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The Potential Effects of Increased
Temperatures and Elevated Ambient
Carbon Dioxide on Loblolly Pine
Prod uctivity:
Results From a Simulation Model
David Arthur Sampson 1
Abstract ' - Loblolly pine forests of the southeastern United States
represent vast, economically and biologically important land base. General
circulation models predict increased temperatures for this region. We used
the process model BIOMASS in conjunction with empirical field data to
explore "potential" loblolly pine productivity simulated under increased
temperatures, increased C02 concentration, and two treatments in low and
high productivity sites.
a
Simulation output suggested a net increase in stand productivity under a
doubling of ambient C02 (700 ppm) and a four degree Celsius increase in
dair temperatures. Low productivity sites increased from 3.5 to 5.7 Mg C
ha- yea(1 while high productivity sites increased from 7.7 to 11.6 Mg C
ha- 1 yea(1 in control plots. This represented a 63% and 51 % increase in
net carbon flux for low and high sites, respectively. A doubling of CO2 under
ambient temperatures in control plots increased net carbon gain by 93%
and 52% for low and high sites, respectively. Maintenance respiration (Rm)
accounted for a 26% loss in net carbon available for growth for low sites.
Gross carbon fixed increased by approximately 18% for high sites in
fertilized plots resulting in a 14% increase in net carbon gain.
INTRODUCTION
1986; Keyes and Grier 1981). Biochemically sensitive
algorithms that intetpret photosynthesis based solely on the
kinetics of Rubisco are available (Farquhar et al. 1980) and must
be used if growth models are to be capable of prediction under
elevated CQ2 (Reynolds and Acock 1985). Additionally, tissue
respiration has been modeled (Ryan 1990; Kinerson 1975).
However, the mechanisms determining carbon partitioning are
still poorly understood (Cregg et al. 1993, Sprugel et al. 1991),
and the influence of resource availability on carbon assimilation
and allocation has not been congruently elucidated (Nadelhoffer
et aI, 1985, Keyes and Grier 1981).
Quantifying caIbon partitioning among tissue components
remains a major impediment to modelling forest productivity.
The formidable task of assessing and incorporating the role of
sink strength on carbon partitioning at the biochemical level
makes empirical surrogates to these biochemical processes
Models incorporating theoretical and empirical algorithms to
simulate growth are used to examine the processes influencing
forest productivity. These process models may be mechanistic;
rate equations that characterize the biophysics of carbon fIXation
and caIbon partitioning are used in a time-step model based on
field experiments (e.g. McMurtrie et al. 1992). Factors that
control or influence photosynthesis and respiration detennine the
amount of carbon fixed (Larcher 1983), while differences in
partitioning of the flXed carbon depend at least in part on sink
strength (Cannell 1985) and resource availability (Gholz et al.
1 David Arthur Sampson is a research associate in forest
ecophysiology in the Department of Forestry, North Carolina State
University, Raleigh, N.C.
337
practical. The role of resource availability on caIbon assimilation
and partitioning can be examined in empirical investigations.
Data are available on the growth response of young and
mid-rotation loblolly pine (Pinus taeda) stands to nutrient
amendments (NCSFNC 1993, NCSFNC 1991). The effects of
soil water availability on loblolly pine growth and phenology
remain unknown, however studies are underway to address these
uncertainties.
Nutrient availability and soil water deficit are the primaty
resource-limiting factors influencing loblolly pine production in
the Southeast (NCSFNC 1993, Teskey et al. 1987). Low nutrient
availability and soil water stress are key factors causing
suboptimal levels of leaf area index (LA!) (Colbert et al. 1990;
Gholz 1986; Vose and Allen -1988). Nutrient amendments
increase LA! and canopy N content (Vose and Allen 1988).
Higher leaf area increases the interception of photosynthetically
active radiation (PAR) and, therefore, the amount of catbon fixed
(Cannell 1989), reflected in increased stemwood growth
increment (Vose and Allen 198~). Elevated canopy N content
would be expected to increase photosynthesis (Zhang and Allen,
in review) and, therefore, production per unit LA!. In addition
to limiting LA!, water stress may reduce loblolly pine production
by promoting early stomatal closure (Teskey et al. 1987;
Bongarten and Teskey 1986). The role of resource availability
on caIbon allocation to branches and roots at the stand level
remains unknown.
Catbon allocation to foliage, stems, branches, and roots will
detennine the relative contribution of these components to total
stand autotrophic maintenance respiration (Rm). Rm may
account for almost 60% of gross caIbon fixed in loblolly pine
forests (Kinerson 1975). For loblolly pine the order of
contribution to total Rm has been estimated as; branches· >
foliage> stems> roots (Kinerson 1975).
