Abstract of PhD Thesis

The Ecological Significance of Water Availability for Burren Plant Communities

Sarah Ryan, University College Dublin (2009)

The Burren is a Karstic limestone area situated on the West Coast of Ireland, noted particularly for its unusual combinations of plant species. The Burren flora experiences an oceanic climate, which is characterised by a high annual rainfall and mild temperatures. While these climatic conditions would not indicate that water availability is a significant environmental factor, this ignores the possible pivotal role of the substrate, which has a poor water-holding capacity. Despite a high annual rainfall (~1600mm per year), field measurements have, paradoxically, indicated a low annual mean volumetric water content (~20% for 2004), for the limestone substrate and plants were found to regularly experience short-term water deficits. These were often accompanied, by high irradiances and increased evaporative losses. The effects of variations in substrate volumetric water content and climatic variables on photosynthetic, chlorophyll fluorescence and shoot water potential measurements were studied over three years. The primary focus was on Corylus avellana and supplemented by work on some other common species. During prolonged periods of reduced substrate volumetric water content (<15%, SWC), decreases in maximum photosynthetic rate (Amax), were associated with reductions in stomatal conductance (Gs). This also coincided with declining values for shoot water potential (Ys). Chlorophyll fluorescence parameters were, however, less sensitive to these changes, indicating that PSII photochemistry is more resistant to water deficits.

Under natural growing conditions plants often experience water deficits in conjunction with other potentially limiting environmental factors, such as high temperatures, high vapour pressure deficits and high irradiances. Despite the insensitivity of PSII photochemistry to water deficits, significant decreases in midday Fv/Fm values were found at moderately low SWCs for C. avellana and P. spinosa perhaps indicating that reductions in Fv/Fm may have been associated with other environmental factors. Rather surprisingly, photosynthetic rates for a number of common species saturated at irradiances substantially lower than those recorded in the field. This could also indicate an environmental limitation in the photosynthetic capabilities of a range of Burren plant species. Although the Burren experiences a temperate climate, increased evapotranspiration and irradiance during the summer could contribute negatively to plant water status, particularly for the plants growing on the exposed limestone pavement. This study shows that due to the minimal storage capacity of the limestone substrate, fluctuations in rainfall have a significant impact on the performance of Burren plant species even under mild climatic conditions.