Water Cycling under Climate Change. Interactions between the water cycle, vegetation and a changing (sub)tropical climate

The water cycle is an essential component of the climate system because the physical properties of water in its liquid, solid and gaseous phases allow for the redistribution of energy in the oceans and atmosphere. At the scale of individual organisms, water and energy are also essential for the biochemical reactions required for life to develop. The terrestrial biosphere may interact with the climate system because plants capture light from the sun and exchange carbon dioxide (CO2) for water vapor and oxygen by photosynthesis. The exchange of these gasses occurs through microscopically pores that perforate the leaf cuticle. These pores are called stomata and allow plants to take up CO2 from the atmosphere while preventing excess water loss through the cuticle. Interactions between the biosphere and climate system may change with plastic and genetic adaptations in the diffusive properties of these stomata that occur in individual plants and throughout the plant community. By adjusting water use and altering carbon sequestration, these plant responses may act to amplify or counteract climatic changes via feedbacks with the water and carbon cycles at various timescales. As plants are an interactive component in the complex climate system, they can also be used to indicate climatic changes. Fossilized plant remains therefore provide so-called paleoecological proxy evidence to infer climate changes that occurred prior to the start of instrumental measurements. This dissertation focuses on the dual role of plants in the climate system by altering and documenting changes in the water and carbon cycles. Our aim was to gain understanding about interactions between various components of the climate system during periods with relatively rapid fluctuations in the water and carbon cycles. The main study site was the Florida peninsula and its surrounding ocean basins. This region is ecologically and climatologically interesting because it is situated on the boundary between the humid tropics and the subtropics. The close proximity to the Atlantic Ocean and Gulf of Mexico makes that changes in ocean circulation responsible for global climate fluctuations are documented by the Florida plant community. Due to the moist climate, (sub)fossil plant remains are stored in sedimentary archives that allowed us to study ecosystem responses to past climate changes. In the Florida region we investigated three coupling pathways that interlink the water and carbon cycles within the climate system: (1) land-atmosphere coupling reflected by changes in leaf gas exchange of plants responding to changes in atmospheric CO2 concentrations, (2) ocean-atmosphere-land coupling reflected by changes in plant community in response to rapid climate fluctuations that occurred during the last glacial period, and (3) ocean-atmosphere-ocean coupling between the Atlantic and Pacific basins as a potential feedback with rapid climate fluctuations that occurred during the last glacial. The coupling between these climate system components was investigated using proxy data and numerical models. Our results revealed novel aspects regarding the interactions between the terrestrial biosphere and the climate system via changes in the water and carbon cycles