Abstract/Summary

According to climatic models, future drought conditions are expected to increase in severity and frequency. Drought is often cited as the most severe stressor of urban trees. It decreases planting survival rates and increases tree stress, resulting in increased susceptibility to biotic pathogens and reduced lifespan. Selecting trees for increased drought tolerance is thus critical for improving their performance within the urban environment. Drought tolerance is a highly desirable trait, particularly in urban areas where limited rooting space, interception of precipitation and urban heat islands accentuate drought stress. Current information on drought tolerance is imprecise, highly variable and, in some cases conflicting, with some studies reporting a tree species to be drought tolerant whilst other studies contradict this. In addition, cultivar-level tolerance is frequently not reported. The purpose of this thesis is to assess the current state of amenity tree selection and suggest how the process could be improved by utilising quantitative in vitro and in vivo trials to evaluate genotypic drought tolerance. The drought tolerance of species and cultivars within the same genus is frequently inferred to be very similar, however, this is known to be untrue. Detailed evaluations of drought tolerance are therefore required in order to cover a wide range of species and cultivars. Eight cultivars from Acer platanoides, A. campestre and A. pseudoplatanus were used in this study. The photosynthetic response to drought and desiccation was evaluated using chlorophyll fluorescence. Continuous excitation chlorophyll fluorescence parameters F0, PIABS and FV/FM, together with additional less often measured parameters, 1-Vi, Vj, T fm, Sm, V0(Bo) and M0, were used to compare whole-tree response to drought, with foliar dehydration. Double-normalised differential kinetics (Vt, ΔVt, ΔWOJ and ΔWOK) were also utilised to evaluate the underlying reasons for parameter response. Whole-tree chlorophyll fluorescence response to drought was found to be similar to the response to foliage desiccation; however, some differences were observed in differential induction kinetics which require further study to elucidate. The parameters PIABS, Fo/Fm and V0(Bo) are recommended for identifying and monitoring drought stress. In vitro methods (with potential to rapidly evaluate drought tolerance) were compared to whole tree methods. The methods tested include measurement of chlorophyll fluorescence during foliar dehydration and measurements of water potential at turgor loss point. The latter were undertaken using the classical, but time-consuming, pressure-volume curve method as well as more rapid, direct methods, calculating the turgor loss point from measurements of the water potential at full turgor via a previously devised regression equation. All methods were able to identify drought tolerance differences, with the exception of the pressure-volume curve method which was unable to identify significant differences within species. Another advantage of rapid in vitro methods is that they facilitated the evaluation of drought tolerance variation by season. Relative tolerance ranking was shown to be consistent between summer and autumn but not spring. This research demonstrates screening methods for tree drought tolerance which are especially relevant to tree selection in urban environments, where droughts occur during both spring and summer. There is considerable potential for tree selectors to save time and money if they have an increased ability to select trees that are fit for purpose. This research demonstrates that the estimation of water potential at turgor loss point, measured by consistent use of an osmometer or psychrometer, is sufficiently sensitive to correspond with findings from a controlled drought trial. Furthermore, this research also defines and identifies a number of reliable chlorophyll fluorescence parameters suitable for monitoring drought stress in vitro and in vivo.