Principles of terrestrial ecosystem ecology.pdf

increasing precipitation (Fig. 6.5) (Sala et al.
1988, Lauenroth and Sala 1992), just as
observed across biomes (Fig. 6.3 and 6.4). In
any single grassland site, NPP also increases in
years with high precipitation and responds to
experimental addition of water, demonstrating
that grassland NPP is water limited (Lauenroth
et al. 1978). However, part of the water limitation
reflects the effects of water on moisturelimited
decomposition and therefore nutrient
supply (see Chapters 7 and 8). Thus at least two
resources (water and nutrients) limit the NPP
of temperate grasslands, and the relative importance
of these resources depends on climate
and soil type. What about other resources?
No one has tested whether addition of light
would stimulate the productivity of any natural
ecosystem. A doubling of atmospheric CO2
stimulates grassland NPP by 10 to 30%, but
most of this stimulation reflects the effects of
CO2 on water and nutrient availability rather
than the direct effects of CO2 on photosynthesis.
Finally, species composition and biomass
influence the response of grassland NPP to
climate.Arid grasslands are never as productive
Aboveground NPP (g m -2 yr -1 )
1000
800
600
400
200
Spatial variation
Temporal variation
0 0 600 1000 1500
Precipitation (mm yr -1 )
Figure 6.5. Correlation of grassland NPP with precipitation
across grassland sites (spatial variation)
and through time for a single site (temporal variation).
A single site responds less to interannual variation
in precipitation than would be expected from
the relationship between average precipitation and
average NPP across all sites, because a single site
lacks the species and productive potential capable of
exploiting high moisture availability. (Redrawn with
permission from Ecological Applications; Lauenroth
and Sala 1992.)
Net Primary Production 131
in wet years as grasslands that regularly receive
high moisture inputs, presumably because
arid grasslands lack the plant species, biomass,
or soil fertility to exploit effectively the years
of high moisture (Fig. 6.5) (Lauenroth and
Sala 1992). In grasslands, therefore, water
appears to be the factor that most strongly
controls NPP, but soil moisture determines
NPP in at least three ways: through its direct
stimulation of NPP, through its effects on nutrient
supply, and through its effect on the species
composition and productive capacity of the
ecosystem.
The controls over NPP in deserts are similar
to those in grasslands: Desert NPP correlates
closely with precipitation among sites, among
years, and in response to water addition
(Gutierrez and Whitford 1987). Even in deserts,
however, NPP is greatest in patches with high
nutrient availability (Schlesinger et al. 1990)
and responds to added nitrogen, especially in
experiments that also add water (Gutierrez and
Whitford 1987), indicating a secondary limitation
of desert NPP by nutrient supply.
In the tundra, where the climate correlations
suggest that NPP should be temperature
limited, NPP increases more in response to
added nitrogen than to experimental increases
in temperature (Chapin et al. 1995, McKane et
al. 1997). Thus, in tundra, the climate–NPP correlation
probably reflects the effects of temperature
on nitrogen supply (see Chapter 9) or
length of growing season more than a direct
temperature effect on NPP (Chapin 1983). Similarly,
NPP in the boreal forest correlates
closely with soil temperature, but soil warming
experiments demonstrate that this effect is
mediated primarily by enhanced decomposition
and nitrogen supply (Van Cleve et al.
1990).
Thus in ecosystems in which climate–NPP
correlations suggest a strong climatic limitation
of NPP, experiments and observations indicate
that this is mediated primarily by climatic
effects on belowground resources. What constrains
NPP in warm, moist climates where
temperature and moisture appear optimal for
growth?
Tropical forests typically have higher NPP
than other biomes (Fig. 6.4). Among tropical