1,720,977 research outputs found
Growth responses and cell characteristics of silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst.) at different altitudes in south- western Germany
No full textClimate response of radial growth of silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst) in the Black Forest, Germany
No full textGrowth responses of silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst.) to climate along an altitudinal gradient in southwestern Germany
No full textDrought resistance of Douglas-Fir in central europe: Future viability and comparison to other temperate tree species
No full textThe climatic changes, which manifest in rising temperatures and increasing dryness, pose a significant challenge to terrestrial ecosystems. Trees, due to their sessile lifestyle and specific adaptation to a given location, are considered particularly vulnerable and are especially affected by these changes, primarily because of the rapid pace at which the environment is changing. Droughts are among the phenomena that are expected to occur more frequently in the future and will become increasingly extreme due to climate change. These are therefore referred to as 'climate change type drought events' or 'hot droughts.' The responses of tree species to drought vary considerably, making it crucial to understand the species-specific strategies for drought tolerance in order to select the most resilient species and provenances for future forests.
Douglas-fir (Pseudotsuga menziesii), originating from North America, is a widely distributed tree species in Europe and is considered a promising species for the future because it exhibits good drought adaptation while also having high economic value. This study aims to investigate the drought tolerance of different Douglas-fir provenances and seeks to describe the fundamental strategic differences in desiccation time between angiosperms and gymnosperms based on their hydraulic capacitance. The study focuses on two central hypotheses: (I) that Douglas-fir provenances from certain regions are more resistant to drought and therefore better suited for climate-resilient forests, and (II) that hydraulic capacity plays a crucial role in the drought tolerance of temperate gymnosperms and angiosperms.
To test these hypotheses, we conducted a gradient study in Central Europe on mature Douglas-fir trees and a drought stress experiment with 16 different temperate tree species. Several ecophysiological, morphological, and growth-related traits were measured, including hydraulic parameters, leaf morphology, carbon isotope ratios in leaves, xylem anatomical characteristics, and dendrochronological measurements.
The results show that Douglas-fir provenances from the northern interior regions grow more slowly and have a lower growth rate compared to provenances from coastal regions. This is associated with an adaptation to higher frost tolerance and appears to be genetically fixed. Provenances from the northern interior regions also demonstrated greater drought tolerance during the pan-European drought events of 2003 and 2018. Interestingly, dendroecological analysis revealed that these provenances had the lowest resistance but equal or even higher recovery rates. This is considered a drought-adapted behavior and is attributed to a more plastic stomatal response.
On a leaf morphological level, we observed a significant reduction in the specific leaf area (SLA) in the northern interior provenances, which correlates with slower growth and reduced photosynthetic area. This trait implies a smaller leaf surface area, and consequently, reduced transpiration area, which contributes to better drought tolerance. However, the physiological values of hydraulic efficiency (Huber value, leaf δ13C as a proxy for intrinsic water use efficiency iWUE) and hydraulic safety (P12, P50) showed no significant differences. For the hydraulic safety related to embolism resistance (P12, P50), this is not surprising, as these values are considered evolutionarily constrained in conifers and therefore show little variation.
When comparing desiccation times in young trees of 16 different temperate tree species, Douglas-fir exhibited the second-longest desiccation time after larch (Larix decidua), closely followed by silver fir (Abies alba). The ranking of the tree species clearly shows longer desiccation times for gymnosperms compared to angiosperms. This is largely due to experimental factors, such as insufficient pot volume for taproot species. However, it is also partly related to strategic behavior. The study on hydraulic capacitance shows that gymnosperms store more water in their stems than angiosperms and have a higher hydraulic capacity. This allows gymnosperms to utilize more stored water during droughts, helping them cope better. Gymnosperms also reduce their hydraulic capacity earlier in the desiccation phase compared to angiosperms.
This study provides strong evidence that northern interior provenances in Central Europe are more resilient to drought compared to coastal provenances. This means that provenance selection is a crucial factor in creating climate-resilient forests. Furthermore, compared to other temperate tree species, Douglas-fir demonstrates high drought tolerance, reflected in longer desiccation times. This suggests that Douglas-fir holds great potential for climate-resilient forests and, in combination with the right provenance choice, can play a key role.
