Kramer, K.

Mohren, G.M.J.

English

NARCIS 

Introduction: forest growth and forest ecosystem carbon budgets in relation to climate change.

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Alterra-report 19-Ch1 13
1 Introduction: forest growth and forest ecosystem carbon
budgets in relation to climate change
G.M.J. Mohren & K. Kramer
The central objective of the LTEEF-II project was to assess climate change impacts
on European forests, in terms of water and carbon fluxes, regional differences, long-
term effects, and the overall carbon budget for forests in Europe. The results of this
assessment were used to identify forest management strategies that account for these
impacts, and that maximise carbon sequestration. The LTEEF-II project mainly
consisted of A) assessment and modelling of the long-term regional impacts of
climate change on European forests, and B) upscaling of these regional responses to
the European scale, to quantify the overall carbon budget and sustainable wood
supply of European forests. As part of the long-term impact assessment, emphasis
was on growth rates and water relations (drought), and on possible adaptive
strategies for forest management. The overall forest sector carbon budget was
quantified under present climate and under expected future climate change.
The LTEEF-II project (1998-2000) comprised the second phase of a major research
effort undertaken as part of the EU Environment and Climate research program under
the 4th Framework Program of the European Commission. In the first phase of LTEEF
(1994-1997), existing forest models where extended and applied to conditions represen-
tative of a range of sites throughout Europe, under present-day and future climate
scenario's. In this part of the project, emphasis was on model development and model
comparison (Mohren, 1999; Mohren & Kramer 1997). In the second phase of the
LTEEF project (LTEEF-II), model performance was evaluated by application to the
EUROFLUX sites, by comparing model output with the eddy-covariance flux
estimates. (EUROFLUX was a largely EU-funded European flux network, in which
carbon and water fluxes over forests were monitored using eddy-covariance techniques,
at more than 15 sites in a range of forest types from northern Sweden to Italy). The
model comparison was done in close collaboration with the EUROFLUX consortium,
and the most recent experimental data, such as from the ECOCRAFT project (Jarvis,
1998), was used as input data to parameterise the process descriptions in the models. In
addition to the comparison against flux data, the models were evaluated against long-
term growth and yield data from permanent plots as used in traditional growth and yield
research. Based on this model evaluation, regional impact studies were carried out to
assess potential climate change impacts for a range of forest ecosystems throughout
Europe. Using the models in combination with climate scenarios as identified from
global climate modelling, forest management response strategies were identified, and
evaluated with respect to the consequences for forest ecosystem carbon budgets. The
regional impacts assessments were scaled up to a European level, using a) national
forest inventory data and b) remote sensing techniques (Karjalainen et al., 1997; Grace et
al., 1999; Veroustraete, 1994; Veroustraete et al. 1996). This resulted in estimates of the
current carbon balance for the European forest sector, as well as assessments of
14 Alterra-report 194-Ch1
timber production and forest carbon budgets per country under future climate
scenarios, including estimates of carbon sink strength.
1.1 Model evaluation
The proces models used were tested using the same site and species information so
that differences as a consequence of differences in parametrisation is ruled out as
much as possible. The criteria for the comparison and selection of models included:
1) model evaluation against short-term flux data; 2) model evaluation against long-
term growth & yield data; 3) a sensitivity analysis, to determine the response of
selected model output to climate change scenario's (temperature, CO2 and
precipitation, independently and jointly); 4) an uncertainty analysis to attribute
uncertainties in the model output to uncertainties in model input; and 5) availability
of technical documentation of the model.
1.2 Forest ecosystem carbon budgets
The components of the forest carbon budget include: the above- and below ground
biomass, the soil organic matter and the forest products. A typical carbon budget of a
managed forest is presented in Figure 1. A total of 15 Mg C ha-1yr-1 is annually fixed
by gross photosynthesis. 7 Mg C ha-1yr-1 of this amount is transferred to the soil
organic matter pool through litter fall and root turnover, 6 Mg C ha-1yr-1 of biomass
C is respired, and an average annual increment of 2 Mg C ha-1yr-1, equivalent to some
8-10 m3 ha-1yr-1 is harvested. The soil organic matter pool in this example releases
somewhat less carbon as compared to the inputs through litter and root turnover,
resulting in a net accumulation of 0.5 Mg C ha-1yr-1. The carbon in wood products
that leave the forest is in the long-term brought back to the atmosphere by product
decomposition. Consequently, the measured Net Ecosystem Exchange (NEE) of 2.5
Mg C ha-1yr-1 (15 Mg C ha-1yr-1 uptake by plant photosynthesis minus 12.5 Mg C ha-