At present no data are available on the effect of chronic,
elevated CO2 and elevated temperatures on the growth and
phenology of mature trees. Short-tenn exposure to a doubling
of CO2 may decrease stomatal conductance by 40010 (Morison
1985). Increased ambient CO2 does significantly increase
photosynthesis in loblolly pine branch chamber experiments
when water is not limiting (Teskey, personal communication).
Increased temperatures will increase dark respiration.
Unfortunately, the complexity of these interactions cannot be
easily resolved in standard factorial experiments which makes
simulation modelling necessary.
The objective of this paper is to examine the potential effect
of increased ambient CO2 and increased temperatures on loblolly
pine productivity in a high and low site under two treatments
using computer simulations. Questions addressed include: 1) can
we expect increased net C assimilation if a 4° C increase in
mean annual temperature and a doubling of ambient CO2
occurs?, 2) to what extent will maintenance respiration offset
any expected gain due to elevated CO2?, and 3) will fertilization
decrease, maintain, or increase forest production over control
sites in a hotter, higher CO2 environment? We used the process
model BIOMASS parameterized for loblolly pine to address
these questions.
METHODS
We parameterized the process model BIOMASS version 12.0
for loblolly pine forests. A complete review of the model has
been described elsewhere (McMurtrie et al. 1992). Model
descriptions includ~ in this paper represent source code changes
made to BIOMASS version 12.0 during model parameterization,
and specific model characterization to clarify process level
interactions.
BIOMASS was written to explore the mechanistic factors
influencing radiata pine (P. radiata) growth response to various
water and fertilization treatments at a physiological process level
(McMurtrie and Landsburg 1992). BIOMASS was developed
using empirical data from the Biology of Forest Growth (BFG)
experiment (see McMurtrie and Landsburg 1992; Benson et al.
1992; Linder et al. 1987).
Study Locations
Simulations used in this analyses were based on empirical
data from two fertilization trials of mid-rotation loblolly pine
plantations of the North Carolina State Forest Nutrition
Cooperative (NCSFNC). The low site was established on the
Piedmont of South Carolina on a Cecil soil series coinciding
with a low site potential and the high site was established on
the upper coastal plain of North Carolina on a Leaf soil series
corresponding to a high productivity site (Table 1). 1\\'0
treatments (control and fertilized) were replicated twice with
fertilized plots receiving a one-time application of 200 Kg N
ha- 1 + 50 kg P ha- 1 in 1987. Simulations presented are for the
1988 growth year (l January through 31 December).
Table 1. - Initial stand characteristics for two mid-rotation
loblolly pine plantations. Projected peak leaf area index data
are for control plots of the growth year.
Basal
Stand
Site Index
Age
Area
Density
(m)
Peak LAI
(stems ha"1) Base age 60 (m2 m-2)
Site (years) m 2 ha"1
Low
14
21
238
18
2.0
High
10
20
244
21
2.4
338
functions. In a similar fashion a threshold sum of mean soil
temperatures detennines the initiation of the fll'St flush of root
development. A maximum threshold for stemwood growth rate
detennines initiation of the second flush for roots. A minimum
threshold in the rate change from time t, to t + 1 controls the
termination of each rate function
Daily relative growth rates for foliage, stems, branches, and
roots are tenned component tissue activity levels (surrogate for
sink strength). All component tissue activity levels are zero
during the donnant season During a growth period one or more
of the activity levels will be greater than zero.
Model Parameterization
We parameterized BIOMASS using data from several
sources. Input parameters were defmed as model run-time
conditions, initial stand chamcteristics and growth parameters
that vaty in time and space, and process parameters. Run-time
conditions set run-constant model parameters. Initial stand
structure parameters were derived from stand inventory data.
Published equations were used to derive estimates of, for
example, initial standing branch and bole biomass and soil water
content. We obtained process parameters (eg. tissue respiIation
rates, maximum photosynthesis, and optimum temperatures for
photosynthesis) and growth parameters from unpublished data,
from the literature, or from personal communication.
Each simulation was run on a daily time step. Daily mean
soil and air temperatures were estimated from daily minimum
and maximum air temperatures. Leaf area index (LAI) for each
plot was estimated from litter-trap, data (Vose and Allen 1988).
The maximum minus the minimUm LA!, converted to mass
•
units, provided an estimate of the yearly foliage production The
empirical estimate of foliage production was used to partition
simulated net catbon assimilated more precisely.
