Exploring Spatial Patterns Of Two Plant Species In Toohey Forest

Objectives of the Study

Vegetation spatial patterns emerge in response to interactions between plant species and their environment. A positive spatial association occurs between two species when they are found together more than expected by random chance given their individual distributions. While a negative spatial association occurs when two species occur together less often than expected by random chance. Analysing these spatial patterns in ecological communities may provide insights into the different processes driving community assembly and dynamics (Castanho, Oliveira & Prado, 2012).  Spatial associations can be influenced by both abiotic and biotic factors; with abiotic factors including local conditions such as nutrient or moisture availability, or disturbances such as fire and flood occurrence or anthropogenic factors. Biotic factors influencing associations include competition, predation, and disease (Day & Wright, 1989).  These disturbances can result in a negative or positive association which is also driven by the individual organism’s physiology and habitat and nutrient requirements (Bertness & Hacker, 1994).

This study explored the spatial distribution of two plant species to investigate if they showed a positive or negative association and speculate on the drivers.  The two species were the Grass Tree (Xanthorrhoea johnsoni) which is known to withstand fires and have a long life span, while the second species, the Red Ash (Alphitonia excelsa), is commonly used in bush regeneration due to their accelerated growth and short life span; it is an early successional species and has a shorter life span than grass trees (Pandey & Prakash, 2014). Both of these species are endemic to Australia and are widely distributed throughout the state of Queensland. 

The aim of this study was to explore the spatial distribution of Xanthorrhoea johnsoni and Alphitonia excelsa to explore any significant associations between the two species and speculate on the drivers.

Toohey Forest is located 10km south of the Brisbane CBD on 260 hectares of bushland. It is classified as an open eucalypt forest, with more than 400 distinct species of flora and fauna. It is a managed forest that is sequentially burnt to prevent bushfire hazards within the urban area.

Data were collected from a total of 155 5x5m² quadrats selected at random within an area in Toohey Forest that spanned the riparian zone of Mimosa Creek to the top of a ridge. The number of grass trees (Xanthorrhoea johnsoni) and red ash trees (Alphitonia excelsa) were counted in each quadrat to build a contingency table to be used for analysis. The approximate distance (in meters) from each quadrat to the creek was also estimated.

Methodology

Raw data was collated into a contingency table which reflected the presence/absence of each species in a quadrat.  This table was used to calculate a chi-squared test of independence. This test was used to determine if there was a spatial association between red ash trees and grass trees.  The hypotheses were as follows;

H₀= the two species are independent (i.e. there is no association between Xanthorrhoea johnsoni and Alphitonia excelsa)

H₁= the two species are not independent (i.e. there is an association between Xanthorrhoea johnsoni and Alphitonia excelsa)

Table 1: Group filed data

Quadrat No.	Number of grass trees	Number of Red Ash	Distance
1	0	1	0
2	0	1	15
3	0	0	30
4	0	3	45
5	0	5	60
6	1	0	75
7	3	0	90
8	4	0	105
9	5	0	120

Table 2: Class Data on presence/absence [Contingency table of presence/absence]

	Grass trees
Present	Absent	Total
Red Ash	Present	46	36	82
Absent	57	16	73
Total	103	52	155

Figure 1: Relationship between two species

The data clearly presents that the in the 9 quadrats selected from the study did not show any association. In the quadrats 1,2,4,5 only had the grass trees (Xanthorrhoea johnsoni) and the quadrats 6,7,8,9 showed the presence of the red ash trees (Alphitonia excelsa). While the quadrat 3 showed did not show any of the of the two trees. The quadrats 4 and 5 have shown larger accumulation of the red ash trees and the quadrats 7, 8, 9 have shown larger accumulation of the grass trees. With respect to distance, the farther the species from the initial quadrat the more concentrated they are. The at a distance of 60 meters, the quadrat 5 have shown the highest number of the red ash tree accumulation. While at a distance of 120 meters, there is a larger accumulation of the grass trees. Both the concentration of the species is found at the when the distances have doubled.

χ² =[(46×16) -(36×57)]²x 155/82x73x103x52

χ²=212637680/32061016

χ²=6.63 (observed value)

The probability (P) value is taken to be P= 0.5

Thus, we reject the null hypothesis and accept the alternate hypothesis

Chi-square test

H₀= the two species are independent (i.e. there is no association between Xanthorrhoea johnsoni and Alphitonia excelsa)

H₁= the two species are not independent (i.e. there is an association between Xanthorrhoea johnsoni and Alphitonia excelsa)

The vital question is there exists the species association or not?

	a	b	c	d
1-40	11	23	3	3
41-80	17	10	10	43
81-120	9	14	10	7
121-140	7	15	9	4
	46	36	57	16

Expected value= (103×82)/155

Expected value = 54.49

The study was conducted on the premises of the of the Toohay forest and which is located 10Km South of the Brisbane central business district. The Toohay forest is about 260 hectares having the bushland. Toohay forest is classified as the eucalyptus forest that has more than 400 different species of fauna and flora. The sampling area is randomly chosen to minimize the manipulation of the data. The study is conducted to study the association between the red ash and the grass trees in the Toohay forest.