1yr-1 by ecosystem respiration) decreases to 0.5 Mg C ha-1yr-1 when product
decomposition outside the forest is taken into account.
Alterra-report 19-Ch1 15
∆ 0.0
∆ 0.0
SOM
∆ 0.5
15.0
5.0
1.0
6.5
12.5
(5.0)
4.0
3.0
0.5
products
∆ 0.02.0
2.0
biomass
biomass
Figure 1.1: A general carbon budget for managed forest, accounting for wood harvest and product decay. All values refer to
annual rates, in Mg C ha-1yr-1. Of the total annual uptake of 15 Mg C ha-1yr-1, 5 Mg C (35 %) is assumed to be allocated
to below-ground biomass. Litter fall is assumed to be 3 Mg C ha-1yr-1, and root turnover is assumed to be equal to 4 Mg C ha-
1yr-1. Average annual wood harvest is assumed to be equal to annual increment, i.e. 2 Mg C ha-1yr-1. The amount of carbon in
soil organic matter (SOM) is assumed to accumulate at a rate of 0.5 Mg C ha-1yr-1. The total amount of carbon in products is
assumed to be constant, hence the average amount of carbon harvested is equal to the amount of carbon released in product
decomposition, i.e. 2 Mg C ha-1yr-1.
16 Alterra-report 194-Ch1
To assess the impacts of climate change on overall carbon budgets one needs to take
into account all components described above, including the role of forest
management. Changes in temperature, atmospheric CO2 concentrations and
precipitation directly affect the rates of gross photosynthesis and the decomposition
of organic matter in the soil, which can be evaluated by process-based forest growth
models. Forest management plays and important factor as it controls the rate at
which carbon in woody products leaves the forest, and as it affects, through thinning
and harvesting, also the growth rates of the remaining trees. Other disturbances of
the forest ecosystem such as fires, storm, flooding and landslides need also be taken
into account to quantify the carbon budget at the regional scale, but are outside the
scope of this project.
1.3 Discussion and conclusions
The availability of fluxes of CO2 and H2O at the stand level provides an
unprecedented opportunity to test stand-level forest growth models. Earlier attempts
of this have been made (e.g. Vermetten et al, 1994), but the flux data currently
collected by the EUROFLUX project and elsewhere in the world provide high
resolution data that make model testing over a wide climatic range and over different
forest types possible. In an earlier model comparison the different models showed
highly divers results because of differences in model assumptions and description of
processes (Sonntag, 1997). The availability of independent data now allowed
selection of those models that are most relevant for specific climatic regions and
forest types. This selection was critical to assess the sensitivity of the different forest
types in Europe to changes in climatic parameters like temperature and precipitation,
and atmospheric CO2 concentration. Carefully formulated and validated models are
the only means to do this as large scale experiments over long time horizons are
impossible to perform. Sensitivity of model output to changes in climatic parameters
provided insight into the relevance of the mechanisms involved in the impact of
climate change on forests. The first of such sensitivity analyses have evaluated the
importance of water and water-stress (Van Wijk et al., 1999; Loustau et al. 1997), and
the importance of differences between species in phenological response to
temperature (Kramer, 1995; Kramer & Mohren, 1996).
The selection of models was also critical for the scaling up from the stand to the
regional level in a future climate. The principle ways of scaling up were aplied in the
LTEEF project: 1) by the adjustment of growth and yield tables of many forest types
based on the sensitivity to climatic parameters of the selected forest growth models,
combined with a large database of the distribution of these forest types over Europe;
2) by formulating simple models that adjust the radiation use efficiency (Bartelink et
al., 1997) also based on the sensitivity to climatic parameters of the selected forest
growth models; and 3) simplified process models. These approaches allowed to
assess the sink strength of European forests in the coming decades (Nabuurs &
Mohren, 1995; Nabuurs et al., 1997; Martin et al., 1998).
Alterra-report 19-Ch1 17
1.4 References
Bartelink, H.H., K. Kramer & G.M.J. Mohren, 1997. Applicability of the radiation-
use efficiency concept for simulating growth of mixed forest stands. Agricultural and
Forest Meteorology 88: 169-179.
Grace, J., F. Veroustraete & T. Karjalainen, 1999. A methodologies for estimating
the forest carbon budget for Europe (in press, these proceedings).
Jansen M.J.W & Withagen J.C.M., 1997. USAGE: uncertainty and sensitivity analysis
in a Genstat environment. Manual. Report CBW, 14 pp.
Jansen M.J.W., Rossing W.A.H. & R.A. Daamen, 1994. Monte Carlo estimation of
uncertainty contributions from several independent multivariate sources. In: J.
Grasman & G. Verstraten (Eds.) Predictability and nonlinear modelling in natural sciences and
economics. Kluwer, Dordrecht, The Netherlands, pp. 334-343.
Jarvis, P.G. (ed.), 1998. European forests and global change. The likely impacts of
rising CO2 and temperature. Cambridge University Press, 380 pp.
Karjalainen, T., G.-J. Nabuurs & S. Kellomäki, 1997. Carbon sequestration in the
forest sector under climate change: upscaling from the plot level to the European
