Structure and Function in Agroecosystem Design and Management - Chapter 14 ppt

CHAPTER 14
The Effect of Elevated Atmospheric
CO
2
on Grazed Grasslands
Paul C.D. Newton, Harry Clark, and Grant R. Edwards
CONTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Properties of Grazed Pasture Ecosystems. . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
Nutrient Cycling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
Physical Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
Preference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
INTRODUCTION
Grasslands cover about a fifth of the terrestrial surface of the world
(Hadley, 1993), and the majority of this area is grazed by animals. The impact
of an increasing concentration of CO
2
in the atmosphere on these grasslands
has assumed importance, first because of the direct effects on food produc-
tion (Gregory et al., 1999), and second because of the influence terrestrial
ecosystems can have on the composition of the atmosphere and therefore on
our climate (Pielke et al., 1998). In the case of grasslands this includes not
only C sequestration, N
2
O release, and CH
4
uptake by soils, but also CH
4
emissions from ruminants. Consequently, many research programs have

been developed to explore these impacts, and our knowledge of the likely
outcomes is progressing rapidly. However, our understanding is based
almost exclusively on cut (as opposed to grazed) grassland (e.g., Wolfenden
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298 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
and Diggle, 1995; Casella et al., 1996a,b; Newton et al., 1996; Clark et al., 1997;
Hebeisen et al., 1997; Potvin and Vasseur, 1997; Taylor and Potvin, 1997; Clark
et al., 1998; Leadley et al., 1999; Navas et al., 1999, but see Edwards et al.,
2000), and grazed swards are very different in their botanical and soil char-
acteristics (Watkin and Clements, 1976; Haynes and Williams, 1993). In addi-
tion, some of these experiments involved the transfer of previously grazed
areas to a cutting management (Newton et al., 1996; Clark et al., 1997; Potvin
and Vasseur, 1997; Taylor and Potvin, 1997; Clark et al., 1998; Leadley et al.,
1999) and therefore do not necessarily display the responses typical of a cut
system but of a system in transition. In these examples it is probable that the
change in management resulted in a process of succession, one consequence
of which would likely be a loss of early successional species. Clearly, any
interpretation of a response to elevated CO
2
in these transitional systems
must be made with this background change in mind.
Altering the frequency and severity of defoliation can have profound
effects on the dynamics of grassland systems (Parsons et al., 1988), and dif-
ferences in the soil and plant properties of cut and grazed swards can often
(in part) be attributed to difference in the timing and severity of harvesting.
However, such comparisons conceal the intrinsic effects introduced by graz-
ing animals. Consequently, in this chapter we concentrate on comparing cut-

ting with grazing at the same frequency and severity of defoliation. We are
concerned with identifying characteristics introduced by grazing animals
that have the potential to alter how pastures might respond to elevated CO
2
.
While the question of CO
2
ϫ grazing interactions has been raised previously
(Wilsey, 1996), we are not aware of any comprehensive treatment of this sub-
ject. Without considerations of different responses of cut and grazed swards
to elevated CO
2
we are not in a position to extrapolate from the considerable
bulk of existing experimental data to grazed grasslands—the predominant
pastoral land use.
Much of what we present is based on temperate pastures; this does not
imply any special importance of this type of grassland but simply reflects
that these systems have been more extensively examined in terms of CO
2
effects than any other grassland type, and there is a long and detailed litera-
ture on responses of these ecosystems to grazing from which we can draw
general principles.
PROPERTIES OF GRAZED PASTURE ECOSYSTEMS
Despite the common practice of using cutting to simulate grazing by
animals, there are clear differences in the ecosystem properties which can be
directly related to these managements. The actions that the grazing animal
introduces involve nutrient cycling, physical damage to plants and soils, and
selective grazing.
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THE EFFECT OF ELEVATED ATMOSPHERIC CO

