Forest dynamics in old growth spruce - fir - beech forests

Research project P 14583 Forest dynamics in old growth spruce-fir-beech forests Georg GRATZER 09.10.2000 The old-growth forest (Urwald) Rothwald is the largest and one of the best-documented remnants of never-logged in Central Europe. It provides an exeptional opportunity to advance the understanding of forest dynamics and to test the applicability of current theories on stand dynamics for Central European forests, Three bodies of Theories on forest dynamics emphasizing different processes or mechanisms in mixed-species forest will be addressed and the underlying hypotheses tested: (1) interspecific differences in the response to different disturbance regimes are driving forest stand dynamics; (2) spatial partitioning, temporal partitioning and/or a competition / colonisation trade off are the mechanisms explaining coexistence of the predominant species and are determining the response to the different disturbance regimes and (3) the knowledge of the spatio-temporal development of a forest stand can be used as a baseline for predicting future stand development. Hypothesis (1) will be studied using the following approaches: (i) Characterisation and comparison of vegetation development after coarse- scale and fine scale disturbances; (ii) assessment of the spatial atid temporal variability of gaps and gap sizes using a time series of aerial photographs covering about 60 years and (iii) reconstruction of the disturbance history using dendroecological methods combined with demographic, parameters, on permanent sample plots, Spatial niche partitioning between species (hypothesis (2)) will be tested using a detailed study of regeneration ecology including effects of differential light levels, snow depth and duration, seedbeds, and competition in intensive plots. Existence of competition/colonization trade off will be tested by comparing the spatial pattern and association of canopy trees within the, four permanent sample plots with literature data of seed dispersal. The presence and importance of temporal partitioning between the predominant species will be examined by characterizing both the different storage potentials of the tree species (e.g., expressed by differences in life histoties) and the variations in resources over time through disturbance (gap creation) or (long-term) climate fluctuation using dendroecological methods. The spatio-temporal development (hypothesis (3)) will be studied by analysis of spatial point patterns using data from repeated recording of 6 permanent sample plots. The application of these three approaches within one study will not only provide a more complete understanding of the dynamics of Central European spruce-fir-beech forests but will also advance the theoretical framework on forest dynamics.

The largest Central European old-growth forests are located in the wilderness area "Dürrenstein". These old- growth forests provide an exceptional opportunity to advance the understanding of forest dynamics and to test the applicability of current theories on stand dynamics for forests. A driving factor for natural forest dynamics are disturbances. Disturbances are defined as temporally discrete events, leading to a change in the resource basis of forests. An important step towards gaining an improved understanding of natural dynamics of mountain forests is the study of the history of disturbance and their role for species composition. In the project "forest dynamics in old grwoth spruce - fir - beech forests", scientists from the Institute of Forest Ecology of BOKU University in Vienna could reconstruct the disturbance history of the last 350 years by analysing tree ring widths. Tree ring widths of trees are influenced by genetic factors, climate and the available light. If shading neigbhours die back due to disturbances, trees can respond with increased growth. The so-called release events were dated and analysed in terms of their spatial distribution in the forest. So far, natural regeneration and also mortality of trees in Central Europe was assumed to be continous in time. In the studied old-grwoth forest however, it could be shown that there is considerable variation in disturbance intensity and severity as indicated by extended gaps and defined peaks in the decadal disturbance histories. Some peaks in the disturbance intensity occur synchronously distributed over the whole 300 ha old-growth forest (1910s, 1930s, 1960s, and 1980s). This indicates that single events or at least periods of high disturbance intensities may affect larger areas of the forest. Most gaps created are small and scattered throughout the stand but at intermediate scales (1 ha) most single events occurred scattered indicating that trees from the forest were rather thinned than cleared, particularly at the slopes with almost pure beech stands. The strong temporal discontinuity in disturbances could be proved by analysing aerial photographs from the years 1962, 1991 and 1996. A detailed analysis of crown heights showed strong spatial and temporal variations in gap formation and gap closure. Gap formation rates were higher than gap closure rates throughout the forest, the fraction of forest in a gap state was 12%. For the first time, the influence of gaps on the surrounding forest could be quantified: closed forest around a gap had a higher probability to be converted into a gap up to a distance of 15 m from the gap. But also the tree species matters for the probability of trees to die: the scientists developed functions showing that beech trees have an increased probability of mortality if their neighbour are also beech trees. Such neighbourhood relationship, which could be proved with analysis of the spatial distribution of trees as well, play and important role for coexistence of trees in the studied forest and thus for the continued existence of natural mixed species forests.