Journal of Environmental Protection, 2011, 2, 769-777
doi:10.4236/jep.2011.26089 Published Online August 2011 (http://www.SciRP.org/journal/jep)
769
Model Estimates of Nutrient Uptake by Red
Spruce Respond to Soil Temperature
J. Michael Kelly1, Frank C. Thornton2, J. Devereux Joslin3
1College of Natural Resources and Environment, Virginia Tech, Blacksburg, USA; 2Tennessee Valley Authority, Muscle Shoals,
USA; 3Belowground Forest Research, Monteverde, Costa Rica.
Received April 28th, 2011; revised June 14th, 2011; accepted July 27th, 2011.
ABSTRACT
A better understanding of the mechanisms that control nutrient acquisition in the context of plant and ecosystem re-
sponses to climate change is needed. Mechanistic nutrient uptake models provide a means to investigate some of the
impacts of temperature change on soil nutrient supply and root uptake kinetics through the simulation of key soil and
plant processes. The NST 3.0 model, in combination with literature values on plant and soil parameters from a red
spruce (Picea rubens L.) site in the southern Appalachians, was used to conduct a series of model simulations fo cused
on the combined effects of changes to the maximal rate of nutrient influx at high concentrations (Imax), root growth rate
(k), concentration of nutrient occurring in the soil solution (Cli), and th e ability of the so il solid phase to buffer chang es
to the soil solution nutrient concentration (b). Previous research has indicated that these four parameters are respon-
sive to changes in root zone temperature. Simulated uptake of NH4 increased by a factor of up to 2.6 in response to in-
creases in soil temperature of 1˚C to 5˚C. The model also projected an increa se in P uptake coupled with up to an 80%
reduction in solution P concentration in response to a 1˚C - 5˚C increase over a 147-d simulation period. These hypo-
thetical changes, if validated, have interesting implications for plant growth and competition and point to a need for
additional studies to better define the impacts of soil tempera ture on soil nutrient supply and root uptake.
Keywords: Mechanistic Modeling, Imax, Root Growth Rate, Soil Buffer Power
1. Introduction
Over the next 100 years, mean global temperature is pro-
jected to increase by 1.5˚C to 4.5˚C [1]. An increase in
atmospheric temperature will eventually lead to an in-
crease in soil temperature [2], and this increase in soil
temperature could lead to changes in soil supply and
plant nutrient uptake rates. Most studies of soil warming
in the forest environment have focused on changes in
plant productivity and soil nutrient availability [3-6] with
only Gessler et al. [7] and Adam et al. [8] assessing the
potential changes in nutrient uptake as a function of
changes in root zone temperature.
In a laboratory study on the influence of temperature
on solution phase nutrient concentrations in soil from a
southeastern US spruce-fir stand, Kelly [9] found higher
concentrations of NH4 and P as soil temperature in-
creased from 4˚C to 24˚C. Conversely, NO3 concentra-
tion in solution was greatest at 4˚C and declined as tem-
perature increased. Since nutrient uptake is thought to
occur largely through root interaction with the solution
phase [10], changes in soil temperature, and associated
changes in soil solution chemistry have the potential to
alter both nutrient availability and plant uptake. And
since nutrient uptake is a physiologically mediated proc-
ess, it also follows that changes in soil temperature could
have an impact on the rate at which plants take up nutri-
ents. For example, Adam et al. [8] in a greenhouse study
found that the uptake of NO3 by red maple plants in-
creased by approximately 2% per degree of temperature
increase as solution temperature increased to an optimum
and then began to decline. Adam et al. [8] also report that
root surface area was influenced by differences in root
zone temperature, with both root length and root radius
values showing a response pattern similar to that of ni-
trate. Similarly, Weih and Karlsson [5] found that moun-
tain birch root-N uptake rate and plant-N concentration
were positively correlated with increases in soil tem-
perature. In a field and laboratory study of Norway
spruce, Gessler et al. [7] found that very little NO3 was
taken up by roots when solution NH4 concentrations
were elevated and that NH4 uptake increased by ap-

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature 770
proximately 10% to 13% per ˚C increase in root zone
temperature.
