Download Temperate Grassland Responses to Climate

Annals of Botany 80 : 205–221, 1997
Temperate Grassland Responses to Climate Change : an Analysis using the Hurley
Pasture Model
J. H. M. T H O R N L E Y and M. G. R. C A N N E L L
Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
Received : 6 January 1997
Accepted : 24 March 1997
The Hurley Pasture Model is process-based and couples the carbon, nitrogen and water cycles in the soil-grass-animal
system. It was used to examine the responses of grasslands in southern, lowland and northern, upland climates in
Britain. Short-term response to step-wise increases in CO concentration (350 to 700 µmol mol−") and temperature
#
(5 °C) were contrasted with long-term equilibrium (the term ‘ equilibrium ’ is equivalent to ‘ steady state ’ throughout
this paper) responses and with responses to gradually increasing [CO ] and temperature. Equilibrium responses to a
#
range of climate variables were also examined.
Three conclusions were drawn regarding the interpretation of experiments : (1) initial ecosystem responses to stepwise changes can be different in both magnitude and sign to equilibrium responses, and this can continue for many
years ; (2) grazing can drastically alter the magnitude and sign of the response of grasslands to climate change,
especially rising temperatures ; and (3) effects of changes in climate, especially temperature and rainfall, are likely to
be highly site-specific. It was concluded that experiments should try to lessen uncertainty about processes within
models rather than try to predict ecosystem responses directly.
Three conclusions were also drawn about the operation of grasslands as carbon sinks : (1) increasing [CO ] alone
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will produce a carbon sink, as long as it continues to accelerate photosynthesis and increase net primary
productivity ; (2) by contrast, increasing temperatures alone are likely to produce a carbon source, because soil
respiration is accelerated more than net primary productivity, even when assuming the same temperature function for
most soil and plant biochemical processes ; and (3) the net effect of projected increases in [CO ] and temperature is
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likely to be a carbon sink of 5–15 g C m−# yr−" in humid, temperate grasslands for several decades, which is consistent
with the magnitude of the hypothesized current global terrestrial carbon sink.
# 1997 Annals of Botany Company
Key words : Grassland, climate change, carbon dioxide, temperature, ecosystem, model, carbon sink.
INTRODUCTION
The problem of predicting the impact of gradually increasing
temperature and [CO ] on the productivity of grasslands,
#
and on whether grassland soils are carbon sinks or sources,
will not easily be solved by experimentation. Firstly, most
experiments are necessarily short-term (less than a decade)
compared with the time constants of many ecosystem
processes, notably those in the soil which determine N
supplies to the grass and carbon storage in the soil.
Consequently, there is a risk that experiments observe
transient responses, unlike the real-world response of
ecosystems to gradual climate change, which is a process of
continual adjustment towards an unattained new equilibrium. Secondly, few experiments include grazing,
although several include periodic cutting (Newton et al.,
1995, 1996 ; Jones, Jongen and Doyle, 1996), which in
fact makes the climate responses difficult to interpret
owing to transient responses to defoliation (Thornley,
1998). Thirdly, all experiments must be constrained to a
limited number of treatments and no two experiments are
exactly alike, making comparisons difficult and leading
to apparently conflicting results which may or may not
be valid.
0305-7364}97}070205­17 $25.00}0
Ecosystem modelling provides a feasible way forward.
Several workers have used models to predict climate change
effects on grasslands, notably Hunt et al. (1991), who
focused on semi-arid prairie, Van den Pol van Dasselaar
and Lantinga (1995) who examined grasslands in the
Netherlands, and a SCOPE project group, which used the
CCGRASS and CENTURY models to quantify the effects
of GCM predicted climates on grasslands worldwide
(Breymeyer et al., 1996). These workers used models that
are substantially less comprehensive than the Hurley Pasture
Model (see below and the Appendix) and generally
concentrated on differences between current and 2¬CO
#
climates.
In this paper, we use the Hurley Pasture Model (Johnson
and Thornley, 1985 ; Thornley and Verberne, 1989 ;
Thornley and Johnson, 1990 ; Thornley 1996, 1998) to
explore differences between transient and equilibrium
climate responses and to quantify the separate and combined
effects of increasing temperature and [CO ] on the net
#
primary productivity (NPP) and total carbon content (in the
plant-soil system, Csys) of grasslands growing in two
contrasting climates in Britain.
bo970430
# 1997 Annals of Botany Company
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Thornley and Cannell—Grassland Responses to Climate Change
MODEL PROPERTIES AND CLIMATE
SCENARIOS
Hurley Pasture Model
The version of the Hurley Pasture Model used here is fully
described by Thornley (1998) including all equations and
parameter values. A synopsis of the processes represented is
given in the Appendix, including some source references,
critical dependencies and parameters.
The model is generic, deterministic, dynamic and largely
mechanistic. It describes the fluxes of carbon (C), nitrogen
(N) and water in a grazed soil-pasture-atmosphere system,
coupling the C and N fluxes, modulating them by the plant
and soil water status and by other features of the
environment. Linked soil, plant and animal submodels
represent the flows of C and N, a water submodel calculates
the flow and status of water in the system, and an
environment and management module provides the driving
variables.
Several other process-based grassland models have been
reported [e.g. ELM (Innis, 1978), CENTURY (Parton et
al., 1987, 1996), ERHYM-II (White, 1987), GEM (Hunt et
al., 1991), CCGRASS (Verberne, 1992) and GEMT (Chen
and Coughenour, 1994)]. The Hurley Pasture Model is
distinctive in its comprehensive treatment of over 50
ecosystem processes, coupling of C, N and water fluxes in
both plants and soil, explicit representation of assimilate
transport and utilization, dynamic involvement of soil
biomass in soil processes, modulation of all soil and plant
biochemical processes by temperature and water status, and
the inclusion of grazing animals. The model is complex
because it includes many processes, none of which we regard
as redundant to its purpose. To minimize complexity and
achieve balance, all equations are as simple as possible—
capturing only the essential responses and interactions—
often more simply than in specialist models (e.g. of
photosynthesis, stomatal conductance, light interception,
soil hydrology and animal behaviour).
Individual processes were modelled to respond to environmental and internal driving variables according to
observations (e.g. photosynthetic responses). Processes were
then assembled into submodels, and submodels into the
whole model, at each stage ensuring that the simulations
matched observed diurnal, seasonal and long term values
and trends in the main measured variables in UK grasslands
(LAI, NPP, root-shoot partitioning, soil N fluxes, C}N
ratios, soil biomass, organic matter etc. ; Whitehead, 1995).
Any discrepancies were remedied by re-examining the
assumptions and altering the ways in which processes were
represented and parameterized, until the dynamics of the
system were stable and precisely determined by the
underlying equations. During this process, the structure of
the soil submodel was substantially modified from an earlier
version (Thornley and Verberne, 1989) in order to simulate
observed long term trends in soil organic matter at
Rothamsted (Arah et al., 1997). However, the model was
not constrained to fit any particular dataset, but was instead
parameterized to give predictions that were qualitatively as
observed and quantitatively within measured ranges, including responses to cutting, grazing, fertilizer application
and changes in weather within a temperate climate
(Whitehead, 1995 ; Thornley, 1998).
It was assumed that the grass was responsive to elevated
[CO ], with no downregulation, as observed for L. perenne
#
and some other grass species (Lutze and Gifford, 1995 ;
T     1. Baseline climates and management conditions used for a lowland site in southern Britain (Hurley) and an upland
site in northern Britain (Eskdalemuir)
Latitude (°N) and [elevation (m)]
N input (kg ha−" yr−")
Windspeed at 2 m height (m s−")
Relative humidity :
Mean a.m.
Annual variation in a.m.
Mean p.m.
Annual variation in p.m.
PAR (MJ m−# d−")
Mean
Annual variation
Air temperature (°C)
Mean max.
Annual variation in max.
Mean min.
Annual variation in min.
