A perspective on the seed bank dynamics of Acacia saligna

[摘要] ENGLISH ABSTRACT: Acacia saligna, or Port Jackson, is an Australian Acacia which has spread throughout the Westernand Eastern Cape provinces of South Africa where it has become invasive and a prominent featureof the vegetation. In areas where A. saligna establishes it replaces natural vegetation, altersecosystem processes and interferes with agricultural practices. Consequently large managementefforts have been made to eradicate this invasive alien tree. However, its large and persistent soilstored seed bank, even in the presence of management and biological control agents, poses aserious obstacle to its successful removal. Furthermore the reproductive output and the size of theseed bank of A. saligna over its whole distribution as well as the variation of seed production andthe seed bank with environmental conditions in time and space is poorly known. Understanding theseed bank dynamics of A. saligna in time and space is essential for reducing invasive success andachieving management objectives. This study has filled this knowledge gap through studying theseed production and seed bank of A. saligna over its invaded range in South Africa, including howenvironmental factors influence these factors in time and space. The seed rain of A. saligna wasassessed at 10 sites across its distribution in South Africa. The seed rain of A. saligna at the siteswas determined through the use of seed rain traps. Twenty five traps were placed out at every siteduring November 2010 (pre-dehiscence) which was collected again during April 2011 (postdehiscence).The seed bank of A. saligna was estimated through sampling at 25 sites across itsdistribution range in South Africa. The seed bank was sampled during April 2010 (postdehiscence),November 2010 (pre-dehiscence) and April 2011 (post-dehiscence) through taking 50litter and soil samples at every site which gave a total sampling size of 3 750 for both the seed inthe soil and litter over its distribution in South Africa. In addition the average tree diameter, treedensity, average number of Uromycladium tepperianum induced galls per tree, the summer aridityindex, De Martonne aridity index, winter concentration of precipitation, temperature of coldestmonth and the soil texture for every site was determined. The damage done by the seed feedingweevil, Melanterius compactus, was also estimated for the seed rain study sites. Water availabilityduring the hot summer months was assessed as the most important factor governing seedproduction and seed bank size. Riparian and non-riparian water regimes were shown to beimportant in understanding the seed bank dynamics of A. saligna over its distribution range in theCape Floristic Region. In non-riparian A. saligna populations the seed production and consequentlythe size of the seed bank and its rate of accumulation is limited by both water and temperature andin riparian A. saligna populations, only by temperature. Therefore, two environmental gradientsinfluence the seed bank dynamics of Port Jackson in South Africa. In non-riparian A. salignapopulations the number of seed produced and the accumulation of seed in the seed bank generallyincreases along the west coast of South Africa from Clanwilliam towards Cape Town and along thesouth coast from Cape Town towards Port Elizabeth. Seed banks are larger closer to the coast,when A. saligna populations of similar age are compared. In riparian A. saligna populations, the number of seed produced generally increases from Port Elizabeth towards Cape Town and fromCape Town towards Clanwilliam, again, with larger seed banks being accumulated closer to thecoast, when populations of similar age are compared. This study provides managers with a usefultool for prioritising management efforts.

[发布机构] Stellenbosch University