forest level. In: G.M.J. Mohren, K. Kramer & S. Sabaté (eds.): Impacts of Global Change on
Tree Physiology and Forest Ecosystems. Proceedings of an International Conference held 26-29
November 1996, Wageningen, The Netherlands. Dordrecht, Kluwer Academic
Publishers, p. 351-361.
Kramer, K., 1995. A modelling comparison to evaluate the importance of phenology on
the effects of climate change on growth of temperate-zone deciduous trees. Climate
Research 5: 119-130.
Kramer, K. & G.M.J. Mohren, 1996. Sensitivity of FORGRO to climatic change
scenarios. A case study on Betula pubescens, Fagus sylvatica and Quercus robur in The
Netherlands. Climate Change 34: 231-237.
Kramer, K., M.T. van Wijk, M.J.W. Jansen, E.M. Falge & G.M.J. Mohren, 1999.
Application of the model FORGRO to 7 EUROFLUX sites: evaluation of climate
change scenarios, long-term effects and uncertainty analysis. Agricultural and Forest
Meteorology, submitted.
Loustau, D., P. Berbigier & K. Kramer, 1997. Sensitivity of the water balance of
South-western France maritime pine forests to climate. In: G.M.J. Mohren, K. Kramer
& S. Sabaté (eds.): Impacts of Global Change on Tree Physiology and Forest Ecosystems.
Proceedings of an International Conference held 26-29 November 1996,
Wageningen, The Netherlands. Dordrecht, Kluwer Academic Publishers, p. 193-205.
18 Alterra-report 194-Ch1
Magnani F., Borghetti M. & J. Grace, 1996. Carbon allocations and tree growth
under hydraulic constraints in Pinus sylvestris L. In Miglietta F., van Laar G. & J.
Goudriaan (eds.): System Analysis and Simulation for Agricultural Science: a contribution of the
RAISA Project. CABO-DLO.
Magnani F., Leonardi S., Tognetti R., Grace J. & M. Borghetti 1998. Modelling the
surface conductance of a road-leaf canopy: effects of partial decoupling from the
atmosphere. Plant Cell and Environment 21: 867-879.
Magnani F., Mencuccini M. & J. Grace, 1999. Age-related decline in stand
productivity: the role of structural acclimation under hydraulic constraints.
Submitted.
Martin, Ph. et al., 1998. New estimate of the carbon sink strength of EU forests
integrating flux measurements, field surveys, and space observations. Ambio 27: 582-
584.
Mohren, G.M.J., 1994. Modelling Norway spruce growth in relation to site conditions
and atmospheric CO2. In: F. Veroustraete, R. Ceulemans et al. (eds.): Vegetation, Modelling
and Climate Change Effects. SPB Academic Publishing, The Hague, Netherlands, pp. 7-22.
Mohren, G.M.J., 1999. Long-Term Effects of CO2-Increase and Climate Change on
European Forests (LTEEF: EV5V-CT94-0468). Proceedings EU-Climate
Conference, Orvieto, in press.
Mohren, G.M.J., H.H. Bartelink, I.T.M. Jorritsma & K. Kramer 1993. A process-based
growth model (FORGRO) for analysing forest dynamics in relation to environmental
factors. In: M.E.A. Broekmeyer, W. Vos & H. Koop (eds): European Forest Reserves.
Proceedings of the European Forest Reserves Workshop, 6-8 May 1992, The
Netherlands. PUDOC, Wageningen, p. 273-279.
Mohren, G.M.J. & K. Kramer, 1997. Simulation of direct effects of CO2 and
temperature increase on forest growth: the LTEEF project. In: G.M.J. Mohren, K.
Kramer & S. Sabaté (eds.): Impacts of Global Change on Tree Physiology and Forest Ecosystems.
Proceedings of an International Conference held 26-29 November 1996,
Wageningen, The Netherlands. Dordrecht, Kluwer Academic Publishers, p. 307-316.
Nabuurs, G.J., A.J. Dolman, E. Verkaik, A.P. Whitmore, W.P. Daamen, O. Oenema,
P. Kabat, G.M.J. Mohren, 1999. Resolving the issues on terrestrial biospheric sinks in
the Kyoto protocol. Dutch National Research Programme on Global Air Pollution and
Climate Change, Report No. 410 200 030, 100 pp.
Nabuurs, G.J. & G.M.J. Mohren, 1995. Carbon sequestering potential of selected
world forest ecosystems. Canadian Journal of Forest Research 25: 1157-1172.
Alterra-report 19-Ch1 19
Nabuurs, G.J., R. Päivinen, R. Sikkema & G.M.J. Mohren, 1997. The role of
European forests in the global carbon cycle - a review. Biomass and Bioenergy 13: 345-
358.
Sonntag, M., 1997. Comparison of forest growth models at the leaf and canopy scale.
In: G.M.J. Mohren, K. Kramer & S. Sabaté (eds.): Impacts of Global Change on Tree Physiology
and Forest Ecosystems. Proceedings of an International Conference held 26-29
November 1996, Wageningen, The Netherlands. Dordrecht, Kluwer Academic
Publishers, p. 299-306.
Van der Voet, H. & G.M.J. Mohren, 1994. An uncertainty analysis of the process-
based growth model FORGRO. Forest Ecology and Management 69: 157-166.
Van Wijk, M., S.C. Dekker, W. Bouten, F.C. Bosveld, W. Kohsiek, K. Kramer &
G.M.J. Mohren, 1999. Evaluation of possibilities to incorporate water stress effects
in three stomatal conductance models when modelling daily ecosystem fluxes of a
Douglas fir forest. Submitted.
Vermetten, A., L. Ganzevel, A. Jeuken, P. Hofschreuder & G.M.J. Mohren, 1994. CO2
uptake by a stand of Douglas-fir: Flux measurements compared to model
calculations. Agricultural and Forest Meteorology 72: 57-80.
Veroustraete F., 1994. On the use of a simple deciduous forest model for the
interpretation of climate change effects at the level of carbon dynamics. Ecological
Modelling 75-76: 221-237.
Veroustraete, F.,R. B. Myneni & J. Patyn, 1996. Estimating Net Ecosystem Exchange
of Carbon Using the NDVI and an Ecosystem Model. Remote Sensing of Environment
58: 115-130.