Finally, the study highlights the importance of understanding the various physiological and morphological strategies employed by different tree species in response to drought. Future research should further investigate the genetic mechanisms behind these adaptations and explore the potential of other species to adapt to changing climate conditions, particularly through additional gradient studies and manipulative experiments.:Table of contents I
Summary V
Zusammenfassung VII
Abbreviations IX
Chapter 1 2
1. General Introduction 2
1.1. Temperate forest ecosystems and droughts 2
1.2. Tree hydraulic and strategies under drought conditions 3
1.2.1. Water transport and the water potential 4
1.2.2. The influence of drought on the tree hydraulic system 5
1.2.3. Tree drought reaction strategies and the role of the water storage capacitance 6
1.3. Phenotypic plasticity and the importance in climate change 8
1.4. Tree ring growth in terms of climate change and droughts 9
1.5. Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) 9
1.6. Project objectives 10
1.7. Experimental design, study sites, and measured parameters 11
1.7.1. Part I: Gradient study 11
1.7.2. Part II: Greenhouse drying out experiment 12
1.7.3. Summary of parameters 14
Chapter 2 16
2. Radial growth reduction during extreme drought of Douglas-fir provenances across a climatic gradient in Central Europe 17
2.1. Introduction 17
2.2. Materials and Methods 19
Study sites 19
Climatic data 21
Provenance clustering 21
Tree structure and aboveground biomass increment 22
Wood core sampling, preparation, and processing 23
Drought response indices 23
Data analysis 23
2.3. Results 24
Long-term aboveground biomass and radial growth increment 24
Growth-response during extreme drought exposure 26
Drought response indices 27
2.4 Discussion 30
Growth behavior of the climate-at-origin groups across the gradient 31
Vulnerability of radial growth to extreme drought events 32
Drought response indices 33
Implementations to future management decisions 33
Conclusion 35
Chapter 3 36
3. The effect of climate at origin on Douglas-fir growth, leaf traits, and embolism resistance along a rainfall gradient in Central Europe 37
3.1. Introduction 37
3.2. Material and Method 39
Study sites and climate 39
Provenance selection and clustering into climate-origin groups 42
Tree structure and aboveground biomass increment 43
Branch sampling 43
Vulnerability curves 43
Leaf traits 44
Statistical analysis 44
3.3. Results 45
Differences between climate-origin groups 45
Variation in functional traits along the climatic gradient 47
Intra-specific trait variability on different levels of organization 50
3.4. Discussion 51
Trait variability between climate-origin groups 51
Effects of water availability along the gradient on trait variability 53
Sources of variation on different levels of organization 54
Conclusion 55
Chapter 4 56
4. Water storage strategies in drought conditions: The fundamental differences between broad-leaf trees and conifers of temperate forests 57
4.1. Introduction 57
4.2. Material and Method 60
Plant material 60
Shoot hydraulic capacitance and water content 60
Wood density and saturated water content 63
Vulnerability curves 63
Vessel anatomical traits from light microscopy 63
Leaf traits 64
Data analysis 64
4.3. Results 65
Water stored, xylem anatomy, and xylem vulnerability 65
Shoot hydraulic capacitance phases 66
Water relations at different stages of drought 69
Shoot capacitance in relation to xylem structure and stored water 71
4.4. Discussion 73
The different water storage and Cshoot strategies in a drought situation 73
Cshoot and the relation to xylem anatomy 74
Conclusion 75
Chapter 5 76
5. Synthesis 77
5.1. Douglas-fir: The drought resistance of different provenances 77
5.2. Temperate conifers and angiosperms under drought 80
5.3. Determination of desiccation times for temperate tree species 82
5.4. Outlook 85
References 87
Appendices 120
Chapter 2 121
Chapter 3 139
Chapter 4 151
Acknowledgments 153
Curriculum Vitae 154
Anlage 1 157
Anlage 2 15
Drought resistance of Douglas-Fir in central europe: Future viability and comparison to other temperate tree species
No full textThe climatic changes, which manifest in rising temperatures and increasing dryness, pose a significant challenge to terrestrial ecosystems. Trees, due to their sessile lifestyle and specific adaptation to a given location, are considered particularly vulnerable and are especially affected by these changes, primarily because of the rapid pace at which the environment is changing. Droughts are among the phenomena that are expected to occur more frequently in the future and will become increasingly extreme due to climate change. These are therefore referred to as 'climate change type drought events' or 'hot droughts.' The responses of tree species to drought vary considerably, making it crucial to understand the species-specific strategies for drought tolerance in order to select the most resilient species and provenances for future forests.