Soil temperature (°C)
Mean
Annual variation
Rainfall (m yr−")
Mean
Annual variation
Grazing (sheep ha−"), continuous
Hurley, England
(southern, lowland)
Eskdalemuir, Scotland
(northern, upland)
51±54° [50]
50
1
55±27° [242]
20
2
0±80
0±1
0±7
0±1
0±84
0±07
0±75
0±13
4±8
4±1
3±8
3±1
14.0
7±0
6±5
5±5
11.0
7±0
4±0
5±0
10±0
6±0
8±5
6±0
0±66
0±22
10
1±53
0±51
5
207
Thornley and Cannell—Grassland Responses to Climate Change
RESPONSES TO STEP CHANGES IN
T E M P E R A T U R E A N D [C O ] C O M P A R E D
#
WITH EQUILIBRIUM RESPONSES
Responses to step changes in climate were best revealed
when simulating a grassland in the southern, lowland
climate, without grazing. Qualitatively similar changes in
underlying ecosystem properties were found in the northern,
upland climate and with grazing, but with different effects
on NPP (see Fig. 3, discussed below).
Figure 1 shows the modelled responses of critical
ecosystem properties to a step-wise increase of 5 °C in
temperature and a doubling of [CO ] from 350 to
#
kg C m–2
kg C m–2
per year
m2 m–2
Annual mean air
and soil temperatures
increased by 5°C
Atmospheric CO2
increased from
350 to 700 µmol/mol
Initial
equil.
Initial
equil.
0
Long-term
equilibrium
A
Total carbon
(Csys)
B
Soil resp.
10
Long-term
equilibrium
H Csys
I
NPP
1.5
1.0
0.5
g N m–2
The model simulated grasslands continuously grazed by
sheep in climates approximating the 30 year average
meteorological conditions at Hurley in southern, lowland
Britain and at Eskdalemuir in northern, upland Britain
(Table 1). The soil was assumed to be a loam, with 10 %
clay, having a soil water content of 40 % at field capacity.
The magnitude of the soil pools at the start of each model
run was that obtained at equilibrium in the climate.
Daylength was calculated from latitude and time of year.
Windspeed and daily N input from the atmosphere and were
assumed to be constant. No fertilizers were applied. Other
quantities were varied sinusoidally throughout the year :
means and annual variations are given in Table 1.
Photosynthetically active radiation (PAR) was maximal on
21 June, temperatures on 26 July and daily rainfall on 20
November. Daily variation in PAR was assumed to be
sinusoidal between dawn and dusk, and in air temperature
between dawn and 1500 h (France and Thornley, 1984). Soil
temperatures were diurnally constant. Relative humidities
varied seasonally and diurnally, with maxima on 30
December and at dawn.
Climate treatments were of three kinds : (1) the model was
run to equilibrium (when total annual C and N outputs
equalled inputs) and was then subjected to a step change in
temperature (5 °C—representing an extreme, in order to
demonstrate the response) or [CO ] (350 to 700 µmol mol−") ;
#
(2) the model was run to equilibrium after changing the
baseline climate in various ways ; and (3) the model was run
to equilibrium in the baseline climate and then [CO ] and
#
temperatures were increased gradually to approximate the
IPCC 1992 a climate change scenarios (Houghton et al.,
1996).
g N m–2
per year
Baseline climates and treatments
20
8
Soil resp.
NPP
C
LAI
J
LAI
Soil
mineral
N
K
Soil
mineral N
L
N leaching
M
S/R ratio
N
System C/N
4
0
6 D
4
2
0
E
8
N leaching
2
1
4
0
kg C kg–1 N kg kg–1
Casella, Soussana and Loiseau, 1996 ; Jones et al., 1996),
unlike alpine and Mediterranean grasses according to
Korner and Miglietta (1994) and Shappi and Korner (1996).
The commonly observed responses of plants to elevated
[CO ], such as decreased stomatal conductance, increased
#
photosynthetic temperature optimum, increased leaf thickness, increased tissue C}N ratio and increased partitioning
to roots in N limiting conditions, are all simulated in the
model (Baxter et al., 1994 ; Idso and Idso, 1994 ; Luo, Field
and Mooney, 1994 ; Lutze and Gifford, 1995 ; Newton et al.,
1995, 1996 ; Wolfenden and Diggle, 1995).
F
S/R ratio
G
System C/N
0
8
4
0
14
12
0
0 10 20 30
Years
0 10 20 30
Years
F. 1. Modelled responses of an ungrazed grassland in southern,
lowland Britain to step-wise changes in temperature and [CO ]. Left,
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air and soil temperatures are increased by 5 °C. Right, [CO ] is doubled
#
−
from 350 to 700 µmol mol ". The starting values are for the steady state
(equilibrium) in the baseline climate (Table 1) and the long-term
equilibrium values are shown in the changed climate. NPP is the
instantaneous net primary productivity above and belowground
accumulated over a year. The other variables are annual averages : total
carbon in the soil-plant system (over 95 % in the soil) Csys ; soil
respiration rate ; leaf area index (LAI) ; amount of soil mineral
nitrogen ; amount of soil nitrogen leached ; shoot}root ratio ; total
system carbon}nitrogen ratio (dominated by the soil).
700 µmol mol−". The model began with the system at
equilibrium in the baseline climate (Table 1) and was run to
equilibrium in the changed climate (Fig. 1). Table 2
summarizes the transient and persistent (equilibrium)
changes in ecosystem properties and Fig. 2 shows, diagrammatically, the sign of equilibrium changes in the main
processes which altered NPP and Csys.
Temperature responses
An increase in temperature affected almost all processes
in the model, but its most powerful effects were (1) to
208
Thornley and Cannell—Grassland Responses to Climate Change
T     2. Contrasting short and long-term responses of grassland ecosystem properties to increases in temperature and
atmospheric CO
#
Immediate response to step-wise
increase in temperature
Increase
Long-term equilibrium
response
N leaching
System C}N ratio
Decrease
Soil mineral N
Shoot}root ratio
Soil respiration
Shoot}root ratio
LAI
Total system C
NPP
Temperature ↑
Leaf longevity ↓
LAI ↓
Drainage ↑
N leaching ↑
(Volatilized N ↑)
Photosynthesis, respiration Soil organic matter ↓
and growth ↑
NPP
(see Fig.3)
System carbon ↓
Soil mineral N ↓
System C/N ratio ↓
CO2 ↑
Stomatal
conductance ↓
Drainage ↓
N leaching ↓
LAI ↑
Photosynthesis ↑
Evapotranspiration ↑
or unchanged
Soil mineral N
concentration
unchanged
NPP ↑
Plant biomass
and litter C/N ↑
Soil organic
matter ↑
(Nitrification
N loss ↑)
System carbon ↑
#
Decrease
Increase
No change
Immediate response to step-wise
increase in CO
System C/N ↑
F. 2. Directional changes, at equilibrium, in grassland ecosystem
properties in response to increases in temperature and [CO ]. These
#
changes occurred in both baseline climates, with and without grazing
(the southern climate without grazing is shown in Fig. 1), but changes
in net annual primary productivity (NPP) in response to temperature
depended on the temperature range, climate and grazing, as shown in
Fig. 3.
