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Ledermann T.,Federal Research and Training Center for Forests
Canadian Journal of Forest Research | Year: 2010

Recent individual-tree growth models use either distance-dependent or distance-independent competition measures to predict tree increment. However, both measures have deficiencies: the latter because the effects of local variation in spacing are not represented, and the former because they cannot be calculated from normal inventory data for lack of spatial information. To overcome these shortcomings, the new class of semi-distance-independent competition indices was proposed. A semi-distance-independent competition index is a distance-independent competition measure that uses only the trees of a single small sample plot that includes the subject tree. Moreover, a semi-distance-independent competition index can be calculated in an analogous way to a distance-dependent competition index by using sample plot size, tree attributes, and intertree distances. However, many semi-distance-independent competition measures are based on simple tree attributes. Therefore, the objective of this study was to analyze if the semi-distance-independent competition indices explain the variation in measurements of tree increment more or less effectively than a set of classical distance-dependent competition indices. The results show that some of the semi-distance-independent competition indices explain at least as much variation in measurements of tree increment as any of the distance-dependent competition indices. Source

Ledermann T.,Federal Research and Training Center for Forests
European Journal of Forest Research | Year: 2011

In this study, a non-linear model was developed that predicts the five-year change of height to crown base (HCB) of Norway spruce (Picea abies [L.] Karst.). Data were available from the Austrian National Forest Inventory and comprised 2,419 trees from 1,637 permanent sample plots measured during 1981 and 2002. The dynamic model explained 36% of the variation in the observed change of HCB. It is well behaved and meets biological expectations. Based on five independent data sets, the predictive ability of the new dynamic model was compared to an already existing static crown ratio model. For this comparison, the models were applied as follows: the new {increment} HCB model was used to predict the change in HCB directly. For the semi-dynamic method, the static model was applied at the end and at the start of the growth period to obtain two estimates of HCB. The difference of these two estimates was then added to the initial HCB. For the static method, the model was only applied at the end of the prediction period to obtain the new estimate of HCB. Except for one plot, the new {increment} HCB model yielded the smallest BIAS and the highest precision, followed by the semi-dynamic and the static method. Because the independent data sets cover a broad range of age classes and thinning regimes, the validation results also indicate that the new {increment} HCB is robust and the effect of stand management is adequately represented. © 2010 Springer-Verlag. Source

Mayer M.,University of Natural Resources and Life Sciences, Vienna | Matthews B.,University of Natural Resources and Life Sciences, Vienna | Schindlbacher A.,Federal Research and Training Center for Forests | Katzensteiner K.,University of Natural Resources and Life Sciences, Vienna
Biogeosciences | Year: 2015

Windthrow-driven changes in carbon (C) allocation and soil microclimate can affect soil carbon dioxide (CO2) efflux (Fsoil) from forest ecosystems. Although Fsoil is the dominant C flux following stand-replacing disturbance, the effects of catastrophic windthrow on Fsoil are still poorly understood. We measured Fsoil at a montane mixed-forest site and at a subalpine spruce forest site from 2009 until 2012. Each site consisted of an undisturbed forest stand and two adjacent partially cleared (stem-fraction-harvested) windthrow areas, which differed with regard to the time since disturbance. The combination of chronosequence and direct time-series approaches enabled us to investigate Fsoil dynamics over 12 years post-disturbance. At both sites Fsoil rates did not differ significantly from those of the undisturbed stands in the initial phase after disturbance (1-6 years). In the later phase after disturbance (9-12 years), Fsoil rates were significantly higher than in the corresponding undisturbed stand. Soil temperature increased significantly following windthrow (by 2.9-4.8 °C), especially in the initial phase post-disturbance when vegetation cover was sparse. A significant part (15-31%) of Fsoil from the windthrow areas was attributed to the increase in soil temperature. According to our estimates, ∼500-700 g C mg'2 yearg'1 are released via Fsoil from south-facing forest sites in the Austrian Calcareous Alps in the initial 6 years after windthrow. With a high browsing pressure suppressing tree regeneration, post-disturbance net loss of ecosystem C to the atmosphere is likely to be substantial unless forest management is proactive in regenerating such sites. An increase in the frequency of forest disturbance by windthrow could therefore decrease soil C stocks and feed back positively on rising atmospheric CO2 concentrations. © 2014 Author(s). Source

