Global Change Abstracts The Swiss Contribution - SCNAT

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96 farming systems. The numbers of earthworm species and juveniles were higher in organic fields. Five species - Lumbricus terrestris (L.), Nicodrilus longus (Ude), Nicodrilus nocturnus (Evans), Nicodrilus caliginosus (Sav.) and Allolobo-phora rosea (Sav.) - were significantly more numerous in the organic fields than in the ICM fields. Multivariate analysis showed that the farming system explained most of the variance and was found to be the key factor in altering the earthworm fauna. Late ploughing in autumn was found to have a major negative effect on earthworm abundance, irrespective of the farming system. Farming practices that differ between these farming systems and may considerably influence earthworm populations and diversity are discussed. (c) 2007 Elsevier B.V. All rights reserved. Applied Soil Ecology, 2007, V37, N3, NOV, pp 184-191. 08.1-156 Combining probabilistic land-use change and tree population dynamics modelling to simulate responses in mountain forests Rickebusch S, Gellrich M, Lischke H, Guisan A, Zimmermann N E Switzerland Agriculture, Soil Sciences , Ecology Altitudinal tree lines are mainly constrained by temperature, but can also be influenced by factors such as human activity, particularly in the European Alps, where centuries of agricultural use have affected the tree-line. Over the last decades this trend has been reversed due to changing agricultural practices and land-abandonment. We aimed to combine a statistical land-abandonment model with a forest dynamics model, to take into account the combined effects of climate and human land-use on the Alpine tree-line in Switzerland. Land-abandonment probability was expressed by a logistic regression function of degree-day sum, distance from forest edge, soil stoniness, slope, proportion of employees in the secondary and tertiary sectors, proportion of commuters and proportion of full-time farms. This was implemented in the TreeMig spatio-temporal forest model. Distance from forest edge and degree-day sum vary through feed-back from the dynamics part of TreeMig and climate change scenarios, while the other variables remain constant for each grid cell over time. The new model, TreeMig-LAb, was tested on theoretical landscapes, where the variables in the land-abandonment model were varied one by one. This confirmed the strong influence of distance from forest and slope on the abandonment probability. Degree-day sum has a more complex Global Change Abstracts – The Swiss Contribution | Terrestrial Ecosystems role, with opposite influences on land-abandonment and forest growth. TreeMig-LAb was also applied to a case study area in the Upper Engadine (Swiss Alps), along with a model where abandonment probability was a constant. Two scenarios were used: natural succession only (100% probability) and a probability of abandonment based on past transition proportions in that area (2.1% per decade). The former showed new forest growing in all but the highest-altitude locations. The latter was more realistic as to numbers of newly forested cells, but their location was random and the resulting landscape heterogeneous. Using the logistic regression model gave results consistent with observed patterns of land-abandonment: existing forests expanded and gaps closed, leading to an increasingly homogeneous landscape. Ecological Modelling, 2007, V209, N2-4, DEC 16, pp 157-168. 08.1-157 Understanding the low-temperature limitations to forest growth through calibration of a forest dynamics model with tree-ring data Rickebusch S, Lischke H, Bugmann H, Guisan A, Zimmermann N E Switzerland Meteorology & Atmospheric Sciences , Forestry , Modelling , Plant Sciences The sensitivity of altitudinal and latitudinal treeline ecotones to climate change, particularly that of temperature, has received much attention. To improve our understanding of the factors affecting tree-line position, we used the spatially explicit dynamic forest model TreeMig. Although well-suited because of its landscape dynamics functions, TreeMig features a parabolic temperature growth response curve, which has recently been questioned. and the species parameters are not specifically calibrated for cold temperatures. Our main goals were to improve the theoretical basis of the temperature growth response curve in the model and develop a method for deriving that curve’s parameters from tree-ring data. We replaced the parabola with an asymptotic curve, calibrated for the main species at the subalpine (Swiss Alps: Pinus cembra, Larix decidua, Picea abies) and boreal (Fennoscandia: Pinus sylvestris, Betula pubescens, P. abies) tree-lines. After fitting new parameters, the growth curve matched observed tree- ring widths better. For the subalpine species, the minimum degree-day sum allowing, growth (kDDMin) was lowered by around 100 degree-days; in the case of Larix, the maximum potential ring-width was increased to 5.19 mm. At the boreal tree-line, the kDDMin for P. sylvestris