Douglas-fir (Pseudotsuga menziesii), originating from North America, is a widely distributed tree species in Europe and is considered a promising species for the future because it exhibits good drought adaptation while also having high economic value. This study aims to investigate the drought tolerance of different Douglas-fir provenances and seeks to describe the fundamental strategic differences in desiccation time between angiosperms and gymnosperms based on their hydraulic capacitance. The study focuses on two central hypotheses: (I) that Douglas-fir provenances from certain regions are more resistant to drought and therefore better suited for climate-resilient forests, and (II) that hydraulic capacity plays a crucial role in the drought tolerance of temperate gymnosperms and angiosperms.
To test these hypotheses, we conducted a gradient study in Central Europe on mature Douglas-fir trees and a drought stress experiment with 16 different temperate tree species. Several ecophysiological, morphological, and growth-related traits were measured, including hydraulic parameters, leaf morphology, carbon isotope ratios in leaves, xylem anatomical characteristics, and dendrochronological measurements.
The results show that Douglas-fir provenances from the northern interior regions grow more slowly and have a lower growth rate compared to provenances from coastal regions. This is associated with an adaptation to higher frost tolerance and appears to be genetically fixed. Provenances from the northern interior regions also demonstrated greater drought tolerance during the pan-European drought events of 2003 and 2018. Interestingly, dendroecological analysis revealed that these provenances had the lowest resistance but equal or even higher recovery rates. This is considered a drought-adapted behavior and is attributed to a more plastic stomatal response.
On a leaf morphological level, we observed a significant reduction in the specific leaf area (SLA) in the northern interior provenances, which correlates with slower growth and reduced photosynthetic area. This trait implies a smaller leaf surface area, and consequently, reduced transpiration area, which contributes to better drought tolerance. However, the physiological values of hydraulic efficiency (Huber value, leaf δ13C as a proxy for intrinsic water use efficiency iWUE) and hydraulic safety (P12, P50) showed no significant differences. For the hydraulic safety related to embolism resistance (P12, P50), this is not surprising, as these values are considered evolutionarily constrained in conifers and therefore show little variation.
When comparing desiccation times in young trees of 16 different temperate tree species, Douglas-fir exhibited the second-longest desiccation time after larch (Larix decidua), closely followed by silver fir (Abies alba). The ranking of the tree species clearly shows longer desiccation times for gymnosperms compared to angiosperms. This is largely due to experimental factors, such as insufficient pot volume for taproot species. However, it is also partly related to strategic behavior. The study on hydraulic capacitance shows that gymnosperms store more water in their stems than angiosperms and have a higher hydraulic capacity. This allows gymnosperms to utilize more stored water during droughts, helping them cope better. Gymnosperms also reduce their hydraulic capacity earlier in the desiccation phase compared to angiosperms.
This study provides strong evidence that northern interior provenances in Central Europe are more resilient to drought compared to coastal provenances. This means that provenance selection is a crucial factor in creating climate-resilient forests. Furthermore, compared to other temperate tree species, Douglas-fir demonstrates high drought tolerance, reflected in longer desiccation times. This suggests that Douglas-fir holds great potential for climate-resilient forests and, in combination with the right provenance choice, can play a key role.