Agronomic studies suggest that when soil P supplies
are low, P availability increases with increasing soil
temperature [11-13]. It stands to reason that this rela-
tionship would also hold in unfertilized forest soils, since
in both agricultural and forest soils, decomposition of
organic matter is the primary source of P. These and
other studies have created a body of knowledge that,
when combined through a mechanistic nutrient uptake
model, provide a means to explore hypothetically the
impacts of soil temperature change on soil nutrient sup-
ply and nutrient uptake by an important tree species.
The NST 3.0 mechanistic nutrient uptake model pro-
vides a means to simulate the short-term impact of soil
temperature change on the concentration of nutrients
occurring in the soil solution as well as the impact of
changes in root growth rate on nutrient acquisition.
Similarly, the impacts of temperature mediated changes
on physiological processes can be explored indirectly
through alterations of the magnitude of the uptake kinet-
ics parameters. NST 3.0 is available for download from
the website of the Department of Crop Sciences at Göt-
tingen University (http://wwwuser.gwdg.de/~uaac/). The
NST 3.0 model, and its predecessor the Barber-Cushman
model [14], have been used with a high degree of success
to explore nutrient uptake under a variety of circum-
stances [15-19].
As noted by Claassen and Steingrobe [18], a validated
mechanistic model provides a means to extrapolate plant
response beyond currently available data and also pro-
vides a way to evaluate potential hypotheses. Thus, the
objective of this study is to use a well-established mech-
anistic nutrient uptake model, NST 3.0, to explore the
potential impacts of 1˚C, 2˚C, 3˚C, and 5˚C increases in
simulated soil temperature on estimates of nutrient up-
take by red spruce seedlings over a growing season. We
will do this by altering the magnitude of the values for
the initial solution concentration (Cli), the ability of the
soil solid phase to resupply the solution phase (b), the
root growth rate (k), and the maximal rate of nutrient
influx at the root surface (Imax). The results of these
comparisons should provide initial theoretical insights
into the potential changes in N and P supply and uptake
that might occur in response to changes in soil tempera-
ture as a consequence of global climate change.
2. Materials and Methods
Published data taken from studies conducted in the
spruce-fir forest at Whitetop Mountain in southeastern
Virginia and reported by Kelly and Mays [20], Thornton
et al. [21], Kelly et al. [22], and Kelly [9] will form the
basic data set used in this analysis (Table 1). These data
sets will be supported by additional information from the
same site reported by Joslin and Wolfe [23,26]. In some
cases the values listed in Table 1 are taken directly from
Table 1. Transport, sorption, and root parameters used in the NST 3.0 model to describe N and P uptake by red spruce seed-
lings under base conditions (Letters in parentheses following each value indicate the data source corresponding to the cita-
tions listed in the footnote).
Parameter Units N P
DL Diffusion coefficient in H2O cm2·s−1 1.9E−5 (a)† 8.9E−6 (b)
Θ Volumetric H2O content cm3 H2O/cm3 soil 0.22 (c) 0.22 (c)
f Impedance factor unitless 0.178 (c) 0.178 (c)
Vo H2O uptake at root cm·s−1 6.3E−8 (d) 6.3E−8 (d)
Cli Initial solution concentration mol·cm-3 1.17E−6 (e) 3.42E−9 (e)
b Buffer power unitless 133 (c) 37 (d)
Imax Maximum influx at high concentration mol·cm−2·s−1 2.1E−11 (f) 2.68E−13 (g)
Km Solution concentration when influx is 0.5 Imax mol·cm−3 2.06E−4 (f) 1.6E−8 (g)
Cmin Solution concentration when influx is zero mol·cm−3 1.11E−9 (f) 6.0E−11 (g)
ro Root radius cm 0.042 (h) 0.042 (h)
r1 Half distance cm 0.31 (h) 0.31 (h)
Lo Initial root length cm 37860 (h) 37860 (h)
k Root growth rate cm·d−1 315 (h) 315 (h)
†
(a) [24]; (b) [25]; (c) [20]; (d) [9]; (e) calculated by model; (f) [19]; (g) [22]; (h) [21].

