The influence of selected environmental factors on the structure of Phragmites australis (Cavanilles) trinius ex steudel stands in the Maltese Islands

Abstract

This study is a test of a hypothesis derived from one of the alternative explanations for the variation in the structure of Phragmites australis stands that was observed in different habitats in the Maltese Islands. Since the structure of stands seemed to vary distinctively when the difference of the environmental conditions of the habitats was greatest as in coastal and inland sites, this study investigated the possible explanation that some environmental factors may be causing the variation in the structure. The hypothesis derived from this explanation is that environmental factors are changing the structure of the P. australis stands. The null hypothesis to this hypothesis is that the structure of the stands is the same in habitats with one or more different environmental factors. The initial stage of this study was to assess a number of habitats in the Maltese Islands where P. australis stands are found in order to select two study sites based on their presumed difference in environmental conditions. The transitional coastal wetland at Wied ta' Kieli and the freshwater wetland at Wied ta' l-lmselliet were selected as the study sites. These sites were then characterised by the Tour du Valat Method. Validation of the presumed differences in selected environmental variables was carried out in order to assess which environmental factors were, in fact, significantly different in the two sites. The seasonal variation of selected environmental variables was monitored in both sites. Statistical analysis of the seasonal variation of selected environmental variables showed that the only two environmental variables which could be considered significantly different were the level of nitrogen and surface water depth. However. the high P values obtained in the analysis of nitrogen levels in the two sites indicate that the difference between the nitrogen levels is not very strong. Statistical corroboration of the existence of the claimed difference in structure showed that whilst the differences in the density of shoots and the shoot height were statistically extremely significant, the difference in the other structural variables was less significant and could be accounted for by the distribution of the shoots in one of the transects. Results suggest that differences in the other structural variables are trade-offs between the density of shoots and the shoot height (Clevering, 1998). Since there was a significant difference in the density and shoot height of P. australis stands in the two sites, and since the habitats differed in the level of nitrogen content and in surface water depth, the null hypothesis can be rejected on these grounds. The rejection of the null-hypothesis supports, though it does not proof, the hypothesis that some environmental factors may contribute to a variation in the structure of P. australis shoots.