A methodologies for estimating the forest carbon budget for Europe (in press, these proceedings).

A modelling comparison to evaluate the importance of phenology on the effects of climate change on growth of temperate-zone deciduous trees.

A process-based growth model (FORGRO) for analysing forest dynamics in relation to environmental factors. In:

Age-related decline in stand productivity: the role of structural acclimation under hydraulic constraints.

An uncertainty analysis of the processbased growth model FORGRO.

Applicability of the radiationuse efficiency concept for simulating growth of mixed forest stands.

Application of the model FORGRO to 7 EUROFLUX sites: evaluation of climate change scenarios, long-term effects and uncertainty analysis. Agricultural and Forest Meteorology,

Carbon allocations and tree growth under hydraulic constraints in Pinus sylvestris L.

Carbon sequestering potential of selected world forest ecosystems.

Carbon sequestration in the forest sector under climate change: upscaling from the plot level to the European forest level.

CO2 uptake by a stand of Douglas-fir: Flux measurements compared to model calculations. Agricultural and Forest

Comparison of forest growth models at the leaf and canopy scale.

Estimating Net Ecosystem Exchange of Carbon Using the NDVI and an Ecosystem Model. Remote Sensing of

European forests and global change. The likely impacts of rising CO2 and temperature.

Evaluation of possibilities to incorporate water stress effects in three stomatal conductance models when modelling daily ecosystem fluxes of a Douglas fir forest.

Long-Term Effects of CO2-Increase and Climate Change on

Modelling Norway spruce growth in relation to site conditions and atmospheric CO2.

Modelling the surface conductance of a road-leaf canopy: effects of partial decoupling from the atmosphere.

Monte Carlo estimation of uncertainty contributions from several independent multivariate sources.

New estimate of the carbon sink strength of EU forests integrating flux measurements, field surveys, and space observations.

On the use of a simple deciduous forest model for the interpretation of climate change effects at the level of carbon dynamics.

Resolving the issues on terrestrial biospheric sinks in the Kyoto protocol.

Sensitivity of FORGRO to climatic change scenarios. A case study on Betula pubescens, Fagus sylvatica and Quercus robur in The Netherlands.

Sensitivity of the water balance of South-western France maritime pine forests to climate.

Simulation of direct effects of CO2 and temperature increase on forest growth: the LTEEF project.

USAGE: uncertainty and sensitivity analysis in a Genstat environment.

Introduction: forest growth and forest ecosystem carbon budgets in relation to climate change.

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Wageningen Yield