Global climate change is expected to manifest itself in Finland as higher temperatures, heavier rainfall and more rain or snow in the winter (Carter et al. 2005, Jylhä et al. 2009). At the same time, forest policies have widened to incorporate new objectives, such as carbon sequestration, biodiversity protection and biofuel production. These changes challenge our methods of predicting forest growth and making decisions about appropriate forest management under the new circumstances.

We are developing a modular system of eco-physiological and ecological models for analysing and predicting various impacts of climate change on the growth of forests. The system should be applicable to questions such as adaptation of rotation lengths and harvest schedules to the new circumstances. The system is based on the PipeQual growth model developed at the University of Helsinki (Mäkelä & Mäkinen 2003, Kantola et al. 2007), currently applicable to boreal coniferous forests. The model is unique in that it provides predictions of both the carbon pools of the tree stand and the detailed structure of the stems for wood quality assessment.

PipeQual derives tree growth from annual carbon acquisition and allocation. The impact of the environmental factors on growth takes places through the rates of the physiological processes included in the carbon balance. In the present model system, the rates of these processes and their dependence on the driving environmental variables are analysed on a daily basis, then integrated over the growing season and mediated to the growth model by means of summary models that operate on an annual basis. The summary models have been devised to be driven by input data that is generally available.

Forest growth is determined, on one hand, by the resources provided by the environment, and on the other hand, by the current state of the stand itself (e.g., species, age, crown cover). These interact with each other, as the availability of resources influences, e.g., the competitive status of different species and the maximum leaf area of a site. Here we separate the impact of environmental resources from the impact of the variable state of the forest stand by defining the concept of potential productivity. It is defined as the gross primary production (GPP) of the stand attained under the prevailing environmental factors, under the hypothetical situation that the canopy is able to absorb all the photosynthetically active radiation (PAR) available during the growing season (Härkönen et al. 2010). In practise, the degree of PAR absorption depends on species and stand structure, e.g. closed pine stands in Southern Finland absorb 80 85 % of PAR (Mäkelä et al. 2006, 2008a).

We have developed methods for predicting potential productivity using a model based on canopy Light Use Efficiency (LUE). The PRELUED model (Mäkelä et al. 2008a, Härkönen et al. 2010) derives potential productivity from the daily courses of PAR, temperature and vapour pressure deficit (VPD). Here we have further developed PRELUED with the objective of incorporating impacts of reduced soil water availability, increased CO2 concentration and increased foliar nitrogen content, all factors predicted to change under climate change (Carter et al. 2005, Jylhä et al. 2009).

The effect of foliar nitrogen on potential productivity was studied by Peltoniemi et al. (2010). However, the correlation between N content and GPP was not found significant, probably because the measurement accuracy was not sufficient for detecting the rather weak signal (Le Maire et al. 2005), so the direct effect of foliar N content on GPP was not included in the model. However, a more important impact of nitrogen availability is believed to take place through carbon allocation, with lower N availability leading to lower carbon allocation to above-ground growth, stem growth in particular (Helmisaari et al. 2007, Mäkelä et al. 2008b).

It is a challenge to study the impact of drought on tree physiology in the field, as the occurrence of drought has been relatively rare in Finland. For example, the SMEAR II station has been running for over 10 years but severe drought has only been observed over a two-week period in August 2006 (Duursma et al. 2008, Ilvesniemi et al. 2010). We developed a simple model for the daily changes of the soil water content, using soil and gas exchange data from SMEAR II (Duursma 2005, Duursma et al. 2008, Linkosalo et al. 2009). The model predicts the occurrence of drought days and how they affect potential productivity. Research on the impacts of drought on stem growth and tree vitality are continued in other on-going projects.

The effect of CO2 concentration on potential productivity was investigated using a detailed simulation model (Mäkelä et al. 2006), the results of which were summarised in PRELUED. Here, the photosynthesis rate of a shoot was calculated using the Farquhar model (Farquhar ym. 1980) combined with the Ball-Berry-Leuning model of stomatal conductance (Leuning 1995), and the effect of temperature on the annual cycle of photosynthetic capacity was incorporated using results from SMEAR I and SMEAR II (Kolari et al. 2007). The simulations suggest that CO2 increases the rate of photosynthesis in a saturating manner. A doubling of CO2 will increase annual GPP by about 30% (with all other things unchanged). In addition, increased CO2 will affect the relationship between photosynthesis and vapour pressure deficit. Based on the simulation results, a summary model was developed and included in PRELUED.

The potential productivity was examined in 10 x 10 km grid for the area of Finland, the time scale being until the end of this century. Years 1971-2000 were used as a reference period.

The potential productivity was predicted to increase towards the end of the century. The impact of increasing CO2 concentration appeared to be more significant for potential productivity than the expected changes in climate. However, it is likely that this increased potential will not be fully realised in the form of increased forest growth unless nutrient availability also increases at the same time.

According to our results, drought will not significantly reduce potential productivity as a whole, although impacts will likely be observed on sites with low water-holding capacity. The timing of drought occurrence should be taken into account when considering the impacts; if drought occurs in a period critical for growth, e.g. at the time of bud development, the effects on potential productivity will continue in the next growth period as well (Lindholm et al. 2009). The occurrence of drought during the active diameter growth period will reduce diameter growth to some extent (Jyske et al. 2010), but this reduction was not quantified in this study. There are other ongoing projects where the effect of weather in growth period on tree physiology and vitality are studied (Schiestl-Aalto 2009, Hölttä et al. 2010).

Finnish guidelines of harvest schedule and rotation length are based on stand basal area at a prescribed height (Tapio 2006), therefore increasing growth rates mean that harvest times are achieved sooner. It can thus be claimed that the guidelines in a way adapt to the new situation automatically. However more research is needed to find out if these guidelines as such are appropriate under new conditions. Our group is now working on these questions using the model system developed.



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