Bugmann Harald
Growth and mortality of oak (Quercus spp.): a combined analysis of monitoring and tree-ring data from Swiss forest reserves
Project Number: Parcs Data Center 9907 / 4D CH-4907
| Project Type: |
Dissertation |
| Project Duration: |
05/01/2009 - 10/31/2012 project completed |
| Funding Source: |
ETH , |
| Leading Institution: |
ETH Zürich |
| Project Leader: |
Prof. Harald Bugmann
Waldökologie
Institut für Terrestrische Ökosysteme (ITES)
ETH Zürich
ETH Zentrum CHN G 76.1
Universitätstrasse 16
8092 Zürich
Phone: +41 (0) 44 632 32 39 ; +41 (0) 44 632 32 17
FAX: +41 (0) 44 632 13 58
e-Mail: harald.bugmann(at)env.ethz.ch
http://www.fe.ethz.ch/ |
Research Areas:
Disciplines:
| |
|---|
| forestry |
| environmental sciences |
Keywords:
Akaike weight; forest dynamics; forest succession; forest reserve; Quercus spp.; tree population; Chapman-Richards growth equation; diameter growth; tree age; dendrochronology; age estimation
Abstract:
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 because of their drought tolerance as climate change proceeds. Hence, the overall goal of my thesis is the assessment of the natural po? tential of Central European oaks by investigating their growth and mortality under the exclu? sion of management. To this end, monitoring and tree?ring data collected in eleven forest reserves that comprise a large environmental gradient in Switzerland are analyzed. Particular emphasis is placed on (1) assessing the performance of oaks in changing stand structures after the abandonment of management, (2) identifying the key drivers of oak growth and mortality, and (3) developing a method for non?destructive estimation of tree age for oak.
In a first step, changes in stand structures after the abandonment of management were investigated, including the consequences for tree mortality (Chapter I). By using repeated diameter measurements from more than 17 600 individually tagged trees, stand dynamics in the reserves over the past 50 years were analyzed. Moreover, generalized linear mixed? effects models were fitted to quantify the effects of tree diameter, stand basal area, precipi? tation and slope on annual mortality rates of oak (Q. petraea, Q. robur, Q. pubescens) and its strongest competitor beech (Fagus sylvatica). Stand basal area strongly increased over time, whereas tree density decreased. The relative importance of oak decreased compared to beech at most sites. Mortality rates increased over time, with the increase being stronger for oak than for beech. Mortality rates of both species strongly decreased with increasing tree diameter, and showed an increasing tendency with precipitation. In addition, oak mortality strongly increased with stand basal area, whereas no such correlation was found for beech. These results indicate that (1) natural oak mortality in Central European oak?beech forests is mainly driven by stand structure, and (2) oaks are at risk of being increasingly outcompeted by beech, unless competition is reduced through management or disturbances.
As a second step in my thesis, the most important climatic drivers of oak diameter growth were identified along a gradient of soil moisture (Chapter II). To this end, tree?ring data from more than 300 oaks representing different vitality classes were sampled in the forest re? serves. Response functions were calculated between tree?ring indices and monthly values of mean temperature, precipitation and a drought index. The time span from June of the previ? ous year until September of the current year was considered as potentially influencing tree? ring widths. In general, tree?ring widths responded more strongly to precipitation than to temperature, with precipitation of the current summer months resulting in the highest posi? tive responses. For the drought index, similar growth responses as for precipitation were
identified. Whereas positive responses to precipitation and the drought index in June were identified along the whole soil moisture gradient, more positive responses for other months were identified at sites with comparably dry soil conditions. These growth responses indicate that, despite their drought?tolerance, oaks show reduced growth during dry summer peri? ods, in particular if they are growing on dry soils.
In a third step, the development of oak diameters with age was investigated (Chapter III). The age?diameter relationships of the oaks growing in the Swiss forest reserves were deter? mined based on the tree?ring samples introduced in Chapter II. To identify the key drivers that shape this relationship, nonlinear mixed?effects models were fitted with environmental variables included as covariates. Furthermore, a mean model was developed and its accura? cy in predicting the age?diameter curves of independent oaks was assessed. The fixed effects elevation, slope and soil water holding capacity were identified as the most important co? variates. Larger maximum diameters were associated with lower elevations, steeper slopes, north?facing aspects, higher water holding capacities and moister summers. Predictions based on the fixed effects were fairly accurate for most trees (root mean squared error between predicted and observed diameter < 6 cm for 75% of the trees) and the inclusion of random effects increased the accuracy for 86% of the trees. These results suggest that runoff plays a key role for oak diameter growth, likely accompanied by limiting temperature effects at higher elevations.
The fourth step of this thesis dealt with developing a method for the non?destructive esti? mation of tree age (Chapter IV). Age estimations derived from a traditional polynomial approach were compared 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 (approach I), sequences of repeated diameter measurements from the monitoring in the re? serves were fitted to a range of systematically varying age?diameter curves. In the second approach (approach II), 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 Chapter III. The polynomial approach and approach II produced similarly accurate age estimations, whereas approach I was less accurate. Additionally including the random effects strongly improved the age estimations from approach II, such that relative errors were below 40% for 98% of the oaks. The included site information and the non? linearity of this approach obviate the need for calibration in further applications, making it an entirely non?destructive alternative for tree age estimation.
Overall, the study of monitoring data revealed competition as the key driver of oak mortali? ty, and the analysis of tree?ring data indicated strong effects of site characteristics and drought on oak growth. These findings indicate that targeted management is needed to retain the current proportion of oaks in Central European forests. The combination of moni? toring and tree?ring data delivered a non?destructive method for tree age estimation, which is suggested to find application in the context of oak?related conservation.
Leading questions:
How does stand structure in oak forests change after the abandonment of management? Do oak and its strongest competitor beech show changing mortality patterns under the development towards near-natural stand conditions? What environmental variables are decisive for the mortality of oak and beech? What are the most important climatic drivers of natural oak growth? What environmental variables limit the growth of oak diameters with age? How accurate are age-diameter relationships of oaks that are predicted based on site characteristics? How accurate are non-invasive tee age estimations for oaks based on nonlinear models?
Publications:
Rohner, Brigitte. 2012. Growth and mortality of oak (Quercus spp.). a combined analysis of monitoring and tree-ring data from Swiss forest reserves. Dissertation ETH Zürich.
PDF Thesis
Last update: 4/5/22
Source of data: ProClim- Research InfoSystem (1993-2024)
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