Open Access

Climate change is expected to outpace the rate at which populations of forest trees can migrate. Hence, in forestry there is growing interest in intervention strategies such as assisted migration to mitigate climate change impacts. However, until now the primary focus when evaluating candidates for assisted migration has been mean or maximum performance. We explore phenotypic plasticity as a potentially new avenue to help maintain the viability of species and populations in the face of climate change. Capitalizing on large, multi-site international provenance trials of four economically and ecologically important forest tree species ( Fagus sylvatica , Picea abies , Picea engelmannii , Pinus contorta ), we quantify growth stability as the width of the response function relating provenance growth performance and trial site climate. We found significant differences in growth stability among species, with P. engelmannii being considerably more stable than the other three species. Additionally, we found no relationship between growth performance and growth stability of provenances, indicating that there are fast-growing provenances with a broad climate optimum. In two of the four species, provenances’ growth stability showed a significant relationship with the climate of the seed source, the direction of which depends on the species. When taken together with data on growth performance in different climate conditions, a measure of growth stability can improve the choice of species and provenances to minimize future risks in forest restoration and reforestation.

Hypoxic environmental conditions such as flooding and the resulting oxygen limitation for plant roots interrupt oxidative phosphorylation and lead to an energy crisis in plants. The energy shortage eventually results in the breakdown of cellular transmembrane gradients and cell death. The degradation of sucrose and starch and an increase in anaerobic glycolysis are well-described mechanisms to provide energy and survive this crisis. Lactic acid and ethanolic fermentation recycle reducing equivalents to maintain ATP synthesis. However, alternative reducing equivalent oxidation mechanisms (Chapter I), alternative energy and carbon sources (Chapter II) and regulation of energy consumption (Chapter III) can be crucial for survival under low oxygen conditions. To answer this hypothesis, emissions of nitric oxide (NO) under low-oxygen conditions (mechanism – Chapter I), changes in lipid composition under hypoxic conditions (energy source – Chapter II) and a redox-reactive and highly conserved cysteine residue of a plasma membrane (PM) P-type H+-ATPase (energy consumption – Chapter III) were investigated. The thesis confirmed the importance of nitrite (NO2-) as an alternative electron acceptor resulting in NO emissions of plant roots during low-oxygen stress. Firstly, the chemiluminescence-based detection of oxygen-dependent NO emissions in in vivo studies for roots of tomato, barley and tobacco is reported using a root reactor (Chapter I). The large amounts of low-oxygen NO emission in the root reactor suggest a role in generation of ATP. NO2- reduction appears to be involved in the mitochondrial electron transfer chain (mETC) and therefore may contribute to ATP generation. In response to oxygen depletion, an escape strategy of the studied species seems likely, including oxygen supply systems. In this strategy, additional ATP is invested to integrate structural changes. However, hypoxic tomato plants seem to rely on glycolysis and fermentation. They do not utilize lipid degradation and β-oxidation as energy source which was confirmed in HPLC-MS-MS analysis studies of hypoxic tomato roots (Chapter II). In terms of energy source, neither a depletion of lipids nor an increase in lipid degradation was observed. Triacylglycerol (TG) was more abundant and the degree of unsaturation of fatty acids increased due to hypoxic conditions (Chapter II). TGs play a role as intermediates in lipid metabolism. Subsequent utilization of carbon and energy sources during prolonged oxygen scarcity is possible. However, an altered activity of PM P-type H+-ATPase due to hypoxic-induced, posttranslational NO modification was not confirmed. A possible S-nitrosylation site at the conserved cysteine residue was not verified in the biochemical mutagenesis study described in Chapter III. Regulation of PM P-type H+-ATPase activity via cytoplasmatic ATP-level may be possible in plants. The maintenance of proteins and cell functions appears to be essential under conditions of low-oxygen and high levels of reactive nitrogen species (RNS) such as NO (Chapter I). The induction of antioxidant systems under low-oxygen stress is well known. In this thesis, the highly conserved cysteine near the P-site of the PM P-type H+-ATPase was shown to act as an endogenous antioxidant (Chapter III). This underlines the importance of this enzyme for the cell. While the changes in lipid composition observed under hypoxia highlight the importance of maintaining membrane integrity and fluidity as in the case of plastids (Chapter II). Membrane adaptation results in increased TG abundance which led to the formation of lipid droplets. TG may scavenge toxic intermediates to prevent cell death. Lipid droplets provide a binding site for enzymes involved in adaptation and may even protect unsaturated fatty acids under high reactive oxygen species (ROS) and RNS environments by being incorporated into their core. Under low-oxygen conditions, various strategies have evolved to provide sufficient energy for adaptation and survival while protecting vital cellular functions in plants. The increase in flooding and heavy rainfall due to climate change poses a challenge to crops and their harvest. Plant-based stress-responses may even be a factor impacting the climate (Chapter I). In order to stabilize food production, plant breeding may need to rethink breading strategies in certain areas. With more heavy rains and same annual rain fall a quiescence strategy via growth retention may sometimes be better than an escape strategy to save crucial energy under time-limited stress conditions. A better understanding of survival mechanisms, as provided in this thesis, may help to develop strategies that strengthen plants under mild low-oxygen conditions.

