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SENSITIVITY OF FORGRO TO CLIMATIC CHANGE SCENARIOS: A CASE STUDY ON BETULA PUBESCENS, FAGUS SYLVATICA AND QUERCUS ROBUR IN THE NETHERLANDS K. K R A M E R and G. M. J. M O H R E N Institute for Forestry and Nature Research, IBN-DLO, P.O. box 23, 6700 AA Wageningen, The Netherlands
Abstract. The impacts of the climate change predictions of four general circulation models (GFDL, GISS, OSU and UKMO) on net primary production (NPP) ofBetula pubescens, Fagus sylvatica and Quercus robur in The Netherlands were analysed using the process-based model FORGRO. FORGRO is a model suitable to simulate growth of managed mono-species stands. For the GCMs mentioned, both transient and equilibrium 2 x CO2 scenarios of temperature and precipitation change were evaluated and compared with responses under current climate. It was found that the NPP increases in the transient scenarios, but remains the same or declines in the 2 x CO2 scenarios. This is because respiration increases more with rising temperature than photosynthesis. During the transient scenarios this effect gradually increases, while in the 2 x CO2 scenario this effect is operating over the entire simulation period. If water limitation is taken into account, then the NPP of the reference scenario is reduced. In both the transient and 2 x CO2 scenarios this water limitation is annulated, resulting in a stronger response of NPP compared to the situation without water limitation. This enhancement of the response is most pronounced in the transient scenario due to the gradual effect of temperature on respiration. Similar results were obtained with a version of FORGRO in which the photosynthesis module of HYBRID (PGEN) is incorporated, although the response in FORGRO-PGEN is usually higher than that of FORGRO. This is because the response of photosynthesis to CO2 rises with increasing temperature as defined in the PGEN-model, but not according to FORGRO.
1. Introduction Most models describing the effects of climate change on the distribution and dynamics of forests operate on a yearly timestep and do not explicitely consider the effects of atmospheric [CO2], temperature and precipitation on photosynthesis. On the other hand, most models incorporating detailed descriptions of light interception and photosynthesis are currently only applicable to coniferous tree species. To evaluate the impact of climatic wanning on deciduous tree species, a model predicting the date of leaf unfolding and leaf fall as a function of temperature is required. This was done in earlier work (Kramer, 1994a, b). In this study the process-based forest growth model FORGRO (Mohren, 1987, 1994; Mohren et al., 199.3) was used to evaluate the potential impacts of climate change scenarios on three deciduous European tree species. Four GCMs were evaluated because the different general circulation models (GCMs) do not agree on the magnitude of the change in temperature and precipitation for a doubles atmospheric concentration of CO2. To compare the sensitivity of FORGRO for the different GCMs, with that Climatic Change 34: 231-237, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.
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of HYBRID (Friend, 1993), the module of HYBRID that calculates the rate of photosynthesis, PGEN (Friend 1991, 1995), was coupled to FORGRO.
2. Models FORGRO (FORest GROwth, Mohren, 1987, 1994; Mohren et al., 1993) is a process-based model suitable to predict growth of even-aged mono-species stands of trees. It simulates growth processes on a daily basis as a function of the environmental variables: incoming radiation, minimum and maximum temperature, humidity, precipitation and wind speed. It uses thinning regimes commonly applied in managed forests rather than simulating natural patterns of tree mortality. Central to FORGRO is the interception of light by the canopy, and the hourly integration of photosynthesis. The vertical profile of the canopy is defined using hourly integration of photosynthesis over five shaded and sunlit layers. The photosynthesis-light response curve is modelled using a negative exponential curve. The initial light-use efficiency, e, of this curve is affected by both the CO2 compensation point, F, and the ambient CO2 concentration, Ca, according to: (Ca - I') / (Ca + 21-'). F rises with increasing temperature according to an exponential function. The asymptote of the light response curve is determined as the minimum of either a species-specific maximum value of photosynthesis, or the CO2-1imited rate of photosynthesis, Fn, c. Fn, c is calculated as: Fc, n = (Ca - F)/(rm + 1.6rs + 1.4rb) with rm, rs and rb the mesophyll, stomatal and boundary layer resistance, respectively, rm is calculated as: rm = (Ci - F)/Fm,m, with Ci the internal CO2 concentration, and Fm, m the photosynthetic capacity. A constant ratio of internal to external CO2 concentration is assumed, rb was set at a constant value, and rs depends on temperature and vapour pressure deficit based on linear interpolation of experimental data. The temperature dependency of the rate of photosynthesis is based on the same approach, whereas an exponential function is used for the temperature dependency of respiration. More