Global Savannah Phenology : Integrating Earth Observation, Ecosystem Modeling, and PhenoCams

Savannahs are land areas where grass and trees co-exists to create a landscape that is neither a grassland nor a forest. It is estimated that savannahs cover approximately one sixth of the global land surface. They also play an important role in the global carbon cycle due to their total size. Most savannahs are characterized with two distinct seasons, one dry season and one wet season where a majority of rainfall and vegetation growth is occurring. This makes the seasonality in climate especially important since it controls the timing of growth by affecting for example leaf emergence and leaf fall. Those timing of growth are most often collectively termed vegetation phenology.Phenology is the science of observing and describing life cycles and events of plants or animals over time. Vegetation phenology has a long history and observations were probably used for the first time thousands of years ago to note down changes in vegetation and harvests. Nowadays we can represent vegetation phenology at different scales using many different techniques. It can, for example, be analyzed and estimated using earth observation, site level measurements, or ecosystem modelling. Earth observation, or remote sensing, is the science of obtaining information about an object or area from a distance. For phenology vegetation indices measured from satellites are normally used. They often take advantage of the fact that plants during photosynthesis absorb the light in specific wavelengths. Site level measurements are instead carried out on the ground to monitor the ecosystem properties at the site level. They typically consist of measurement of temperature, soil moisture, precipitation, and carbon fluxes. But they could also be expanded with PhenoCams which is term referring to a digital camera producing images of the vegetation at a regular interval. Finally, ecosystem models are a mathematical representation of the ecological system and are used to understand and predict the real system. However, there exists a knowledge gap regarding the precise mechanisms affecting the vegetation phenology on savannahs. The aim of this thesis was therefore to use all of the above mentioned fields to improve our understanding and ability to estimate savannah vegetation phenology. The main results showed that soil water is of great importance for estimation of savannah vegetation phenology. It further showed the potential of using remotely sensed soil moisture and PhenoCams to improve our estimates of savannah vegetation phenology