2
ON GRAZED GRASSLANDS 299
Table 14.1 Proportion of Nutrients Returned by Milking
Cows Grazing at Pasture.
Element % Returned % Returned Total
in Feces in Urine Returned (%)
K11 81 92
Na 30 56 86
Ca 77 3 80
P66 0 66
N26 53 89
From Hutton, J.B., Jury, K.E., and Davies, E.B., NZ. J. Ag. Res.,
10:367–388, 1967.
Nutrient Cycling
Grazing animals return nutrients to the pasture, and it is in the composi-
tion and spatial arrangement of these nutrient returns that lies the major dif-
ference between cut and grazed systems (Haynes and Williams, 1993).
Animals use only a small proportion of the nutrients they ingest; 60–99% are
returned to the pasture as dung and urine (Barrow, 1987). The actual amounts
returned are dependent on the species of animal and the stage of its devel-
opment (Haynes and Williams, 1993). Some typical values for dairy cows are
shown in Table 14.1. There are some differences between animal types in the
proportion of nutrients returned; for example, sheep return greater amounts
of N in the urine than cattle (about 70–75% of the excreted N in urine; Sears
et al., 1948). However, as a general principle, the concentration returned
depends upon the concentration in the food ingested. In the case of the N in
urine and the P in dung, the relationship with the feed composition is linear
(Barrow and Lambourne, 1961).
Unlike cutting, which removes nutrients from the whole of a paddock
and then returns them evenly by fertilizer application, grazing removes

nutrients from the whole paddock but returns them heterogeneously in the
excreta. A sheep may have 18–20 urinations in a day, each event returning
nutrients over an area of 0.03–0.05 m
Ϫ2
. A typical value for cattle would be
8–12 urinations, each covering an area 0.16–0.49 m
Ϫ2
(Haynes and Williams,
1993). In the excretal areas, the nutrients are at very high concentrations
(Table 14.2). There are three consequences of this localized return at high con-
centrations. First, the pasture becomes a mosaic of patches ranging from very
high to very low nutrient status. The outcome of such a distribution is that
pasture growth is very high in a small area of the paddock; for example,
Saunders (1984) found that under cattle grazing, 75% of the dry matter was
produced from 38% of the pasture area. Second, losses of nutrients through
gaseous emissions, leaching, and runoff are all exaggerated in the high nutri-
ent patches. Third, plants in the excretal areas can be damaged or killed (e.g.,
by urine scorch or buried under dung) making immediate recovery of the
nutrients less likely (Haynes and Williams, 1993). In addition, animals may
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300 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 14.1 Potential N response curves at ambient (solid line) and elevated
(dashed line) CO
2
.
Table 14.2 Typical Application Rates of Nutrients (kg ha
؊1
)
Contained in Single Urination or Defecation Events
Sheep Cattle

Urination Defecation Urination Defecation
N 500 130 1000 1040
S 18 13 35 100
K 450 50 900 400
P — 35 — 280
Data sources and assumptions given in Haynes, R.J. and Williams, P.H., Adv. Agon.,
49:119–190, 1993.
avoid grazing areas close to excreta (Haynes and Williams, 1993) resulting in
ungrazed patches of herbage that may be at ceiling yield interspersed with
grazed areas in the early stages of regrowth.
The long-term effects of excretal return are to increase organic matter (C
and N) storage largely because of the return of organic matter as dung
(Carran and Theobald, 1998). This outcome, that grazing management can
influence the equilibrium organic matter content (Haynes and Williams,
1993), has important implications for C storage and therefore greenhouse gas
emissions from pasture. One negative consequence of grazing is the lower
soil Ca and Mg contents due to the high rate of cycling of K through excretal
returns (Carran and Theobald, 1998).
How might we expect pasture response to elevated CO
2
to be modified
by grazing-mediated changes in biogeochemical cycles? The distribution of
nutrients into high and low patches is the characteristic that has the greatest
potential to interact with CO
2
. Let us consider a hypothetical example of the
distribution of N which gives the same average application of 240 kg ha
Ϫ1
in
both cut and grazed swards, but in the cut sward, the N is distributed evenly

and in the grazed sward it is at a rate equivalent to 1000 kg ha
Ϫ1
in 20% of the
area and at 50 kg ha
Ϫ1
in 80% of the area. If plant responses to N are linear at
both ambient and elevated CO
2
(Figure 14.1a), then it makes no difference to
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THE EFFECT OF ELEVATED ATMOSPHERIC CO
2
ON GRAZED GRASSLANDS 301
Figure 14.2 N response curves for Lolium perenne plants grown at 390 or 690 ppm
CO
2
. (From Schenk et al., J. Pl. Nutr. 19:1423–1440, 1996.)
the response whether the N is distributed homo- or heterogeneously.
Consequently, despite a strong response to elevated CO
2
, there is no differ-
ence in response between a cut (homogeneous N) and grazed (heterogeneous
N) management. If there is no CO
2
effect—despite nonlinear N response
curves (Figure 14.1b)—then it makes no difference whether the pastures are
cut or grazed. However, if the plant/community responses to N are nonlin-
ear, and if they are different between CO
2
levels (Figure 14.1c), then the rela-

