Dendroecological analysis of climate, site, growth, mortality and disturbances

In my research projects, I investigate spatio-temporal processes of stand dynamics of forests. I am particularly interested in growth and mortality processes of individual trees and forest stands as well as the impact of natural disturbances – such as fire, insect outbreaks and drought – on processes of stand and landscape development. In my studies, I work often with dendrochronological data (see Tree-ring lab).

Ongoing projects

Currently, I work on the following projects:

Climatic and topographic impacts on mortality processes of mountain pine in the Swiss National Park

Tree mortality processes may have rapid and long-lasting effects on the structure and species composition of forest stands as well as on demographic properties of tree populations. In the Swiss National Park, different studies have focused mainly on influences of competition and pathogens on mortality processes of mountain pine (Pinus montana). However, effects of climate variability and particularly extreme climate events (drought, heatwaves, frost) on growth patterns and tree mortality have rarely been investigated, although it is known from other studies that mortality rates may increase for example during and following drought years. In the Swiss National Park with its continental climate, extreme climate events such as low precipitation or high temperatures have occurred relatively frequently in the last few years.

The objective of this project is to investigate the effects of climate variability and topography on growth patterns and mortality processes of mountain pine in the Swiss National Park. Dead standing and living mountain pine trees will be cored at several sites on different aspects. To reconstruct growth patterns and death dates, tree rings will be measured and dated in the tree-ring lab. The impact of climate on site-specific growth patterns and the temporal distribution of death dates will be investigated using descriptive and analytical methods.

Contact: Christof Bigler

Influence of site characteristics and climate on tree growth

Tree growth is influenced by a multiplicity of factors, amongst others by variables such as tree species, stand dynamics, topography, soil characteristics, and climate variability. Calculation and prediction of growth, e.g. tree height or diameter, at a certain age and at a certain site are of high relevance for forestry.

In this stuy, I investigate the influence of different site characteristics (elevation, slope steepness, aspect, soil depth) and climate variability (temperature, precipitation, drought) on the development of diameter growth. Tree-ring series of dead trees of three ecologically different tree species form the data base: subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii) and bristlecone pine (Pinus contorta) from subalpine forests in Colorado (USA). By means of so called mixed-effects, non-linear growth functions I model the influence of different site characteristics on diameter at a certain age.

The resulting growth models allow the calculation and prediction of the expected age-dependent diameter at a certain site. These empirical growth models might complement yield tables that are still in use, which are based on data from even-aged stands. Furthermore, the influence of climate change (increased temperature, change in precipitation) on tree growth might be quantified at a later stage by means of these models.

Contact: Christof Bigler

Finished projects

The following projects are finished:

Relationship between growth rates, site characteristics and longevity of trees

Rapid growth and large tree height seem to be a benefit for trees, because amongst others the competitiveness is larger compared to smaller trees and the generation time is reduced. Generally it is known that rapidly growing organisms tend to live shorter and vice versa slowly growing organisms get often older. One reason for the shorter longevity of rapidly growing trees seems to be the smaller allocation of resources into defense, which results in rapidly growing trees being more frequently attacked by insects and fungi. The relationship between growth rates and longevity of trees has been shown empirically only in a few studies. Apart from growth rates, spatial effects seem also to influence longevity. For certain tree species such as bristlecone pine (Pinus aristata) in the southwest of the USA, it is known for quite a long time that the oldest trees grow often at exposed sites and they differ morphologically from younger tree individuals.

In this study, I investigate the influence of growth rates and site characteristics (elevation, slope steepness, aspect, soil depth) on longevity of trees for five ecologically different tree species in the Rocky Mountains and in the Swiss Alps: subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii) and bristlecone pine (Pinus contorta) from subalpine forests in the north of Colorado (USA), Norway spruce (Picea abies) from subalpine forests in Davos (Switzerland) and Scots pine (Pinus sylvestris) from low-elevation forests in the Valais (Switzerland).

References:

• Bigler C. and Veblen T.T. 2009. Increased early growth rates decrease longevities of conifers in subalpine forests. Oikos 118: 1130-1138
• Rötheli E., Heiri C. and Bigler C. 2012. Effects of growth rates, tree morphology and site conditions on longevity of Norway spruce in the northern Swiss Alps. European Journal of Forest Research 131: 1117-1125.

Contact: Christof Bigler

Changes in litter and dead wood loadings following tree mortality of subalpine conifer species in northern Colorado

Litter and dead wood are important elements of nutrient and carbon cycling, they further influence tree regeneration and enhance biodiversity. In fire-prone forest ecosystems, the quality, quantity and spatial distribution of litter and dead wood may crucially determine fire hazard. Thus, a better understanding of the dynamics of fuel complexes following tree mortality will allow more accurate projections of their effects on fire hazard.

In this study, I investigate loadings of litter and dead wood of > 200 dead and living Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in subalpine forests of northern Colorado. Litter loadings are estimated based on dried litter samples, dead wood loadings are calculated for different diameter classes. To relate the time since tree death and litter loadings, I determined death dates based on tree rings. Changes in litter loadings following tree mortality are analyzed by applying a negative exponential regression model and an asymptotic regression model to chronosequences of death dates.

Reference:

• Bigler C. and Veblen T.T. 2011. Changes in litter and dead wood loads following tree death beneath subalpine conifer species in northern Colorado. Canadian Journal of Forest Research 41: 331-340.

Contact: Christof Bigler