Environmental Engineering Reference
In-Depth Information
trees that escaped fire, or were killed or damaged by fire, or newly established after fire, is
a major feature in such regions. If it suits the interests of the researchers, a large landscape
containing different patches of intact, burned, killed, or new trees can be considered a sin-
gle ecosystem. Such a model might be appropriate to understanding the distribution of a
large migrating herbivore. An alternative model would be to take the different burned and
unburned patches as different ecosystems, assuming that their function would depend on
the structure set by the fire history at each site. Sites representing different ages since fire
will differ in key determinants of ecosystem functioning, such as nutrients, soils, light,
temperature, and moisture.
Pit and Mound Microtopography
In very old forests in moist temperate regions, a characteristic soil surface heterogeneity
can be found. The forest floor is punctuated by mounds and nearby pits, often differing in
elevation by 1 to 2 m from lowest to highest point. The pits were formed as root mats
were wrenched from the ground when high winds blew down large trees. The adjacent
mounds result from sloughing off of soil that was originally held in the web of exposed
roots. Spatial heterogeneity in the topography created by pits and mounds is reflected in
heterogeneity of moisture content, soil texture, stability, and organic matter ( Peterson and
Pickett 1990 ). Pits are subject to deep accumulations of plant litter, and of flooding during
wet periods. These factors are all capable of affecting microbial activity, soil nutrient and
water availability, and, therefore, the establishment dynamics of seedlings and saplings.
Pits and mounds in terrestrial systems are also created by animal burrowing or digging
for below-ground storage organs of plants ( Figure 10.5 ). For example, echidnas
( Tachyglossus aculeatus ) pits and mounds can have effects on ecosystem processes. An
experiment by Eldridge (2011) found that increasing the proportion of an area disturbed
increased the time for runoff to begin after rain, and increased the production of sediment
FIGURE 10.5 A pit dug by a foraging animal in Australian arid landscape. Such pits are common features in
desert landscapes where animals dig to find the underground storage organs of perennial plants. Seeds, organic
matter, and sediment accumulate in such pits, and the disruption of the microphytic soil crust permits enhanced
water infiltration during rainfall events that are large enough to generate overland flow. Hence, pits are resource
and regeneration hotspots. (Photo copyright S.T.A. Pickett.)
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