Terrestrial plants face a multitude of physiological and ecological trade-offs, and require a compromise between hydraulic efficiency and safety in the water transport system, as well as a balance between gas exchange for photosynthesis and water loss. To survive in dry conditions, plants can adjust their hydraulic vulnerability to some degree, e.g. via anatomical modification, conservative stomatal regulation, or efficient repair mechanisms. However, our knowledge of the water transport system is strongly focussed on trees of temperate ecosystems, and few studies about the vulnerability of plants to drought explore the effect of within-plant coordination in hydraulic parameters.
In this PhD thesis, I studied the vulnerability to drought in trees, shrubs, and herbaceous species using hydraulic and acoustic methods, and examined its coordination with related parameters such as stomatal regulation and cellular adjustments across species, across conspecific populations, and within plants. Measurements were made in forests, meadows, and at the alpine timberline in Austria, and in tropical and temperate biomes in Australia. I also aimed to optimise acoustic emission analysis, and compared the obtained results with hydraulic reference measurements. In further experiments, I investigated the timing of important physiological events such as stomatal closure and cavitation during drought, patterns of acoustic emissions during freeze-thaw-cycles at the alpine timberline, and within-species acclimation to drought stress in an invasive species.
Based on methodical aspects of conducted studies, a novel acoustic parameter for efficient assessment of xylem hydraulic vulnerability using acoustic emission analysis was proposed, and new hydraulic data was collected for three plant groups (tropical trees, woody shrubs, and herbaceous species) that are significantly under-represented in the literature. While woody species (trees, shrubs) were similar in their hydraulic parameters, small herbs differed considerably regarding their hydraulic vulnerability and adaptation to drought stress. We showed that internal coordination helps protect plants from drought-induced hydraulic failure at moderate drought levels, and that leaves protect the more vital stems when drought further increases. These patterns and hydraulic strategies were also observed in studied trees at the timberline, an invasive herbaceous species, and in trees of temperate Australia.
Overall, this study demonstrated that drought stress is a fundamental, shaping factor for terrestrial plants, regardless of growth form. Hydraulic vulnerability and stomatal regulation are two main parameters of drought resistance, and are therefore useful characteristics to evaluate and compare the drought resistance of plants.