Finally, the study highlights the importance of understanding the various physiological and morphological strategies employed by different tree species in response to drought. Future research should further investigate the genetic mechanisms behind these adaptations and explore the potential of other species to adapt to changing climate conditions, particularly through additional gradient studies and manipulative experiments.:Table of contents I
Summary V
Zusammenfassung VII
Abbreviations IX
Chapter 1 2
1. General Introduction 2
1.1. Temperate forest ecosystems and droughts 2
1.2. Tree hydraulic and strategies under drought conditions 3
1.2.1. Water transport and the water potential 4
1.2.2. The influence of drought on the tree hydraulic system 5
1.2.3. Tree drought reaction strategies and the role of the water storage capacitance 6
1.3. Phenotypic plasticity and the importance in climate change 8
1.4. Tree ring growth in terms of climate change and droughts 9
1.5. Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) 9
1.6. Project objectives 10
1.7. Experimental design, study sites, and measured parameters 11
1.7.1. Part I: Gradient study 11
1.7.2. Part II: Greenhouse drying out experiment 12
1.7.3. Summary of parameters 14
Chapter 2 16
2. Radial growth reduction during extreme drought of Douglas-fir provenances across a climatic gradient in Central Europe 17
2.1. Introduction 17
2.2. Materials and Methods 19
Study sites 19
Climatic data 21
Provenance clustering 21
Tree structure and aboveground biomass increment 22
Wood core sampling, preparation, and processing 23
Drought response indices 23
Data analysis 23
2.3. Results 24
Long-term aboveground biomass and radial growth increment 24
Growth-response during extreme drought exposure 26
Drought response indices 27
2.4 Discussion 30
Growth behavior of the climate-at-origin groups across the gradient 31
Vulnerability of radial growth to extreme drought events 32
Drought response indices 33
Implementations to future management decisions 33
Conclusion 35
Chapter 3 36
3. The effect of climate at origin on Douglas-fir growth, leaf traits, and embolism resistance along a rainfall gradient in Central Europe 37
3.1. Introduction 37
3.2. Material and Method 39
Study sites and climate 39
Provenance selection and clustering into climate-origin groups 42
Tree structure and aboveground biomass increment 43
Branch sampling 43
Vulnerability curves 43
Leaf traits 44
Statistical analysis 44
3.3. Results 45
Differences between climate-origin groups 45
Variation in functional traits along the climatic gradient 47
Intra-specific trait variability on different levels of organization 50
3.4. Discussion 51
Trait variability between climate-origin groups 51
Effects of water availability along the gradient on trait variability 53
Sources of variation on different levels of organization 54
Conclusion 55
Chapter 4 56
4. Water storage strategies in drought conditions: The fundamental differences between broad-leaf trees and conifers of temperate forests 57
4.1. Introduction 57
4.2. Material and Method 60
Plant material 60
Shoot hydraulic capacitance and water content 60
Wood density and saturated water content 63
Vulnerability curves 63
Vessel anatomical traits from light microscopy 63
Leaf traits 64
Data analysis 64
4.3. Results 65
Water stored, xylem anatomy, and xylem vulnerability 65
Shoot hydraulic capacitance phases 66
Water relations at different stages of drought 69
Shoot capacitance in relation to xylem structure and stored water 71
4.4. Discussion 73
The different water storage and Cshoot strategies in a drought situation 73
Cshoot and the relation to xylem anatomy 74
Conclusion 75
Chapter 5 76
5. Synthesis 77
5.1. Douglas-fir: The drought resistance of different provenances 77
5.2. Temperate conifers and angiosperms under drought 80
5.3. Determination of desiccation times for temperate tree species 82
5.4. Outlook 85
References 87
Appendices 120
Chapter 2 121
Chapter 3 139
Chapter 4 151
Acknowledgments 153
Curriculum Vitae 154
Anlage 1 157
Anlage 2 15
Growth, vitality and stability: Spatio-temporal responses of European beech and Scots pine to climate change
Full text linkRecent climatic extreme events, such as the 2018-2020 drought period, demonstrate that ongoing climate change has a significant impact on our plant ecosystems, resulting in a variety of consequences such as temporal shifts in the growing season, biodiversity loss, and increased tree mortality. Forest ecosystems are especially endangered because the trees’ long lifecycles and their sessile nature impairs the potential to adapt or evade negative impacts in time. Nonetheless, forests are particularly essential because they accomplish key functions in our economic, ecological, and social lives, such as supplying timber, regulating carbon- and water cycling, or providing recreational benefits. Consequently, we need to investigate and comprehend the climatic impact on forest growth at both temporal and spatial scales. Additionally, we must examine the current state of forest vitality and productivity in order to make predictions about forest growth under changing climate.
