Below is a summary of the data collection, analysis process, and primary results of the report. The full report, raw data and R script can be accessed via: https://drive.google.com/drive/folders/13pmQmiNh3kolfrnyLaJ-ka4JP3ruuX2q?usp=drive_link 

Aim

The objectives of this study were to determine whether certain abiotic and biotic factors varied between ecosystems at the Bunya Mountains, as well as compare wet and dry ecosystems. The ecosystems investigated were the araucarian rainforest, notophyll forest, dry rainforest, dry schlerophyll forest, and grassy bald. These ecosystems were also categorised into wet and dry with the aruarcarian rainforest and notophyll forest considered wet, and the other three as dry. The biotic factors explored were tree density, basal area, tree volume and seedling density. The abiotic factors investigated were ambient temperature, relative humidity, light availability, wind speed, soil pH, litter depth, and soil moisture. 

Data Collection

The study area was five sites at the Bunya Mountains, one for each ecosystem. At each ecosystem, ten sample plots were randomly selected. Each plot was 5m by 6m and contained four randomly selected 1m2 quadrats. Within each plot, all trees with a diameter at breast height greater than 4cm were counted and the height was measured. Diameter at breast height was measured using a diameter tape. The ambient temperature, relative humidity, light lux, and wind speed was also measured once per plot which was measured using a multimeter. Within each quadrat, the soil pH, soil moisture, seedling count, litter depth, and ground cover was recorded and averaged except for the seedling count which was totalled per plot. The soil pH was measured using a CSIRO soil pH test kit containing pH indicator dye and barium sulphate. The soil moisture was measured using a TDR probe and litter depth was measured using a ruler.

The following calculations were conducted to derive the tree density, basal area, seedling density and tree volume from the diameter at breast height (DBH), tree height, seedling count, and tree count per plot:

Data Cleaning

To ensure all ecosystems were categorised correctly as wet or dry. It was found using the following script that one of the wet rainforest samples was labelled as a dry ecosystem. Once this issue was rectified, it was found that there were no further issues with the dataset and was ready for analysis.

Analysis

To determine whether tree volume, density, seedling density, and basal area varied between the five ecosystems, the Kruskal-Wallis test was used as all four variables were found to lack homogenous variance using the Bartlett’s test and lack a normal distribution using the Shapiro-Wilk test. If the results were found to be significant, the Pairwise Wilcoxon Rank Sum test was used to determine which ecosystem pairs were different. To determine whether tree volume, seedling density, basal area, and tree density varied between the wet and dry habitats, the Kruskal-Wallis test was also used. To determine whether abiotic factors varied depending on whether the ecosystem was wet or dry, principal component analysis (PCA) was conducted to analyse the abiotic variables simultaneously through dimension reduction.

Results

Tree density varied significantly between ecosystems with the dry rainforest having the greatest tree density and the grassy bald having the lowest tree density. While it was expected that the grassy balds would have the lowest tree density, it was expected that the more wet araucarian microphyll rainforest would have the greatest tree density over the dry rainforest. A possible reason for this increased tree density may be the absence of cultural burnings in recent history, however more research with greater sample sizes would be required to corroborate this.

Post-hoc testing showed that seedling density was significantly lower in the grassy balds. This was expected as the grassy balds was labelled after the absence of trees. However, the homogeneity of seedling density between the other four ecosystems was unexpected. This seedling density homogeneity could also be attributed to a lack of cultural burnings of the dry rainforest in recent years howeber further research would be necessary to corroborate this.

The principal component analysis of abiotic conditions found that ambient temperature, relative humidity, and ambient light were highly correlated in the first principal component and that relative humidity was inversely related to ambient temperature and ambient light. 

The results of the second principal component found that mean litter depth and mean soil moisture were highly correlated. This observation meets the expected relationship between litter depth and soil moisture based on exiting literature due to leaf litter preventing soil water evaporation.

Principle component analysis of the abiotic factors found that dry ecosystems had higher ambient temperatures and ambient light than wet ecosystems. Wet ecosystems were shown to have higher relative humidity, soil moisture, and litter depth.

Conclusion

In conclusion, the analysis of biotic factors found that all of the biotic factors except tree volume varied between the five ecosystems, but not between wet and dry ecosystems with the exception of seedling density. The analysis of abiotic factors in wet and dry ecosystems found the expected relationships between ecosystems and between the abiotic variables themselves.