increase the rate of leaf senescence and hence decrease
leaf longevity, and (2) to increase soil respiration}
decomposition}mineralization rates. In the model, these
two primary effects had a number of interrelated short and
long-term consequences (Figs 1 and 2) : (a) At equilibrium,
Increase
Decrease
NPP, LAI, System C}N
Soil respiration
Total system C
—
—
—
Soil mineral N
Shoot}root ratio
N leaching
NPP is the sum of the above and belowground litter inputs
to the soil (plus animal intake in grazed systems) and is
exactly balanced by soil respiration. In the model, a step
increase in temperature always increased soil respiration
more than NPP, even at high LAI. The result was a net loss
of soil carbon, which occurred rapidly for a few years and
then progressed slowly until a new lower equilibrium total
system carbon level (Csys) was reached, when soil respiration
was once again in balance with NPP (Fig. 1 A and B). The
difference in temperature response of soil respiration and
NPP occurred although the rates of all plant and soil
biochemical processes (other than quantum efficiency of
photosynthesis) were modified by the same cubic temperature function (see Appendix). This difference is critical
in determining the magnitude of the carbon sink (Rotmans
and den Elzen, 1993 ; Kirschbaum, 1995). (b) Decreased leaf
longevity caused an immediate and sustained decrease in
LAI (Fig. 1 C). When LAI fell below about 5 (which would
be the equilibrium state at warmer or drier sites, or of
grazed swards) both NPP and evapotranspiration were
decreased. (c) In the short term, faster mineralization
produced large amounts of soil mineral N, substantial
amounts of which were leached, partly because the drop in
LAI resulted in decreased evapotranspiration and increased
drainage (Fig. 1 D and E). (d ) In the longer term (at
equilibrium), higher temperatures decreased the amount of
soil mineral N. Although mineralization occurred faster at
higher temperatures, the total soil organic matter pool was
smaller, more N was continually being lost by leaching, and
higher temperatures increased N losses by ammonium
volatilization (Figs 1 D and 2). (e) In the short term, high
soil mineral N levels resulted in increased assimilate
partitioning to shoots and an increase in shoot}root ratio,
but in the long term this ratio decreased or changed
relatively little (Fig. 1). ( f ) In the short term, increased
mineralization caused N enrichment and a fall in the system
C}N ratio, but in the long term the depletion of N by
leaching and volatilization resulted in an increase in the
system (mainly soil) C}N ratio (Fig. 1 G).
CO responses
#
In the model, the only effects of increasing [CO ] were to
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(1) increase photosynthesis, and (2) decrease stomatal
conductance. However, as for temperature, there were
Thornley and Cannell—Grassland Responses to Climate Change
profound consequent changes in N relations and other
ecosystem properties (Fig. 2). Figure 1 shows that most
changes were the opposite of those caused by an increase in
temperature : (a) A step-wise increase in photosynthesis
immediately increased NPP and litter input to the soil,
followed by an increase in soil respiration (Fig. 1 I). This lag
between increased NPP and soil respiration resulted in an
increase in soil carbon (including soil biomass) and hence in
Csys towards a new equilibrium value (Fig. 1 H). This lag
makes it inevitable that continually increasing [CO ] will
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always be accompanied by an increase in ecosystem carbon
storage, that is, in a carbon sink (Lloyd and Farquhar,
1996). (b) In the model, increased [CO ] and photosynthesis
#
also increased LAI (Fig. 1 J). (In grazed swards, elevated
CO can decrease LAI in the short term, especially in N#
poor conditions, owing to increased allocation to roots, but
at equilibrium LAI is generally increased.) The increased
LAI further increased NPP in swards with LAIs less than
about 5 (see further discussion below). (c) Although stomatal
conductances decreased in elevated [CO ], evapotrans#
piration increased because the increase in LAI caused an
increase in total canopy conductance. (d ) In the short term,
an increase in carbon substrate in the plants resulted in
increased allocation to roots (Fig. 1 M), greater N uptake
and depletion of the soil mineral N pools (Fig. 1 K). Less N
was leached (Fig. 1 L), partly because of this depletion and
partly because greater evapotranspiration resulted in less
drainage (the opposite of what occurred in response to
increased temperature). (e) In the longer term, both soil
mineral N levels (total amount of mineral N m−#) and
shoot}root ratios recovered towards a level similar to
their original equilibrium values (Fig. 1 K and M). That
is, a balance was reestablished between processes which
increased mineral N levels (e.g. more soil organic matter for
mineralization and less drainage loss) and those which
decreased them (e.g. greater plant uptake and greater
nitrification losses because of greater soil biomass). ( f )
Elevated CO increased plant and litter C}N ratios (which
#
contributed to the lag in soil respiration) and increased the
system C}N ratio to a new equilibrium value (Fig. 1 N).
Interim conclusions
Three points should be noted at this stage. First, the
immediate changes in ecosystem properties following stepwise increases in temperature and CO were not only
#
different in magnitude to equilibrium changes, but were also
sometimes different in sign. This was particularly true for
temperature and for variables such as soil mineral N and
shoot}root ratio (see Table 2). Secondly, the model predicted
that a rise in temperature increased total soil respiration
more than NPP, producing a net carbon source, whereas a
rise in [CO ] increased NPP more than soil respiration,
#
producing a net carbon sink. Thirdly, effects of temperature
and [CO ] on leaf area index were potentially critical in
#
determining grassland responses to elevated temperature
and [CO ] (see Fig. 2). Decreasing LAI in response to
#
temperature had the potential to lower NPP, because less
light was intercepted and more N was leached (owing to
lowered evapotranspiration, increased drainage and faster
209
mineralization). By contrast, increasing LAI in response to
elevated [CO ] had the potential to increase NPP, because
#
more light was intercepted and less N was leached—
remembering that canopy evapotranspiration could be
increased despite lowered stomatal conductance. The importance of changes in LAI indicated the potential importance of grazing in determining responses to climate
change.
EQUILIBRIUM RESPONSES OF NPP AND
C A R B O N S T O R A G E (Csys) T O
T E M P E R A T U R E A N D [CO ] : S I T E
#
DIFFERENCES AND EFFECTS OF
GRAZING
Clearly, the magnitude of the long-term equilibrium values
to which grasslands tend, in response to slowly changing
climate, depend on the magnitude of the increases in
temperature and [CO ]. Figure 3 A shows the equilibrium
#
values of NPP, LAI and Csys predicted in response to
temperature, varying 5 °C above and below the baseline
annual mean air temperatures (shown with arrows ; with
equivalent changes in soil temperatures) at both sites, with
and without grazing. Figure 3 B shows the equilibrium
values in response to [CO ] varying from 350 to
#
800 µmol mol−".
Ungrazed swards
In the absence of grazing, in the baseline climates, the
model predicted equilibrium LAIs of 6 in the southern
climate and over 10 in the northern climate. (The model did
not simulate flowering, and so allowed ungrazed swards to
reach higher LAIs than would occur in nature.) LAIs of 5
are sufficient to intercept most solar radiation. LAIs fell
below this value in ungrazed swards only when the southern
climate was warmed to mean air temperatures above 12 °C
(Fig. 3 A). Thus, with the latter exception, changes to NPP
in ungrazed swards in response to temperature and [CO ]
#
were not very sensitive to changes in LAI. They occurred
primarily in response to temperature—through effects on
photosynthesis, respiration, growth, soil N and water status
(the southern climate was markedly drier)—and in response
to elevated [CO ] through effects on photosynthesis and
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stomatal conductance.
In both climates, the NPP of ungrazed swards increased
in response to warming to maxima at 12–13 °C. Over the
range 5–12 °C, NPPs were higher in the upland climate, and
increased more in response to temperature over this range
(38 % compared with 20 %, Fig. 3 A), because stomatal
closure occurred less in the upland climate as a result of
higher rainfall and humidity. The decrease in Csys with
temperature, over the range 5–12 °C, was correspondingly
greater in the lowland climate (2±0 compared with
1±5 kg C m−# loss per °C).
Without grazing, NPP increased less in response to a
doubling of [CO ] in the upland climate than in the lowland
#
climate (34 % compared with 45 %, Fig. 3 B) because of the
greater response of photosynthesis to elevated [CO ] at
#
higher temperatures. Csys increased correspondingly less in
210
Thornley and Cannell—Grassland Responses to Climate Change
Without grazing
Northern, upland
2.0 30
NPP
20
Without grazing
20
1.0
1.0
10
Csys
>10
2.0
10
>10
LAI
With grazing
With grazing
1.0 20
20
NPP
10
1.0
0.5 10
Csys
0.5
LAI
0
5
10 15 20
Temperature (°C)
0
5
10
15
Temperature (°C)
Southern, lowland
Leaf area index (LAI); System carbon (Csys) (kg m–2)
Leaf area index (LAI); System carbon (Csys) (kg m–2)
Southern, lowland
30
30
Without grazing
NPP
Northern, upland
2.0 30
Without grazing
1.5
20
1.5
20
Csys
1.0
10
LAI
1.0
10
LAI > 10
0.5
With grazing
20
1.5
NPP
0.5
With grazing
20
1.0
10
2.0
Csys
1.5
1.0
10
0.5
0.5
LAI
0
400
600
800
[CO2] µmol mol–1
0.0
0
400
600
800
–1
[CO2] µmol mol
0.0
Net annual primary productivity (NPP) (kg C m–2 per year)
B
Net annual primary productivity (NPP) (kg C m–2 per year)
A
F. 3. Modelled equilibrium responses of British grasslands to changes in temperature (A) and [CO ] (B), with and without grazing.