This thesis adds to our understanding of the climate-growth responses of two economically and ecologically important tree species in Central Europe within their low elevational and central distribution ranges: European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.). We examine patterns in climate-driven growth responses at different spatiotemporal scales, ranging from regional to site-specific extents, and from retrospective to near real-time monitoring. In addition, we look at the possibility of employing tree-ring width (TRW) and remote-sensing (RS) data to assess forest vitality and productivity. A deeper knowledge of climate-growth responses in European beech and Scots pine will provide a foundation for decision making and forest management, assisting in the development of a resistant and resilient forest of the future.
Chapter 1 provides an overview of the research objectives by situating them in the context of the present state of the art, framing the research objectives, introducing the study design, and finally formulating the research questions for this thesis. For that reason, we employ two tree-ring networks with varying spatial scales: the regional-scale Baltic Sea Network and the site-specific BDF-F-Network. The Baltic Sea Network includes TRW data from 119 pine and 55 beech study sites spread throughout the southern Baltic Sea region, which is distinguished by its predominantly medium nutritious soils, low elevation, and transitional climate ranging from maritime to more continental conditions. The BDF-F-Network, situated within the spatial extents of the Baltic Sea Network, spans along a precipitation gradient in northern Germany. It comprises 54 permanent monitoring plots with substantial information on soil and tree status dating back 40 years. During this PhD project, we extended the exhaustive data base of site-specific information by collecting TRW data for the entire network. As a result, the newly established BDF-F-Network acts as the thesis' centering point.
In Chapters 2 and 3, we investigate the spatio-temporal growth responses of beech and pine in their low-elevational and central distribution ranges. Both species exhibit species-specific climate-growth responses with similar patterns at different spatial scales, i.e. when comparing the Baltic Sea and BDF-F-Network. While beech growth is predominantly impacted by summer drought conditions, winter temperature has the greatest impact on pine. We show that the main climatic drivers stay stable across spatial scales, whereas secondary climatic drivers, or climatic drivers with weaker correlations, may vary. Further, we investigate temporal instabilities in climate-growth responses for both networks by applying spatial segregation analyses and comparing growth responses for an early and a later period. We show that during the last few decades, both beech and pine have responded instable to their main climatic drivers, with increased sensitivity to summer drought and winter temperature, respectively. These temporal instabilities are visible at both regional and site-specific scales.
Furthermore, Chapter 3 addresses how non-climatic and site-specific soil- and stand characteristics may influence tree growth across the BDF-F-Network's precipitation gradient. We use multilinear regression modeling to examine how stand parameters such as average tree height, diameter at breast height, and TRW differ across the gradient, and if they are impacted by soil water availability or soil type. However, our findings indicate no significant differences in site-specific soil- and stand-characteristics, with the exception of a minor effect on average tree height of European beech.
In Chapter 4, we estimate the potential of TRW to assess long-term trends in beech vitality. At 9 sites, we compare the growth trends, climate sensitivities, and drought resistance of 10-20 pairs of vital and non-vital trees that are visually classified by crown condition. Moreover, we use individual heterozygozity as a proxy to determine if differences in growth behavior are caused by genetic predisposition. Surprisingly, growth responses and individual heterozygozity are similar in non-/vital trees. At several study sites, some as vital classified trees exhibit an even greater reduction in TRW than non-vital trees. In summary, we show that TRW is a better proxy for assessing long-term trends in tree vitality, compared to crown condition assessments that are defined by a high year-to-year dynamic.