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature771
the published work cited in Table 1 or re-calculated from
these values when not available as a direct result of these
studies. For parameters that could not be calculated or
extrapolated from on-site data directly available to the
authors, missing parameter values were obtained from
the literature sources noted or calculated with the model
using a process similar to that described in Kelly et al.
[17]. Values for the NH4 uptake kinetics parameters Imax,
Km, and Cmin were taken from a study of white spruce
conducted by Hangs et al. [19]. Phosphorus values for
the same parameters came from a study of loblolly pine
conducted by Kelly et al. [22]. While it would be prefer-
able to have actual values for red spruce, values from
these two conifer species are thought to provide a rea-
sonable first approximation.
In this study we used the NST 3.0 model to focus on
the impacts of temperature change on the level of nutri-
ent occurring in the soil solution (Cl), the ability of the
soil solid phase nutrients to sustain the solution phase
concentration (b), the root growth rate (k), and the max-
imal rate of nutrient uptake under unlimited nutrient
availability (Imax). These four parameters were chosen for
evaluation because of their demonstrated sensitivity to
change as a function of temperature [8,9]. Values used
for these and the other parameters required for the base
case simulations of N and P using NST 3.0 are listed in
Table 1 and their source identified.
Percentage modifications (+/−) to the base level values
used for k and Imax to simulate changes in temperature
(Table 2) are based on the relations observed by Adam et
al. [8] for red maple. In a study of root zone temperature
influence on root growth and nitrate uptake by red maple,
Adam et al. [8] found that root growth rate increased by
an average of 4.3% per degree of temperature change as
the root zone temperature was increased from 14˚C to
24˚C and that the nitrate Imax value increased by 1.9% per
degree increase over the same temperature range. Given
the absence of actual measurements for red spruce, these
two percentage values were assumed to represent the
relative change rate in k and Imax for both nutrients (Ta-
ble 2). It should be noted however, that the temperature
range used by Adam et al. [8] exceeds the range of soil
temperature increase likely to occur in soils currently
supporting the growth of red spruce.
Similarly, in a controlled environment investigation of
changes in soil solution NH4 and P in the surface horizon
of a soil from a spruce-fir stand on Whitetop Mountain,
Kelly [9] found that NH4 concentration increased by 10%
per degree as temperature increased from 4˚C to 24˚C
while soil solution P concentration increased by 13.1%
per degree C. Using the relationship between Cli and b
reported by Kelly et al. [27], the corresponding b values
for NH4 and P were estimated to decrease by 10% for
Table 2. Percentage change factors for Tests I and II (+/− %)
per degree centigrade increase in simulated soil tempera-
ture for Cli, b, k, and Ima x for simulations by NST 3.0 of N
and P uptake by red spruce seedlings.
N P
Parameter
Test I Test II Test I Test II
Change %
Cli +10 † +13.1
b −10 † −6.5
k +4.3 +10 +4.3 +10
Imax +1.9 +10 +1.9 +10
†Cli and b values were unchanged in Test II, only the k and Imax values were
modified according to the Q10 relationship.
NH4 and 6.5% for P per degree of temperature increase
(Table 2). For the purposes of this analysis all nitrogen
uptake is assumed to be in the form of NH4 because of
the established preference of conifers for this form of N
[7,28]. Using the change values listed in Table 2 for the
four model parameters, a suite of uptake values was cal-
culated for each nutrient to represent simulated tempera-
ture increases of 1˚C, 2˚C, 3˚C, and 5˚C. All four values
for each nutrient-temperature scenario were entered
concurrently while the values for the remaining nine pa-
rameters in the model were held constant at the level
specified for each nutrient in Table 1.