details of this approach can be found in Goudriaan and Unsworth (1990), and Goudriaan and van Laar (1994). Table I presents the relevant characteristics for the species included in FORGRO. PGEN (Friend, 1993, 1995) is a model aiming to predict leaf photosynthesis at the biochemical level (Farquhar and yon Caemmerer, 1982), and assumes stomatal conductance to be optimized instantaneously as a trade-off between CO2 gain and water losses via the stomates. The demand for CO2 is determined either by carboxylation limitation of Rubisco, or by regeneration limitation of RuBP, which is a substrate of Rubisco. Whether the CO2 supply meets the photosynthetic demands depends on the resistance to CO2 along the pathway from outside the leaf boundary layer to the mesophyll cells. Explicit functions for both the boundary layer and mesophyll resistance are presented in PGEN, while the stomatal resistance is optimized numerically. PGEN was coupled to FORGRO by adapting it such that all abiotic variables and the leaf water potential, which are calculated daily by
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Table I Characteristics in which the species differ in FORGRO. SLA: specificleaf area (m2 kg-l), Lma~:maximal leaf area index (m2 m-2), Pn, ma~:maximal rate of net photosynthesis (mg CO2 m - 2 S - t ) , ~U/~T: shift in the date of leaf unfolding, U, with changing temperature, T (d ~C-Z), as predicted by the sequential model (Kramer, 1994b) B. pubescens
SLA
25 4 P,, max 0.69 ~U/6T -5.0 Lmax
F. sylvatica
Q. robur
20 6 0.56 -3.6
15 4 0.42 --4.6
FORGRO, are input to PGEN. Output of PGEN is the rate of gross photosynthesis in each leaf layer. For FORGRO-PGEN only one species, B. pubescens, was evaluated. Parameters were not adjusted from those presented in Friend (1995). To evaluate the relative importance of the change in either temperature or precipitation, situations both with and without water limitation were analyzed. In FORGRO, photosynthesis is reduced proportionally to the ratio of actual and potential transpiration. In PGEN both photosynthesis and dark respiration are reduced proportionally to the leaf water potential (lwp). The reduction factor decreases linearly from unity when lwp = 0 MPa, to zero if lwp = -1.65 MPa. The date of bud burst of B. pubescens, F. sylvatica and Q. robur was modelled using the approach of Hanninen (1990) in which chilling and forcing temperatures are required sequentially in time to release dormancy. The sequential model was fitted to data of leaf unfolding of B. pubescens, F. sylvatica and Q. robur observed in the Netherlands (Kramer, 1994b). It explained 86, 68 and 82% of the variance in the date of leaf unfolding of B. pubescens, F. sylvatica and Q. robur, respectively. The date of leaf fall was set at a constant, but species-specific, date. To summarize the results a logarithmic response function of net primary production to [CO2] was used (Goudriaan, 1993):
NPP NPPo
-
1 + / 3 In
/ ICOn] k [C02]0/
(1)
The subscript' 0' indicates the current climate. In this study the 100-year average of the annual NPP was used. For the current climate [CO2] = 350 ppm(v), and for both the transient and 2 x CO2 scenarios [CO2] -- 700 ppm(v) was used. The response factor/3 is about 0.7 when [CO2] doubles under good growing conditions (Goudriaan and Unsworth, 1990).
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3. Data
Four climate change scenarios as predicted by the general circulation models GFDL, GISS, OSU and UKMO (see Lauenroth, this issue) were evaluated for one site in The Netherlands (52 ~ N, 5 ~ E). For this location (grid cell) the mean annual temperature increase is 5.3, 4.0, 3.0 and 6.5 ~ according to the GFDL, GISS, OSU and UKMO scenario, respectively. The mean annual increase in precipitation for these scenarios is 7, 15, 7 and 22%, respectively. Because FORGRO requires daily input of meteorological data, a synthetic weather series was generated using the statistical model WGEN (Richardson and Wright, 1984). The parameters for this model were estimated using 30 years of daily observations at De Bilt (52 ~ N, 6 ~ E). With WGEN 100 years of synthetic weather was generated and taken as the current climate. Four transient scenarios were made by superimposing on this series a linear increase in both precipitation and temperature from the current to the 2 • CO2 scenarios as predicted by the GCMs, to occur in a time span of 100 years. Furthermore, 100 years of 2 • CO2 weather was constructed by superimposing the change in temperature and precipitation as predicted by the four GCMs in a 2 x CO2 world on the same weather series. Thus, for each species and for each GCM- scenario, three simulations were undertaken: 1. 100-year current (i.e. the generated weather series), 2. 100-year transient (i.e. linear change of the generated weather series from current to 2 x CO2, for each GCM-scenario), and 3. 100-year 2 x CO2 weather (i.e. generated weather series adjusted to climate for equilibrium 2 x CO2 scenario, for each GCM-scenario). Because FORGRO does not simulate population dynamics of forests but a thinning regime, the values of the biomass of foliage, branches, stem, coarse roots and fine roots were reset to the initial values at the start of each 100-year run.
