Refine
Document Type
- Doctoral Thesis (2)
- Bachelor Thesis (1)
Language
- English (3) (remove)
Has Fulltext
- yes (3)
Is part of the Bibliography
- no (3)
Keywords
- Ecology (1)
- Global change (1)
- Plant physiology (1)
- Remote sensing (1)
- Savanna (1)
- Species distribution modelling (1)
Institute
Long-term average groundwater recharge representing the sustainable groundwater resources is modeled as a 0.5° by 0.5° grid on global scale by the WaterGAP Global Hydrology Model. Due to uncertainties of estimating groundwater recharge, especially in semiarid and arid regions, independent estimates are used for calibrating the model. This work compiled a new set of independent groundwater recharge estimates based on a work of Scanlon et al. (2006). The 59 independent estimates, together with an already existing independent estimates compilation, are used for the evaluation of two WGHM variants; one variant is modeling with an improved more realistically distributed daily precipitation dataset.
The objective of this thesis is the evaluation of the modeled data of the WaterGAP Global Hydrology Model (WGHM). The analysis of the impact of the new Watch Forcing Data (WFD) precipitation dataset on the modeled groundwater recharge tends to result in lower values in humid and higher values in (semi-)arid regions compared to the WGHM standard variant. Comparing both WGHM variants to the independent estimates compilations, representing (semi-)arid regions, the WGHM variant shows over- and underestimations especially of the low values and the WGHM WFD variant shows a bias toward overestimation especially for values below 4 mm/yr. The analysis of texture, hydrogeology and vegetation/ land cover could not give satisfying explanations for the discrepancies, but derived from the groundwater recharge measurement methods analysis indirect/ localized recharge seems to be a significant factor causing underestimations, as resulted in the comparison of the independent estimates based on Scanlon et al. 2006 with the WGHM variants.
The timing and duration of leaf deployment strongly regulate earth-atmosphere interactions and biotic processes. Leaf dynamics therefore have major implications for life on earth, including the global energy balance, carbon and water cycles, feedbacks to climate, species extinction risk and agriculture. Evidence of shifts in the timing of leaf deployment and senescence (leaf phenology) as a result of climate change has been accumulating over the past decades, particularly in relation to spring phenology in the northern hemisphere. However, leaf phenological change in other parts of the world has received less attention. This thesis quantifies global phenological change over the past three decades using remotely sensed data. Phenological change was found to be widespread and severe, also in the southern hemisphere. While the detected change testifies of the phenological plasticity of many plant species, it is not clear if the duration of leaf deployment (leaf habit) is equally sensitive to environmental change. Since evergreen and deciduous leaf habits are often distinctly sorted along environmental gradients, ecologists have hypothesised that these patterns result from natural selection for an optimal leaf habit, under a given environmental regime. Such evolutionary convergence can be examined by testing if the physiological niche that is occupied by a particular leaf habit (evergreen or deciduous) is similar among regions with distinct evolutionary histories. Using a process-based model of plant growth and a constructed map of evergreen and deciduous vegetation, the physiological niche of leaf habits was quantified in four global biogeographic realms. Substantial niche overlap was found between the same leaf habit in different realms, suggesting evolutionary convergence of the physiological niche. This implies a sensitivity of leaf habit to environmental change, as environmental variables determine the geographic space where the physiological niche allows a positive carbon balance, and therefore occurrence of the leaf habit. Since the physiological niche consists of the integrated effects of physiological traits and trade-offs, environmental dependencies and leaf habit and phenology, an understanding of the carbon economy of individual plants requires decomposing the physiological niche into its components. Using empirical data on leaf phenology, leaf habit and physiological processes from woody species in a seasonally dry African savanna, a simple carbon balance model was parametrised. Carbon gain varied considerably between species as a result of substantial variation in leaf habit, leaf phenology and physiological traits. The multiple lines of evidence in this thesis therefore suggest that, while convergent selective forces may determine the dominant leaf habit in a particular environment, inter-specific variation is substantial, potentially as a consequence of historical contingencies or competitive interactions.
