Spatial Change in Subtropical Queensland Mangrove Distribution Related to Landscape and Rainfall Patterns

Abstract

In eastern Australia, the changing patterns of mangrove distribution are commonly associated with upslope encroachment into saltmarsh. This phenomenon could be related to climate change (e.g., change in precipitation, CO2 concentrations and sea-levels), as the result of human-induced changes in the wetlands and surrounding landscape or due to the interactions among these factors. This thesis investigated relationships between rainfall, land use, and landscape structure of selected wetlands in northern Moreton Bay, Queensland, Australia, from 1972 to 2004 over multiple temporal and spatial scales through the use of remote sensing, GIS and statistical analysis.

To assess the relationship between rainfall patterns and the spatial distribution of mangrove forests, periods of relatively consistent rainfall patterns were identied using the non-parametric Pettitt-Mann-Whitney-Statistic and the Cumulative Sum technique. This identied change-points in rainfall distribution and these were then used to dene the temporal periods over which changes to mangrove area were assessed. Historical aerial photos acquired in 1972, 1990 (the year at which there was the most signicant change-point in rainfall pattern), and 2004 were used to estimate the rate of expansion (gross increase) and change (net increase) in mangrove spatial extent for the wetter period (pre- 1990) and the drier period (post-1990). A signicant positive relationship was demonstrated between rainfall variables and landward mangrove expansion and change.

A Mangrove-Saltmarsh Interface (MSI) index, which was developed as part of this research, quantied the relative opportunity for mangrove to expand into saltmarshes, based on the shared boundary between them. There was a consistent relationship between the MSI and mangrove spatial change. The data for land use/cover at three spatial scales (catchment, sub-catchment, and land adjacent to wetlands) were derived from Landsat satellite imagery for the study area. Also, human population density has been estimated. The results indicate that the contribution of landscape variables (land use/cover and population density) to spatial changes in mangroves at each spatial scale is aected by the rainfall pattern. The research also indicates that there is a scale-dependent eect of development on mangrove spatial change during wet and dry periods.

Integrating the results of investigations into the relation between the mangrovesaltmarsh interface (MSI) index, rainfall, land use/cover and population density with mangrove spatial change led to comprehensive models explaining mangrove expansion and change in the inter-tidal coastal wetland of northern Moreton Bay, south east Queensland. Findings from this study have shown that a combination of natural and anthropogenic factors was related to the distribution of mangroves at the local scale. Rainfall appeared to be the driving variable in the system and determined the scale at which anthropogenic factors aect mangroves. The contribution of environmental variables to mangrove spatial dynamics varied in accordance with rainfall patterns. With the potential effects of climate change on rainfall and sea-levels, this research has identied an approach that may assist in identifying and understanding related changes in mangrove distribution.