PRODUCTIVITY OF BOREAL TREES IN CHANGING CLIMATE
P. KOLARI and E. NIKINMAA
Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland
Keywords: climate change, photosynthesis, Scots pine, birch
INTRODUCTION
We estimated the potential increase in photosynthetic productivity in Scots pine (Pinus sylvestris) and
silver birch (Betula pendula) due to direct effects of increasing temperature and CO2. Secondly, we
studied how changes in the soil nutrient cycling are reflected in the biomass production of pine.
METHODS
Photosynthetic production was calculated half-hourly using biochemical model of photosynthesis
(Farquhar et al., 1980) along with stomatal model of Leuning (1990). The seasonality of photosynthetic
capacity and quantum yield in pine were described as delayed temperature response (Mäkelä et al., 2004).
The model parameters were estimated from multiannual time series of shoot and leaf gas exchange in pine
shoots in Hyytiälä, Southern Finland. For birch, the annual cycle model was replaced by simple
temperature-driven model of leaf unfolding and daylength-triggered leaf senescence that determined the
seasonal development of the leaf area index of the stand. The parameters of the photosynthesis model
were estimated from gas exchange of aspen leaves in Hyytiälä.
The stand photosynthesis was calculated with SPP (Mäkelä et al., 2006). In the simulations tree
dimensions, leaf area index and tree density were typical for a middle-aged stand. The model was run with
a climate change scenario that corresponds to approx. 50% increase in the emissions of CO2 from fossil
fuel combustion by 2050 and a slow decline after that (Table 1). Modified weather data from Hyytiälä was
used as the model input. All half-hourly records of air temperature and atmospheric CO2 were increased
by the mean annual temperature rise and CO2 increase, respectively. Water vapour concentration in the air
was altered so as to keep relative humidity unchanged.
Biomass production of pine was further studied with MicroForest (Hari et al., 2008) that incorporates soil
nitrogen (N) cycling and changing allocation into foliage, wood and roots. The key parameters of the
model are annual photosynthetic production in unshaded conditions, decomposition rate of proteins in the
soil, and nitrogen deposition. The annual photosynthesis was obtained from simulations with increased
CO2 and temperature. The rate of decomposition was increased by 6% per °C rise in temperature. Nitrogen
deposition was assumed to remain at the present level.
Year
2025
2055
2085
CO2 (ppm)
430
540
650
T increase (deg C)
1
2
3
Table 1. Projected increase in atmospheric CO2 and mean annual temperature in Finland according to
Jylhä et al. (2009).
RESULTS
Annual photosynthetic production will increase more in birch due to steeper instantaneous temperature
response of photosynthesis than in pine (Figure 1). Most of the increase can be attributed to longer
growing season, in midsummer the simulated momentary photosynthetic rates in 2085 are only 10–15%
higher than in the present climate. Enhanced N cycling and change in within-tree biomass allocation
allowed for additional increment of approximately 20% in pine stemwood production. Increasing CO2
enhances water-use efficiency as the stomata tend to open less at elevated CO2 than in present CO2. This
means that drought will probably remain minor risk in Finnish conditions in the future.
Relative productivity (%)
160
140
120
100
80
60
Pine
Birch
Pine with enhanced N cycling
40
2000
2020
2040
Year
2060
2080
2100
Figure 1. Predicted relative (year 2000 = 100) annual photosynthetic production in middle-aged pine and
birch stands (only direct effect of temperature and CO2 considered) and in the stemwood production of
pine (enhanced N cycling and changed allocation patterns taken into account) until year 2085.
ACKNOWLEDGEMENTS
This work was supported by the EU-funded project VACCIA (Vulnerability assessment of ecosystem
services for climate change impacts and adaptation).
REFERENCES
Farquhar, G.D., S. von Caemmerer and J.A. Berry (1980). A biochemical model of photosynthetic CO2
assimilation in leaves of C3 species. Planta 149, 78.
Hari, P., M. Salkinoja-Salonen, J. Liski, A. Simojoki, P. Kolari, J. Pumpanen, M. Kähkönen, T. Aakala,
M. Havimo, R. Kivekäs and E. Nikinmaa (2008). Growth and development of forest ecosystems;
The MicroForest Model. In: Hari, P. and L. Kulmala (eds.) Boreal forest and climate change.
Advances in Global Change Research, Vol. 34 (Springer Verlag), 433.
Jylhä, K., K. Ruosteenoja, J. Räisänen, A. Venäläinen, H. Tuomenvirta, L. Ruokolainen, S. Saku and T.
Seitola (2009). Changing climate in Finland: estimates for adaptation studies. Finnish
Meteorological Institute, Reports 2009: 4.
Leuning, R. (1990). Modelling stomatal behaviour and photosynthesis in Eucalyptus grandis. Australian
Journal of Plant Physiology 17, 159.
Mäkelä, A., P. Hari, F. Berninger, H. Hänninen and E. Nikinmaa ( 2004). Acclimation of photosynthetic
capacity in Scots pine to the annual cycle temperature. Tree Physiology 24, 369.
Mäkelä, A, P. Kolari, J. Karimäki, E. Nikinmaa, M. Perämäki and P. Hari (2006). Modelling five years of
weather-driven variation of GPP in a boreal forest. Agricultural and Forest Meterology 139, 382.