Coupled Behaviour of Soil and Plant Water Dynamics: Effects of Vegetation Cover, Soil Type and Fire Disturbance
Plant and soil water dynamics are crucial for terrestrial carbon uptake, agriculture and human livelihoods. As plants primarily rely on water uptake from the soil, soil and plant water dynamics are strongly coupled over most of the land surface. Soil and plant water dynamics also play a crucial role in linking water, energy and carbon cycles, as they are underlying plant transpiration, latent heat transport between land surface and atmosphere as well as plant photosynthesis. The future trajectories of soil and plant water dynamics will also shape the availability of water across the land surface. However characterising soil and plant water dynamics is challenging because the mechanisms controlling them are inter-related. Currently a lot of the knowledge of soil and plant water dynamics is built on field-scale and experimental work, which limits the understanding regarding the mechanisms controlling kilometre-scale soil and plant water dynamics. In this thesis, I use microwave remote sensing observations of soil and plant water dynamics at a kilometre scale to investigate the relationship between them and several other land surface processes and variables. I ask the following primary research question: how do vegetation cover, soil type and fire disturbance interact with and affect soil and plant water dynamics? Each factor in this question will be assessed in an individual chapter. First, I assess vegetation cover using a long-term soil moisture dataset, a vegetation index and temperature data. I investigate periods of decreasing soil moisture, finding a strong control of both vegetation cover and temperature on soil moisture losses. Increases in vegetation lead to slower soil moisture loss, across most of the land surface. This effect persists independent of temperature conditions. Only in cold boreal areas increases in vegetation lead to faster soil moisture loss, but this is limited to high soil moisture conditions. A simple water and energy balance model can reproduce the VI observational results, giving insights into how absorption of radiation in the canopy and water uptake across the root profile can lead to this simple globally emerging behaviour. Second, I assess the role of soil type as well as plant and soil water dynamics for water limitation. Using a newly developed remote sensing datasets, a water and energy balance model, flux tower data and sap flux data to quantify ecosystem productivity and transpiration rate I show the importance of soil type in controlling the onset, as well as the progression of water limitation. Soil type also influences the relative importance of soil moisture versus atmospheric water demand in controlling water limitation. This result contributes to an ongoing scientific debate. Third, I assess the effects of fire disturbance on soil and plant water dynamics. Combining microwave remote sensing soil moisture and vegetation water content with fire detections from optical remote sensing, this chapter finds previously unknown effects of fire on soil and plant water dynamics. Relative to pre-fire conditions, post-fire soil moisture loss, vegetation growth and plant water uptake are accelerated. Plant water strategies also change, with post-fire vegetation exhibiting less control on its water content. Post-fire, the coupling between soil moisture loss and vegetation water uptake and growth is also stronger. All these effects are strongest in forests, suggesting that fire induced shifts in plant composition could be the underlying driver of the discovered effects. The discovery of strong fire effects on plant and soil water dynamics shows the importance of fire disturbance for larger scale land-atmosphere coupling. This thesis identifies several previously unknown mechanisms affecting the behaviour of plant and soil water dynamics. Understanding these mechanisms increases the knowledge of how ecosystems work and can also inform us of how ecosystems might respond to future climate change. The findings prove the usefulness of microwave remote sensing to observe plant and soil water dynamics on a global scale, providing new research cases for future microwave satellite missions.
Remote Sensing, Hydrology, Soil Moisture
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