Fischer J.-T.,Federal Research and Training Center for Forests
Natural Hazards and Earth System Sciences | Year: 2013

An innovative approach for the analysis and interpretation of snow avalanche simulation in three dimensional terrain is presented. Snow avalanche simulation software is used as a supporting tool in hazard mapping. When performing a high number of simulation runs the user is confronted with a considerable amount of simulation results. The objective of this work is to establish an objective, model independent framework to evaluate and compare results of different simulation approaches with respect to indicators of practical relevance, providing an answer to the important questions: how far and how destructive does an avalanche move down slope. For this purpose the Automated Indicator based Model Evaluation and Comparison (AIMEC) method is introduced. It operates on a coordinate system which follows a given avalanche path. A multitude of simulation runs is performed with the snow avalanche simulation software SamosAT (Snow Avalanche MOdelling and Simulation - Advanced Technology). The variability of pressure-based run out and avalanche destructiveness along the path is investigated for multiple simulation runs, varying release volume and model parameters. With this, results of deterministic simulation software are processed and analysed by means of statistical methods. Uncertainties originating from varying input conditions, model parameters or the different model implementations are assessed. The results show that AIMEC contributes to the interpretation of avalanche simulations with a broad applicability in model evaluation, comparison as well as examination of scenario variations. © Author(s) 2013. Source

Berger T.W.,University of Jordan | Inselsbacher E.,University of Jordan | Zechmeister-Boltenstern S.,Federal Research and Training Center for Forests
Soil Biology and Biochemistry | Year: 2010

Soil respiration is the largest terrestrial source of CO2 to the atmosphere. In forests, roughly half of the soil respiration is autotrophic (mainly root respiration) while the remainder is heterotrophic, originating from decomposition of soil organic matter. Decomposition is an important process for cycling of nutrients in forest ecosystems. Hence, tree species induced changes may have a great impact on atmospheric CO2 concentrations. Since studies on the combined effects of beech-spruce mixtures are very rare, we firstly measured CO2 emission rates in three adjacent stands of pure spruce (Picea abies), mixed spruce-beech and pure beech (Fagus sylvatica) on three base-rich sites (Flysch) and three base-poor sites (Molasse; yielding a total of 18 stands) during two summer periods using the closed chamber method. CO2 emissions were higher on the well-aerated sandy soils on Molasse than on the clayey soils on Flysch, characterized by frequent water logging. Mean CO2 effluxes increased from spruce (41) over the mixed (55) to the beech (59) stands on Molasse, while tree species effects were lower on Flysch (30-35, mixed > beech = spruce; all data in mg CO2-C m-2 h-1). Secondly, we studied decomposition after fourfold litter manipulations at the 6 mixed species stands: the Oi - and Oe horizons were removed and replaced by additions of beech -, spruce - and mixed litter of the adjacent pure stands of known chemical quality and one zero addition (blank) in open rings (20 cm inner diameter), which were covered with meshes to exclude fresh litter fall. Mass loss within two years amounted to 61-68% on Flysch and 36-44% on Molasse, indicating non-additive mixed species effects (mixed litter showed highest mass loss). However, base cation release showed a linear response, increasing from the spruce - over the mixed - to the beech litter. The differences in N release (immobilization) resulted in a characteristic converging trend in C/N ratios for all litter compositions on both bedrocks during decomposition. In the summers 2006 and 2007 we measured CO2 efflux from these manipulated areas (a closed chamber fits exactly over such a ring) as field indicator of the microbial activity. Net fluxes (subtracting the so-called blank values) are considered an indicator of litter induced changes only and increased on both bedrocks from the spruce - over the mixed - to the beech litter. According to these measurements, decomposing litter contributed between 22-32% (Flysch) and 11-28% (Molasse) to total soil respiration, strengthening its role within the global carbon cycle. © 2010 Elsevier Ltd. All rights reserved. Source

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