Principles of terrestrial ecosystem ecology.pdf

5
Carbon Input to Terrestrial Ecosystems
Photosynthesis by plants provides the carbon and energy that drive most biological
processes in ecosystems. This chapter describes the controls over this carbon input.
Introduction
The energy fixed by photosynthesis directly
supports plant growth and produces organic
matter that is consumed by animals and soil
microbes. The carbon derived from photosynthesis
makes up almost half of the organic
matter on Earth; hydrogen and oxygen account
for most of the remainder. Human activities
have radically modified the rate at which
carbon enters the terrestrial biosphere by
changing most of the controls over this process.
We have increased by 30% the quantity of
atmospheric CO2 to which all terrestrial plants
are exposed. On a regional scale we have
altered the availability of water and nutrients,
the major resources that determine the capacity
of plants to use atmospheric CO2. Finally,
through changes in land cover and the introduction
and extinction of species, we have
changed the regional distribution of the
carbon-fixing potential of the terrestrial biosphere.
Because of the central role that carbon
plays in the climate system (see Chapter 2) and
the biosphere, it is critical that we understand
the factors that regulate its cycling through
vegetation and ecosystems. We address carbon
inputs to terrestrial ecosystems through photosynthesis
in this chapter and inputs to aquatic
ecosystems in Chapter 10. In Chapters 6 and 7,
we explore the carbon losses from plants and
ecosystems, which, together with photosynthesis,
govern the patterns of accumulation and
loss of carbon in ecosystems.
Overview
The availability of water and nutrients is the
major factor governing carbon input to ecosystems.
Photosynthesis is the process by which
most carbon and chemical energy enter ecosystems.
The proximate controls over photosynthesis
by a single leaf are the availability of
reactants such as light energy and CO2; temperature,
which governs reaction rates; and
the availability of nitrogen, which is required
to produce photosynthetic enzymes (Fig. 5.1).
Photosynthesis at the scale of ecosystems is
termed gross primary production (GPP). Like
photosynthesis by individual leaves, GPP varies
diurnally and seasonally in response to changes
in light, temperature, and nitrogen supply.
Differences among ecosystems in annual
GPP, however, are determined primarily by the
quantity of leaf area and the length of time that
this leaf area is photosynthetically active. Leaf
area and photosynthetic season, in turn, depend
on the availability of soil resources (water and
nutrients), climate, and time since disturbance.
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