In addition to the change factors for k and Imax identi-
fied for Test I in Table 2, an additional set of values was
calculated for these two physiologically mediated pa-
rameters based on a simple Q10 relationship. Thus for the
data sets based on the Q10 relationship (Test II, Table 2),
k and Imax were each increased by 10% per ˚C of tem-
perature increase. This level of change for Test II was
chosen in part based on the study of Norway spruce by
Gessler et al. [7] who found an increase in NH4 uptake of
approximately 10% - 13% per ˚C increase in root zone
temperature. The values for Cli and b in the Q10 simula-
tions (Test II) continued to change at the rates indicated
for Test I in Table 2, while the remaining values for the
other nine parameters listed in Table 1 were again held
constant at the levels indicated. Nine scenarios were run
for each nutrient-temperature combination for a total of
18 simulations covering both nutrients.
3. Results and Discussion
3.1. Validity of Assumptions
Before proceeding further it is appropriate to consider the
assumptions that have been made concerning the magni-
tudes and directions of change associated with the Cli, b,
k, and Imax values used for each nutrient. Unfortunately,

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature 772
studies of soil temperature influences focused directly on
these parameters are very limited for woody species.
Therefore we have depended heavily on the study of
Adam et al. [8] even though their study was conducted
with a deciduous species and used a temperature range
likely to exceed temperatures in red spruce forest soils.
We combined the insights from Adam et al. [8] with ac-
tual seasonal totals of N and P uptake by red spruce
seedlings from the work of Thornton et al. [21]. The Test
I values are taken to be at the lower end of the potential
response spectrum. The observations of Gessler et al. [7]
were used to inform the other end (Test II) of what we
feel is a conservative spectrum of input parameters that
are consistent with observed growing season uptake
based on the data of Thornton et al. [21]. Therefore, the
focus of this study is to develop an assessment of “rela-
tive change” rather than providing “absolute values”.
While these uptake estimates are hypothetical, they have
the potential to raise important research questions. These
questions in turn could lead to the formulation of future
studies specifically designed to address possible changes
in nutrient uptake in response to changing soil tempera-
ture.
3.2. Nitrogen
Using the parameter values listed in Table 1, the model
estimate of NH4 uptake over a 147-d period was on the
order of 24.0 mmols. For the purpose of this study this
value is considered to be the base level of N uptake and
is equivalent to the level of uptake actually observed for
the red spruce seedlings described by Thornton et al. [21]
during their second year of growth. This level of corre-
spondence between the model calculation and observed
uptake was achieved by adjusting the Cli value to a level
that, in conjunction with the other values entered into the
model (Table 1), would combine to produce a level of
model predicted uptake equivalent to the observed uptake.
The same procedure was used to establish the base level
of uptake for P as well.
In the case of NH4, this led to a higher value for Cli
(1.17E−6 mol·cm−3) being used in the model than was
reported by Kelly [9] for a spruce-fir soil at the Whitetop
Mountain site (5.27E−8 mol·cm−3). However, it is not
unreasonable to assume that even though the soil used in
the pot study conducted by Thornton et al. [21] came
from a similar site on Whitetop Mountain, the handling
of the soil and the warmer environment in the chambers
used by Thornton et al. [21] led to a higher level of N
availability to the study seedlings. This supposition is
consistent with the observation of Kelly [9] that soil so-
lution NH4 levels increased as soil incubation tempera-
ture increased, as well as the findings of Rustad and Fer-
nandez [4] who reported that a 4˚C - 5˚C increase in soil
temperature in a red spruce forest stand stimulated de-
composition and the subsequent release of N from or-
ganic sources.
Estimates of N uptake derived by the model in re-
sponse to each of the temperature increase scenarios in-
dicate steady increases in uptake with the highest esti-
mate using the Test I parameters being 1.75 times that
occurring under base conditions (Table 3). When Q10
values are used to represent further increases in Imax and k
(Test II), the level of N uptake estimated at 5˚C increases
by a factor of 2.6 over base conditions. This level of in-
crease, if realized, could stimulate the growth of estab-
lished red spruce, assuming that current growth rate is N
limited. The predicted level of increased uptake could
also have other impacts such as the creation of imbal-
ances with other nutrients that could slow growth, or
increased susceptibility to freeze damage due to early
bud out or delayed senescence [29]. Alternatively, an
increase in nitrogen availability could stimulate the
growth or establishment of other species that might be
better suited to a more favorable nitrogen supply and
warmer soil than red spruce and ultimately lead to the
displacement of red spruce by another species over time
[30].
