Report – BS2001 – E. Platyphylla at two different habitats

BS2001, 2000-05-02

Study of E. Platyphylla at two different habitats

Abstract

Two populations of E. platyphylla were studied, at James Cook University (J.C.U.) in tropical north Queensland, to determine whether the mean density and mean diameter on mature E. Platyphylla trees varied between the two populations based on tree girth measurement. The two populations were divided into 10m2 quadrats, and of those 15 were chosen at random. In each quadrat the number of mature E. platyphylla (taller than 2.5 m)

The results show that the mean density are the same in both populations but the mean diameter differ. This tend to indicate that the number of trees doesn’t increase, but the trees present grow larger at cultivated areas.

Introduction

To overseas visitors the Eucalyptus is as Australian as Koalas and Kangaroos. Eucalypts dominate Australian landscapes, except for the rainforests and the arid interior. Despite the present-day dominance of eucalypts, little is known about their evolutionary history from the fossil record. It is thought, however, that Eucalyptus is an ancient Australian group that has increased in dominance relatively recently as the climate developed a marked dry season, and the fire became more frequent (Knox et al. 1999).

When the E. platyphylla has grown up to a diameter of about 8-10 cm the tree is tall enough, and the bark thick enough to cope with fires, but trees smaller than that will burn down, and give the other trees nutrients.

This study was conducted at James Cook University, Townsville, at a cultivated site called the Lawn site and an uncultivated site referred as the Natural site. The aims of the study were to investigate if the mean density and mean diameter, of mature trees, of two populations of Eucalyptus platyphylla varies.

The two hypothesises tested are:

H0: There is no difference between the mean densities of the two populations.

H1: There is a difference between the mean densities of the two populations.

And

H0: There is no difference between the mean diameters of the two populations.

H1: There is a difference between the mean diameters of the two populations.

3. Method

Data collection

Two population of E. platyphylla on the J.C.U. campus was investigated. These two sites are referred to as Lawn site and the Natural site and are both 13 200 square meters. The sites were divided into 132, 10m2 quadrats (See maps, Appendix 1).

Each quadrat was labeled with a number, starting at the northwestern corner (number 1) to northeastern corner and finishing at southeastern corner (number 132). To randomly chose 15 quadrats a calculator was used. The quadrats chosen were: 3, 25, 52, 6, 31, 104, 130, 35, 36, 24, 98, 61, 109, 115, and 103.

The girth of all Eucalyptus platyphylla trees of each site was measured at breast height (1.5m) on all mature trees taller than 2.5 meters. To avoid re-measurement, each tree was marked with chalk.

B. Data analysis

The data were then put into a table and in the case of for multiple stem trees, a composite girth was determined by the following formula; Girth = Ö (cir12 + cir22 + cir32 + etc.).

The samples were visual tested to see if the samples were normally distributed, which have to be an assumption for the t-test. The mean diameter did not fit the normal distribution and the sample was transformed using natural log (Ln). Two 2-tailed t-tests were run, to determine if the mean density and the mean diameter were the same at the two populations. The alpha level was set to 0.05.

Results

Mean densities of E. platyphylla at the two study sites.

Figure 1, Indicating mean density of E. platyphylla at the Natural site.

Figure 2, Indicating mean density of E. platyphylla at the Lawn site.

Table 1, Indicating the result of a 2-tailed t-test, with equal variance assumed, comparing mean densities of E. platyphylla at the two study sites.

Levene’s Test for Equality of Variances

t-test for Equality of Means

F Sig. t df Sig. (2-tailed) Mean difference Std. Error Difference 95% Confidence Interval

of the Difference

Lower Upper
.147 .705 1.382 28 .178 -.6000 .4342 -1.4895 .2895

The Hypothesis is accepted, there is no significant difference between the Natural and the Lawn populations’ mean density of E. platyphylla.

Mean diameters of E.platyphylla at study sites.

Figure 3, Indicating mean diameter of E. platyphylla at the Natural site.

Figure 4, Indications mean diameter of E. platyphylla at the Lawn site.

Figure 5, Indicating natural log of mean diameter at the Natural site.

Figure 6, Showing natural log of mean diameter at the Lawn site.

Table 2, Indicating the result of 2-tailed t-test, with equal variance assumed, comparing natural log of mean diameters of E. platyphylla at the two study sites.

Levene’s Test for Equality of Variances

t-test for Equality of Means

F Sig. t df Sig. (2-tailed) Mean difference Std. Error Difference 95% Confidence Interval

of the Difference

Lower Upper
.089 .767 -5.456 28 .000 -.4883 8.949 E-02 -.6716 -.3050

The hypothesis is rejected, there is a difference between the two populations mean diameter.

Discussion

The Lawn site is aggregated and supplied with fertilizers, any seedlings are cut back by ground staff regularly. This results long-lived and healthier trees, however there is no chance of regeneration. An important factor resulting in the greater mean diameter is the protection from fire. Fire is considered a ‘secondary modifier’ of the distribution of vegetation communities (Crowley, 1995). The cultivated area has not been burnt in over 30 years.

The Natural site is neither provided with water nor fertilizers, and therefore the E. platyphylla will periodically lack water and nutrients, which will affect the health and the growth of the tree. The area is exposed to natural fires every 5-10 years; meaning the seedlings have time to grow large between the fires. Different fire regimes will affect different plant species different. Intense fires are more likely to favour those of the ground layer, and mild fires are more likely to favour canopy species (Crowely, 1995).

The Natural site was burnt in 1999, after the fire there were significant less numbers of seedlings and middle-aged trees compared to 1998. A massive seed release occurs after a fire event (Crowley, 1995), that results in a high number of seedlings the following years. Some of which will be killed at the next fire and only a few will make it through mature age.

The number of trees present was not affected by the cultivation and protection from fire but do impact on the mean diameter.

References

Crowley, G. M. (1995) Fire on Cape York Peninsula Land Use Strategy. CYPLUS, Office of the Co-ordinator General of Queensland, Brisbane and the Department of Environment, Sport and Territories, Canberra

Knox, B., Ladiges, P. and Evans, B. (1999) Biology. McGraw-Hill Book Company Australia Pty Limited, Roseville.

Acknowledgements

The sampling was conducted together with Jessica Simonsson, Alex Tan Eu Leong and Samantha West.

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