| Popis: |
The key to the survival of every organism on the planet is the optimal timing of essential life cycle events. This allows them to first avoid unfavourable periods, which would be harmful to the metabolic functioning or directly threatening the survival of a species (e.g. drought or freezing events). Second, it allows them to synchronize sexual reproduction among individuals of the same species to ensure gene flow in a population. In the case of plants, prominent examples of such ‘phenological events’ include the emergence of leaves in spring as well as flowering, fruit ripening and the abscission of leaves in autumn (senescence). Sensing the optimal time for vegetation onset and completing the phenological cycle requires regular feedback with the environment to adjust investments in vegetative growth, reproduction or preparation of the dormancy – the state in which trees can persist cold periods during winter in temperate and boreal latitudes. The mechanisms for predicting the onset of spring include daylength (photoperiod; PP) and temperatures as the two driving factors. The accumulation of low temperatures ('chilling') followed by an accumulation of a certain amount of thermal forcing in combination with critically PP, at least in some species, are such 'predictive mechanisms' or at least models, that proved to be well usable. The optimal timing of the growing season onset (e.g. leaf-out) marks a trade-off between maximizing the growing season length including competition for resources (e.g. light, nutrients, water) and reducing the risk of damaging late spring frost events. Global warming has changed this trade-off established over a long evolutionary history, increasing the risk of damaging late frosts in many regions of the world, due to a more rapid advancement of leaf emergence compared to the advancement of last spring frost events. This has recently led to large-scale defoliation of the forest canopy in many regions worldwide, which in turn has massive impacts on water, nutrient, and carbon cycling as well as on the overall food web of the forest ecosystem. This PhD aims to address the following three objectives, each presented in one experimental study corresponding to one chapter: i) (Re-)assess the effective chilling temperature range and their efficiency to release temperate deciduous trees from winter dormancy. ii) Disentangle the effects of PP and temperature during the growing season on the timing of senescence and assess the leaf emergence under limiting chilling conditions for provenances from the southern and northern distribution ranges. iii) Quantify the classic trade-off involved in leaf-out timing, i.e. estimating the cost of freezing damages versus the potential gains associated with advanced leaf emergence. An additional chapter is dedicated to a project which aims to foster the exchange of phenological knowledge between researchers, Swiss rangers and the public to promote environmental awareness. In the first study of my thesis (chapter 2), I placed 1,020 twig cuttings of six common European tree species in climatic chambers and exposed them to various chilling temperatures and durations. The experiment revealed a much wider chilling range than was previously assumed, spanning temperatures from minimum -2 up to 10 °C with freezing temperatures revealing equal or highest efficiencies in releasing the dormancy of deciduous trees. Moreover, I showed that species strongly differ in their chilling requirement and that the chilling mediated dormancy progression and release is characterized by three indicators, namely the maximum dormancy depth (a measure for the inability of a species to flush without prior chilling), the temperature-dependent rate of dormancy release (shape of the forcing-chilling-curve) and the minimum forcing requirement after saturating chilling (corresponding to the asymptote of the forcing-chilling-curve). IV In the second study (chapter 3), I exposed 324 saplings of three deciduous tree species originating from the northern and southern distribution ranges to a combination of 3 PP x 2 temperature treatments between the spring and fall equinoxes. Additionally, I extended the warming treatments to exclude critical portions of the chilling range during the subsequent dormancy period. This experiment revealed that a pure PP effect (not altering the amount of light energy for photosynthesis) was evident, generally advancing the timing of senescence in birch and beech (but not in oak) under shorter PPs. The exclusion of temperatures below 5 or 10 °C throughout winter dormancy resulted in consistent advancement of the budburst timing in oak, but weakened or prevented further advances in birch and beech with the latter showing first signs of incomplete leaf development under the most limiting chilling conditions. Provenances from the northern distribution range revealed both earlier senescence as well as budburst dates compared to the southern provenance in birch and beech, reflecting higher temperature sensitivities at higher latitudes. Oak, in contrast, showed the reverse trend with the southern provenance flushing earlier compared to the northern one. In the third study (chapter 4), I used 960 saplings of four deciduous tree species to artificially induce a wide range of realistic leaf-out dates and to subsequently apply a damaging frost event. Except for hornbeam, mortality after the induced damaging frost was negligible for all species. The timing of leaf-out significantly affected the regreening and growth of frozen and non-frozen saplings, with the lowest performance found at the most delayed leaf-out date. I demonstrate that species have different ways to withstand and recover from frost damage, reflecting their risk-taking strategy. Strategies to recover included a clear prioritization of the recovery of sugar reserves over growth (all species), the deployment of dormant reserve buds (oak), regrowth from the stem basis (cherry) and delayed leaf senescence (beech). The ability to recover from frost damage thus becomes an increasingly important functional trait for species persistence in the coming decades, as the frequency of extreme climatic events such as late spring frosts or droughts is expected to increase for wide areas across Europe. In addition to these research goals, I have initiated an Agora Project (still ongoing) entitled ‘PhenoRangers’ (chapter 6). Within this framework, a network has been established to collect high quality data for research to assess how the seasonal activity of different taxa and trophic levels will change in the future (scientific objective). At the same time, this network serves as a basis for communication between researchers, Swiss rangers, and the public to increase awareness of climate change and promote positive attitudes about how to address environmental problems at an individual level (communication objective). |