Relative growth rate, and catbon partitioning and storage
modules were written to model the seasonal patterns in loblolly
pine growth phenology. Empirical data from the Southeast Tree
Research Education Site (SElRES) were used to develop the
phenology routines (SElRES 1993).
Assimilation
BIOMASS can use either an empirical model of assimilation
based on light absorption, or a biochemical model based on
enzyme kinetics. For these simulations we used the biochemical
model (Farquhar et al. 198Q). This model interprets C3
photosynthesis from the kinetics of Rubisco. The rate of
carboxylation obeys Michaelis-Menten kinetics, and depends on
the partial pressures of the competing gaseous substrates, CO2
and 02, and on the ratio of ribulose-l,5-bisphosphate (RuBP)
concentration to enzyme active sites. This structure makes the
model sensitive to changes in CO2 concentrations. Net caIbon
assimilation is predicted from gross photosynthesis minus
construction and maintenance respiration
Phenological Rates
Carbon Pools
The closed fonn logistic equation was fit to the growth data
and scaled to sum to one for initiation and cessation of stem
and branch diameter growth, and leaf area development. The
Daily net carbon assimilation enters either active or passive
labile carbon pools. The component activity level and net caIbon
assimilated detennines movement of carbon into or out of these
pools. For example, when the activity level is zero and net
assimilation is greater than zero, carbon enters the passive pool
to be stored in component tissue. Carbon storage begins with
foliage. When the foliage storage reaches maximum capacity,
carbon is stored in roots. This process continues with the
remaining tissue components and the hieI3IChy of storage is:
foliage > roots > branches > stems. If daily net cmbon is
negative, an equal amount of carbon is removed from storage
beginning with foliage. The carbon removal hierarchy follows
the carbon storage ranking.
During an active growth period net carbon assimilated enters
the active pool. If net carbon assimilated is negative during an
active growth period, carbon is removed from storage.
Additional catbon proportional to the maximum activity level
must be removed from storage to meet the growth demand.
Available carbon is partitioned among the tissue components
during positive net carbon availability. During an active growth
period with positive net carbon availability, carbon is removed
from storage at a rate proportional to the sum of the tissue
component rates. The foliar nitrogen concentration modifies this
carbon flux.
fonn of the equation is:
RGR
=«e(BO+ 81 • T»)I(1
+ e(BO + 81 • T»)
(1)
Where:
e
=
2.71828,
RGR Relative growth rate,
BO = Scaling parameter,
B1
Inflection parameter, and
T Year day (1 to 366).
=
=
=
The BO and B1 model parameters are estimated from foliar
nutrient concentration at the beginning of the growth year. A
hypothetical model of the same fonn was used to simulate root
activity. The timing of root growth initiation and cessation was
approximated using data from Harris et al. (1977).
The first derivative of equation 1 provided daily growth rate
functions for the foliage, stem, branch, and root phenologies.
Day length detennines the initiation of foliage development. A
threshold sum of consecutive daily mean air temperatures
beginning with the first day of the growth year, along with day
length, determines the commencement of stem and branch rate
339
Carbon Partitioning
Table 2. - Comparison of net carbon production for
temperate coniferous forests from this study with
published literature1•
Net Carbon production
(Mg C ha- 1 year-1)
Source
The relative component tissue activity levels, when expressed
as a fraction of one, detennine the partitioning of net catbon to
foliage, stems, branches, and roots. Daily partitioning rates must
therefore sum to :zero or one. Carlx)ll flux to foliage must be
met first before carlx)ll can be made available to other tissue
components. If the demand for foliage production is less than
daily net assimilated, cmbon is removed from storage in an
amount equal to the deficit. CaIbon storage occurs when daily
production become less than net catbon assimilated.
Simulated carbon from this study
Seven-year-old P. elliottii stand from
Florida
Twenty seven-year-old P. elliottii stand
from Florida
Sixteen-year-old P. taeda stand from
North Carolina
1 Vogt,
3.5 and 11.6
2.4
8.2
9.8
K. 1991.
Model Assl:Imptions
The following assumptions pertain to source code changes
made during model parameterization. General model
assumptions can be found elsewhere (McMurtrie et al. 1992).
• Daily root production cabnot exceed one-half of
current standing root carbon (see Gholz et al.
1986).
• Maximum storage of carbon for stems and branches
is 4% of current standing carbon.
• Maximum carbon storage in foliage is 14.5 % of
current standing carbon (Birk and Matson 1986).
• Maximum carbon storage in roots is 14.0 % of
current standing carbon (Adams et al. 1986).
• Initial root biomass is equivalent to initial foliage
biomass (see Gholz et al. 1986).