tive responses to CO
2
will depend on the nutrient distribution; i.e., they will
be different depending on whether the pasture is cut or grazed and the man-
ner of the difference will depend on the relative shape of the curves. In fact,
nonlinear curves of the kind shown in Figure 14.1c are frequently seen in
experimental data for a range of variables, such as dry matter (Schenk et al.,
1996; Figure 14.2), photosynthesis (Bowler and Press, 1996), and competitive
ability (Navas et al., 1999). Obviously the argument made for N can also
apply to other nutrients that are returned in high concentration by animals
(e.g., P, K, or S) and for which there are likely to be nonlinear response curves
and CO
2
ϫ nutrient interactions.
Soussana and Hartwig (1996) have described the consequences of ele-
vated CO
2
for N cycling in cut systems but were not able to speculate on
aboveground transfer of N by grazing animals at elevated CO
2
due to a lack
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302 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 14.3 Nitrogen content (%) of the leaves of legume species exposed to ele-
vated CO
2
for different durations and under different management
regimes. Values described as Face are for Trifolium repens and T. sub-
terraneum plants sampled after 18 months exposure to ambient (360
ppm) or 475 ppm CO

2
using free air carbon dioxide enrichment; the
plants were in an established permanent pasture and were grazed
intermittently by sheep (see Edwards et al., 2000). Short term data are
for T. repens plants harvested after exposure to 350, 500, 650, or 800
ppm for a period of 4 weeks in controlled environment rooms; the light
level was 700
␮
mol m
Ϫ2
s
Ϫ1
for the 14 h photoperiod and the day/night
temperatures 22/16°C. Long term exposure data are for Lotus uligi-
nosus plants growing at different distances from a natural CO
2
spring
and presumed to have been exposed to elevated CO
2
for many
decades; the CO
2
concentration the plants experienced was estimated
as the mean of spot measurements taken at canopy height over a
period of three years (see Ross et al., 2000); the plants were subject to
intermittent cutting.
of experiments under grazing. The arguments for changes in N cycling
revolve in part around the well-documented decrease in N content of plant
leaves at elevated CO
2

(Poorter et al., 1997) and the increase in the fixed N
contribution by legumes (Soussana and Hartwig, 1996). These arguments
also apply to grazed swards (although see later section on legume content
under grazing), but we must also consider the aboveground return through
excreta. The reduction in leaf N content at elevated CO
2
appears to be main-
tained over the long term; i.e., over lengths of time during which adaptation
could occur (Körner and Miglietta, 1994) and in grazed as well as cut systems
(Figure 14.3). Consequently, less N will be returned by animals at elevated
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THE EFFECT OF ELEVATED ATMOSPHERIC CO
2
ON GRAZED GRASSLANDS 303
CO
2
because the N in the urine is directly proportional to the N in the herbage
eaten (Barrow and Lambourne, 1961). The lower N in herbage could be com-
pensated for by a greater volume of returns but as dry matter yield (and
therefore the potential to increase animal numbers) is not markedly increased
at elevated CO
2
(Hebeisen et al., 1997; Edwards et al., 2000) this compensa-
tion is not likely. If the N returned in each urination is reduced we might
expect lower losses through leaching and volatilization as these are concen-
tration dependent (Haynes and Williams, 1993), and greater uptake by plants
which are able to use the lower concentrations more effectively. These
changes should result in tighter N cycling and greater N efficiency under
grazing at elevated CO
2