#
Temperatures were varied equally for the soil and air ; those shown are annual mean air temperatures and arrows mark the baseline values (Table 1).
The quantities shown are annual net primary productivity (NPP), total system carbon (Csys ) and leaf area index (LAI).
Grazing
Part of plant N
returned to soil
in urine and faeces
LAI ↓
Part of NPP eaten
and respired
Evapotranspiration ↓
NPP ↓ System carbon ↓
Drainage ↑
N leaching ↑
N volatilization ↑
System C/N ratio ↓
F. 4. Directional changes in grassland ecosystem properties in
response to grazing.
the upland climate (21 % compared with 42 % in the
lowland climate).
In short, the NPP of ungrazed grasslands in the cool, wet,
northern climate responded most to temperature (reducing
the loss of carbon relative to that at the southern site) and
least to elevated [CO ] (increasing the gain in carbon relative
#
to that at the southern site).
Grazed swards
As expected, grazing per se (in the absence of climate
change) had major effects on the ecosystem, principally by
(1) decreasing LAI (grazing was a function of LAI,
independent of temperature) ; (2) consuming part of the
plant NPP and losing it in animal respiration rather than
transferring it to the soil ; and (3) returning most of the
consumed N as urine and faeces (see Fig. 4). The consequent
effects on the ecosystem were to lower NPP substantially
and to increase N losses by leaching (because of greater
drainage) and volatilization (from animal excreta) so that,
at equilibrium, Csys and system C}N ratios were lower in
grazed than in ungrazed swards (Fig. 4).
Figure 3 A shows that, with grazing, NPPs were generally
much lower than without grazing, but increased at low
temperatures up to 5–7 °C, because of increased rates of
photosynthesis and growth (as in ungrazed swards). But
above 5–7 °C, the NPP of grazed swards decreased because
the decline in LAI (resulting from shorter leaf longevity)
outweighed the positive effects of temperature on photosynthesis and growth. Consequently, NPP was 25 % lower
in the warm, southern baseline climate than in the northern
baseline climate (marked with arrows in Fig. 3), and, in
contrast to the ungrazed swards, any increase in temperature
above the baseline values decreased NPP in both climates
(by about 0±06 kg C m−# per year per °C). As expected, (1)
the grazed swards contained markedly less carbon than the
ungrazed ones (Csys) ; (2) the upland grazed grassland
contained much more carbon than the lowland one ; and (3)
both suffered a dramatic decrease in Csys with increase in
mean annual air temperature above 5 °C (over 1 kg C m−#
decrease per °C in the upland climate).
Figure 3 B shows that, in grazed swards, increases in NPP,
LAI and Csys in response to elevated [CO ] were greater at
#
the upland than the lowland site. (Whereas, without grazing,
NPP, LAI and Csys increased most at the lowland site.) This
occurred because the increase in LAI in response to elevated
[CO ] was greater at cool temperatures, and increases in
#
LAI above the low values in grazed swards increased light
interception, NPP and Csys. (By contrast, without grazing,
increases in LAI had little effect on NPP and Csys.)
Thornley and Cannell—Grassland Responses to Climate Change
EQUILIBRIUM RESPONSES TO A RANGE
OF CLIMATE AND MANAGEMENT
VARIABLES WITH GRAZING
Figure 5 gives the equilibrium values of NPP, LAI, Csys and
annual animal intake (Ci,an) in the baseline climates (row 6)
and shows the effects on equilibrium values of changes in a
number of climate and management variables. The rows are
ordered according to equilibrium Csys in the southern
climate. In all conditions, the northern, upland grassland
was more productive in NPP (though not in Ci,an) owing
mainly to larger LAIs at lower temperatures (and with only
five sheep ha−" compared with ten sheep ha−" at the southern
site), despite lower N inputs and solar radiation levels
(Table 1).
Increasing rainfall and windspeed (affecting the water
balance) had little effect on the grasslands—a 30 % increase
in summer rainfall in the southern climate did not prevent
water stress. But it is interesting that decreasing rainfall had
a deleterious effect at the southern site (because of increased
water stress) but a beneficial effect at the northern site
(because of decreased N leaching). Increasing water stress
by decreasing relative humidity by 10 % decreased productivity at both sites, and disabling the model to prevent N
leaching increased productivity at both sites.
The treatments which decreased Csys most were increases
in temperature, because of decreased LAI, NPP and
accelerated soil organic matter decomposition, and animal
grazing, because of severely reduced LAI and NPP. The
treatments which increased Csys most were increases in CO ,
#
owing to greatly increased NPP (without a proportionate
increase in LAI) and N inputs, especially in the growing
season, owing to large increases in LAI and NPP.
Southern, lowland
PRODUCTIVITY AND CARBON STORAGE
IN RESPONSE TO GRADUALLY
I N C R E A S I N G [C O ] A N D T E M P E R A T U R E
#
The upper half of Fig. 6 shows the simple linear increases in
[CO ] and mean annual air temperature which were imposed
#
on grazed swards in the lowland and upland climates for 50
years, after bringing the ecosystems to equilibrium in the
baseline climates at time zero.
As mentioned, at equilibrium, the NPP of the upland
grassland was greater than that of the lowland grassland
(Fig. 3 A), so the two ecosystems had different starting
values in Fig. 6. A linear increase in [CO ] increased NPP at
#
both sites, but, as explained above (Fig. 3 B) the upland
grassland responded most. Also, at the upland site, a linear
increase in temperature (above the annual mean of 7±5 °C)
slightly increased NPP, whereas at the lowland site an
increase in temperature (above the annual mean of 10±25 °C)
slightly decreased NPP (Fig. 6). (Note that, in the long term,
at equilibrium, an increase in temperature decreased NPP at
the upland site.)
The net effect was that, at the upland site, increasing
[CO ] and temperature together increased NPP more than
#
increasing [CO ] alone, and the absolute rate of increase in
#
NPP in response to climate change was greater at the upland
site. Also, decreasing the rate of temperature rise from 0±03
to 0±02 °C yr−" decreased the overall response of the upland
grassland but had a small beneficial effect on the lowland
grassland (Fig. 6).
The effect of gradual climate change on changes in total
carbon storage (Csys) is shown in Fig. 7. As expected, rising
temperature caused a loss of carbon, at an ever-increasing
rate with increase in temperature, whereas elevating [CO ]
#
Northern, upland
Row
Csys LAI NPP
Ci,an
Csys
LAI NPP
1
3.14 0.77 0.46 0.127
6.24
1.38 0.67 0.120
Temperature, +2°C
2
3.26 0.60 0.38 0.118
5.99
0.92 0.47 0.117
Animal stocking, +50%
3
4.20 0.88 0.52 0.153
8.25
2.33 0.76 0.155
Relative humidity, –0.1
4
4.41 0.91 0.55 0.159
8.46
2.51 0.79 0.160
Rainfall –30%
5
4.48 0.93 0.57 0.164
8.22
2.28 0.75 0.152
Rainfall +30%
6
4.51 0.94 0.57 0.165
8.36
2.42 0.77 0.157
Standard conditions
7
4.63 0.95 0.59 0.168
8.27
2.31 0.76 0.154
Windspeed × 2
8
4.97 1.08 0.65 0.187
9.04
3.10 0.87 0.178
No leaching
9
5.93 1.24 0.80 0.203
9.92
3.43 1.00 0.184
Radiation (PAR), +20%
10
7.94 2.79 1.10 0.298
13.58
9.95 1.35 0.196
+300 kg N ha–1 year–1, continuous
11
8.14 3.00 1.13 0.306
13.69 10.18 1.36 0.196
3 * 100 kg N ha–1, 2 Mar, 1 May, 30 Jun
12
9.14 1.79 1.28 0.241
14.33
CO2, 700 µmol mol–1
0 5 10 15
Sequestered C
(kg C m–2)
211
Ci,an
8.15 1.53 0.199
Environment and management
0 5 10 15
Sequestered C
(kg C m–2)
F. 5. Modelled equilibrium changes in the amount of carbon sequestered in the system (Csys, the histograms) and some other properties of
grazed British grasslands in response to changes in environmental and management variables relative to the baseline conditions in Table 1. The
other properties are leaf area index (LAI), net annual primary production (NPP, kg C m−# per year), and the annual carbon input to grazing
animals (Ci,an, kg C m−# per year).
