Dataset: Mangrove forest structure, forest primary production and soil factors at Dickson Inlet, Port Douglas, north Queensland


Surveys of the Dickson Inlet mangrove system were conducted over the period 20-23 February 1989. A limited survey of the mangrove forests in the nearby Mowbray River system was carried out in October 1989.

Mangrove forest structure was determined by the "angle count cruising" method, which employs an optical measuring device (relascope) designed especially for rapid basal area estimates of individual tree species and whole forests. At each site, a 360° sweep was carried out using the relascope, with trees falling within the chosen angle scale being counted. The total count for each species or all trees was then simply multiplied by the appropriate factor corresponding to the chosen angle scale to give basal area (m²/ha).

Forest potential primary production (Pn) estimates, which provide a good index of the present state of forest health and growth rate, were determined using the light attenuation method. At each of 9 sites (F1-F9), depending on the patchiness of the forest canopy, between 40 and 100 measurements of the intensity of photosynthetically active radiation (PAR) were made using a PAR quantum sensor. These measurements, when compared with the external PAR intensity, provide a measure of the amount of chlorophyll in the forest canopy and, assuming an assimilation constant derived from gas-exchange studies and literature values, provide an estimate of the potential primary production of the forest (kg carbon fixed/ha/day).

The long-term, plant-available, nutrient status of the forest soils was estimated by sampling mature leaves of Rhizophora spp. (3 replicate sets of composite samples at each of the 9 sites) and analysing for the major macro nutrient elements which are most likely to be growth-limiting, phosphorus (P), nitrogen (N), iron (Fe) and manganese (Mn). Other soil properties of potential importance in determining forest health such as pH, redox potential (Eh) and salinity were measured in situ using appropriate probes inserted into the soil to a depth of 5-10 cm (pH and Eh) and by measurement of the electrical conductivity of soil water which filled 20-30 cm depth core holes. Soil samples were also taken for determination of sand, silt and clay content and for determination of total nitrogen and organic carbon. Both of these latter parameters provide information on the nutrient retention capacity and present nutrient status of the soil.

Creek water samples (W1-W10) were taken at 10 approximately equally-spaced stations along the main channel, with a further sample (sample W11) taken in the same tributary as for forest site F8. Samples were filtered through 0.2 µm Nucleopore filters to provide samples (in duplicate) for later laboratory analyses of dissolved organic carbon and dissolved inorganic nutrients. Dissolved oxygen concentrations were measured using standard polarographic oxygen electrodes calibrated (at zero and saturated levels) just prior to use. One measurement was made at each station by direct immersion of the probe to a depth of about 1 m. Surface water salinity was measured at each station using a conductivity meter. A temperature sensor, built into the conductivity probe, was used to measure the water temperature.

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