Deciphering mechanisms and implications of temperate tree phenology: A treatise of physiological processes, water dynamics, and tree-induced feedback to the ecosystem
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Date
2024
Publication Type
Doctoral Thesis
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Abstract
The collection of experiments and studies that I conducted during my PhD significantly advanced the current understanding of temperate tree phenology, particularly in the context of projected climate warming. The thesis focused on vital physiological processes regulating dormancy and enabling water flow into the visually dormant buds during winter. I also investigated resource allocation and growth of temperate deciduous saplings during summer and the effect of full-grown trees’ spring and autumn phenology on carbon, water, and nutrient cycling of the soil in three Swiss beech forests. These findings provide significant insights into how chilling, forcing, and photoperiod influence the spring phenology of tree species and evaluate how strongly species are adapted to the local climate and which role the phylogenetic history of the species plays. In the first part of chapter 2, I exposed tree cuttings from six European tree species to different chilling (low and high), forcing temperatures (5, 10, 15, and 20 °C), and photoperiodic conditions (8 h and 16 h daylength) during winter. At the same time, the same conditions were also applied to twig cutting of East Asian tree species by a research group in China on three tree species that were phylogenetically related to the tree species investigated in Switzerland. Further, I calculated the thermal time required to budburst and interpreted its forcing temperature dependence. In this chapter, I substantiate that East Asian species react more responsively to forcing temperatures regardless of chilling exposure than their Central European counterparts, suggesting that regional climatic predictability has shaped the evolution of chilling and forcing requirements. Therefore, I expect East Asian tree species to advance spring phenology more strongly in a future warmer climate than European tree species. Retrospectively, it would have been great to also include collaborators from Eastern North America, as the climate during winter seems to be even more unpredictable there than in Central Europe. However, unfortunately, this did not happen and we missed the opportunity to explore the phenology-climate predictability interaction further. In the second part of chapter 2, I used the phenological data of the twig cuttings exposed to high chilling and long photoperiod (i.e., we wanted twig cuttings without spring phenological constraints) from the first part of this chapter to challenge the prevailing belief that warminginduced changes in tree physiology are responsible for the observed decline in phenological sensitivity to temperature. Additionally, my claims were substantiated using long-term spring phenological records (>100 years) of cherry trees growing in Liestal, Switzerland. By this means, I could show that the observed decline in phenological sensitivity to temperature is a mathematically expected consequence of increasing temperatures, assuming that a thermal accumulation of temperature triggers budburst. In the end, I advocate for further critical assessment of established methods and results in the future. In the first part of chapter 3, I sampled twig cuttings of five deciduous temperate tree species continuously from autumn senescence to spring budburst at two sites (Muttenz and Uetliberg) with an elevational difference of approximately 500 m to simulate different chilling and forcing exposure during winter. This procedure was chosen to link short-term water uptake with the dormancy status of the bud using 2H-labeled water. Therefore, two twigs of each investigated tree were sampled at every sampling campaign. One twig was used to determine dormancy depth, while the other was taken to measure short-term water uptake. I hypothesized a continuous degradation of obstacles in the symplasts with increasing chilling. However, contrary to my expectation, short-term symplastic water uptake from twigs into buds did not progressively increase before budburst. Yet, a link between bud water content and dormancy status was found, suggesting that water into the bud is happening during winter despite the bud being visually dormant. Interestingly, there were strong differences between investigated tree species at both sites, while the reaction of all tree species was largely unaffected by the site. Thus, all tree species showed a species-specific but not site-specific reaction regarding all investigated parameters despite the consistent temperature difference of about 2.5 °C throughout the entire winter. Although this experiment did not allow me to track the exact path of the 2H-label, twig-to-bud water transport could be sustained through the apoplastic way and by low aquaporin activity during winter. In summary, the experiment indicates that bud water content is a more reliable indicator of dormancy release than short-term water uptake and provide insightful information about water dynamic during dormancy. In the second part of chapter 3, potted saplings of four temperate tree species were exposed to a below-ground 2H-labeling in experiment 1 and then exposed to forcing conditions in a greenhouse and continuously harvested every other day for two weeks to track root water uptake and plant internal water reallocation from stem to twigs and buds. In experiment 2, saplings of three temperate tree species were exposed to aboveground 2H-labeling for six hours to track atmospheric water uptake into tree tissues in early spring before budburst. The combined results of both experiments illustrate that trees rehydrate aboveground tissues during winter by absorbing soil and atmospheric water, which likely represent mechanisms to compensate for water losses by transpiration. However, significant differences between species regarding internal water allocation were found, which I attributed to differences in the tree’s phenological status and species-specific wood anatomy. In chapter 4, tree saplings of three European species (i.e., beech, oak and linden) were planted as monocultures or mixtures of two into 18 raised beds at the research institute WSL and exposed to passive warming during spring and precipitation reduction all year long for two subsequent years. Precipitation reduction increased leaf bulk δ13C (which I interpreted as increasing water use efficiency) and decreased the growth of beech and linden, but not oak. Nevertheless, beech saplings remained competitive for soil water and nitrogen against the other two more drought-tolerant species under precipitation reduction despite the recent decade’s widespread dieback of old-grown beech trees. This finding illustrates well that beech saplings likely remain a strong competitor during the recruitment phase of forests in a warmer and dryer climate. In chapter 5, we investigated the combined effects of temperature and phenology of mature trees on carbon, water and nutrient cycling in three temperate beech forests with distinctively different soils and climates. Our research revealed that soil respiration increased with warmer temperatures, while gravimetric water content and plant-available nitrate and sulphate content decreased at all three sites, aligning with our expectations. However, when the data generated during spring and autumn were investigated separately, we uncovered complex interactions between site, temperature and tree phenology for soil respiration, and plant-available nitrogen and ammonium. This finding indicates that trees are affecting soil respiration and nutrient cycling substantially. Depending on site-specific soil conditions, trees could increase available inorganic nitrogen forms after budburst and before 50% leaf senescence contrary to our expectation. Furthermore, we found some evidence that roots cease their activity during autumn depending on temperature rather than the phenological status of the leaves. Collectively, the work represented in my doctoral thesis deepens the understanding of the interplay between climate factors, tree phenology, tree physiology and the soil microbial community. This research offers new perspectives on what could happen in a warmer and dryer climate in Switzerland and Central Europe as a whole. Thus, my research mainly aimed at taking basic tree phenology research further. However, I am convinced, that my research does also provide insights for current challenges that forest practitioners are facing with ongoing climate change and I think that my research together with other findings could help to initiate more sustainable forest management.
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Examiner: Gessler, Arthur
Examiner : Hille Ris Lambers, Janneke
Examiner : Delpierre, Nicolas
Examiner : Vitasse, Yann
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ETH Zurich
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Subject
Phenology; FOREST BIOLOGY + FOREST ECOLOGY (ECOLOGY)
Organisational unit
02352 - Departementsadministration D-USYS / Dep. Administration D-USYS