To explore these model results further, a series of sin-
gle factor sensitivity analyses were conducted using the
approach described by Silberbush and Barber [31] to see
if a particular one of the four parameters evaluated was
driving the response. Using the 1˚C increase scenario as
an example, the results of the sensitivity analyses in-
dicted that for NH4 the Imax and Cli values were the most
sensitive to change and both produced essentially the
same level of response (Figure 1). Root growth rate (k)
responded to a lesser degree with NH4 uptake exhibiting
a 19% decline when the k value was reduced by 50% and
a 40% increase when the k value was doubled (Figure 1).
Table 3. Estimates of N and P uptake by red spruce seed-
lings over a 147-d period as a function of simulated soil
temperature changes of 1˚C, 2˚C, 3˚C, or 5˚C based on NST
3.0 calculations.
Uptake (mmols)
N P Scenario
Test I Test II Test I Test II
Base 24.0 1.48
+1˚C 27.3 29.7 1.61 1.66
+2˚C 30.8 37.0 1.77 1.87
+3˚C 34.4 44.8 1.86 2.01
+5˚C 42.2 63.2 2.06 2.26

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature773
Change Ratio
0.5 1.0 1.5 2.0
Estimated NH4 Uptake (mmol)
0
10
20
30
40
50
60
Imax
Cli
b
k
Figure 1. Sensitivity analysis of predicted NH4 uptake in
response to a simulated 1˚C increase in temperature using
the NST 3.0 model showing the effect on predicted NH4
uptake of varying individually each of the four parameters
illustrated while holding all remaining model parameters
constant. Each parameter has been varied from a 50% re-
duction (0.5 change ratio) to a doubling (2.0 change ratio).
The buffer power (b), although very much related to Cli,
was not responsive and NH4 uptake remained essentially
unchanged in its response (Figure 1). This suggests that
NH4 supply exceeds plant demand as defined by the Imax
value used.
To explore this further, the change in the solution NH4
concentration profile along the radius of a cylinder of soil
around a typical root, as represented by the ratio of the
ending solution concentration to the initial solution con-
centration (Cl/Cli) was plotted (Figure 2). This depiction
generated by the model is from the 2˚C increase using
the Test I values for Imax, Cli, b, and k. This example il-
lustrates that there was no change in solution NH4 con-
centration at the end of 147-d of simulated plant uptake
for the 2˚C scenario or any of the other soil temperature
change simulations of NH4. Figure 2 clearly illustrates
that under the conditions investigated, that N uptake had
very minimal impact on the concentration of N in the soil
solution and further suggests that plant growth would be
unlikely to be N limited on an annual basis under these
conditions. This conjecture is also consistent with the
N-saturation hypothesis of Joslin and Wolfe [26] for
southern red spruce stands.
3.3. Phosphorus
Uptake of P ranged from 1.48 mmol under base condi-
tions to a maximum of 2.26 mmol for the 5˚C scenario
(Table 3). In contrast to NH4, the solution concentration
profile for P was substantially depleted at the end of the
147-d simulation period (Figure 3). The response de-
picted in this figure is similar to that observed for all of
r (cm)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
NH4 Concentration Ratio (Cl/Cli)
0.0
0.2
0.4
0.6
0.8
1.0
Figure 2. Effect of distance from the root surface on the
ratio of the ending solution concentration to the initial solu-
tion concentration (Cl/Cli) of NH4. The gradient in solution
perpendicular to the root illustrates the combined impact of
a change in Imax, b, k, and Cli for a 2˚C simulated change in
soil temperature using Test I data.
