Browsing by Person "Land, Alexander"

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Complex imprint of solar variability on tree rings
(2020) Land, Alexander; Kromer, Bernd; Remmele, Sabine; Brehm, Nicolas; Wacker, Lukas
Many studies have investigated the role of solar variability in Holocene climate. Beyond sunspot observations, solar activity can be reconstructed from 14C in tree rings. Due to the lack of sub-decadal resolution of 14C records, these studies focused on long-term processes. In this study, we use an annually-resolved 14C record to examine solar variability (e.g. 11-year Schwabe solar cycle) and its connection to European seasonal climate inferred from tree-ring records during the entire past millennium with spectral and wavelet techniques. The 11-year Schwabe solar cycle shows a significant impact in European moisture- and temperature-sensitive tree-ring records. Complex ’top-down’/‘bottom-up’ effects in the strato-tropoatmospheric system are assumed to affect European spring and summer climate with a temporal-shift as evident from observed changes in phase behavior. Significant evidence is also found for the ∼60- and ∼90-year band during the first half of the past millennium.
Holzanatomische Veränderungen als Reaktion auf extreme Umweltereignisse in rezenten und subfossilen Eichen und deren Verifizierung im Experiment
(2014) Land, Alexander; Küppers, Manfred
This study focuses on changes in wood anatomy of recent (living study), container-grown (verification phase) and subfossil (subfossil study) oaks responding to severe biotic and abiotic events, as well as on “classical” tree-ring analysis (e.g. climate-growth relationships, signature-year analysis and age-trend analysis). Further, non-typical cell formations in wood anatomy with respect to the number, the shape and the size of xylem cells have been investigated. At this juncture, it was distinguished between continuous proxies (year-to-year), like total ring width or earlywood-vessel size, and non-continuous proxies (not year-to-year), like collapsed earlywood vessels or highly enhanced latewoodcell diameters. Measurement of annually resolved cross-section areas of earlywood vessels in recent and subfossil oak trees has been carried out by a specific developed semi-automatic tree ring image-analysis software (TRIAS). Basic objectives of this study were to connect the affecting biotic or abiotic events to specific changes in wood anatomy. Investigations on living (from locations with different side conditions in southern Germany) and container-grown (in the botanical garden, University of Hohenheim) oaks were performed to validate (verification phase) the results independently between the living and container study, getting clearer insights into the wood anatomical reaction of oaks and the homogeneity of the signal. Finally, the potential of qualitative and quantitative wood-anatomy for palaeo-climatology/-ecology is shown by applying the results to subfossil oaks (Holocene Oak Chronology Hohenheim). An event analysis was performed within two long-lasting periods (130 years and 1.000 years) in the mid-Holocene. Frequency distribution of extreme events, like high-magnitude floods or severe late-frost events, within these two periods in the mid-Holocene was compared with frequencies during the past 100 years. One result of this study is that one signature wood-anatomical change could be related to an extreme biotic or abiotic event, if the wood-anatomical feature occurred not only on local side level but on regional or national scale. Key evidence could be found for, that 1. out-of-true and collapsed earlywood vessels combined with traumatic tissue (wood-anatomical feature F/T) occurred after very low temperatures at the time of bud break, 2. intra-annual density bands are developed after rapid drops of temperature at an early stage (feature C) of the vegetation period or within (feature D) (depending on the position within the tree ring), 3. a long-lasting summer-flood event results in highly enhanced latewood-cell diameters (feature G), 4. a long-lasting spring-flood event reduces significantly the cross-section area of all vessels within earlywood (feature SEV), 5. latewood formation is absence or highly reduced in years with severe drought periods, 6. rapid drops of temperatures among winter periods (winter frost-events) reduce cross-section areas of initially built earlywood vessels (MVA5) and 7. continuous tree-ring proxies, like total ring width or earlywood-vessel variables, are significantly influenced by different climate factors. Wood-anatomical features from the recent study for 1., 3. and 4. could also be validated in the verification phase. The link between a biotic or abiotic event and the related specific wood-anatomical feature for 2., 5. and 6. could be found either in the recent study or the verification phase. Conducted measurements of diametrical stem growth, stem-growth dynamic and water deficit demonstrate big differences between the simulations (drought, frost, water logging, defoliation) that have been carried out on container-grown oaks. The simulations show massive changes in stem-growth dynamics of container-grown oaks affected by drought, frost and defoliation, whereas the waterlogged and control oaks reacted quite similar in their behaviour, and document the good adaptation of pedunculate oak trees to flooding. Pointer years were calculated and analysed for latewood and earlywood growth at the region Main and upper Rhine. Negative latewood-pointer years show great concurrence to years of annual summer-drought periods respectively long-lasting perennial droughts. Extreme events have been dated and analysed within two mid-Holocene periods of 2527-2396 BC (upper Rhine region) and 3279-2201 BC (Main region). High-magnitude floods and winter-frost events were dated using wood-anatomical features in subfossil oak trees (HOC Hohenheim). A “period of high river dynamic” in the Main valley could be analysed around 2750 BC. This result points out that the number of flood events in this period could be much higher than in the twentieth century whereas the number of detected winter-frost events at the upper Rhine region seems not to be increased compared to the past one hundred years.
