Ecology of plant communities in Ku-Ring-Gai Chase National Park, New South Wales: an examination of vegetation and environmental patterns.

Abstract

Patterns in the floristic composition, stand structure, species richness and environmental characteristics of plant communities at a number of spatial scales were examined in Ku-ring-gai Chase National Park, New South Wales. Vegetation patterns in eastern Australia have often been related primarily to a single environmental variable, soil phosphorus concentration. This study examines the 'single nutrient' hypothesis regarding the distribution of plant species. If soil phosphorus concentration is the majot factor affecting the distribution of plant species, then this should be highly correlated with spatial patterns in the floristic composition of plant communities within the study area. Floristic composition was determined as the frequencies of all vascular species occuring within duplicate 500 m2 quadrants from fifty sites within the Park. Environmental data consisted of 21 variables, including soil physical and chemical characteristics, from each quadrant. The patterns in floristic composition and environmental factors are described and the relationships between composition and environment were examined. through indirect and direct gradient analyses. The measured environmental factors showed strong correlations with floristic patterns; however, two scales of species distributions were apparent: between and within soil type. Direct gradient analyses of composition and environmental data showed soil phosphorus concentration was inadequate in explaining the observed patterns in floristic composition of plant communities are a response to complex multivariable environmental gradients. The structural characteristics of vegetation stands were examined through the development of a multivariate approach to the classification of stand structure. This multivariate approach is essentially a modification of an existing scheme utilising foliage projective cover of various recognisable strata within the strands. Multivariate classification and ordinations of plant communities based on structural characteristics showed strong correspondence to that obtained by compositional analyses. The complex multivariable gradient hypothesis is supported from results examining stand structure-environment relationships. The relationship between species richness and richness of three growth-form types (tree, shrubs, and herbs) and environmental variables were examined through generalised linear models. The measured environmental variables showed strong relationships with species richness, consistent with the results found multivariately with floristic composition and stand structure. No single measured environmental variable adequately predicted the observed patterns in species richness, rather species richness showed strong relationships with complex multivariable environmental gradients. This study clearly demonstrates the the 'single-nutrient' or 'phosphorus' hypothesis is inadequate in explaining all the patterns in the various components of vegetation within Ku-ring-gai Chase National Park. The complex multivariable environmental gradient hypothesis suggested by the relationships between floristic composition and environmental variables is supported by the relationships exhibited by two other attributes of vegetation communities: stand structure and species richness. Further studies testing the significance of the complex multivariable environmental gradient hypothesis are required.