r (cm)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
P Concentration Ratio (Cl/Cli)
0.0
0.2
0.4
0.6
0.8
1.0
Figure 3. Effect of distance from the root surface on the
ratio of the ending solution concentration to the initial solu-
tion concentration (Cl/Cli) of P. The gradient in solution
perpendicular to the root illustrates the combined impact of
a change in Imax, b, k, and Cli for a 3˚C simulated change in
soil temperature using Test I data.
the P scenarios with the concentration profile being re-
duced further in the 5˚C scenario. The sensitivity analysis
depicted in Figure 4 illustrates that the model is more
sensitive to changes in the Cli value than the b value. This
seems reasonable given the availability of P for root up-
take is controlled by the diffusion rate of P through the
soil [10]. Since P supply is relatively low in most unfer-
tilized soils, it is common to see steep solution concen-
tration profiles in the soil surrounding a root [18].
The growth of new roots and mycorrhizae can also
play an important role in P uptake given the relatively
slow rate of P diffusion and the negligible contribution of

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature 774
Change Ratio
0.5 1.0 1.5 2.0
Estimated P Uptake (mmol)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Imax
Cli
b
k
Figure 4. Sensitivity analysis of predicted P uptake in re-
sponse to a 1˚C increase in temperature using the NST 3.0
model showing the effect on predicted P uptake of varying
individually each of the four parameters illustrated while
holding all remaining model parameters constant. Each
parameter has been varied from a 50% reduction (0.5
change ratio) to a doubling (2.0 change ratio).
mass flow to P delivery. Under base case conditions, P
uptake over the study period attributed to existing roots,
as calculated by the model, was on the order of 912
µmols with new roots responsible for an additional 563
µmols. At the 5˚C level using Test II data, existing roots
accounted for 1.17 mmols of uptake and new roots 1.09
mmols of P uptake. In the latter case, new roots account
for approximately half of the uptake, while in the base
case simulation new roots provide approximately 38% of
the P uptake. Increasing the degree to which roots ramify
the soil has a definite impact on the uptake of P as a re-
sult of shorter diffusion distance to a root.
Hyphae of mycorrhizae can serve much the same
function as roots and are known to be important to P ac-
quisition in P limited situations. We did not have infor-
mation on mycorrhizal growth rates or mycorrhizal up-
take kinetics values for P or NH4. Consequently, we
choose not to consider their potential contribution. Provi-
sions have been made in NST 3.0 to estimate the contri-
bution of root hairs to uptake. To do so, additional values
for k and Imax are entered into the model. This same pro-
vision could be used to get an estimate of mycorrhizal
contributions as well, assuming appropriate k and Imax
values were available. However, under the conditions
considered here it is likely that the addition of my-
corrhizae would have increased further the rate of P up-
take and contributed to the further decline of plant avail-
able P.
3.4. Implications of Results
In this study, we simulate NH4 and P uptake using data
from a southeastern US red spruce forest stand where
there is evidence that growth is not consistently N-li-
mited. The absence of red spruce responses to N fertili-
zation on Whitetop Mt. [23] and the observation of net
N-mineralization rates that exceed tree uptake require-
ments in various other high-elevation red spruce-fir for-
est sites [32,33] would indicate the absence of an N limi-
tation for red spruce in some southeastern US locations.
In light of this evidence, it appears prudent to question
related assumptions given our limited knowledge of the
impact that a lack of N-limitation might have.
While we were able to calibrate the model to existing
data on the total uptake of N by red spruce seedlings over
a 147-d period, direct information on uptake responses to
changes in Cl under current conditions in the southern
Appalachians was not available and the assumptions was
made that NH4 concentration in the soil solution would
increase in response to increasing soil temperature in a
manner similar to that observed by Kelly [9]. Both
Karlsson and Nordell [3] and Weih and Karlsson [5]
found an increase in N uptake and tissue concentration in
mountain birch as soil temperature increased. In a study
of oats, Nielsen et al. [34] found an increase in N uptake
as soil temperature increased. Similarly, both Whitfield
and Smika [35] and Gavito et al. [1] found an increase in
N uptake in several wheat varieties as soil temperature
increased. All of these responses are consistent with the
findings of Adam et al. [8]. However, none of these
studies, except for the work of Adam et al. [8], was con-
ducted in a manner that allows the role of increased soil
supply and increased root uptake to be assessed sepa-
rately. Consequently, we assumed that Imax for modeled
NH4 would increase with temperature either at rates ob-
served by Adam et al. [8] (Test I) or according to a Q10
relationship (Test II).