Similarly, Chapter 5 seeks to study the potential of satellite-derived leaf area index (LAI) series to monitor and evaluate forest productivity using European beech as an example. We employ an interdisciplinary approach by combining medium resolution LAI time series derived from two separate satellite sensors (SPOT-VGT/PROBA-V and MODIS), as well as long-term masting monitoring and TRW data from BDF-F-Network sites. By applying site-specific and across-network correlation analysis, we analyze the link between these three target parameters and identify common climatic drivers. While SPOT-VGT/PROBA-V LAI is negatively correlated with masting and positively correlated with TRW, finer resolved MODIS data does not show any significant relationships. We show that RS data from the SPOT-VGT/PROBA-V sensor could be a useful tool for assessing forest vitality and productivity if the LAI time series are sufficiently long. Furthermore, our findings indicate that masting and TRW are both influenced by summer climate conditions, whereas RS LAI appears to be climatically de-coupled. Our findings suggest that RS data has the potential to explore masting and hence forest productivity, but it should always be evaluated in light of the restrictions of different RS products.
Finally, Chapter 6 summarizes the preceding chapters' findings and discusses them in the context of the research questions provided at the beginning of the thesis.Die jüngsten klimatischen Extremereignisse, wie die Dürreperiode 2018-2020, zeigen, dass sich der anhaltende Klimawandel erheblich auf unsere Pflanzenökosysteme auswirkt. Dies führt zu einer Vielzahl weitreichender Folgen, wie der zeitlichen Verschiebung von Wachstumsperioden, Verlusten in der Biodiversität oder einer erhöhten Waldsterblichkeit. Besonders gefährdet sind hierbei Waldökosysteme, da die langen Lebenszyklen von Bäumen eine schnelle Anpassung an sich ändernde Klimabedingungen stark beeinträchtigen. Jedoch erfüllen vor allem Wälder wichtige Funktionen in unserem wirtschaftlichen, ökologischen und sozialen Leben, z. B. als Holzlieferanten, Regulatoren des Kohlenstoff- und Wasserkreislaufs oder auch als Erholungsraum für die Menschen. Dementsprechend ist es unerlässlich die klimatischen Auswirkungen auf das Waldwachstum sowohl auf zeitlicher als auch auf räumlicher Ebene zu untersuchen und zu verstehen. Zudem müssen wir auch den aktuellen Zustand der Vitalität und Produktivität der Wälder überprüfen, um aussagekräftige Prognosen zum Waldwachstum unter dem Einfluss des Klimawandels treffen zu können.
Diese Dissertation trägt zu unserem Verständnis der Klimawachstumsreaktionen zweier wirtschaftlich und ökologisch relevanter Baumarten in ihrem ökologisch optimalen Verbreitungsgebiet in Mitteleuropa bei: der Rotbuche (Fagus sylvatica L.) und der Waldkiefer (Pinus sylvestris L.). Hierbei werden die klimabedingten Wachstumsreaktionen auf verschiedenen räumlichen und zeitlichen Skalen untersucht, welchen von regionalen bis standortsspezifischen Ausmaßen und von retrospektiven Analysen bis nahezu Echtzeit-Monitoring reichen. Darüber hinaus erörtert diese Arbeit das Potenzial Jahrringdaten (TRW) als auch Fernerkundungsdaten (RS) zur Bewertung der Vitalität und Produktivität von Wäldern beispielhaft an der Rotbuche zu verwenden. Ein tiefreichendes Verständnis der Klima- Wachstumsreaktionen der Rotbuche und der Waldkiefer bietet somit eine gute Basis um bei Fragen zur zukünftigen Waldbewirtschaftung und der Entscheidungsfindung zu unterstützen, sodass wir einen widerstandsfähigen Wald der Zukunft aufbauen können.