Results

The study has indicated that the data itself is unable to show the association between the grass trees and the red ash. Although the data received from the chi square test has revealed that the chi- square value has no significance to the probability value of 0.5. The figure 1 shows that there is not even a single quadrat that shows the presence of both the trees within a single quadrat. The quadrat 3 even doesn’t have a single species of the red ash and grass trees. Whereas one remarkable finding is the at the distance of the quadrats have increased, the number of the species in each of the quadrats also shows an increasing tendency. Even the number of the grass trees have increased gradually as the distance has increased between among the quadrats. The quadrats 6,7,8,9 shows an increasing trend of the grass trees.

The chi square test has clearly rejected the H₀= the two species are independent (i.e. there is no association between Xanthorrhoea johnsoni and Alphitonia excelsa). However, the comparison made in the studies have proved that the species have no associations. Which means that both the grass trees and the red ash are independent of each other and are not found together in the eucalypt forest of Toohey. The 155 quadrats studied have shown that both the plant species have no such spatial association with each other. Although considering a larger number of the quadrats could have provided a different and more coherent result.  

It has been found that the two different environments that have identical climate and physiography may be occupied by plant species of different associations. The duration of association is also found to be limited because the plant communities are constantly undergoing succession. The existence of the association can be both for the long and the shorter periods. The association sometimes gets difficult to analyse and locate satisfactorily because the boundaries of association are not clear and prominent. The only thing that is recognisable is the core of the association that both has the historical and the geographical aspects. Within the perspectives of ecology, the associations that are of longer durations are wide in extent. There are other factors like the fire that play a major role in bringing up succession. Communities are so different, heterogeneous and discrete that are no demarcation of the boundaries and the communities fail to show an equilibrium state (Gleason, 1926).

Discussion

The major factors that have led to generate the natural communities are the presence of the environmental factors like the recruitment, disturbance, predation, competition. The main driving forces in the communities are the positive interactions. One of the established ideas is that the positive interactions in nature occur due to the harsh conditions that prevail within the environment. This is so because one neighbour will always act as a buffer for another plant species besides it. Studies have revealed that there exist positive interactions between the species that are undergoing physical stress (Bertness & Hacker, 1994).

The plant population that is existing in a suitable habitat can highlight the aspects of seed dispersal. The factors that promote the factors of seed dispersal helps in establishing a plant to grow in size and in turn produce the seeds themselves. It has been highlighted by the ecologists that the plants have their habitat preferences and it is seen that the plants prefer the chalk down, well drained and the shady habitats. The competitive communities are seen to be simple and it might not hold the pathogens, parasites, and other predators but instead the community will have the element of competition (Thomson et al., 1996).

Conclusion

From the above study, it can be concluded that spatial associations between the plant species are dependent on both the biotic and the abiotic factors. The biotic factors include the moisture availability and the nutrient availability, fire, anthropogenic factors, flood. While the biotic factors are the disease, predation and competition. It has been seen that the tree (Xanthorrhoea johnsoni) and the Red Ash (Alphitonia excels) and their spatial distribution in a natural community. Alphitonia excels is known to be used in the bush regeneration and the Xanthorrhoea johnsoni is known to withstand the fire and have a long span of life. The two hypothesis set for the study are: H₀= the two species are independent (i.e. there is no association between Xanthorrhoea johnsoni and Alphitonia excelsa); H₁= the two species are not independent (i.e. there is an association between Xanthorrhoea johnsoni and Alphitonia excelsa). The study has indicated that the data itself is unable to show the association between the grass trees and the red ash. Although the data received from the chi square test has revealed that the chi- square value has no significance to the probability value of 0.5. The figure 1 shows that there is no even a single quadrat that shows the presence of both the trees within a single quadrat. The quadrat 3 even doesn’t have a single species of the red ash and grass trees. Whereas one remarkable finding is the at the distance of the quadrats have increased, the number of the species in each of the quadrats also shows an increasing tendency. Even the number of the grass trees have increased gradually as the distance has increased between among the quadrats. The quadrats 6,7,8,9 shows an increasing trend of the grass trees.

Reference

Bertness, M. D., & Hacker, S. D. (1994). Physical stress and positive associations among marsh plants. The American Naturalist, 144(3), 363-372.

Bertness, M. D., & Hacker, S. D. (1994). Physical stress and positive associations among marsh plants. The American Naturalist, 144(3), 363-372.

Castanho, C. T., Oliveira, A. A., & Prado, P. I. (2012). The importance of plant life form on spatial associations along a subtropical coastal dune gradient. Journal of Vegetation Science, 23(5), 952-961.

Day, T. A., & Wright, R. G. (1989). Positive plant spatial association with Eriogonum ovalifolium in primary succession on cinder cones: seed-trapping nurse plants. Vegetatio, 80(1), 37-45.

Gleason, H. A. (1926). The individualistic concept of the plant association. Bulletin of the Torrey botanical club, 7-26.

Pandey, D. N., & Prakash, N. P. (2014). Tropical Dry Forest Restoration. Science and Practice of Direct Seeding in a Nutshell. Deep Narayan Pandey. Neha Pandey Prakash. Climate Change and CDM.

Tálamo, A., Barchuk, A., Cardozo, S., Trucco, C., MarÁs, G., & Trigo, C. (2015). Direct versus indirect facilitation (herbivore mediated) among woody plants in a semiarid C haco forest: A spatial association approach. Austral Ecology, 40(5), 573-580.

Thomson, J. D., Weiblen, G., Thomson, B. A., Alfaro, S., & Legendre, P. (1996). Untangling multiple factors in spatial distributions: lilies, gophers, and rocks. Ecology, 77(6), 1698-1715.