However, under conditions where N is not limiting
growth, source-sink relationships could result in reduced
NH4 uptake capacity and lower Imax values than those
used in our modeling effort. Studies have documented
reductions in uptake capacity for NH4 with increases in N
availability for a variety of tree species, including two
species closely related to red spruce—Norway spruce [36]
and Engelmann spruce [28]. If these observations hold
for red spruce under conditions of high N availability, the
values for the increase in N uptake with increases in
temperature would be lower than those predicted here.
Possible secondary effects of a reduction in N uptake by
red spruce include: 1) increases in the availability of NH4
for other plant species, 2) increased production of NO3,3)
attendant elevation in cation concentrations in soil solu-
tion, and 4) leaching losses of cations.
In the case of P, we again assumed an increase in soil
supply with increasing temperature based on the findings

Model Estimates of Nutrient Uptake by Red Spruce Respond to Soil Temperature775
of Kelly [9]. This assumption is supported by the work of
Sheppard and Rocz [11] who found that the availability
of P increased as soil temperature increased from 10˚C to
25˚C. They attributed this increase to desorption of
bound P. Studies of P uptake by oats [13,34] and wheat
[35] found a general increase in P uptake with increases
in soil temperature. Del Valle and Harmon [37] found a
300% increase in P uptake in the shoots and a 200% in-
crease in the roots of turnip as soil temperature increased
from 7˚C to 23˚C. Conversely, Nielsen and Cunningham
[38] did not find an increase in P uptake by ryegrass in
response to increased soil temperature while De Lucia et
al. [39] found that foliar P in big bluestem increased but
root P declined in response to soil temperature increase.
Given that the P response is mixed we chose to use an
increasing scenario given that in most forest soils supply
of P is generally low and thus an increase in availability
is most likely to lead to an increase in uptake.
4. Conclusions
Model predictions from the study indicate that even
modest changes in soil temperature in response to global
warming could lead to increased levels of N and P uptake
in the near term. While the actual magnitude of the po-
tential response is uncertain, if the underlying assump-
tions prove to be correct, there is reason to feel that
trends depicted by the model are credible given that the
NST 3.0 model is based on mechanistic principles and its
outputs have proven reliable in a variety of situations
[18,22].
Possibly the most useful outcome of this analysis is
that model results point to a number of areas that deserve
further exploration through experimentation. For exam-
ple, the sensitivity analyses indicate that a better under-
standing of the impact of increasing temperature on plant
nutrient uptake kinetics could be informative. This ap-
pears particularly true in the case of NH4, since red
spruce may respond differently to increases in soil supply
if it is not N-limited. Similarly, changes in soil supply as
reflected by changes in soil solution nutrient concentra-
tions might be the most informative parameter and the
least complicated to explore experimentally. As noted by
Bassirirad [40], there is an urgent need to generate more
data in this area and future studies of plant response to
global warming should integrate measurements of soil
nutrient supply and absorption into study plans.
Although it was not the intent of this study to explore
the ecological or whole plant implications of the simu-
lated changes, the potential ecological response to re-
duced P supply, in conjunction with an increase in P up-
take, seems the most provocative. The potential impact
of increased nitrogen availability on competition with
other species for site dominance, possible changes in red
spruce cold tolerance, and/or increases in nitrate produc-
tion and nitrate leaching of cations also have important
ecological implications and warrant further evaluation.
5. Acknowledgements
The authors express their appreciation to Dr. Norbert
Claassen for making the NST 3.0 Model available for
public use. Thanks also to Dr. J. K. Kelly for his assis-
tance in the preparation of the figures and to Alan Mays
for his assistance with most of the field studies that un-
derlie the papers from which these data were taken.
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