• Root biomass and production refers to fine roots
(l mm).
• Root sloughing is proportional to needle litter-fall.
• No internal acclimation to elevated C02.
• A four degree increase in minimum and maximum
daily temperatures approximates a four degree
increase in mean annual temperatures.
10
t..CIS
8
~ Simulated
~ EmpiriCal Estimate
CD
>-
.~
.c
0
CI
6
~
><
:J
..J
U.
z
0
a:a
a:
4
(§
~
w
Ien
2
C
F
LOW SITE
C
F
HIGH SITE
Figure 1. - Comparison of simulation output and empirical
estimates for annual stem carbon production (Mg C ha-1
yea(1) for two mid-rotation loblolly pine stands of the
southeastern United States. C and F deSignate control and
fertilizer (one-time application of 200 kg N ha-1 and 60 kg P
ha-1) plots for low and high productivity sites.
RESULTS AND DISCUSSION
Simulated net cmbon production was comparable to the
literature for southern pine species (Table 2). Additionally, net
carbon allocated to stemwood growth was similar to the
empirical estimates (Figure 1). CaIbon budgets presented here
are likely feasible given the parameterization procedure.
Simulation results indicated a net increase in stand
productivity under a doubling of ambient CO2 (700 ppm) and
a four degree Celsius increase in daily temperatures (Figure 2).
Low productivity sites increased from 3.5 to 5.7 Mg C ha- I
I
year- while hi~ productivity sites increased from 7.7 to 11.6
I
Mg C ha- year- I in control plots. This represented a 63% and
51 % ~crease in net carbon flux for low and high sites,
respectively (Figure 2).
A doubling of CO2 under ambient temperatures in control
plots increased net carbon gain by 93% and 52% for low and
high sites, respectively (Figure 2). The differences in net catbon
gain in the 2x CO2 simulations are not maintained when
increased temperatures are imposed. Maintenance respiration
(Rm) accounted for a 26% loss in net cmbon available for
growth for low sites. High sites under elevated C02 did not
change appreciably in net carbon gain with increased
temperatures.
An increase in gross catbon fixed accounted for the negligible
effect of increased temperatures on net carbon gain for high sites
in control plots. Gross carbon fixed increased by 8% in these
plots. The 8% increase off-set an almost identical increase in
Rm between the 2x C02 and the 2x CO2 with the imposed four
340
20
Control Plots
i
~ Net Carbon Gain
_Rm
24
15
~ Foliage
Stems
~ Branches
c=J Roots
Rm
':"
1i
CI)
>-
!2"ZZZJ
-
20
';,
.c
0
a
~
16
X
::J
..J
u..
z
0
12
aJ
a:
<
0
I-
zw
z
AI A
2xl A
LOW SITE
2xl+4
AlA
2xl A
8
0
2x/+4
Q.
~
8
HIGH SITE
Figure 2. - Simulation output of annual carbon flux (Mg C ha-1
yea(1) for two mid-rotation loblolly pine stands of the
southeastern United States. AlA represents ambient CO2
concentrations (360 ppm) and ambient temperatures; 2x1A
represents twice ambient C02 with ambient temperatures;
and 2x/+4 designates twice ambient CO2 and a plus 4 degree
Celsius increase in mean annual temperatures for low and
high productivity sites.
4
0
C
F
F
C
LOW SITE
HIGH SITE
Figure 3a. - Simulation output of annual component carbon flux
(Mg C ha-1 year-1) for two mid-rotation loblolly pine stands
of the southeastern United States. C and F deSignate control
and fertilizer (one-time application of 200 kg N ha-1 and 50
kg P ha-1) plots for low and high productivity sites.
degree Celsius increase in daily tempemtures (Figure 2). Gross
catbon fixed remained unchanged for the 2x CO2 and 2x CO2
plus increased temperature scenario for low sites.
The effect of fertilization on total carbon flux and embon
partitioning varied by site. Control and fertilized plots for low
sites did not differ in either gross catbon fIXed or net embon
gain (Figure 3a). Catbon partitioning among foliage, branches,
stems, and roots remained unchanged with treatment for these
sites. Conversely, gross catbon fixed for high sites increased by
approximately 18% in fertilized plots resulting in a 14% increase
in net catbon gain The net result of increased catbon availability
was increased foliage, stem, branch, and root production when
compared to control plots in high productivity sites. Foliage
production did not increase in fertilized plots for low sites which
can explain the lack of growth response to treatment in these
sites (Figure 3a).
