Growth and mortality of oak (Quercus spp.): a combined analysis of monitoring and tree-ring data from Swiss forest reserves

Oaks (Quercus spp.) are of high ecological value since they feature the highest associated biodiversity of all Central European tree species. Due to their centuries-long facilitation through management, little is known about the current and future role of oaks in natural dynamics of Central European forests. On the one hand, reduced management may lead to oaks being increasingly outcompeted by more shade-tolerant tree species; on the other hand, oaks may gain relative competitiveness be¬cause of their drought tolerance as climate change proceeds. Hence, the overall goal of this Ph.D. project was the assess¬ment of the natural potential of Central European oaks by investigating their growth and mortality under the exclusion of management. To this end, monitoring and tree-ring data collected in eleven forest reserves that comprise a large environmental gradient in Switzerland were analyzed.

Part I: Changes in stand structure and mortality rates of oak and beech
Qualitative descriptions of succession have suggested an increasing dominance of the shade-tolerant beech after management stops in Central European oak-beech forests. Under climate change, this trend may reverse as a consequence of the higher drought tolerance of oak compared to beech. However, structural changes and mortality processes in oak-beech forests developing towards natural conditions have only rarely been quantified. Thus, the first part of this project dealt with iden¬tifying changes in stand structure and mortality rates – and their drivers – in twelve forest sites in ten forest reserves located along a large environmental gradient in Switzerland. Based on repeated measurements from more than 17 600 individually tagged trees, the development of stand basal area, stand density and the relative proportion of oak and beech were analyzed. In addition, generalized linear mixed-effects models were fitted to determine the differing effects of tree diameter, stand basal area, precipitation and slope on mortality rates of oak and beech. Reference: Rohner B., Bigler C., Wunder J., Brang P. & Bugmann H. 2012. Fifty years of natural succession in Swiss forest reserves: changes in stand structure and mortality rates of oak and beech. Journal of Vegetation Science 23: 892-905.	Individual tag of a tree growing on a permanent plot.
Part II: Annual growth responses of oaks to temperature, precipitation and a drought index along a gradient of soil moisture
Knowledge on the relationship between climate and growth is a prerequisite for assessing the natu-ral potential of oak, particularly under climate change. The second part of this project therefore aimed at identifying the most important climatic drivers of natural oak growth in Central Europe. Further¬more, the question was addressed how these growth responses differ with site characteristics. To this end, increment cores from a total of 333 oaks representing different vitality classes were taken within the forest reserves, which represent a gradient of soil moisture conditions. Response functions of annual tree-ring indices to monthly temperature means, monthly precipitation and a monthly drought index were calculated. The time frame from June of the previous year until September of the current year was considered as potentially influencing oak growth. Furthermore, relating the responses to soil moisture conditions allowed for identifying characteristically varying response intensities.	Oak stems marked for data collection in the forest reserve Les Follatères.
Part III: Estimating the age-diameter relationship of oaks using nonlinear mixed-effects models
The development of tree height growth with increasing age has been studied intensively in the con-text of traditional yield tables. However, the relationship between age and diameter has reached less attention sofar, although a tree’s diameter is a fundamental measure in forest science and forestry. There¬fore, the main goal of the third part of this project was to identify the key environmental variables that shape the age-diameter relationship of oaks. The tree-ring samples introduced above were used to determine the age-diameter relationship for the oaks growing in the Swiss for¬est reserves. Nonlinear mixed-effects models were fitted, with environmental variables included as covariates. In addition, a mean model was developed and its accuracy in predicting age-diameter curves of independent oaks was assessed.	Section of an oak core prepared for tree-ring measuring.

Part IV: Non-destructive estimation of tree age

Accurate tree age information is a prerequisite for many conservational, silvicultural and scientific questions such as assessing expected future growth. Although dendrochronological methods for tree age estimation have proved to be accurate, they are invasive or even destructive and thus often not applicable, e.g. in protected forests. The fourth part of this project therefore aimed at developing a method for the non-destructive tree age estimation for oaks. To this end, age estimations derived from a traditional polynomial approach were com¬pared to those derived from two alternative approaches that allow for bypassing tree-ring sampling in the future. In the first of these two alternative approaches, sequences of repeated diameter measurements from the monitoring in the reserves were fitted to a range of systematically varying age-diameter curves. In the second approach, the diameter sequences were fitted to the age-diameter curves that were predicted site-specifically for every tree from the fixed effects of the mean nonlinear model developed in the previous step of this project.

Contact: Brigitte Rohner, Christof Bigler