156
compensatory regrowth by increasing the frequency of movement will need a parallel increase
in sampling effort to avoid statistical errors.
Clipping inside cages or exclosures to simulate
grazing or browsing has the advantages and disadvantages of any simulation: simplicity is gained
and realism is lost. Because the same plots are sampled throughout the season, movement is not necessary and bias associated with false peaks and
troughs generated by spatial heterogeneity is not a
source of error. However, several difficulties arise
when trying to simulate grazing or browsing by
clipping: the spatial and temporal pattern of consumption is difficult to replicate and other effects
of grazing different from consumption, such as nutrient return and trampling, are not included (McNaughton et al. 1996).
Moving the animals to allow for regrowth determinations during times of vacancy may not be practical while working with large herbivores, but can
be successful with small herbivores. The frequency
of animal movements will have here the same effect
on production estimates as the frequency of cage
movement in the moveable-cage method (McNaughton et al. 1996). An additional application of
this approach is in livestock production systems
managed with rotational grazing. This management
generates a pattern of utilization and vacancy that
may be timed with growth determinations during
times of vacancy.
These three approaches share the need for herbivore removal and biomass determinations. The
"grazed" vegetation being sampled is then different
from the real grazed vegetation. Assuming a full
accuracy in biomass determinations, the effect of
the difference between the real and the sampled situations on productivity will result in a bias. Controlling for this bias, which means controlling for
the difference between real and sampled situations,
goes against the assumption of full accuracy in biomass determination unless sampling effort is proportionally increased.
The observation that spectral data may be translated into productivity because they are an integration of absorbed photosynthetic active radiation
(see Chapter 3) (Paruelo et al. 1997) suggests a way
out of this tradeoff. Net primary productivity is the
product between absorbed photosynthetic radiation
and a coefficient that represents the conversion of
radiation into biomass growth (see Chapter 3). SeMartin Oesterhe1d and Samuel J. McNaughton
quential determinations of canopy reflectance from
remote sensing devices or hand-held radiometers
may be integrated and provide estimates of production values of systems being grazed. The accuracy
of these determinations will depend on the empirical calibration between production and reflectance
(see Chapter 3). Also, differences in canopy structure, tissue composition, and water status between
the grazed and ungrazed systems to be compared
may require differential calibrations for each treatment.
Acknowledgments An anonymous reviewer made
valuable comments on a previous version of this
manuscript. Supported by University of Buenos
Aires and Consejo Nacional de Investigaciones
Cientificas y Tecnicas (CONlCET).
References
Ballard, R.A.; Simpson, R.I.; Pearce, G.R. Losses of the
digestible components of annual ryegrass (Lolium rigidum Gaudin) during senescence. Aust. J. Agric. Res.
41:719-732; 1990.
Basset, Y. Influence of leaf traits on the spatial distribution of insect herbivores associated with an overstorey
rainforest tree. Oecologia 87:388-393; 1991.
Belsky, A.J. Does herbivory benefit plants? A review of
the evidence. Am. Natur. 127:870-892; 1986.
Biondini, M.E.; Lauenroth, W.K., Sala, O.E. Correcting
estimates of net primary production. Are we overestimating plant production in rangelands? J. Range
Manage. 44:194-198; 1991.
Bondi, A. Nutricion Animal. Zaragoza: Acribia; 1989.
Bray, J. Primary consumption in three forest canopies.
Ecology 45:165-167; 1964.
Brown, V.K.; Gange, A.C. Secondary plant succession:
How is it modified by insect herbivory? Vegetatio
101:3-13; 1992.
Choong, M.P.; Lucas, P.w.; Ong, J.S.Y.; Pereira, B.; Tan,
H.T.W.; Turner, I.M. Leaf fracture toughness and
sclerophylly: Their correlations and ecological implications. New Phytol. 121:597-610; 1992.
Coleman, J.S.; Leonard, A.S. Why it matters where on a
leaf a folivore feeds. Oecologia 101:324-328; 1995.
Demment, M.W.; Van Soest, P.I. A nutritional explanation for body-size patterns of ruminant and nonruminant herbivores. Am. Natur. 125:641-672; 1985.
Edenius, L. Browsing by moose on Scots pine in relation
to plant resource availability. Ecology 74:2261-2269;
1993.
compensatory regrowth by increasing the frequency of movement will need a parallel increase
in sampling effort to avoid statistical errors.
