Mountain regions and the important ecosystem services they provide are considered to be very vulnerable to the current warming, and recent studies suggest that high-mountain environments experience ...more rapid changes in temperature than environments at lower elevations. Here we analysed weather records for the period 1975-2010 from the Eastern Italian Alps that show that warming occurred both at high and low elevations, but it was less pronounced at high elevations. This negative elevation-dependent trend was consistent for mean, maximum and minimum air temperature. Global radiation data measured at different elevations, surface energy fluxes measured above an alpine grassland and above a coniferous forest located at comparable elevations for nine consecutive years as well as remote sensing data (MODIS) for cloud cover and aerosol optical depth were analysed to interpret this observation. Increasing global radiation at low elevations turned out to be a potential driver of this negative elevation-dependent warming, but also contributions from land use and land cover changes at high elevations (abandonment of alpine pastures, expansion of secondary forest succession) were taken into account. We emphasise though, that a negative elevation-dependent warming is not universal and that future research and in particular models should not neglect the role of land use changes when determining warming rates over elevation.
Vegetation has a substantial impact on the local climate. Land cover changes through afforestation or deforestation can amplify or mitigate climate warming by changes in biophysical and ...biogeochemical mechanisms. In the montane to subalpine area of the Eastern Alps in Europe, where forests have constantly expanded in the last four decades, data of meteorological stations show a consistent reduction in incoming global radiation for the period 2000-2015. To assess the potential role of forests in contributing to such a reduction, three site pairs in Central Europe with neighbouring forest and non-forest sites were analysed. In all the pairs, a lower amount of incoming radiation was recorded at the forest site. When biophysical mechanisms such as albedo, surface roughness and Bowen ratio changes were modelled together with changes in global radiation, the total radiative forcing accounted for a rate of change in air temperature was equal to 0.032 °C ± 0.01 °C per Wm−2. These results suggest that local climate is influenced by land cover change through afforestation both via albedo and radiation feedbacks but also by means of indirect biophysical and species-dependent mechanisms. The data obtained for the site pairs in Central Europe are finally discussed to infer the occurrence of similar forest-driven effects in the Eastern Alps which may explain part of the solar dimming observed in high elevation weather stations.
•Tree transpiration deficits and bark beetle attacks were induced experimentally.•Water balance routine was developed to update a practical risk rating tool.•Transpiration deficit demonstrated as ...meaningful proxy for host susceptibility.•However, stress proxy-host susceptibility relationship appears complex.•The tool PHENIPS-TDEF reports potential beetle flight and tree stress periods.
The projected increase in the frequency and severity with which bark beetle disturbances occur is forecasted to be partially driven by increases in drought episodes. Drought is widely considered to predispose host conifer trees to bark beetle attack; however, experimental data supporting this hypothesis are scarce. This study revisits the Rosalia Roof Project, the first throughfall manipulation experiment to investigate how attack by the Eurasian spruce bark beetle (Ips typographus) on mature Norway spruce (Picea abies) trees is affected by drought stress. Using the in situ “attack box” method, this study explores whether increased host acceptance by I. typographus and/or reduced host defense against attack coincide with increased tree transpiration deficits (i.e. the reduction from a potential transpiration caused by soil water limitation). To estimate transpiration deficits of the respective control and drought stress-induced (full-cover) trees, sap flow measurements were combined with simulations from a simple forest water balance routine. The model, which was calibrated against in situ hydrological measurements, has been developed for a hazard rating tool (PHENIPS-TDEF) which simulates both potential I. typographus phenology and tree drought stress in Norway spruce stands. While host acceptance appeared unaffected by tree transpiration deficits, acute and chronic transpiration deficits did lead to reduced host defense. Full cover trees for instance, which experienced an estimated 93 mm transpiration deficit in the previous May-Sep, could only defend against <10% of the total individual attack attempts between spring and midsummer compared to the control trees which experienced a corresponding deficit of 9 mm and defended >70% of attacks. However, similar defended attack percentages on the full-cover and control trees during late summer demonstrate the difficulty in deriving simple stress proxy-infestation risk relationships. The experiment therefore highlights the utility and limitations of transpiration deficits within I. typographus disturbance models and hazard assessment tools, such as PHENIPS-TDEF.
