154
mined and translated into biomass according to the
leaf specific area, the area per unit of leaf weight.
Although this value can be obtained at the ecosystem level by standard methods, its pattern of
variability among species and individuals, together
with behavioral preferences by herbivores on
leaves with different leaf specific area may be an
important source of error. Specific leaf area of Amazonian trees ranges from 50 to 200 cm 2 g - 1 (Reich
and Walters 1994), it shows about a twofold variation between northern pin oak (Quercus ellipsoidalis) and sugar maple (Acer saccharum) (Reich
et al. 1991), it may vary seasonally within a species
by 50 or 70% (Reddy et al. 1989; Reich et al. 1991),
and it may vary by almost 100% within single individuals because of leaf age or position within the
canopy (Basset 1991). If this variation is superimposed on dietary preferences of herbivores toward
leaves according to their leaf specific weights (Basset 1991; Choong et al. 1992), it becomes clear that
gross average translation of leaf area consumed to
biomass or energy consumed may be biased.
Consumption by browsers is also estimated by
surveying marks left by the animals on plants. The
number of twigs consumed may be counted because apical growth stops after consumption. The
number and diameter of these marks is recorded
and then transformed into biomass from previously
determined regressions between twig biomass and
diameter (e.g., Edenius 1993). The type of scar left
by different browsers is sometimes so characteristic
that consumption may be quantified by species
(McInnes et al. 1992).
Differential Use of the Two Approaches
For practical reasons, animal-based and plant-based
methods are tightly related to spatial scale. Because
of the data they require and the sort of errors they
handle relatively well, they are used with different
purposes. The animal-based methods require data
on animal density which usually have a spatial resolution that integrates different components of a
landscape or even different landscapes of a region.
Instead of being affected by structural patchiness
and spatial heterogeneity, they integrate it. Methods
based on establishing cages or sampling leaf or twig
marks are usually applied to homogeneous environmental units, such as single plant communities,
Martin Oesterheld and Samuel J. McNaughton
in which plant traits can be accurately estimated
with a reasonable effort.
Effect of Herbivores
on Primary Production
The effect of herbivores on primary production
must be estimated by comparing the rates of production of grazed and ungrazed systems. However,
since most methods of estimating primary production are based on determinations of plant biomass
(see Chapter 2), the presence of herbivores, which
consume plant biomass, may introduce an underestimation bias. For example, if herbivore consumption exactly matched daily primary production, sequential determinations of plant biomass
would reveal no accumulation and would erroneously return a production value equal to zero. As a
result, most approaches to assess the effect of herbivores on primary production involve the exclusion of herbivores. However, if production is not
estimated from biomass determinations but from
some measurements of canopy functioning, such as
gas exchange rates or intercepted radiation (see
Chapters 2 and 3), there would be no need to remove the herbivores from the grazed treatment.
In this section we discuss advantages and disadvantages of these approaches. However, we first
review a few central concepts regarding the effect
of herbivores on productivity. As in the previous
section, our text is complemented by the reviews
on methods to estimate aboveground and belowground production in Chapters 2,3, and 4.
Compensatory Growth
There are a number of alternative responses of productivity to grazing, depending on how grazing affects the growth rate per unit of biomass or relative
growth rate (McNaughton 1979; Hilbert et al. 1981;
Belsky 1986; Oesterheld and McNaughton 1991).
When relative growth rate decreases as a consequence of grazing, the response is called damage.
When it increases, the response is called compensatory growth. Depending on the effect that this
increase in relative growth has on the absolute
growth rate or productivity, three different compensatory responses may be distinguished. When the
mined and translated into biomass according to the
leaf specific area, the area per unit of leaf weight.
Although this value can be obtained at the ecosystem level by standard methods, its pattern of
variability among species and individuals, together
with behavioral preferences by herbivores on
leaves with different leaf specific area may be an
important source of error. Specific leaf area of Amazonian trees ranges from 50 to 200 cm 2 g - 1 (Reich
and Walters 1994), it shows about a twofold variation between northern pin oak (Quercus ellipsoidalis) and sugar maple (Acer saccharum) (Reich
et al. 1991), it may vary seasonally within a species
by 50 or 70% (Reddy et al. 1989; Reich et al. 1991),
and it may vary by almost 100% within single individuals because of leaf age or position within the
canopy (Basset 1991). If this variation is superimposed on dietary preferences of herbivores toward
leaves according to their leaf specific weights (Basset 1991; Choong et al. 1992), it becomes clear that
gross average translation of leaf area consumed to
biomass or energy consumed may be biased.
Consumption by browsers is also estimated by
surveying marks left by the animals on plants. The
number of twigs consumed may be counted because apical growth stops after consumption. The
number and diameter of these marks is recorded
and then transformed into biomass from previously
determined regressions between twig biomass and
diameter (e.g., Edenius 1993). The type of scar left
by different browsers is sometimes so characteristic
that consumption may be quantified by species
(McInnes et al. 1992).
Differential Use of the Two Approaches
For practical reasons, animal-based and plant-based
methods are tightly related to spatial scale. Because
of the data they require and the sort of errors they
handle relatively well, they are used with different
purposes. The animal-based methods require data
on animal density which usually have a spatial resolution that integrates different components of a
landscape or even different landscapes of a region.
Instead of being affected by structural patchiness
and spatial heterogeneity, they integrate it. Methods
based on establishing cages or sampling leaf or twig
marks are usually applied to homogeneous environmental units, such as single plant communities,
Martin Oesterheld and Samuel J. McNaughton
in which plant traits can be accurately estimated
with a reasonable effort.
Effect of Herbivores
on Primary Production
The effect of herbivores on primary production
must be estimated by comparing the rates of production of grazed and ungrazed systems. However,
since most methods of estimating primary production are based on determinations of plant biomass
(see Chapter 2), the presence of herbivores, which
consume plant biomass, may introduce an underestimation bias. For example, if herbivore consumption exactly matched daily primary production, sequential determinations of plant biomass
would reveal no accumulation and would erroneously return a production value equal to zero. As a
result, most approaches to assess the effect of herbivores on primary production involve the exclusion of herbivores. However, if production is not
estimated from biomass determinations but from
some measurements of canopy functioning, such as
gas exchange rates or intercepted radiation (see
Chapters 2 and 3), there would be no need to remove the herbivores from the grazed treatment.
In this section we discuss advantages and disadvantages of these approaches. However, we first
review a few central concepts regarding the effect
of herbivores on productivity. As in the previous
section, our text is complemented by the reviews
on methods to estimate aboveground and belowground production in Chapters 2,3, and 4.
Compensatory Growth
There are a number of alternative responses of productivity to grazing, depending on how grazing affects the growth rate per unit of biomass or relative
growth rate (McNaughton 1979; Hilbert et al. 1981;
Belsky 1986; Oesterheld and McNaughton 1991).
When relative growth rate decreases as a consequence of grazing, the response is called damage.
When it increases, the response is called compensatory growth. Depending on the effect that this
increase in relative growth has on the absolute
growth rate or productivity, three different compensatory responses may be distinguished. When the