Global Change Abstracts – The Swiss Contribution | Terrestrial Ecosystems 97 was lowered by 210 degree-days and its maximum ring-width increased to 2.943 mm; for Betula (new in the model) kDDMin was set to 325 degree- days and the maximum ring-width to 2.51 mm; the values from the only boreal sample site for Picea were similar to the subalpine ones, so the same parameters were used. However, adjusting the growth response alone did not improve the model’s output concerning species’ distributions and their relative importance at tree-line. Minimum winter temperature (MinWiT, mean of the coldest winter month), which controls seedling establishment in TreeMig, proved more important for determining distribution. Picea, P. sylvestris and Betula did not previously have minimum winter temperature limits, so these values were set to the 95th percentile of each species’ coldest MinWiT site (respectively -7, -11, -13). In a case study for the Alps, the original and newly calibrated versions of TreeMig were compared with biomass data from the National Forest Inventor), (NFI). Both models gave similar, reasonably realistic results. In conclusion, this method of deriving temperature responses from tree-rings works well. However, regeneration and its underlying factors seem more important for controlling species’ distributions than previously thought. More research on regeneration ecology, especially at the upper limit of forests. is needed to improve predictions of treeline responses to climate change further. Forest Ecology and Management, 2007, V246, N2-3, JUL 31, pp 251-263. 08.1-158 Effects of grazing and soil micro-climate on decomposition rates in a spatio-temporally heterogeneous grassland Risch A C, Jurgensen M F, Frank D A Switzerland, USA Plant Sciences , Ecology , Agriculture, Soil Sciences Grazing and seasonal variation in precipitation and temperature are important controls of soil and plant processes in grasslands. As these ecosystems store up to 30% of the world’s belowground carbon (C), it is important to understand how this variability affects mineral soil C pools/fluxes, and how C cycling might be affected by changes in precipitation and temperature, due to climate change. The aim of this study was to investigate the effects of grazing and differences in soil temperature and moisture on standard organic matter (OM) decomposition rates (cotton cloth) incubated in the top 10 cm soil of grasslands with variable topography in Yellowstone National Park (YNP) during the 2004 growing season. Grazing did not affect soil temperature, moisture, cotton cloth decomposition rates, soil bulk density, soil C and N concentrations, or soil C:N ratios. However, a large spatio-temporal variability in decomposition was observed: cotton cloth decomposition was positively related to soil moisture and soil C and N concentrations, and negatively to soil temperature. Highest decomposition rates were found in wetter slope bottom soils (season averages of decomposition given as rate of decomposition (cotton rotting rate=CRR)=23-26%) and lower rates in drier, hill-top soils (season averages, CRR=20%). Significantly higher decomposition rates were recorded in spring, early summer and early fall when soils were moist and cool (spring, CRR=25%; early summer, CRR=26%; fall, CRR=20%) compared to mid-summer (CRR=18%) when soils were dry and warm. Our findings suggest that climate-change related decreases in precipitation and increases in temperature predicted for North American grasslands would decrease soil OM decomposition in YNP, which contrasts the general assumption that increases in temperature would accelerate OM decomposition rates. Plant and Soil, 2007, V298, N1-2, SEP, pp 191-201. 08.1-159 Natural avalanche disturbance shapes plant diversity and species composition in subalpine forest belt Rixen C, Haag S, Kulakowski D, Bebi P Switzerland, USA Plant Sciences , Biodiversity , Ecology , Forestry , Cryology / Glaciology Background: Disturbances by avalanches have created unique habitats for animals and plants in subalpine ecosystems worldwide, but at the same time avalanches can pose a major threat to humans. Thus, avalanches are suppressed by means of avalanche barriers to protect settlements and infrastructures in populated areas of the European Alps. As a consequence, the disturbance regime in avalanche tracks has fundamentally changed. Methods: In the present study we address ecological consequences of avalanche suppression on plant diversity. We analysed plant diversity and species composition in recent and old avalanche tracks with and without avalanche suppression and in undisturbed adjacent forests at high and low elevations. Results: The number of species was higher in both active and inactive avalanche tracks as compared to undisturbed subalpine forest. The species composition indicated a wider range of ecological niches in active than in inactive avalanche tracks. The vegetation from active tracks showed lower indicator values for temperature and nitrogen availability. The pro-
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