.
Physical Effects
By the action of their hooves, animals have the potential to physically
alter (usually detrimentally) properties of soils and plants. The hoof of an
ungulate exerts a pressure that can be calculated from the area of contact and
the mass of the animal. Typical static load values per hoof of domestic ani-
mals would be 192 kPa for a cow, 83 kPa for a sheep, and 60 kPa for a goat
(Willatt and Pullar, 1983). In practice, the pressure applied is almost always
greater than this as the hoof is rarely applied flat to the ground. The result of
treading can be seen in soil properties; there is a positive relationship
between treading intensity and soil bulk density and a negative relationship
with hydraulic conductivity (Willatt and Pullar, 1983). In addition, treading
alters surface properties, leaving patches (gaps) of bare ground (Watkin and
Clements, 1976; Betteridge et al., 1999). Plants are also susceptible to damage
from treading, either by crushing or through cutting of plant parts by sharp
hooves. The consequences of these physical aspects of grazing are not always
separated from the effects of other grazing influences. However, Edmond
conducted a comprehensive series of trials to study the effects of treading
alone on pastures (see Brown and Evans, 1973, for a review of this work).
Edmond (1970) showed that treading could reduce herbage yields by
30–40%, with the yield reduction being dependent on the plant species pres-
ent (Figure 14.4). Lolium perenne is particularly resistant to treading (Edmond,
1964) and is observed to increase in abundance as treading pressure
increases.
During the process of biting it is not just that leaves are removed—as
they would also be under cutting—but there is also the potential for damage
to meristems and other plant parts resulting in a loss of function, e.g., photo-
synthetic capacity, transport of nutrients, or increased susceptibility to pest
and diseases. Part of the reason for this is that the biting process also involves
pulling, which lifts plant parts, such as stolons, leaving meristems vulnera-

ble to being eaten. Pulling can also lift plant roots from the soil. Typically, 9%
of the apical meristems of
Trifolium repens are removed during a rotational
grazing event (Hay et al., 1991). The consequences of the different mechanical
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304 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 14.4 Sensitivity of some pasture species to treading by sheep, expressed as
yield relative to an untrodden control. (From Edmonds, D.B., N.Z. J. Ag.
Res., 7:1–16, 1964.)
effects of cutting and grazing are rarely considered and can be compared only
at the same defoliation interval, at the same severity of defoliation, and with
the same nutrient returns. Sears (1953) conducted a five year study of graz-
ing effects on pastures which included a number of subtreatments. From
these, we can find a comparison of the mechanical effects of grazing; this does
not exclude the treading effects sensu Edmond, but in this case observation
showed the most marked effects were through the biting process. In particu-
lar there was a sharp decline in Trifolium pratense under grazing because the
animals were able to remove plant crowns, whereas the cutting process left
them intact (Figure 14.5).
We can envisage CO
2
interacting with many of these physical conse-
quences of grazing. First, the damaging effects of treading on soil structure—
compaction, loss of drainage capacity—may have different effects at elevated
CO
2
in which greater allocation of C below ground is frequently observed.
Changes in soil biota have also been reported at elevated CO
2
(O’Neill,

1994; Yeates et al., 1999), and these can strongly influence soil structural prop-
erties (O’Neill, 1994). Second, the creation of gaps by the grazing animals
has important consequences for population processes as these promote
both recruitment from seed (Panetta and Wardle, 1992) and vegetative
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THE EFFECT OF ELEVATED ATMOSPHERIC CO
2
ON GRAZED GRASSLANDS 305
Figure 14.5 Effects of mechanical damage by grazing animals on the botanical
composition of a pasture. (From Sears, P.D., N.Z. J. Sci. Tech.,
35A:1–29, 1953.)
development—e.g., branching and tillering (Arnthórsdóttir, 1994)—and both
of these regenerative processes have been shown to be influenced by elevated
CO
2
. Many studies have shown elevated CO
2
can alter the number of seeds
produced (Lawlor and Mitchell, 1991; Farnsworth and Bazzaz, 1995) which
can lead to changes in recruitment in seed-limited species (Edwards et al.,
2000). It may also be the case that more seed heads are left intact after defoli-
ation by grazing rather than cutting, allowing greater expression of any CO
2
effects on seed characteristics. Other studies have shown changes in seed
quality at elevated CO
2
(e.g., in C:N ratio and seed mass) which have the
potential to alter germination and establishment rates. If the likelihood exists
of different seed behavior in gaps at elevated CO
2

, as shown by Spring et al.
(1996), then there is a strong possibility that the increased gap frequency
under grazing will result in a grazing management ϫ CO
2
interaction.
Increased vegetative propagation has been shown to be an important mech-
anism driving changes in species abundance in response to elevated CO
2
in a
wide range of situations, such as temperate pasture (Clark et al., 1997) and
alpine meadows (Leadley et al., 1999). It should achieve even greater expres-
sion in the presence of more gaps (regeneration niches) and be of more criti-
cal importance given the losses of meristems experienced in a grazed system.
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306 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Preference
The botanical composition of pastures (species identity and abundance)
is determined in part by the method of harvesting. Even a consistent, uniform
process, such as cutting, has a selective effect which reflects the vertical dis-
tribution of plant species in the canopy. For example, cutting removes pro-
portionally more clover (Trifolium repens) laminae than grass (Lolium perenne)
laminae in a mixed grass/clover sward because the clover leaves are held
higher in the canopy (Woledge et al., 1992). It has been argued that grazing
animals such as sheep have a selective effect on sward composition simply by
the kind of passive selection imposed by cutting (Milne et al., 1982). In fact,
Parsons and co-workers have shown that sheep have a preference (i.e.,
actively select) for white clover; in this case, it is a partial preference, with
sheep preferring a diet of 70% clover and 30% grass (Parsons et al., 1994).
Note that this also means that animals might select against clover when the
clover percentage in the sward exceeds 70%. As a consequence, the clover