212
Thornley and Cannell—Grassland Responses to Climate Change
r
C pe
0.03°
year
0.02°C per
450
11
10
CO2
400
CO2
C
0.03°
9
ear
per y
8
ear
per y
0.02°C
350
10
20
30
40
50
0
10
20
30
40
50
7
1.1
1.1
CO2 and temp
1.0
1.0
0.9
0.9
CO2
0.8
0.8
CO2
Temp
0.7
CO2 and temp
0.7
0.6
Temp
0.6
0.5
0
10
20
30
Years
40
50
0
10
20
30
Years
40
50
0.5
Mean annual air temperature
(°C)
12
year
0
Net annual primary productivity
(kg C m–2 per year)
Northern, upland
Net annual primary productivity
(kg C m–2 per year)
Atmospheric [CO2] µmol mol–1
Southern, lowland
500
F. 6. Modelled changes in annual net primary productivity of grazed grasslands in lowland and upland Britain in response to increasing
temperature (0±03 and 0±02 °C yr−") and CO (3 µmol mol−" yr-"). The systems were at equilibrium at year zero in the baseline climates defined
#
in Table 1.
System carbon gain/loss
(g C m–2 per year)
Southern, lowland
Sink
20
CO2
Northern, upland
20
CO2
10
CO2
and 0.02°C
per year
CO2 and 0.02°C
per year
10
CO2 and 0.03°C
per year
0
CO2 and 0.03°C
per year
0
0.02°C per year
0.02°C per year
–10
Source 0.03°C per year
0
10 20 30 40 50
Years
–10
0.03°C per year
0
10 20 30 40 50
Years
F. 7. Modelled annual changes in the amount of carbon in lowland
and upland British grasslands (plants and soil) in response to increasing
temperature (0±03 and 0±02 °C yr−") and CO (3 µmol mol−" yr−"). The
#
systems were at equilibrium at year zero in the baseline climates defined
in Table 1.
caused an accumulation of carbon towards an asymptote at
high [CO ]. The net effect of increasing [CO ] and
#
#
temperature together was to produce a carbon sink of
around 10 g C m−# yr−", which was greater with slow rather
than rapid warming, and which decreased with time as the
response to [CO ] levelled off. Differences in the responses
#
of the two sites can be explained in the same way as the
equilibrium responses discussed above.
CONCLUSIONS AND DISCUSSION
This analysis advances discussion on two aspects of the
impacts of climate change on grasslands, namely on the
interpretation of experiments at the ecosystem level,
especially on temperature effects, and the role of temperate
grasslands as carbon sinks in response to increasing
temperature and [CO ].
#
Interpretation of experiments
Although experiments on elevated [CO ] and temperature
#
effects on photosynthesis and other individual processes
may give interpretable results, it is more difficult to determine
responses at the ecosystem level by experimentation because :
(1) short-term experiments may observe misleading transient
responses ; (2) it is difficult to simulate animal grazing ; and
(3) effects on NPP can be highly specific to the conditions of
the experiment. These three points are elaborated below.
Initial ecosystem responses to step changes in temperature
and [CO ] can be Šery different in magnitude, and eŠen be
#
different in sign, to equilibrium responses or responses to
gradually changing climate (Fig. 1 and Table 2). Our model,
and all others which couple vegetation and soil processes
(e.g. Hunt et al., 1991 ; Friend et al., 1996 ; McGuire,
Kicklighter and Melillo, 1996 ; Parton et al., 1996 ; Thornley
and Cannell, 1996) show that ecosystem responses to most
Thornley and Cannell—Grassland Responses to Climate Change
aspects of climate change are highly dependent upon soil
processes which control the supply of mineral nutrients.
These processes take decades to reach near-equilibrium.
Consequently, experiments which impose sudden changes in
temperature or [CO ] on an ecosystem in the field or on
#
monoliths brought into the laboratory, and which last only
a few years, will rarely produce quantitative changes in
NPP, Csys or other ecosystem properties which indicate
likely long-term trends in response to gradual climate
change (see also Rastetter and Shaver, 1992). In our view,
this fact makes it practically impossible to quantify realworld responses of NPP and Csys to climate change other
than by modelling.
Grazed swards react Šery differently from ungrazed swards
to changes in temperature and [CO ], because the productiŠity
#
of grazed swards responds to changes in LAI as well as direct
effects on other processes (Figs 3 and 4). Some experiments
on grasslands simulate mowing (Newton et al., 1995, 1996 ;
Jones et al., 1996) but few experiments or models simulate
grazing (Ojima et al., 1996). Almost all grasslands in the
world are subject to grazing, which has profound effects on
ecosystem properties—lowering LAI, NPP, Csys and system
C}N ratio (Fig. 4). Most importantly, light interception is
often incomplete, so that grazed swards are responsive to
anything that changes LAI—NPP being roughly proportional to intercepted photosynthetically active radiation
at low LAIs (Monteith, 1977). If it is recognized that leaf
longevity is decreased by warming, and assumed that
grazing is independent of temperature, then increasing
temperatures above about 5 °C can lower the LAI of
temperate, grazed swards to such an extent that NPP is
actually decreased—despite accelerated photosynthesis, nutrient release by mineralization and growth (Fig. 3 A).
The response of ungrazed swards to temperature is quite
different because of their large LAIs (see Fig. 3 A). For the
same reason, ungrazed swards respond less to nitrogen
addition (Fig. 5 ; Van den Pol van Dasselaar and Lantinga,
1995).
Effects of climate change on grasslands are likely to be
highly specific to particular climatic conditions, intensity of
grazing and nutrient supply. The SCOPE study, reported by
Parton et al. (1996), predicted very different responses of
grassland to increasing [CO ] and changes in temperature
#
and rainfall at the biome level. Elevated [CO ] generally
#
increased NPP and soil organic matter levels (approx.
Csys)—but to different extents in different biomes—whereas
changes in temperature and rainfall had dramatically
different effects depending on the current climate and
feedbacks involving, primarily, nutrient mineralization and
water use.
This analysis goes a step further in showing that responses
of grasslands within a biome are likely to be very variable,
depending on the current climate and the intensity of
grazing. Again, the main sources of variation were responses
to temperature and rainfall, rather than to [CO ]. Rising
#
temperatures could increase or decrease NPP, depending on
the current climate (cf the two sites in Fig. 6) and the level
of grazing (Fig. 3). The optimum temperature for NPP was
12–13 °C without grazing, but only 5–7 °C with the levels of
grazing chosen in this study (Fig. 3). Increasing rainfall
213
increased NPP where it relieved summer droughts, but
decreased NPP where it increased N leaching (Fig. 5).
Similarly, amounts of N applied to grasslands can alter
climate change responses (Thornley, 1998). Clearly, differing
responses of NPP to climate change mean different amounts
of litter input to the soil and different effects on Csys and the
operation of grasslands as carbon sinks. It is not surprising
that experiments give a variety of responses, some of which
may be correctly attributed to species, but others are likely
to be explained by differences in environmental and
management conditions (e.g. Owensby et al., 1993 ; Lutze
and Gifford, 1995 ; Newton et al., 1995 ; Casella et al., 1996 ;
Jones et al., 1996 ; Soussana, Casella and Loiseau, 1996).
