Principles of terrestrial ecosystem ecology.pdf

Lightning
Fires
Anthropogenic
Fires
Figure 14.4. Locations of naturally and humancaused
fires in Alaska (Gabriel and Tande 1983).The
human-caused fires mirror the road and river trans-
cleared patches created by shifting agriculture.
Similarly, road access is the major factor determining
the distribution of fire ignitions in the
boreal forest of interior Alaska (Fig. 14.4). In
general, road access is one of the best predictors
of the spread of human-induced disturbances
in relatively natural landscapes (Dale
et al. 2000).
Socioeconomic factors, such as farmer
income, interact with site characteristics to
influence human effects on the landscape
pattern. Heterogeneous landscapes are often
converted to fine-scale mosaics of agricultural
and natural vegetation, whereas large areas
suitable for mechanized agriculture are more
likely to be deforested in large blocks. In northern
Argentina, for example, patches of dry
deciduous forests on the eastern slopes of the
Andes were converted to small patches of cropland
or modified by grazing into thorn-scrub
grazing lands and secondary forests (Cabido
and Zak 1999) (Fig. 14.5). On the adjacent
plains, however, larger parcels were initially
deforested for grazing and more recently converted
to mechanized agriculture. Large holdings
on the plains are owned by companies that
make land use decisions based on the global
economy. Small family producers in the mountains
maintain a more traditional lifestyle that
involves smaller, less frequent changes in land
use.
Disturbance is increasingly used as a management
tool to generate more natural stand
and landscape structures. Forest harvest varies
Causes of Spatial Heterogeneity 313
Major
Roads
portation corridors, indicating the importance of
human access in altering the regional fire regime.
from 0 to 100% tree removal, and the sizes and
shapes of clearcuts can be altered from the
standard checkerboard pattern to mimic more
natural disturbances (Franklin et al. 1997).
Forest harvest regimes can also be designed to
retain some of the functional attributes of late
successional forests, such as the filtering function
of riparian vegetation, the presence of
large woody debris, and the retention of a few
large trees as seed source and nesting habitat.
Protection of these features can significantly
reduce the ecological impact of forest harvest.
Prescribed fire is increasingly used as a management
tool, particularly in areas where a
century of Smoky the Bear policy of complete
fire suppression has led to unnaturally large
fuel accumulations. Prescribed fires are typically
lit when weather conditions are such that
fire intensity and severity are low, so the fire can
be readily controlled. In populated mediterranean
regions, vegetation may be physically
removed as a substitute for fire, because prescribed
fires are considered unsafe. Natural fire,
prescribed fire, and physical removal of vegetation
are likely to differ in their impacts on
ecosystem processes due to differences in
the quantity of organic matter and nutrients
removed; these differences affect subsequent
regrowth. Ecologists are only beginning to
understand the long-term consequences of
different disturbance regimes for the structure
and functioning of ecosystems and landscapes.
As this understanding improves, more informed
decisions can be made in using distur-