In 2011, MoorFutures® were introduced as the first standard for generating credits from peatland rewetting. We developed methodologies to quantify ecosystem services before and after rewetting with a focus on greenhouse gas emissions, water quality, evaporative cooling and mire-typical biodiversity. Both standard and premium approaches to assess these services were developed, and tested in the rewetted polder Kieve (NE-Germany). The standard approaches are default tier 1 estimation procedures, which require little time and few, mainly vegetation data. Based on the Greenhouse gas Emission Site Type (GEST) approach, emissions decreased from 1,306 t CO2e in the baseline scenario to 532 t CO2e in the project scenario, whereas 5 years after rewetting they were assessed to be 543 t CO2e per year. Nitrate release assessed via Nitrogen Emission Site Types (NEST) was estimated to decrease from 1,088 kg N (baseline) to 359 kg N (project), and appeared to be 309 kg N per year 5 years after rewetting. The heat flux − determined with Evapotranspiration Energy Site Types (EEST) – decreased from 6,691 kW (baseline) to 1,926 kW (project), and was 2,250 kW per year 5 years after rewetting. Mire-specific biodiversity was estimated to increase from very low (baseline) to high (project), but was only low 5 years after rewetting. The premium approaches allow quantifying a particular ecosystem service with higher accuracy by measuring or modelling. The approaches presented here have been elaborated for North-Germany but can be adapted for other regions. We encourage scientists to use our research as a model for assessing peatland ecosystem services including biodiversity in other geographical regions. Using vegetation mapping and indicator values derived from meta-analyses is a cost-efficient and robust approach to inform payment for ecosystem services schemes and to support conservation planning at regional to global scales.

The initiation of spring leaf-out is a critical determinant of the growing season in trees, affecting primary production and species interactions in forest ecosystems. Variations in the timing of leaf-out among tree species are linked to their differential progression of bud dormancy. However, identifying reliable markers for bud dormancy has been challenging, leaving the connection between the timing of autumn leaf senescence, bud dormancy and spring leaf-out unclear. To test whether species initiating dormancy release earlier also exhibit earlier leaf senescence in autumn and earlier leaf-out in spring, we estimated the dates of peak dormancy depth (PDD), senescence timing and spring leaf-out across various species, locations and experimental conditions in Central Europe. PDD was defined as the date when the maximum thermal sum was required for leaf-out, whereas leaf senescence was assessed through the decrease in leaf greenness. Our findings reveal that PDD timing is a more accurate predictor of species-level differences in spring leaf-out dates than the timing of leaf senescence, the latter being a poor proxy for PDD. The observed temporal asynchrony between PDD and senescence was linked to dormancy induction showing species-specific sensitivity to temperature variations. Conversely, the timing of leaf senescence showed a consistent reaction to temperature changes across all species. These findings suggest that the physiological processes within buds and leaves during autumn are governed by distinct environmental cues, with the bud dormancy process serving as a more reliable predictor of spring phenological differences among forest tree species than does autumn leaf senescence.

The EU Nature Restoration Law (NRL) is critical for the restoration of degraded ecosystems and active afforestation of degraded peatlands has been suggested as a restoration measure under the NRL. Here, we discuss the current state of scientific evidence on the climate mitigation effects of peatlands under forestry. Afforestation of drained peatlands without restoring their hydrology does not fully restore ecosystem functions. Evidence on long-term climate benefits is lacking and it is unclear whether CO2 sequestration of forest on drained peatland can offset the carbon loss from the peat over the long-term. While afforestation may offer short-term gains in certain cases, it compromises the sustainability of peatland carbon storage. Thus, active afforestation of drained peatlands is not a viable option for climate mitigation under the EU Nature Restoration Law and might even impede future rewetting/restoration efforts. Instead, restoring hydrological conditions through rewetting is crucial for effective peatland restoration.