removed under grazing is proportionally larger than the clover removed by
the passive selection of a lawnmower (Woledge et al., 1992).
The outcome for a plant species that is a preferred component of the diet
is clearly not favorable in comparison to a nonpreferred species. Indeed, ani-
mals may reduce the abundance of their preferred species in the sward until
it becomes a small component of their diet (Parsons et al., 1991b)—the
“Paradox of Imprudence” (Slobodkin, 1974). The only way in which a plant
species can overcome the deleterious consequences of being preferred (in
relation to other plant species) is if it holds some advantage in growth over
its companion species. By this means, a preferred species can maintain its
presence in a grazed sward until a point at which the grazing pressure out-
weighs the growth advantage (Parsons et al., 1991b). It has been suggested
that the growth advantage held by clover is a greater specific leaf area
(Parsons et al., 1991a).
At elevated CO
2
, there is strong evidence that legumes are advantaged in
comparison to grass species (Newton et al., 1994; Clark et al., 1997; Hebeisen
et al., 1997; Leadley et al., 1999); although this evidence comes from cutting
experiments, we might anticipate that, in the absence of any change in ani-
mal preference, CO
2
would also result in greater legume growth under graz-
ing. In a Face experiment grazed by sheep we compared the effects of grazing
(Figure 14.6a) and cutting (Figure 14.6b) on pasture responses to elevated
CO
2
. The ambient values show that clover was deleteriously affected by graz-
ing (compare Figure 14.6a and b); however, CO
2

enrichment gave the clover
sufficient advantage to compensate for the grazing effect so that clover
growth under grazing at elevated CO
2
(Figure 14.6a) was comparable to
clover growth under cutting at ambient CO
2
(Figure 14.6b). In this instance,
clover responded positively to elevated CO
2
only when grazed; under cut-
ting, there was only a minimal stimulation of clover, suggesting that in this
environment factors other than CO
2
set a limit to the growth of clover.
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THE EFFECT OF ELEVATED ATMOSPHERIC CO
2
ON GRAZED GRASSLANDS 307
Figure 14.6 Modification by grazing management of the response to CO
2
of
Trifolium spp. (principally T. repens, T. subterraneum, T. glomeratum).
The pasture was enriched to 475 ppm CO
2
using Face technology (see
Edwards et al., 2000 for further details) and sheep were either (a)
allowed to graze periodically or (b) excluded from grazing by metal
cages. Dry matter was measured from cuts taken just prior to grazing to
a height of 2 cm above the soil surface. The response profiles over time

were analyzed using the concept of antedependence (Kenward, 1987);
there were significant CO
2
ϫ grazing management interactions in the
early spring periods (P Ͻ 0.05) with a P value of 0.06 for the interaction
term for the whole series.
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308 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
CONCLUSIONS
We already know that management (fertilizer, cutting frequency) can
modify pasture responses to CO
2
(Hebeisen et al., 1997) and “no cutting treat-
ment can satisfactorily reproduce the defoliation regime in a grazed pasture”
(Watkin and Clements, 1976), there is every expectation that grazed pastures
may not respond to elevated CO
2
in the same manner as cut pastures. In this
chapter we have attempted to identify factors and processes that are funda-
mentally different under grazing and found many that might be expected to
interact with elevated CO
2
. Because of this, and because there is accumulat-
ing evidence (e.g., Figure 14.6) to show that grazing management can mod-
ify pasture responses, it is by no means certain that we can extrapolate the
results of CO
2
enrichment experiments from cut to grazed situations. In fact,
what is required is not only a greater understanding of how changes in pas-
ture might alter animal performance at elevated CO

2
but also, more impor-
tantly, a clearer picture of the CO
2
response of ecosystems in which grazing
animals are an integral part.
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