Kapitel 1 gibt einen Überblick über diese Dissertation, indem es Forschungsfragen in den Kontext des aktuellen Stands der Wissenschaft einordnet, die Forschungsziele erläutert, das Studiendesign vorstellt und schließlich die Forschungsfragen formuliert. Hierbei verwenden wir zwei Jahrringnetzwerke mit unterschiedlichen räumlichen Ausmaßen: das regionale Baltic Sea Netzwerk und das standortspezifische BDF-F-Netzwerk. Das Baltic Sea Netzwerk umfasst TRW-Daten von 119 Kiefern- und 55 Buchen-Standorten, welche sich über die gesamte südliche Ostseeregion erstrecken. Diese Standorte sind durch überwiegend mittelnährstoffreiche Böden, niedrige Höhenprofile und einem Übergangsklima von maritimen zu kontinentalen Bedingungen charakterisiert. Das BDF-F-Netz erstreckt sich entlang eines Niederschlagsgradienten in Norddeutschland und befindet sich somit innerhalb der geographischen Ausdehnungen des Baltic Sea Netzwerkes. Es umfasst 54 permanente Monitoring-Standorte zu denen umfangreichen Informationen zum Boden- und Baumzustand der letzten 40 Jahre vorliegen. Im Rahmen dieses Dissertationsprojektes wurden die umfassenden standortspezifischeren Informationen mit TRW-Daten für das gesamte Netzwerk ergänzt. Das daraus resultierende BDF-F-Netzwerk bildet somit den Dreh- und Angelpunkt der Dissertation.
In den Kapiteln 2 und 3 werden die räumlichen und zeitlichen Wachstumsreaktionen von Rotbuche und Waldkiefer in ihren zentralen Verbreitungsgebieten untersucht. Unabhängig von der räumlichen Skala, sprich beim Vergleich von Standorten beider Netzwerke, zeigen beide Baumarten artspezifische Klimawachstumsreaktionen. Während die Rotbuche primär von Sommertrockenheit beeinflusst wird, ist das Wachstum der Waldkiefer maßgeblich von den Temperaturen im Winter geprägt. Zudem zeigt diese Arbeit, dass die primären Klimatreiber über verschiedene räumliche Skalen hinweg stabil bleiben. Sekundäre Klimatreiber, welche durch schwächeren Korrelationen zwischen Klima und Wachstum charakterisiert sind, können jedoch über geographische Ausbreitungen hinweg variieren. Weiterhin wurden zeitliche Instabilitäten in den Klimawachstumsreaktionen für beide Netzwerke mittels räumlicher Segregationsanalysen und dem Vergleich einer frühen und einer späteren zeitlichen Periode untersucht. Die Ergebnisse zeigen, dass sowohl Rotbuche als auch Waldkiefer Instabilitäten in ihren Hauptklimatreibern aufweisen. In den letzten Jahrzehnten stieg die Sensitivität der Rotbuche auf Sommertrockenheit signifikant an, während die Waldkiefer immer stärker auf Wintertemperaturen reagiert. Diese zeitlichen Instabilitäten können in beiden Netzwerken, sowohl auf überregionalen als auch auf standortortsspezifischer Skala, nachgewiesen werden.
Kapitel 3 beleuchtet den Einfluss nichtklimatischer und standortspezifische Merkmale des Bodens und Bestandes auf das Baumwachstum. Hierbei werden entlang des Niederschlagsgradienten des BDF-F-Netzwerks Unterschiede in der durchschnittlichen Baumhöhe, des Brusthöhendurchmesser und TRW mittels multilinearer Regressionsmodellierung untersucht. Zudem werden die Einflüsse der Bodenwasserverfügbarkeit als auch der Bodenart erörtert. Abgesehen von einem marginalen Einfluss auf die durchschnittliche Baumhöhe der Rotbuche bestehen keine signifikanten Unterschiede in den Bestands- und Bodencharakteristika des Standorts. Die zugrundeliegenden Ursachen werden in Hinblick auf Erkenntnisse der aktuellen wissenschaftlichen Literatur und des Studiendesigns des BDF-F-Netzwerkes besprochen.