The overarching goal of the thesis was to create a holistic predictive framework, a vegetation model, by improving the representations of and interactions between the biosphere, hydrosphere, atmosphere and pedosphere. Vegetation models rep- resent a crucial component of Earth system model since the properties of the land surface, via interactions with the atmosphere, can have extremely large climatic effect. Yet, there remains great uncertainty associated with the dynamics of the vegetated land surface. Various vegetation models have been critiqued for numerous reasons including overly simplistic representations of vegetation, prescribed vegetation, poor representations of diversity, inaccurate representations of competition, non-transparent model calibration, and poor responses to drought. The purpose of the creation of this "next generation" model was to address deficiencies common to current vegetation modelling paradigms.
The representation of the biosphere within this framework was improved via two separate development axes. First, ecological realism was improved by integrating concepts from community assembly theory, co-existence theory, and evolutionary theory. Explicitly, rather than defining teleonomic rules to define plant behaviour the process of natural selection is modelled. By modelling the pro- cess of natural selection and its affect on relative fitness, myriad "rules" which continually adapt to biotic and abiotic conditions "come out" as a consequence of the modelled dynamics rather than being "put in". In aDGVM2 (adaptive Dynamic Global Vegetation model 2) communities of plants and their trait values evolve through time, this evolution is constrained by trade-offs between traits. Poorly performing individuals are more likely to die and produce fewer copies of themselves, this results in a filtering of trait values. Further, the community and species’ trait values can evolve through successive generations via reproduction, mutation and crossover which we approximate by using a genetic optimisation algorithm. Thus, a plant community consisting of individuals and species with potentially novel and diverse trait values is assembled iteratively through time.
We tested the assertion that improved integration of concepts from community assembly, evolutionary, and co-existence theory could address limitations of DGVMs in Chapter 2. We demonstrated that such an approach does indeed allow diverse communities of plants to emerge from the modelling framework. We showed that the position of the emergent communities in trait space differed along abiotic gradients and that, in simulations where reproductive isolation was simulated, communities emerged which were composed of multiple co-existing clusters in trait-space. Simulated trait values of co-existing strategies emerging from aDGVM2 were often multimodal, indicative of the emergence of multiple life- history strategies.
Second, to successfully model how natural selection forms a community requires accurate representation of how resource availability affects fitness. In the majority of dynamic global vegetation models (DGVMs) there is no real representation of plant hydraulics with plant water availability being calculated as a simple function of relative soil moisture content and root fractions across a number of soil layers. Worryingly, a number vegetation models appear to under represent the magnitude of these observed responses to drought. This was deficiency was ad- dressed in Chapter 3 by designing a simplified version of the cohesion tension theory of sapwood ascent where elements determining plant conductances are considered in series and implementing a set of trait trade-offs which influence a plant’s hydraulic strategy whereby hydraulic safety trades-off against xylem and leaf conductivity.
Interactions between the biosphere, pedosphere, and hydrosphere can also potentially mediate water resource availability and thus fitness. In the majority of DGVMs the volume of soil explored and explorable by plant roots in fixed glob- ally and usually constrained to a depth not greater than 3m. However, we know that soils can have a strong effect on vegetation distributions, that soil depth is not constant globally, and that plants root to variable depths.
In Chapter 4 I explored interactions between soil depth, plant rooting and the emergent properties of communities and highlighted the importance of considering interactions between the biosphere, hydrosphere, pedosphere, and fire. Here I demonstrated that, in addition to fire and precipitation, edaphic constraints on the volume of soil explorable by plant roots (e.g. by shallow soils, lateritic layers, anoxic conditions due to water logging, toxicity resulting from heavy metal concentrations) can affect the process of plant community assembly, alter the mean values of multiple traits in communities, and the trait diversity of communities.
...