Our overall conclusion from the three points made above
is that research should be focused on the development of
models which represent the essential feedback processes
that are altered by climate. Experiments should be directed
towards lessening uncertainty about the operation and
parameterization of those processes, and rather than
towards answering questions at the ecosystem level directly
(see also Rastetter, 1996).
Temperate grasslands as carbon sinks in response to
increasing temperature and [CO ]
#
One of the most important and difficult problems in
climate change research is to quantify the net effect on
carbon sequestration of the opposing effects of increasing
[CO ] and temperature. It is a difficult problem, because the
#
net flux is a small difference between two very large
numbers. Also, it is not always realized that the hypothesized
terrestrial carbon sink of 0±5–2±0 Gt C yr−" due to CO and
#
nitrogen fertilization (Mellilo et al., 1996) represents a sink
of only 5–20 g C m−# y−" if spread evenly over the vegetated
surface of the earth (8–9¬10* ha). This is a small number,
well within the error of direct estimates of CO fluxes and
#
detectable changes in carbon pools. Our analysis indicates
the rising [CO ] alone produces a sink, rising temperatures
#
alone produce a source, and the net effect is likely to be a
sink for several decades. These three points are elaborated
below.
As long as rising [CO ] continues to increase gross
#
photosynthesis and grassland NPP, then total system carbon
(Csys, mainly in soil organic matter) will increase, representing
a carbon sink, because of the time lag between increasing
NPP and increasing soil respiration. This point was
elaborated by Taylor and Lloyd (1992) and Lloyd and
Farquhar (1996). The argument is that there are time lags
between the instantaneous rise in rates of photosynthesis
with increase in [CO ], consequent increases in NPP, litterfall
#
and soil organic matter pools. In general, the total litter and
soil respiration CO fluxes are proportional to the sizes of
#
their decomposing carbon pools, so that the increase in loss
of carbon from litter and soils is not in equilibrium with the
increase in NPP. This lag between NPP and litter}soil
respiration produces a transient carbon sink.
The Hurley Pasture Model, and most other process-based
ecosystem models, simulate this effect (Fig. 1 H and I) and
make it inevitable that rising [CO ] results in rising Csys—that
#
214
Thornley and Cannell—Grassland Responses to Climate Change
is, in a carbon sink (Fig. 7, CO effect)—provided
#
photosynthesis does not totally downregulate, or become
CO saturated, and that NPP responses are not severely
#
limited by nutrient and water supplies (i.e. are not lessened
by greater nutrient and water capture and}or use efficiencies ;
Idso and Idso, 1994). Any effect of elevated [CO ] on
#
decreasing maintenance respiration rates (not represented in
our model ; Gonzales-Meler, Drake and Azcon-Bieto, 1996)
or soil respiration rates, by changing litter quality (which is
represented) will exaggerate the effect.
By contrast, increasing temperature generally decreases
Csys, although NPP may be increased or decreased (Figs 1–3).
As mentioned, ecosystem NPP can increase or decrease in
response to increasing temperature depending on the starting
temperature (the current climate), level of grazing, nutrient
input and the extent of positive and negative feedbacks on
nutrient supplies and water stress (McGuire et al., 1996 ;
Thornley and Cannell, 1996).
However, even when NPP increases, the net result is
generally a decrease in Csys—that is, a carbon source (Figs
1 A, B, 7). This means that soil respiration rates generally
increase more rapidly than rates of NPP with increase in
temperature—a fact supported by their different Q values
"!
(Kirschbaum, 1995) and by the observation that Csys tends
to decrease from the tundra to the equator (Post et al.,
1982). In this analysis, both NPP and Csys were greater at
the cool, upland site than at the warmer, lowland site (Fig.
5).
The important point is that, in the Hurley Pasture Model,
the expected result of decreasing Csys with increase in
temperature was obtained although the same temperature
response function was used for all soil and plant biochemical
processes (other than quantum efficiency of photosynthesis,
and with additional modifiers for photosynthesis ; see
Appendix). This temperature function was similar to that
used for soil decomposition in the CCGRASS and CENTURY models (Van den Pol van Dasselaar and Lantinga,
1995 ; Parton et al., 1996) and, when applied to maintenance
respiration and other plant processes, it predicted a relatively
small change in the fraction of gross photosynthate lost by
respiration over a large range of temperatures (see Thornley,
1998) as found experimentally by Gifford (1994). We
recognize that temperature response functions will differ for
plants and microbes adapted to different environments, but
we have less reason to believe that the response functions of
biochemical processes in plants and soils differ greatly
within any one environment.
The net effect of current and projected increases in [CO ]
#
and temperature (oŠer the next few decades) is likely to be an
increase in NPP and Csys in humid, temperate grassland, with
a carbon sink of 5–15 g C m−# yr−". This sink will decrease to
zero and become negatiŠe after seŠeral decades (Figs 6 and 7).
Most researchers have addressed the problem of determining
the magnitude of the carbon sink}source produced by
grasslands in response to climate change by running models
in a 2¬CO climate either to equilibrium (Thornley, Fowler
#
and Cannell, 1991) or for 50–100 years—using much simpler
vegetation models than the Hurley Pasture Model (e.g.
grassland models : Hunt et al., 1991 ; Ojima et al., 1993 ;
Parton, Ojima and Schimel, 1994 ; Hall et al., 1995 ; Van den
Pol van Dasselaar and Lantinga, 1995 ; McGuire et al.,
1996 ; Parton et al., 1996). Some models predict a net source
for most grasslands (Ojima et al., 1993 ; Hall et al., 1995 ;
Van den Pol van Dasselaar and Lantinga, 1995) others a
small sink, depending mostly on the magnitude of increased
nutrient availability in response to warming and the
respiration response (McGuire et al., 1996).
The Hurley Pasture Model predicts that the combined
rates of increase in [CO ] and temperature that are occurring
#
now, and are likely to continue for a few decades, will
produce a sink of about 5–15 g C m−# yr−" in temperate
grasslands for several decades (Fig. 7). This is the right
order of magnitude, if it occurs globally (see above).
However, it is inevitable that the positive effects of increasing
[CO ] will decrease as the photosynthetic system saturates,
#
whereas the negative effects of temperature will continue to
increase in non-tropical regions. Clearly, the timescale over
which the global terrestrial sink weakens is critical for
current negotiations on the control of greenhouse gas
emissions and requires further modelling research on realworld transient responses.
A C K N O W L E D G E M E N TS
The work has been supported by NERC through its TIGER
(Terrestrial Initiative in Global Environmental Research)
programme, the Department of the Environment Contract
No EPG1}1}3 on carbon sequestration, the European
Community EPOCH (EPOC-CT90–0022) and Espace
(EV5V-CT93–0292) grassland programmes, and the BBSRC
Institute of Grassland and Environmental Research at
Northwyke, Devon, UK.
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369–380.
APPENDIX
A synopsis of processes and properties represented in the
Hurley Pasture Model
A full description of the Hurley Pasture Model is given by
Thornley (1998) and the ACSL source program, pasture.csl,
is available by ftp from buua.nbu.ac.uk in pub}tree}Pasture,
or e-mail to johnt!buua.nbu.ac.uk.
Temperature and water modifiers of biochemical processes
Temperature. The rates of all biochemical processes in
plants and soil are modified by the same cubic temperature
function, which increases sigmoidally from zero at 0 °C to
maximal activity at 30 °C, with the inflexion at 15 °C, and
falling to zero activity at 45 °C. Exceptions are the
photosynthetic processes.
Water status. The rates of all biochemical processes in the
plant and soil are made dependent on the ‘ chemical activity ’
of water, which is derived from the tissue or soil water
potential (Maron and Prutton, 1965 ; see Thornley, 1996,
1998). Shoot water potential is used as a surrogate to derive
surface litter water activity.
Plant submodel (21 state Šariables, 60 parameters)
Species and oŠerŠiew. The grassland is assumed to be
composed entirely of a C grass such as Lolium perenne. The
$
grass plant model (Fig. A1) is driven by C input from
photosynthesis and N input from soil mineral N pools
(ammonium and nitrate). The soil submodel influences
plant growth only by altering the supply of mineral N
supply and water.