On a mass basis, foliage, stems, branches, and roots contribute
disproportionately to Rm. For instance, foliage may represent 4
to 6% of the standing biomass yet may contribute > 34% of
Rm (Kinerson 1975). Stem mass may exceed 65% of standing
biomass, and, if we assume a live cell volume of 8 to 10% for
bole wood (Ryan 1990), live stem tissue may represent 5.2 to
6.5% of standing biomass yet may contribute only 13% to Rm
(Kinerson 1975). Increased foliage and root production mther
than increased stem and branch production explained the roughly
4% increase in total Rm in fertilizer plots for high sites (Figure
3b). Data suggest that for these stands the order of contribution
to total Rm for loblolly pine systems would be; roots > foliage
> branches = stems.
5
...
'I
•
>-.
o
"'0
0
4
'I
I::.
o
Foliage
Stems
Branches
Roots
I::.
High Site
Fertilizer
Effect
.c
o
0'1
~
3
Z
o
~
2
CL
(J)
W
0:::
W
o
z
4.:
z
w
~
4:
0
2
-1
o
100
200
300
400
Year Day
Figure 3b. - Simulation output of the component maintenance
-respiration (kg C ha-1 day-1) difference between control and
fertilized plots at 700 ppm C02 concentrations and a plus
four degrees Celsius increase in temperature for two
mid-rotation loblolly pine stands of the southeastern United
States. Simulation data are for the fertilizer (one-time
application of 200 kg N ha-1 and 60 kg P ha-1) plots of the
high productivity site.
341
Gholz, H.L. 1986. Canopy development and dynamics in
relation to primary production. P. 224-242 IN: Fujimori, T.
and Whitehead, D. (eds.)., Proc. Crown and Canopy Structure
in Relation to Productivity. Forestry and Forest Products
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Harris, W.F.; Kinerson, R.S.; and Edwards, N.T. 1978.
Comparison of below-ground biomass of natural deciduous
forests and loblolly pine plantations. Pedobiologia. 17;
369-381.
Keyes, MR; and Grier, C.C. 1981. Below- and above-ground
biomass and net production in two contrasting 40-year-old
Douglas-fir staI$. Canadian Journal of Forest Research. 11:
599-605.
Kinerson, R.S. 1975. Relationships between plant surface area
and respiration in loblolly pine. Journal of Applied Ecology.
12: 965-971.
Latcher, W. 1983. Physiological plant ecology. Springer-Verlag,
New Yorlc
Linder, S.; Benson, MJ.; Mey-ers, BJ.; and Raison, R.I. 1987.
Canopy dynamics and growth of Pinus radiata. L Effects of
irrigation and fertilization during a drought. Canadian Journal
of Forest Research. 17: 1157-1165.
McMurtrie, R.E.; and Landsburg, 1.1. 1992. Using a simulation
model to evaluate the effects of water and- nutrients on the
growth and catbon partitioning of Pinus radiata. Forest
Ecology and Management (in press).
McMurtrie, R.E.; Leuning, R.; Thompson, W.A.; and Wheeler,
A.M. 1992. A model of canopy photosynthesis and water use
incoIporating a mechanistic formulation of leaf CO2
exchange. Forest Ecology and Management. 52: 261-278.
Morison, 1.LL. 1985. Sensitivity of stomata and water use
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467-474.
Nadelhoffer, K.; Aber, 1.; and Melillo, 1. 1985. Fine roots, net
primary production, and soil nitrogen availability: a new
hypothesis. Ecology. 66: 1377-1390.
NCSFNC. 1993. Six-year growth and foliar nutrient responses
of midrotation loblolly pine plantations to Nand P
fertilization NCSFNC Report No. 29. College of Forest
Resources. North Carolina State University, Raleigh.
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weed control on the growth and nutrition of loblolly pine.
NCSFNC Report No. 27. College of Forest Resources. North
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CONCLUSIONS
Simulation output suggested a net increase in stand
productivity under a doubling of ambient CO2 (700 ppm) and
a four degree Celsius increase in daily temperatures for loblolly
pine stands of the southeastern United States. Site potential will
likely effect the response of trees to these perturbations, with
low sites responding greater than high sites. If fertilizer
treatments are used, the response of trees to treatment under
elevated CO2 and temperatures may also depend on site
potential.
Maintenance respiration comprises a latge portion of the
caIbon budget for loblolly pine systems. Because foliage, stems,
branches, and roots due not col\tnbute proportionately to total
Rm, estimates of component biomass production will strongly
influence simulated net catbon assimilated in these modelling
exercises.
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342
Vogt, K. 1991. Catbon budgets of temperate forest ecosystems.
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343