Vulnerability of selected native and invasive woody species to streamflow variability in Western Cape fynbos riparian ecotones

[摘要] ENGLISH ABSTRACT: Riparian ecosystems of the Mediterranean south-western Cape region are projected to experiencesignificant decreases in streamflow due to climate change and increased demands for water associatedwith human demographic trends and increasing living standards. Aggravating this problem are woodyinvasive alien plants, such as Acacia mearnsii, whose impacts, including those on catchment wateryields, have justified extensive eradication programmes such as Working for Water (WfW). WfW hasbeen highly successful in managing invasive alien species in some areas, while at the same timeenhancing the quality of life of poor South Africans. Unfortunately, the managers of these eradicationinitiatives often experience a lack of information on the species- and site-specific ecological propertiesthat may aid in prioritising sites more prone to invasion, essentially inhibiting accurate managementoutput. Knowledge, such as that associated with the possible future distribution of invasive species ina changing environment, may help to optimise eradication initiatives. I therefore determined whetherwoody plants portray different physiological (vulnerability to cavitation) and wood anatomical traits(wood density, vessel resistance to implosion, vessel lumen and wall diameters) across threeprominent riparian zones in the south-western Cape that each differ in streamflow quantity – to gain amechanistic understanding of how woody species, especially invasive species, adapt their hydraulicstrategy across this proxy for water availability. Using factorial ANOVA's, to distinguish anydifferences in plant physiological and wood anatomical responses to water availability within andbetween species, showed Acacia mearnsii having consistently higher drought-tolerance (lower P50values and denser wood) compared to native species under reduced water availability. These resultssupported the significant variation in drought-tolerance strategies that exist within and betweentaxonomically different species across different environments. Water availability thus has a strongselective effect on functional traits of species; however, minimum water potentials were more usefulin describing in situ hydrological conditions than streamflow. Additionally, a non-causal relationshipbetween wood anatomical traits and drought-induced cavitation was observed. Therefore, using onlywood anatomical measurements to assess drought-tolerance of species might not be accurate across allspecies. This mechanistic approach to assess the invasive potential of species under projected drierconditions has great practical value. It can be used to improve species selection for restorationinitiatives, and is of great value for future use in prioritizing eradication programmes.

[发布机构] Stellenbosch University