Principles of terrestrial ecosystem ecology.pdf

98 5. Carbon Input to Terrestrial Ecosystems
LONG-TERM
CONTROLS
STATE
FACTORS
BIOTA
TIME
PARENT
MATERIAL
CLIMATE
Interactive
controls
Plant
functional
types
Soil
resources
Figure 5.1. The major factors governing gross
primary production (GPP) in ecosystems. These
controls range from the direct controls, which determine
the diurnal and seasonal variations in GPP,
to the interactive controls and state factors, which
are the ultimate causes of ecosystem differences
in GPP. Thickness of the arrows associated with
In this chapter we explore the mechanisms
behind these causal relationships.
Carbon and energy are linked as they move
through ecosystems because the same processes
govern their entry into ecosystems in
photosynthesis, transfer within ecosystems, and
loss from ecosystems. Photosynthesis uses light
energy (i.e., radiation in the visible portion of
the spectrum) to reduce CO 2 and produce
carbon-containing organic compounds. This
organic carbon and its associated energy are
then transferred among components within the
ecosystem and are eventually released to the
atmosphere by respiration or combustion.
The energy content of organic matter differs
among carbon compounds, but for whole tissues,
it is relatively constant at about 20kJg -1
of ash-free dry mass (Golley 1961, Larcher
1995). The carbon concentration of organic
matter is also variable but averages about 45%
in herbaceous tissues and 50% in wood (Gower
et al. 1999). Both the carbon and energy con-
SHORT-TERM
CONTROLS
Direct
controls
Leaf area
Nitrogen
Season
length
Temperature
Light
CO 2
tents of organic matter are greatest in materials
such as seeds and animal fat that have high
lipid content and are lowest in tissues with high
concentrations of minerals or organic acids
(Fig. 5.2). Because of the relative constancy of
the carbon and energy contents of organic
matter, carbon, energy, and biomass have
been used interchangeably as currencies of the
carbon and energy dynamics of ecosystems.The
preferred units differ among fields of ecology,
depending on the processes that are of greatest
interest or are measured most directly. Production
studies, for example, typically focus on
biomass; trophic studies, on energy; and gas
exchange studies, on carbon.
Photosynthetic Pathways
C 3 Photosynthesis
GPP
direct controls indicates the strength of the effect.
The factors that account for most of the variation in
GPP among ecosystems are leaf area and length of
the photosynthetic season, which are ultimately
determined by the interacting effects of soil
resources, climate, vegetation, and disturbance
regime.
The rate of carbon input to ecosystems depends
on the response of photosynthetic biochemistry