Plant state Šariables. These are leaves (lamina), leaf area,
sheath plus stem, and roots ; all have four age categories.
Carbon (C) and nitrogen (N) substrate pools in the shoot
(leaves, sheaths and stem) and root are defined separately
from the structure.
Light interception. Calculated using the Beer-Lambert
law, with light extinction coefficient 0±5 and leaf transmission
coefficient 0±1.
Photosynthesis. Uses a non-rectangular hyperbola dependence of gross photosynthesis on light, with three
parameters. The sharpness of the knee of the hyperbola is
constant at 0±95. The initial slope (α, the inverse of quantum
requirement) is set to a value equivalent to 19 quanta per
molecule of CO (Jones, 1992), with modifiers for [CO ]
#
#
(representing photorespiration suppression), temperature
217
Thornley and Cannell—Grassland Responses to Climate Change
Plant submodel
Removal of shoot structure and substrates
Harvesting
Animal grazing
Animal submodel
SHOOT
Lamina area
Lamina
Sheath + stem
Four age categories:
i = 1 → 2→ 3 → 4
Senescence
and litter
submodel
Recycling of C and N
Photosynthesis
Soil
Grazing/degradation by other fauna
Shoot C
substrate
Shoot N
substrate
Growth
Maintenance
CO2
respiration
Transport
Transport
CO2 Growth respiration
Growth
Root C
substrate
CO2
Respiration
with N
uptake
Exudation
Root N
substrate
Recycling of C and N
i = 1 → 2→ 3 → 4:
Four age categories of
root mass
Root density
ROOT
N
uptake
Soil
and
Senescence
Litter
Exudation
sub-
Grazing/degradation by other fauna
model
F. A1. Hurley Pasture Model : the plant submodel.
(decreasing α by 1±5 % per °C above 15 °C) and shoot water
status ; α is not dependent on N concentration (Evans and
Terashima, 1988). The asymptote (Pmax) has a notional
maximum at 350 µmol mol−" CO of 1¬10−' kg CO m−# s−",
#
#
equivalent to 23 µmol CO m−# s−", typical of C grasses,
$
#
with cubic modifiers for temperature and water status as
above, and modifiers for leaf N level (with minimum and
maximum set at 1 and 2 g N m−#, Hirose and Werger, 1987),
CO and stomatal conductance. The optimum temperature
#
for Pmax is [CO ] dependent (Long, 1991) although this is
#
unimportant in a temperate climate. The model gives
responses similar to the biochemical model of Evans and
Farquhar (1991, see Pachevsky, Haskett and Acock, 1996).
Values of α and Pmax are not ‘ downregulated ’ with increase
in [CO ], but leaf photosynthesis decreases as a result of
#
increased leaf thickness, C}N ratio and stomatal resistance.
Canopy photosynthesis. Calculated using a single analytical expression where the contributions of all leaves are
added by using an algebraic expression for the integral
through the canopy. This method gave similar results to two
alternative methods evaluated by Thornley (1998) who
showed that canopy photosynthesis was not appreciably
affected by decreasing foliage N concentration from the
youngest to oldest age category. Therefore uniform canopy
N concentrations were assumed (cf. Friend et al., 1996).
Stomatal conductance. Stomata are open only in the day.
Conductance varies linearly, between fully open and closed,
with shoot relative water content (i.e. water content at
prevailing water potential divided by water content when
the potential is zero). Conductance is modified by [CO ], so
#
that doubling [CO ] from 350 to 700 µmol mol−" CO
#
#
caused a 40 % reduction (Morison, 1985). There is no root
message control of stomatal conductance.
Allocation of assimilates. The transport}utilization
method of Thornley (1972) is used. Allocation depends on
the growth rates of the plant parts (see below) and the
conductances to C and N substrate transport along opposing
root-shoot concentration gradients within the plant. Transport conductances increase with temperature and water
status and are scaled with plant mass. This mechanism
simulates widely observed behaviour of shoot}root allocation related to plant C}N status, including responses of
L. perenne to [CO ] (Cannell and Dewar, 1994 ; Shenck et
#
al., 1995).
Shoot and root growth. Shoot and root growth are
proportional to their structural mass times the product of
local C and N substrate concentrations, modulated by
temperature and water status. With neither limiting, the
relative shoot growth rate is 0±2 d−" (Robson, Ryle and
Woledge, 1988). Leaf expansion is proportional to shoot
218
Thornley and Cannell—Grassland Responses to Climate Change
relative water content, as well as being affected by the
dependence of shoot growth and shoot-root partitioning on
water status.
Specific leaf area. The maximum specific leaf area is
25 m# kg−". Actual incremental specific leaf area decreases
linearly with increase in C substrate concentration (in high
light and [CO ], at low temperatures) and with increasing
#
water stress.
Respiration. Constant fractions of C substrate are used
for growth respiration, using established conversion factors
(Penning de Vries, Brunsting and van Laar, 1974). Maintenance respiration rates are proportional to structural
mass, weighted by age category, and modified by temperature and water status. N dependence operates through
the increase in C}N ratio with age category rather than as
a direct N dependence (cf. Ryan, 1995). There is also no
direct [CO ] dependence of maintenance respiration rate
#
coefficients (cf. Gonzalez-Meler et al., 1996).
Root N uptake. N uptake rate from soil ammonium and
nitrate pools has Michaelis-Menten dependence on the
mineral pool sizes [weighted towards ammonium at low
temperatures, Clarkson, Jones and Purves (1992)] and is
modified by temperature, root water status, and soil relative
water content (m$ water}m$ soil). Uptake is proportional to
root mass (weighted by age), is increased by increasing root
C substrate concentration and is decreased by increasing
root N substrate concentration.
Root exudation. Roots exude a maximum 2 % of their C
substrate and 0±5 % of their N substrate per day, modulated
by root structural mass, C and N substrate concentration,
temperature and water status.
Root density (kg m−$ soil). This modifies water uptake,
applies to new root and is dependent on soil water content,
being maximal at field capacity.
Shoot and root aging. Transfer to older age categories is
accelerated with increase in temperature and water stress.
At 20 °C, with water non-limiting, leaves have a half-life of
24 d and roots 26 d (Eason and Newman, 1990). Material
from the oldest categories is transferred to the three litter
pools (see below).
Retranslocation from senescing shoot and root. The lower
the N substrate concentration, the more N is withdrawn and
recycled before the tissue dies (Hunt, 1983). This is assumed
to occur in both shoot and root (cf. Nambiar and Fife,
1991). The fraction of litter that is recyclable (proteinaceous
material) has a C}N ratio of 2±7 kg kg−".
Soil and litter submodel (15 state Šariables, 68 parameters)
OŠerŠiew. There is only one soil horizon. The original soil
submodel of Thornley and Verberne (1989) has been
expanded to include more litter and soil organic matter
(SOM) pools, giving more realistic SOM values (Arah et al.,
1997) and making the model more akin to the CENTURY
model (Parton et al., 1987). However, unlike soil models by
Jenkinson (1990), Verberne et al. (1990) and Parton et al.
(1993) the soil biomass is actively involved in dynamic C
and N transfer processes, made possible with the representation of a small soluble C pool. The compartment
flows are shown in Fig. A2 ; individual pool inputs and
outputs are described in equations given by Thornley
(1998).
Soil type and clay content. The soil is a loam with 10 %
clay. The clay content regulates the relative fractions of
dying biomass carbon and decomposing lignin entering the
unprotected and protected SOM pools. Also, the C}N ratio
of the stabilized SOM pool depends on clay content.
Litter pools. Shoot litter and faeces enter surface litter
pools ; root litter enters soil litter pools. Both surface and
soil litter pools are divided into metabolizable (met),
cellulose (cel) and lignin (lig) subpools. The percentages of
litter entering each subpool are 20, 65 and 15 % to each
pool, respectively. The C}N ratio of the met. subpool is
variable, depending on the litter ; the C}N ratios of the cel.
and lig. subpools are 150 and 100, respectively. The rates of
decomposition at 20 °C are : met. 0±2 d−", cel. 0±1 d−" and lig.
