Principles of terrestrial ecosystem ecology.pdf

284 13. Temporal Dynamics
Small rodents (index)
50
25
0
Flowers
1972 1975 1978 1981
Year
Figure 13.2. Interannual variation in flowering
density of an understory shrub (Vaccinium myrtillus)
and of small rodents in northern Finland (Laine and
of years. During the Tertiary they occupied a
warm moist environment throughout much of
western North America. Their range is now
restricted to valleys where coastal fog minimizes
summer drought stress. Aspen clones in
the Rocky Mountains range in age from a few
years to as much as 10,000 years (Kemperman
and Barnes 1976, Tuskan et al. 1996). The
current distribution of redwoods and aspens
is therefore a product of past population and
community processes and cannot be fully
understood with reference only to the present
environment. Many species are still migrating
poleward in response to the disappearance of
continental ice sheets 10,000 years ago. The
soils beneath these recent arrivals may still
reflect the properties of earlier communities
rather than being completely a function of
current vegetation.
The current functioning of the East Siberian
coastal plain also reflects processes that occurred
thousands of years ago. During the
Pleistocene, this region was a steppe grassland
that accumulated highly organic loess soils and
ice lenses that now occupy 50 to 70% of the soil
volume (Zimov et al. 1995). As the ice melted
in the warmer Holocene climate, the soils subsided,
forming lakes with organic-rich sediments.
Pleistocene-age organic matter is now
the major carbon source for methane production
by these lakes (Zimov et al. 1997). In other
words, the current processing of carbon in these
lakes is strongly influenced by processes that
occurred 10,000 to 100,000 years ago.
Current ecosystem processes also respond
to changes that have occurred more recently.
Small
rodents
160
Flowers (number m -2 )
Henttonen 1983). These herbivores and their food
plants show approximately 4-year cycles of abundance.
Large areas of Europe and northeastern North
America were deforested for agriculture in
recent centuries and have more recently
reverted to forests. Even forests older than 200
years still exhibit composition and dynamics
that reflect their earlier history (Foster and
Motzkin 1998) (Fig. 13.3). A plow layer is still
evident in these forests, for example, resulting
in a sharp vertical discontinuity in soil processes
and nutrient supply. Net primary production
(NPP) substantially exceeds the rates
of heterotrophic respiration in these ecosystems,
so vegetation and soils are actively accumulating
carbon (Goulden et al. 1996). Any
study of carbon dynamics that ignored these
historical legacies would seriously misinterpret
the relationship between carbon balance of
Landscape composition (% of land)
100
80
60
40
20
Forest
Pasture
80
0
0
1650 1750 1850 1950
Year
0
Cultivated
100
Figure 13.3. Changes in land use and population
density in central Massachusetts (northeastern
United States) since European colonization. Most
forests in this region were previously croplands
or pastures. (Redrawn with permission from
Ecosystems; Foster and Motzkin 1998.)
80
60
40
20
Population density (people km -2 )