Soil drought sets site specific limits to stem radial growth and sap flow of Douglas-fir across Germany
(2024) Niessner, Armin; Ehekircher, Stefan; Zimmermann, Reiner; Horna, Viviana; Reichle, Daniel; Land, Alexander; Spangenberg, Göran; Hein, Sebastian; Niessner, Armin; Department of Silviculture, University of Applied Forest Sciences, Rottenburg am Neckar, Germany; Ehekircher, Stefan; Department of Silviculture, University of Applied Forest Sciences, Rottenburg am Neckar, Germany; Zimmermann, Reiner; Ecological Botanical Gardens ÖBG, University of Bayreuth, Bayreuth, Germany; Horna, Viviana; Department of Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Jena, Germany; Reichle, Daniel; Institute of Biology, University of Hohenheim, Stuttgart, Germany; Land, Alexander; Department of Silviculture, University of Applied Forest Sciences, Rottenburg am Neckar, Germany; Spangenberg, Göran; Department of Silviculture, University of Applied Forest Sciences, Rottenburg am Neckar, Germany; Hein, Sebastian; Department of Silviculture, University of Applied Forest Sciences, Rottenburg am Neckar, Germany
Introduction: Soil drought during summer in Central Europe has become more frequent and severe over the last decades. European forests are suffering increasing damage, particularly Norway spruce. Douglas-fir ( Pseudotsuga menziesii (Mirbel) Franco), a non-native tree species, is considered as a promising alternative to build drought-resilient forests. The main goal of this study was to investigate the intraannual radial stem growth and sap flow performance of Douglas-fir along a precipitation gradient across Germany under severe drought. Material and methods: Sap flow and stem radial changes of up to ten trees each at four sites with different precipitation regimes were measured in combination with volumetric soil water content during the growing season of 2022. Measurements of stem radial changes were used to calculate the trees’ stem water deficit, a proxy for tree water status and drought stress. Results: The severe summer drought of 2022 led to an early growth cessation and a significant reduction in daily sap flow at all four sites monitored. We could identify a site-specific threshold in soil water availability ranging between 21.7 and 29.6% of relative extractable water (REW) under which stem water reserves cannot be replenished and thereby inhibiting radial growth. We could also demonstrate that at this threshold, sap flow is heavily reduced to between 43.5 and 53.3%, and for a REW below 50%, sap flow linearly decreases by 1.1–2.0% per 1% reduction in REW. This reduction tends to follow the humidity gradient, being more pronounced at the most oceanic characterized site and suggesting an adaptation to site conditions. Even though Douglas-fir is considered to be more drought stress resistant than Norway spruce, growth and sap flow are greatly reduced by severe summer drought, which became more frequent in recent years and their frequency and intensity is likely to increase. Conclusions: Our results suggest that timber production of Douglas-fir in Central Europe will decline considerably under projected climate change, and thus pointing to site specific growth constraints for a so far promising non-native tree species in Europe.