain ranges
and volcanoes
Granite
Crumpled sedimentary
and metamorphic rocks
Trench
Figure 3.2. Cross-section of a zone of plate collision
in which the oceanic plate is subducted beneath a
continental plate, forming an ocean trench in the
zone of subduction and mountains and volcanoes in
Temperature and moisture influence rates of
chemical reactions that in turn govern the
rate and products of weathering and therefore
the development of soils from rocks. Tempera-
Lithosphere
the zone of uplift. (Redrawn with permission from
Earth by Frank Press and Raymond Siever ©
1974, 1978, 1982, and 1986 by W.H. Freeman and
Company; Press and Siever 1986.)
ture and moisture also influence biological
processes, such as the production of organic
matter by plants and its decomposition by
microorganisms and therefore the amount and
quality of organic matter in the soil (see Chapters
5 to 7). Soil carbon, for example, increases
along elevational gradients as temperature
decreases (Vitousek 1994b) and decreases in
rain shadows downwind of mountain ranges
(Burke et al. 1989). Precipitation is a major
pathway by which many materials enter ecosystems.
Oligotrophic (nutrient-poor) bogs are
isolated from mineral soils and depend entirely
on precipitation to supply new minerals. The
movement of water is also crucial in determining
whether the products of weathering accumulate
or are lost from a soil. In summary,
climate affects virtually all soil properties at
scales ranging from local to global.
Topography
Topography influences soils through its effect
on climate and differential transport of fine soil
particles. The attributes of topography that are
important for ecosystem processes include the
site’s topographic position on a catena or hillslope
complex, the aspect of the slope, and the
relationship between the site and hydrologic
pathways (Amundson and Jenny 1997).
Characteristics such as soil depth, texture, and
mineral content vary with hillslope position.
Erosional processes preferentially move fine-