0±02 d−", modified by temperature, water status and (for the
met. subpool) C}N ratio, as in the CENTURY model. The
temperature of the surface litter pools is the mean between
soil and air temperature. Outputs are shown in Fig. A2.
Soil organic matter (SOM) pools. There are unprotected,
protected (by clay particles) and stabilized SOM pools. All
have varying C}N ratios because the C}N ratios of their
inputs vary, depending on the size of the soil mineral N
pools (ammonium and nitrate). Inputs and outputs are
shown in Fig. A2. Decomposition rates depend on soil
temperature and water status. At 20 °C, and with ample
water, the half lives of the pools are : unprotected 1 year,
protected 12 years and stabilized 100 years. Stabilization
transfers occur from unprotected to protected to stabilized
SOM at rates modified by soil temperature and water status.
Mineral N pools. The ammonium mineral N pool receives
N from atmospheric deposition, any N fertilizers applied,
non-symbiotic N fixation, animal urine, root exudates and
dying soil biomass, as well as from mineralizing litter and
SOM (Fig. A2). Ammonium N can be taken up by plants,
nitrified to nitrate, volatilized, immobilized or used in soil
biomass growth. The nitrate N pool receives N from the
ammonium pool, atmospheric deposition and fertilizers and
loses N by plant uptake, immobilization, leaching and
denitrification.
Soil biomass. This consists of microbes, invertebrates and
all other soil organisms. The biomass has a constant C}N
ratio of 8. It grows, using substrates from the soluble C pool
and two mineral N pools, at a semi-exponential rate,
modified by soil temperature and water status, to a
maximum biomass proportional to the size of the three
SOM pools (Brookes et al., 1985). Predicted growth rates
are similar to those observed (Lundgren and Soderstrom,
1983). Death rates are also dependent on soil temperature
and water status.
Soluble C pool. This small pool, which derives C from
root exudates, dying soil biomass and mineralizing litter and
SOM pools, provides the C substrate for soil biomass
growth, and can be depleted by leaching and respiration.
Mineralization}immobilization. The definitions of
mineralization and immobilization are always somewhat
arbitrary. Here, mineralization is defined as N release from
the three decomposing SOM pools. Immobilization is the N
219
Thornley and Cannell—Grassland Responses to Climate Change
Soil and litter submodel
Shoot litter
Surface
litter
Animal faeces
metabolic
cellulose
C, N variable C/N = 150
lignin
C/N = 100
CO2
Unprotected
SOM
C, N variable
CO2
Root litter
Surface
litter
metabolic
cellulose
C, N variable C/N = 150
C/N = 100
CO2
Protected
SOM
C, N variable
CO2
Soluble C
Leaching
Volatilization
CO2
CO2
Animal urine
Death
Soil biomass
Ammonium N
Growth
Fertilizer, root exudate
Atmospheric deposition
Nitrogen fixation
N
lignin
Root exudate
CO2
CO2
C/N = 8
N
Stabilized
SOM
C, N variable
Nitrification
Immobilization
Fertilizer
Nitrate N
Atmospheric deposition
Plant uptake
CO2
Leaching
Denitrification
Mineralization
CO2
F. A2. Hurley Pasture Model : the soil and litter submodel.
required from the mineral N pools so that newly created
SOM possesses a C}N ratio that depends on the concentration of mineral N. Thus, the immobilization flux
depends on the C}N ratio of the material which is currently
entering the SOM pools as well as the mineral N
concentration.
Nitrification and denitrification. Nitrification, converting
ammonium to nitrate, occurs at a rate dependent on the
ammonium concentration and soil biomass, modified by
soil temperature and water status. At 20 °C, with 0±2 kg m−#
soil biomass, the nitrification flux is about
300 kg N ha−" yr−", 2 % of which is lost as N gases.
Denitrification occurs only when the soil is above field
capacity (in anaerobic conditions) at a rate dependent on
the nitrate concentration and soil biomass, modified by soil
temperature. At 20 °C, with 0±2 kg m−# soil biomass, the flux
is about 7 kg N ha−" yr−". Nitrification and denitrification
fluxes are within measured ranges for N-rich grasslands
(Williams, Hutchinson and Fehsenfeld, 1992 ; Whitehead,
1995).
Volatilization. Volatilization to ammonia gas depends on
the ammonium concentration and increases with tem-
perature. At 20 °C, with 0±001 kg N m−# in the ammonium
pool, the flux is 73 kg N ha−" yr−" (Sutton, Pitcairn and
Fowler, 1993).
N fixation by non-symbiotic soil microbes. Fixation occurs
#
at a rate proportional to soil biomass, modified by soil
temperature and water status, inhibited by high level of
mineral N, with Michaelis-Menten dependence on the
soluble C pool. With 0±2 kg m−# soil biomass, the maximum
fixation rate at 20 °C is 37 kg N ha−" yr−".
Leaching. N and C are leached from the nitrate N and
soluble C pools, respectively, depending on the concentrations of N and C in those pools and the amount of
drainage. A N concentration of 0±001 kg N m−# is equivalent
to about 10 mg l−" in leachate (Rowell, 1994).
Animal submodel (2 state Šariables, 9 parameters)
Species. The pasture is assumed to be grazed by mature,
non-pregnant, non-lactating sheep.
Grazing. Shoot removal by grazing is a sigmoidal
asymptotic function of LAI, with a maximum rate of
220
Thornley and Cannell—Grassland Responses to Climate Change
Animal submodel
Outputs to environment:
Maintenance respiration
Methane production
Growth respiration
C retained for
animal growth
C
Inputs of C, N from
pasture submodel
by grazing
shoot material
N
N retained for
animal growth
C
N
N
Urine
C
Faeces
Outputs to the soil and litter submodel
F. A3. Hurley Pasture Model : the animal submodel.
Water submodel
Transpiration
SHOOT
Water content
Relative water content
Water potential
Rain
Rainfall
interception
by and
evaporation
from canopy
ROOT
Water content
Relative water content
Water potential
Soil
evaporation
(assumed zero)
SOIL
Water content
Relative water content
Water potential
Surface
run-off
(assumed zero)
Drainage
F. A4. Hurley Pasture Model : the water submodel.
2±5 kg DM per animal d−" and a half-maximum rate attained
at LAI ¯ 1 (Penning et al., 1991).
Animal growth. It is assumed that 1 % of N ingested is
retained and that the C}N ratio of animal protein is 3±5.
Gaseous losses. These occur by maintenance respiration
(60 % of C), methane production (5 % of C) and growth
respiration.
Urine and faeces. This output is obtained by difference.
The fraction of N excreted as urine increases with increase
in C}N ratio of ingested material.
Water submodel (3 state Šariables, 40 parameters)
OŠerŠiew. This submodel is fully described by Thornley
(1996). It applies to a closed canopy and so has no soil
evaporation. It calculates the amount and status of water in
Thornley and Cannell—Grassland Responses to Climate Change
the soil (single compartment), roots and shoots. Water
fluxes are driven by water potential gradients.
Interception. The fraction of rainfall intercepted by the
canopy depends on the LAI and canopy extinction
coefficient. The fraction that evaporates each month is given
a numerical value each month depending on the intensity
and duration of rainfall.
Soil water. Soil water potentials and hydraulic conductivities are calculated using the equations of Gregson,
Hector and McGowan (1987) and Campbell (1974), respectively.
Soil-root-shoot flux. The flux of water from the soil to the
roots depends on root mass, root density and the resistance
221
in the bulk soil, which depends on soil hydraulic conductivity
and soil relative water content. The flux from root to shoot
depends on the water potential difference divided by a rootshoot resistance to water flow, which is scaled to root and
shoot mass.
EŠapotranspiration. The Penman-Monteith equation is
used.
Plant water status. The separation of substrates from
structure permits detailed treatment of tissue water relations.
The components of water potential—osmotic potential and
pressure potential—are calculated, as well as relative water
content. Osmoregulation occurs in response to increased C
substrate concentration.