Environmental Engineering Reference
In-Depth Information
Figure 2.6 in Chapter 2), and is not easily
accounted for by Jenny's framework. In
addition, although time is undoubtedly
important to ecosystems, it is not always
easy to offer an unambiguous definition of
what exactly we mean by “time.” Time
since the underlying bedrock was formed?
Time since the glaciers retreated? Time
since the last fire or rainfall? Time since
the last herbivorous insect outbreak? All of
these are important to different aspects
of ecosystem function, and they are not eas-
ily collapsed into a single value for “time.”
One useful way to look at the utility of
Jenny's state factors is to see them as the
large-scale constraints within which the
next-lower-level controls have to work.
These in turn set the constraints within
which the next-level controls have to oper-
ate, and so on.
In the end, the “best” framework depends
on the scales on which the ecosystem is
defined and the purposes for which the
framework will be used. For example, terres-
trial primary production at global and millen-
nial scales is controlled primarily by latitude
and altitude within the context of Earth's cli-
matic history, while within regions at decadal
scales Jenny's state factors are good predic-
tors. Within an individual field or forest at
daily scales, photosynthetically active radia-
tion and soil water
content are good
predictors.
We know of no framework equivalent to
Jenny's state factors for aquatic ecosystems,
but the corresponding aquatic equivalents
might be climate, organisms, hydrologic
gradients, the physical and chemical charac-
teristics of the fluid medium, and time. Can
you apply these to an aquatic system you
know? Are they sufficient, or do you need
other state factors? As in terrestrial systems,
these factors would be useful at some spa-
tial and temporal scales and not at others.
Evergreen
Deciduous
Grassland
Canopy surface area
FIGURE 11.4
Biotic (vegetation structure) and abiotic (wind speed, cloud frequency, cloud liquid water con-
tent) factors interact to affect fog water capture. For a given elevation and climatic region, greater vegetation
height and surface area will increase the fog capture.