Evapotranspiration of a mixed European beech - Norway spruce forest was measured by means of the eddy covariance technique during a 19 day fine weather period in August 2000. Site conditions were ...non-ideal as to conventional micrometeorological standards. Sloping terrain and heterogeneously composed, small fetch required a thorough evaluation of the results by (1) the degree of energy balance closure (EBC), (2) footprint analysis and (3) cross-checking against soil water depletion estimates of evapotranspiration. Spatially distributed soil moisture measurements (194 permanent measuring points across a 70 x 70 m plot) guaranteed extended representativeness of the soil water budget method. Due to the convectively active atmosphere during the observation period and to the low measuring height, the source area of the latent heat flux remained small. Footprint analysis revealed that in 95% of the situations with positive net radiation the cumulative latent heat flux reached 50% of the total flux at a maximum upwind distance of 135 (+/-27) m. Fifty five percent of the peak footprints occurred within the soil moisture measuring plot, providing good spatial comparability between the two methods for determining evapotranspiration. They differed only by 0.2 mm for the whole period, showing better agreement in the second half of the period, when water redistribution in the subsoil had ceased. EBC evaluated on a daily basis ranged between 73 and 92%. No clear relation between magnitude of the closure gap and meteorological parameters could be identified. Overall, 46% of the incident net radiation drove evapotranspiration and 28% were released as sensible heat, the Bowen ratio being sensitive to weather conditions and wind direction, though. The results of this study give confidence that the eddy covariance technique is a useful tool to measure forest evapotranspiration also in complex terrain. PUBLICATION ABSTRACT
Automated time domain reflectometry (TDR) measurements in high resolution over soil depth and over time were performed in a mixed beech-spruce and a spruce stand during two hydrologically contrasting ...seasons. Soil drying was more intensive and reached deeper soil layers in the mixed stand, which on the other hand allowed more stand precipitation, compensating for the higher evapotranspiration rates. These results were confirmed by a large number of spatially distributed TDR measurements along grids of different spacing, which additionally covered a beech stand. Spatial water depletion patterns of the topsoil in spring appeared to be largely congruent with tree species distribution and reflected the higher water consumption of fully foliated beech. Variability was highest in the mixed stand, where a spatial correlation within a range of about 7 m was observed. The pure stands lacked spatial correlation. The effect of the mixed stand on soil water depletion and recharge turned out to be non-additive as compared to the pure stands of beech and spruce: changes of soil water storage under the mixed stand almost equalled the values measured in the beech stand. During selected drying periods in 2000 average daily water extraction rates from the uppermost 60 cm of soil amounted to 1.65 mm in the beech as well as in the mixed stand, which is about 45% more than under pure spruce. Maximum differences of up to 84% occurred in periods with high evaporative demand. The over-proportionate evapotranspiration of the mixed stand was exclusively attributable to beech, which deepened and intensified its fine-root system in mixture, while spruce rooted more shallowly. The mixed stand extracted a higher percentage of water from deeper soil layers than the pure stands.
The spatial dynamics of soil water-recharge in a forest stand is the product of a number of interacting processes. This study focuses on the role of tree species and antecedent soil water content ...upon horizontal and vertical patterns of soil water recharge in heavy clay soils of a mixed European beech-Norway spruce stand and of a pure Norway spruce stand after rewetting periods with different rain quantities and intensities. Volumetric water content (VWC) was measured at 194 locations across 0.5-ha plots in each stand using time-domain reflectometry (TDR) with fixed 30- and 60-cm vertical waveguides. This was repeated 28 times (as close as possible) before and after rewetting periods during the vegetation seasons in 2000 and 2001. In addition, the locations of all trees within the plots were recorded. Geostatistics was used to describe the spatial correlation between VWC measurements and to interpolate soil water recharge in space. Spatial patterns of soil water recharge were then evaluated according to antecedent soil water-content and tree species distribution. Open-field precipitation of 30 mm (maximum intensity 10 mm h^sup -1^) on extremely dry initial soil conditions resulted in higher subsoil (30-60 cm soil depth) recharge and erratic recharge patterns. This was presumably due to preferential flow in opening shrinkage cracks of the heavy clay soil. A comparable quantity and intensity of rainfall under moderately dry antecedent soil water conditions resulted in almost exclusively topsoil (0-30 cm soil depth) water recharge and patterns of recharge that were clearly related to tree species distribution. The higher recharge around beech trees can be attributed to the lower interception rates there. Spatial patterns of soil water recharge reflect patterns of antecedent soil water conditions.PUBLICATION ABSTRACT