Kapitel 4 untersucht das Potenzial TRW-Daten zu verwenden, um langfristige Trends in der Vitalität der Rotbuche zu bewerten. Hierbei wurden in 9 Beständen 10-20 Baumpaare, bestehend aus je einem vitalen und einem nicht vitalen Baum, visuell nach Kronenzustand klassifiziert. Somit können Unterschiede in den langfristigen Wachstumstrends, der Klimasensitivität und der Trockenheits-resistenz zwischen nicht-/vitalen Individuen analysiert werden. Zudem wird die individuelle Heterozygotie als Proxy für genetische Vielfalt der Rotbuche herangezogen, um genetisch bedingte Unterschiede im Wachstumsverhalten zu beleuchten. Überraschenderweise unterscheiden sich weder die Wachstumsreaktionen noch die individuelle Heterozygotie bei nicht-/vitalen Rotbuchen. Zudem zeigen einige vitale Bäume eine höhere TRW-Reduktion im Vergleich zu nicht vitalen Bäumen. Dementsprechend zeigen die Ergebnisse, dass TRW ein guter Proxy für die Bewertung langfristiger Trends in der Vitalität von Bäumen zu sein scheint, wohingegen Vitalitätsansprachen mittels visueller Abschätzung des Kronenzustands aufgrund einer hohen Jahr-zu-Jahr-Dynamik eine Momentaufnahme der Baumvitalität darstellen.
Kapitel 5 erörtert die Frage, inwieweit wir satellitenbasierte Daten (RS) mit mittlerer Auflösung zur Überwachung und Bewertung der Waldproduktivität verwenden können und untersucht dies am Beispiel der Rotbuche. In diesem interdisziplinären Ansatz werden Fernerkundungsdaten des Blattflächenindex (LAI) von zwei Satellitensensoren (SPOT-VGT/PROBA-V und MODIS) mit Daten zum Mastverhalten und der TRW der Rotbuche innerhalb des BDF-F-Netzwerks kombiniert. Durch standortspezifische und netzwerkweite Korrelationsanalysen wird der Zusammenhang dieser drei Zielparameter analysiert und anhand gemeinsamer klimatischer Treiber diskutiert. Während SPOT-VGT/PROBA-V LAI negativ mit Mast und positiv mit TRW korreliert, gibt es keine signifikanten Korrelationen in höher aufgelösten MODIS-Daten. Die Ergebnisse zeigen, dass bei ausreichend langen Zeitreihen SPOT-VGT/PROBA-V-Daten ein nützliches Werkzeug zur Bewertung der Waldvitalität und -produktivität sein können. Darüber hinaus scheint RS LAI klimatisch entkoppelt zu sein, während sowohl Mastverhalten als auch TRW hauptsächlich vom Sommerklima beeinflusst werden. Die Ergebnisse zeigen, dass satellitenbasierte LAI-Daten das Potential besitzen das Mastverhalten und somit die Waldproduktivität zu erfassen und zu analysieren. Jedoch sollten etwaige Interpretationen und Rückschlüsse immer in Bezug zu technologischen und methodologischen Limitationen verschiedener RS-Produkte bewertet werden.
Abschließend fasst Kapitel 6 die Ergebnisse der vorherigen Kapitel zusammen und diskutiert sie im Zusammenhang mit den Forschungsfragen, die zu Beginn der Arbeit gestellt wurden
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Towards a global understanding of tree mortality
No full textRates of tree mortality are increasing globally, with implications for forests and climate. Yet, how and why these trends vary globally remain unknown. Developing a comprehensive assessment of global tree mortality will require systematically integrating data from ground-based long-term forest monitoring with large-scale remote sensing. We surveyed the metadata from 466 865 forest monitoring plots across 89 countries and five continents using questionnaires and discuss the potential to use these to estimate tree mortality trends globally. Our survey shows that the area monitored has increased steadily since 1960, but we also identify many regions with limited ground-based information on tree mortality. The integration of existing ground-based forest inventories with remote sensing and modelling can potentially fill those gaps, but this requires development of technical solutions and agreements that enable seamless flows of information from the field to global assessments of tree mortality. A truly global monitoring effort should promote fair and equitable collaborations, transferring funding to and empowering scientists from less wealthy regions. Increasing interest in forests as a natural climate solution, the advancement of new technologies and world-wide connectivity means that now a global monitoring system of tree mortality is not just urgently needed but also possible
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