Patterns and trends in growth of beech and oak. Does biodiversity improve the resilience of forests to environmental change?

Forests deliver invaluable ecosystem services and products to humanity. The provision of forest biomass is one of the important ecosystem services delivered by forests. Woody biomass is used as building material, for the production of industrial products and bio-energy. In addition, woody biomass is an important carbon dioxide sink, therefore forests are seen as a climate mitigation strategy. However, recent studies link climate change (e.g. increased occurrence of drought events) and emissions (e.g. nitrogen and carbon dioxide) with changes in tree growth in forests. Changes in forests will have consequences for the delivery of these forest ecosystem services.

In this thesis we aim to get more insight in recent change in beech and oak growth and its drivers in order to guarantee the delivery of forest ecosystem services in the future. For this purpose, retrospective tree growth data obtained from tree-ring width measurements on tree cores are used. The year-to-year growth variability but also the growth fluctuation on longer time scales (i.e. 50 year or longer) is studied. For the study of long-term tree growth trends multilevel mixed models are used. These models allow us to answer the question if a tree, independent of its size, grows faster, slower or similar compared to x years ago and link the change in growth to changes in the environment (i.e. climate or emissions). In a second stage the effect of biodiversity on the growth response pattern to drought (only for beech) and on long-term tree growth trends in beech and oak is evaluated. As biodiversity is known to result in a higher ecosystem functioning, biodiversity effects might have a positive effect on the production function of forests and mitigate negative effects of climate change and increased emissions on forest growth. The overall goal of this research is to understand the future of forest production under climate change conditions and broaden the knowledge of biodiversity effects on forest functioning.

This research is focused on the growth of beech (Fagus sylvatica) and oak (both Quercus robur and Quercus petraea) trees, important broadleaf tree species, growing in Belgium. Two study areas were set up, one in the South and one in the North of Belgium. In the study area in the South of Belgium Quercus petraea and Fagus sylvatica are studied. The study area in the North of Belgium was designed in this way that the studied Fagus sylvatica and Quercus robur trees grew in neighborhoods of different tree species diversity (i.e. from monocultures to neighborhoods up to three tree species).

In a first step, a newly developed method to measure tree-ring widths based on CT scan images was evaluated. The additional measurement of wood density makes this newly developed method interesting. No significant differences between tree-ring widths measured with the CT scan and the conventional LINTAB measuring method were found. We conclude that tree-ring widths measured on CT scan images can be used to model long-term changes in tree growth.

Next, we demonstrate that wood density and radial growth have different long-term trends for both beech and oak. This finding indicates that wood density needs to be taken into account in studies on the carbon sequestration capacity of forests. Overall an increasing long-term trend in aboveground biomass was found for both beech and oak trees growing in the study area in the South of Belgium.

When we look at the effect of biodiversity, we see that beech trees growing in diverse neighborhoods have higher growth compared to beech trees growing in monocultures, even in dry years. Based on carbon and oxygen stable isotope measurements, we can conclude that the stomatal conductance of beech trees growing in diverse neighborhoods was not impacted greatly during drought years compared to beech trees growing in monoculture neighborhoods. The effect of diversity on oak tree growth was found to be negative. This negative effect of diversity on oak growth can be explained by the shade tolerance of the studied species in mixture, with oak having the lowest shade tolerance, and the observed higher competition on oak growing in diverse neighborhoods. For beech and oak trees growing in the study area in the North of Belgium an increasing long-term growth trend was found. The shape of this long-term growth trend was found not to be influenced by diversity, however a lower year-to-year growth variability was found for both beech and oak trees growing in diverse neighborhoods.

Overall, a 20th century growth increase was found for beech and oak in the study areas located in the North and South of Belgium. Drought and water availability shortages, especially when occurring during the growing season, were found to affect beech growth negatively. Nevertheless, until now drought and water availability shortages did not result in beech growth decline in the studied areas, however this might change in the future. Diversifying forests can be a good management strategy to alleviate these negative effects of drought in beech. Oak growth was found to be affected negatively by late frost and positively by water availability. Further research on long-term growth trends along an environmental gradient and on other tree species is needed to further improve our understanding of the effect global change has on tree growth. In addition, there is a need to further develop our knowledge on how diversity influences the long-term growth trends in other climatic conditions in order to understand how biodiversity can influence the effect global change has on tree growth.