4. Results
The net primary production (NPP) calculated by FORGRO was found to increase for all three species over the 100 year period of transient climate changes. However, for the 2 • CO2 equilibrium climate conditions NPP either remains approximately at the same level or is reduced compared to the current climate (Figure 1). When comparing the different GCMs, UKMO consistently shows the smallest increase of NPP for the transient climate, or the largest reduction of NPP for the 2 • CO2 climate. The scenarios most profitable for growth are GISS and OSU, while GFDL takes an intermediate position (Figure 1). In the situation where water is not limiting it appears that, according to this parametrization of FORGRO, F. sylvatica increases its NPP in all scenarios except for the 2 x CO2 UKMO climate (Table II). B. pubescens responds similar as F. sylvatica, but shows a decreased NPP for the 2 • CO2 climate of GFDL and
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Net Primary Production 15
B. pubescens
F. sylva .a
Q robtr
m
ourront
E2~ GFDL
QISS
NN OSU
m
T0
] transient 2 x CO=
[ transient ] 2 x CO 2
[ transient [ 2 x CO 2
Figure 1. Net primaryproductionofB. pubescens, E sylvatica and Q. robur of the current,transient and 2 x CO2 scenarios,calculatedby FORGROwithoutwaterlimitations.
UKMO. Q. robur shows the least increase in NPP in all scenarios, and strongly reduces its NPP in all the 2 x CO2 scenarios, as well in the transient UKMO scenario. A similar pattern of response of NPP to the scenarios was found for FORGRO-PGEN. Although the response according to FORGRO-PGEN is higher than that of FORGRO, even a reduction in NPP was found for the 2 x CO2 UKMO scenario. When water can be limiting then the response of NPP to the scenarios is higher compared with the not water-limited situation (Table II). This effect is most pronounced in the transient scenarios, for both FORGRO and FORGRO-PGEN.
5. Discussion and Conclusions
The results show that the magnitude of the response of net primary production to a doubling of the CO2 concentration is increasingly hampered with rising temperature. This is caused by the stronger increase of the respiratory costs with increasing temperature, compared to the increase of photosynthesis with both CO2 and temperature. In a more detailed model comparison between FORGRO and FORGRO-PGEN, but on a shorter time-span, Kramer (1995) found that the breakeven temperature given a doubled [CO2] is between 4 and 6 degrees temperature rise for FORGRO, depending on the parametrization. Goudriaan and Unsworth (1990) found similar results for potential yield of grain. The break-even temperature given a doubled [CO2] for this crop was 4.5 Kelvin. However, for FORGRO-PGEN the response of gross photosynthesis to a doubled [CO2] was enhanced at higher temperatures. This CO2 x temperature interaction is such that the break-even temperature exceeded 7 degrees temperature rise, if the direct effects of leaf water
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Table II Values of the response factor/3 of the different scenarios relative to the current climate for FORGRO and PGEN coupled to FORGRO,with and without water limitations Transient GFDL GISS OSU B. pubescens F. sylvatica Q. robur FORGRO-PGEN
0.34 0.37 0.10 0.81
0.39 0.39 0.25 0.76
0.41 0.39 0.26 0.77
B. pubescens F. sylvatica Q. robur FORGRO-PGEN
1.23 1.34 0.55 1.65
1.30 1.38 0.75 1.69
1.32 1.38 0.77 1.72
UKMO
2 x CO2 GFDL GISS OSU
Without water limitations 0.24 --0.06 0.07 0.37 0.03 0.12 -0.13 -0.50 -0.06 0.69 0.37 0.36 With water limitations 1.07 -0.14 1.34 -0.08 0.26 -1.39 1.51 0.12
0.25 0.29 0.14 0.32
UKMO
0.11 -0.45 0.14 0.00 -0.02 -0.95 0.41 -0.18 0.27 -0.46 0.30 0.02 0.18 -1.43 0.39 -0.73
potential on both photosynthesis and dark respiration are not taken into account (Kramer, 1995). Thus the order, from high to low, of the response of NPP of the GCMs (OSU, GISS, GFDL, UKMO) was caused by the increase in the predicted temperature (3.0, 4.0.5.3 and 6.5 Kelvin, respectively). In the transient scenarios the growth reducing effect of an higher temperature are imposed gradually, resulting in a higher NPP than in the 2 • CO2 scenarios, where a high temperature affects growth during the entire simulation period. In some occasions, the cost of respiration exceeded the gains of photosynthesis in the last years of the 2 x CO2 scenarios. Consequently, the reserve level is rapidly emptied, the build-up of the canopy is not possible anymore and growth stops. This explains the low values of Q. robur in the 2 x CO2 GFDL and UKMO scenarios. In the transient scenarios this die-off of the forest does not occur. Thus the value of fl depends on the duration of the simulated time span and on the initial value of the biomass of the stand. Furthermore, it was found that when water can be limiting, then the response to the scenarios is enhanced compared with the not water-limited situation. This is the consequence of the fact that in the reference climate, photosynthesis and consequently NPP, is frequently reduced due to water shortage. In the GCM scenarios this reduction occurs much less frequently due to the predicted increase of precipitation by 7 to 22%. In an absolute sense the NPP of the scenarios of the not water-limited situation is similar as the water-limited situation, while the reference NPP in the water-limited situation is less compared with the not water-limited situation. This aspect agrees with earlier findings (Goudriaan, 1993; Mohren, 1994). The most striking aspect of the results presented in Table II is the variability of the response factor for different species and different scenarios. This makes it difficult to use one fl value to characterize an entire ecosystem, and evaluate the impacts of climatic change on a global level.
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