Bulk precipitation, throughfall (TF) and stemflow (SF, for beech only) were monitored from May 02 to October 03 in adjacent stands of pure spruce, pure beech and mixed spruce–beech in Lower Austria. ...The main objective of this study was to specify regulating factors of TF and to address the impact of tree species composition on atmospheric input. We were especially interested, whether components of TF in the mixed stand indicate non-linear effects that would not be obvious in the single species stands. The mean annual nitrogen (NH
4
+
NO
3) and sulfur (SO
4) TF (+SF) fluxes were in the low to moderate range, increasing from the beech (N: 9.1 versus S: 5.5
kg
ha
−1) over the mixed (N: 11.6 versus S: 6.1
kg
ha
−1) to the spruce stand (N: 16.6 versus S: 9.2
kg
ha
−1). Two different methods were used to separate net throughfall (NTF) into the dry deposition (DD) and canopy exchange (CE) components. The regression analysis method on the event-to-event variation in NTF was useful for explaining the regulating factors for H
+ and K but failed for other substances, since we used multiple rain event collections, which mixed up both components. According to these results it is concluded that cation leaching was mainly driven by organic anions and bicarbonate but insignificantly by a cation exchange reaction with H
+, what may be considered a natural process in the absence of heavy acidic deposition. The filtering approach method, applying sulfate instead of widely used sodium as tracer for calculation of DD, yielded reasonable estimates. This model revealed that during the dry summer 03 DD rates were much higher than during the wet summer 02 for all studied elements. However, canopy leaching rates of K, Ca and Mg showed the opposite trend, since this process is mainly controlled by the amount of precipitation. Calculated total deposition (TD) fluxes as estimates of true inputs to the system were different from measured TF fluxes and justify the conclusion that the mixed spruce–beech stand showed an approximately linear response for H
+, K, NH
4, NO
3, SO
4, Cl and PO
4, if compared to the TD fluxes of the single species stands, but an antagonist response for Ca and Mg, since TD fluxes of these cations were almost identical for the beech and the mixed stand.
For a representative measurement of soil water content and its changes at the scale of a forest stand, information about the spatial and temporal variability has to be taken into account. The scale ...on which single trees and tree species influence the variation in space and time was investigated in a mixed Norway spruce–European beech stand in Lower Austria. On a 0.5 ha plot the volumetric soil water content (VWC) was measured at 194 sample locations along square grids of different spacing, using a Trase1 TDR system with waveguides installed vertically over a soil depth of 0–30 cm. Discontinuous measurements were taken in approximately 14 day intervals during the growing season of 2000. Semivariogram analysis was used to summarize the spatial variation of VWC at selected dates. Kriging interpolation plots of soil water depletion (recharge) between these dates served to compare extraction (rewetting) patterns to tree (species) distribution. During a long drying cycle in spring, species specific transpiration behaviour turned out to be the main source of variation. Light rainfalls of medium intensity on moderately dry soil caused a rewetting pattern, which reflected the different interception efficiency of the contrasted species resulting from architectural properties. Rain events of high intensity on dry soil led to an erratic distribution pattern of water in the soil due to preferential flow in shrinkage cracks, which had opened up during drying. The effect of tree architecture was largely covered in this case. The extent to which a clear spatial correlation of VWC could be detected varied within a radius of 4 to 20 m, depending on VWC as well as on the drying and rewetting history.
The development of vascular features in stems of Populus x euramericana was studied in two adjacent floodplain forest stands near the Danube in Austria. The construction of a hydropower plant caused ...one of the stands to loose contact with the groundwater at the age of 12 years, whereas the other stand remained uninfluenced. The investigation estimated the extent of vascular acclimation to altered site hydrology. Vessel lumen areas were separately measured for 26 tree-rings from six trees. Vessel densities (VD) and the percentages of the total vessel-lumen area per tree-ring area (PLA) were determined. Our results suggest that vessel size and density are correlated with circumferential stem growth, which is in turn governed by site water supply. In relation to the control trees, basal area increment (BAI) of the hydrologically altered trees was 66% lower. BAI was positively correlated to vessel size and negatively to VD and PLA. Vessel diameters of the mature control trees were large (with a mean of 95 μm and maxima around 160 μm), while average VD (42 no. mm^sup -2^) and PLA (31%) were within the normal ranges. In comparison to controls, the hydrologically altered trees reduced their vessel-lumen area by 15% (i.e. 8% in terms of diameter); average VD increased by 43%, and average PLA increased relatively by 19%. Vessel density proved to be the most sensitive indicator of hydrological alterations. Intra-annual development of vessel size indicated a rapid response to the hydrological conditions of the current growing season.PUBLICATION ABSTRACT
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