Clipping inside cages or exclosures to simulate
grazing or browsing has the advantages and disadvantages of any simulation: simplicity is gained
and realism is lost. Because the same plots are sampled throughout the season, movement is not necessary and bias associated with false peaks and
troughs generated by spatial heterogeneity is not a
source of error. However, several difficulties arise
when trying to simulate grazing or browsing by
clipping: the spatial and temporal pattern of consumption is difficult to replicate and other effects
of grazing different from consumption, such as nutrient return and trampling, are not included (McNaughton et al. 1996).
Moving the animals to allow for regrowth determinations during times of vacancy may not be practical while working with large herbivores, but can
be successful with small herbivores. The frequency
of animal movements will have here the same effect
on production estimates as the frequency of cage
movement in the moveable-cage method (McNaughton et al. 1996). An additional application of
this approach is in livestock production systems
managed with rotational grazing. This management
generates a pattern of utilization and vacancy that
may be timed with growth determinations during
times of vacancy.
These three approaches share the need for herbivore removal and biomass determinations. The
"grazed" vegetation being sampled is then different
from the real grazed vegetation. Assuming a full
accuracy in biomass determinations, the effect of
the difference between the real and the sampled situations on productivity will result in a bias. Controlling for this bias, which means controlling for
the difference between real and sampled situations,
goes against the assumption of full accuracy in biomass determination unless sampling effort is proportionally increased.
The observation that spectral data may be translated into productivity because they are an integration of absorbed photosynthetic active radiation
(see Chapter 3) (Paruelo et al. 1997) suggests a way
out of this tradeoff. Net primary productivity is the
product between absorbed photosynthetic radiation
and a coefficient that represents the conversion of
radiation into biomass growth (see Chapter 3). SeMartin Oesterhe1d and Samuel J. McNaughton
quential determinations of canopy reflectance from
remote sensing devices or hand-held radiometers
may be integrated and provide estimates of production values of systems being grazed. The accuracy
of these determinations will depend on the empirical calibration between production and reflectance
(see Chapter 3). Also, differences in canopy structure, tissue composition, and water status between
the grazed and ungrazed systems to be compared
may require differential calibrations for each treatment.
Acknowledgments An anonymous reviewer made
valuable comments on a previous version of this
manuscript. Supported by University of Buenos
Aires and Consejo Nacional de Investigaciones
Cientificas y Tecnicas (CONlCET).
References
Ballard, R.A.; Simpson, R.I.; Pearce, G.R. Losses of the
digestible components of annual ryegrass (Lolium rigidum Gaudin) during senescence. Aust. J. Agric. Res.
41:719-732; 1990.
Basset, Y. Influence of leaf traits on the spatial distribution of insect herbivores associated with an overstorey
rainforest tree. Oecologia 87:388-393; 1991.
Belsky, A.J. Does herbivory benefit plants? A review of
the evidence. Am. Natur. 127:870-892; 1986.
Biondini, M.E.; Lauenroth, W.K., Sala, O.E. Correcting
estimates of net primary production. Are we overestimating plant production in rangelands? J. Range
Manage. 44:194-198; 1991.
Bondi, A. Nutricion Animal. Zaragoza: Acribia; 1989.
Bray, J. Primary consumption in three forest canopies.
Ecology 45:165-167; 1964.
Brown, V.K.; Gange, A.C. Secondary plant succession:
How is it modified by insect herbivory? Vegetatio
101:3-13; 1992.
Choong, M.P.; Lucas, P.w.; Ong, J.S.Y.; Pereira, B.; Tan,
H.T.W.; Turner, I.M. Leaf fracture toughness and
sclerophylly: Their correlations and ecological implications. New Phytol. 121:597-610; 1992.
Coleman, J.S.; Leonard, A.S. Why it matters where on a
leaf a folivore feeds. Oecologia 101:324-328; 1995.
Demment, M.W.; Van Soest, P.I. A nutritional explanation for body-size patterns of ruminant and nonruminant herbivores. Am. Natur. 125:641-672; 1985.
Edenius, L. Browsing by moose on Scots pine in relation
to plant resource availability. Ecology 74:2261-2269;
1993.