Effects of environmental changes on the vitality of forest stands

Using the physiological single tree growth model BALANCE, vitality of forest stands was simulated in dependence of the site related factors climate and stand structure. At six level II plots in southern Germany with the main tree species beech (Fagus sylvatica L.), oak (Quercus robur L.), spruce (Picea abies [L.] Karst.) and pine (Pinus sylvestris L.) simulated results were compared to measured values (soil water content, bud burst and leaf colouring, diameter at breast height, tree height and crown density) in order to validate the model. Sensitivity tests were done to examine the influence and the interactions of the environmental parameters. The validation results show that BALANCE is capable of realistically simulating growth and vitality of forest stands for central European regions for medium term time spans (several years).. The validation of the water balance module produces mean absolute errors based on field capacity between 2.7% and 6.9% in dependence of sites and forest stands. Senescence of foliage as well as crown density is reproduced with a correlation coefficient of 0.70 compared to measurements. Differences between measured and simulated diameter values were smaller than 1% for spruce and smaller than 6.5% for beech after 7 years of simulation, and smaller than 1% for oak after 8 years of simulation. On the other hand, the simulations for pine trees conform less with the measurements (difference: 22.6% after 8 years). The sensitivity of the model on environmental changes and on combinations of these parameters could be demonstrated. The responses of the forest stands were quite different.

Models for Forest Ecosystem Management: A European Perspective

Background: Forest management in Europe is obliged to sustainability. In the face of climate change and accompanying risks however, planning in order to achieve this aim becomes increasingly challenging, underlining the need for new and innovative methods. Models potentially integrate a wide range of system knowledge and present scenarios of variables important for any management decision. In the past, however, model development has been mainly focused on specific purposes whereas today we get more and more aware that the whole range of information that can be provided by models is needed. It is therefore necessary to review the various approaches that are available for specific tasks and to discuss how they can be used for future management strategies.

Scope: In this paper we develop a concept for the role of models in forest ecosystem management based on historical analyses. Five paradigms of forest management are identified: 1) multiple uses, 2) dominant use, 3) environmentally sensitive multiple uses, 4) full ecosystem approach, and 5) eco-regional perspective. For each of these paradigms, an overview of corresponding model approaches is given. It is discussed how these models can contribute to goal setting, decision support and development of guidelines for forestry operations. Furthermore, it is shown, how scenario analysis including stand and landscape visualization can be used to depict alternatives, make long-term consequences of different options transparent and ease participation of different stakeholder groups and education.

Conclusions: In our opinion, the current challenge of forest ecosystem management in Europe is to integrate system knowledge from different temporal and spatial scales and from various disciplines. For this purpose, using a set of models with different focus that can be selected from a kind of toolbox according to the particular needs is probably more realistic than developing one overarching model, covering ecological, production, and landscape issues equally well.

Effects of droughts on the carbon dynamics of forest stands in Central Europe

Physiological growth models are powerful instruments for analysing the influence of environmental changes as for example droughts on plant. This way, possible adaptation strategies e.g. a change of the stand structure or a change of the tree species composition on a changed environment e.g. more drought periods can be analysed by model simulations. Growth simulations based on the physiological model BALANCE show that future climate conditions with decreased precipitation rates and higher temperatures will lead to reductions in the productivity of forest stands in Central Europe. However, the degree of the growth reduction strongly depends on the distribution of precipitation and temperature over time. More extreme events such as droughts will have a stronger influence on the annual biomass increment than evenly distributed precipitation reductions (and with it temperature increases). Effects of droughts on biomass production of different forests are reported in a number of articles (e.g. Lebourgeois et al. 2005, Geßler et al. 2007). However, additionally to closer investigations of the influence of a changing climate on tree growth and forest production, also forest adaptation measures to a changing climate and studies on their influence on the productivity have to be done. We could show that with a changed stand structure, which could result in a less dense stand, in a mixed stand or in a stand with changed tree species, the drop of the biomass increment caused by a changing climate can be attenuated or equalised.

Modelling above and below ground carbon dynamics in a mixed beech and spruce stand influenced by climate

Tree growth and carbon dynamics are important issues especially in the context of climate change. However, we essentially lack knowledge about the effects on carbon dynamics especially in mixed stands. Thus, the objective of this study was to test the effects of climatic changes on the above and below ground carbon dynamics of a mixed stand of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) by means of scenario simulations. To account for the typical tree interactions in a mixed-species stand a spatial explicit tree growth model based on eco-physiological processes was applied. Three different climate scenarios considering altered precipitation, temperature, and radiation were calculated for an unthinned and a thinned stand. The results showed significant changes of above and belowground biomass over time, especially when temperature and radiation were increased additionally to decreased precipitation. The reduction in biomass increments of Norway spruce were more attenuated above than below ground. In contrast, the results for beech were the opposite: The belowground increments were reduced more. These results suggest a shift in the species contribution to above and belowground biomass under dryer and warmer conditions. Distinct effects were also found when thinned and unthinned stands were compared. A reduced stand density changed the proportions of above and below ground carbon allocation. As a main reason for the changed growth reactions the water balance of trees was identified which lead to changed biomass allocation pattern.

Simulating stand climate, phenology and photosynthesis of a forest stand with a process based growth model

In the face of climate change and accompanying risks forest management in Europe is becoming increasingly important. Model simulations can help to understand the reactions and feedbacks of a changing environment on tree growth. In order to ensure correct forest growth simulations based on future climate change scenarios, we tested the basic processes underlying the growth model BALANCE, simulating stand climate (air temperature, PAR: photosynthetically active radiation and precipitation), tree phenology and photosynthesis. A mixed stand of 53 to 60-year-old Norway spruces (Picea abies) and European beeches (Fagus sylvatica) in Southern Germany was used as reference. The results show that BALANCE is able to realistically simulate air temperature gradients in a forest stand from air temperature measurements above the canopy and PAR regimes at different heights for single trees inside the canopy. Interception as an important parameter for the water balance of a forest stand was also estimated satisfyingly. Tree phenology, i.e. bud burst and leaf colouring, was reproduced well. Simulated photosynthesis rates were in good accordance with measured values for beech, both in the sun and the shade crown. For spruce, however, some discrepancies in the rates were obvious, probably due to changed conditions after bud break, which had not been taken into account by the model. Overall, as shown in an example, BALANCE can be used for scenario simulations of forest growth regarding a changing environment (e.g. climate change, change of forest stand structure).