Geoscience Reference
In-Depth Information
Live tree
biomass
15
Scots pine
10
0.6
Atmospheric
deposition
Slash pine
0.3
5
Above-
ground
litter
Lodgepole pine
Fixation
0.01
1.2
0
25
50
75
0.9
STAND AGE (years)
Fig. 12.1. Rate of forest plant growth, mostly tree growth
but also that of understory shrubs and herbaceous plants, is
known as net primary productivity (nPP)—expressed here as
metric tons per hectare per year. nPP changes significantly as
forests age. note the low growth rate for Wyoming lodge-
pole pine forests compared to slash pine in Florida and Scots
pine in northern europe. one metric ton = 2,205 pounds;
1 hectare = 2.47 acres. Adapted from Knight (1991).
Soil
organic
matter
Fixation
0.01
tion can be observed in the annual rings of trees and
is caused mostly by annual variation in growing sea-
son temperature and precipitation. Also, younger for-
ests tend to have higher growth rates than older forests.
Growth rate, also known as net primary productivity
(often abbreviated as nPP), is commonly expressed as
the average rate at which all new plant biomass is added
to the forest annually, whether in the form of trees,
Living trees, soil organic matter, and dead plant mate-
rial on the forest floor are the largest components of
the forest biomass and change through time after a dis-
turbance (figs. 12.2 and 12.3). the forest floor biomass is
composed almost entirely of leaves, twigs, branches, and
the boles of fallen trees. Some of it—not all—is burned
during a forest fire, but the litter accumulates in total
weight for many years. this litter buildup occurs because
winters are long; summers are relatively dry; and most
plant tissues in Rocky Mountain environments have high
lignin content and high carbon-to-nitrogen ratios, both
of which slow the rate at which bacteria and fungi—the
primary decomposers—can convert organic materials
to inorganic molecules. Microbial decomposition is also
limited by the relatively cool and sometimes dry condi-
tions that prevail in the summer. notably, considerable
Outflow
0.01
Fig. 12.2. carbon and nitrogen distribution in a lodgepole
pine forest. the size of the boxes indicates the relative
amounts of organic matter in live tree biomass, aboveground
litter, and soil organic matter (which includes bacteria and
fungal biomass but not live root biomass). Roots are part
of the live tree biomass box. Shrub and herbaceous plant
biomass is not shown but is less than 1 percent of the tree
biomass. Annual nitrogen flows are shown by the arrows;
the numbers are grams per square meter per year. Much of
the nitrogen in the rooting zone is in organic matter that
is not readily decomposable. note that nitrogen inputs are
larger than nitrogen losses, suggesting that nitrogen is ac-
cumulating in the tree biomass, litter, and/or soil organic
matter, probably because it is a limiting factor for plants and
microbial organisms. Also, note that the tree uptake estimate
is larger than the sum of the input estimates to the rooting
zone, which suggests that the amount of soil nitrogen is
gradually depleted in this forest as biomass accumulates. the
soil nitrogen pool is replenished as the forest ages further or
following such disturbances as fire. Based on data in Fahey et
al. (1985) and Fahey and Knight (1986).
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