Report – BZ1030 – Age structure of Eucalytus platyphylla

BZ1030- 990916

A study of the age structure of four populations of

Eucalyptus platyphylla subjected to

different patterns of disturbance.

1. Abstract

Four populations of E. platyphylla were studied to determine the effects of fire and other disturbances on age structure, based on tree girth measurement and other population parameters. It was found that fire frequency had a significant effect on recruitment, tree density and species diversity. Artificial management had a similar effect to fire by eliminating recruitment and addition of nutrients in a Lawn population consisting of predominantly large, mature trees. It is likely that the Firebreak, Protected and Lawn populations of E. platyphylla will continue to decline either due to too high fire frequency in the Firebreak or absence of fire germination effecting recruitment Protected / Lawn site.

2. Introduction

To overseas visitors the eucalypts are 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.

The study investigated effects of disturbance (fire / artificial management) on population of E. platyphylla that are occurring on the JCU campus in tropical northern Queensland, with girth as an index of age.

Aims:

To test the hypothesis (H0) that there is no significant difference in mean girths of four populations of E. platyphylla.

2. To discuss the results obtained in relation to:

The different patterns of disturbance (fire history, “gardening”) and the effect of these disturbances on age structure, competition, recruitment within each population

3. Method

A. Data collection

Four population of E. platyphylla on the JCU campus was investigated. These sites are referred to as the Protective site, Lawn site, Firebreak site and Natural site. In the four different areas the fire frequency varies from annually to more than 30 years. The Lawn and the Protected area have not been burned for over 30 years while the Natural area is burned naturally every 5-10 years and the Firebreak is burned annually by ground staff.

In each site, the students were assigned tasks: site boundary marking, and area determination, using a measuring tape and plastic ribbons to indicate were the borders went, and tree girth measurement and recording, using a 2 meter tape measure, and pen and paper.

The girth of all Eucalyptus platyphylla trees of each site was measured at breast height (1.4m), for small saplings the girth was measured 10 cm from the base. 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 trees were first divided up into 4 groups, seedlings (1st year recruits) 0-10cm in girth, young trees with a girth of 10-50 cm, middle aged trees, 50-110 cm and old trees between 110-250 cm.

4. Results

I. Age structure

The results are presented in figure 1-4, which show girth frequency distribution (i.e. age structure) of the Eucalypts platyphylla population on the JCU campus

Statistical analysis is indicated in the second section

Figure 1.

In Figure 1 show that the dominating trees (14%) are middle-aged trees.

The number of middle aged trees were almost two thirds of the number recorded, and the area had barely no seedlings at all (3 %).

Figure 2.

In the Protected area, there were younger and middle aged trees, almost 91% of number recorded and almost no old trees (2.5%) or saplings (6.8%).

Figure 3.

In the Firebreak the number of saplings are 48%. The number of young trees was quite low (8.5%). The middle aged and old trees combined makes out almost 43%.

If the saplings not were taken to account, the numbers would look like something like this; young trees 16.7%, middle aged trees 70%, and old trees 13.3%.

Figure 4.

In the Natural area the low proportion of saplings (about 7 %) was about the same again as encountered at the Lawn and the Protected sites. The majority of trees were young to middle-aged trees (about 88% combined).

Table 1. Percentage of class data for population of E. platyphylla on JCU campus 1999.

Lawn Protected Firebreak Natural Mean
Saplings 3.0% 6.8% 48.7% 7.2% 12.7%
Young tree 14.4% 48.7% 8.6% 35.7% 26.3%
Middle aged 67.0% 41.9% 35.9% 52.3% 52.7%
Old trees 15.7% 2.6% 6.8% 4.7% 8.3%

II. Statistical analysis

Results of ANOVA

f calculated = 43.5270 (p0.05- 2 tailed; num. df = 3, denom. df ¥ , where n= 698)

f critical = 3.12

Therefore reject the null hypothesis H0 : there is a significant difference between mean girths. The result of the ANOVA does not, however, indicate which populations are different, therefor the Tukey- HSD test was used to determine this.

Results of the Tukey-HSD test:

Populations ranked by means:

Site Firebreak = Protected = Natural < Lawn

Girth mean (cm) 39.2 50.7 55.8 80.1

Tukey-HSD ———————— ———————–

The test shows that the mean tree girths in the Firebreak and Protected areas are statistically similar and the Protected and Natural populations are also statistically similar.

The mean girths for the Firebreak and Natural are significantly different. The mean girth for the Lawn site is significantly different from the mean girths for all the other sites.

5. Discussion

The results indicate that fire and artificial management have a significant effect on the age structure of the populations of E. Platyphylla.

The Lawn, has the lowest amount of small plant recruitment (3%, see Results, Table 1), but the largest amount of old trees (67%, see Results, Table 1). This is due to, that the area is under constant artificial management, it is regularly watered, especially in the dry season, and taken care of by ground staff all year.

In the Protected area, there was an even distribution between the young trees and the middle-aged, and a low number of saplings, as shown in Figure 2.

This suggests that there is competition between the plants for space, nutrients and sunlight. The Protected area was the site with the largest diversity of plant species (25 spp). Most of the additional species compared with Natural site (10 spp) are fire intolerant – they are able to establish in this site due to low fire frequency.

The Firebreak had a lot of new regeneration, but had a lack of young trees, as shown as Figure 3. The fast re-growth of saplings is due to the fire that releases a lot of nutrients. The new saplings take advantage of this and try to grow big, but they will only get about a year old, and they are killed when the Firebreak is burned; though ground staff as mentioned before, burns off the area every year. The Firebreak has also the lowest numbers of species (5 spp)

The Natural area, had almost 20 saplings, and compared to the other site (except the Firebreak) this tends suggest a re-growth and that the E. platyphylla has some time to regenerate sufficient before the next fire disturbance. I.e. saplings able to grow large enough and develop thick enough bark to protect them from ground fires. The Natural area had 10 species of plants (see Results, Table 1).

Lawn and the Protected sites had the highest populations of middle aged & young trees and fewer “recruits”. Interestingly, the Natural area had very few “recruits” while the Firebreak had high proportion of “recruits”. (Not really what would normally be expected).

The explanation is that in July 1999, fire burnt through the Natural area, but did not burn into the Firebreak. Also the ground staff did not burn the Firebreak (for safety) before the sample was taken. Consequently there were no recruits in the Natural area but a many in the Firebreak in 1999.

In 1998 – the reverse was true – The sample was collected after ground staff had burned the Firebreak, therefore no “recruits” (saplings) while there were “recruits” in the Natural area.

The tree density in the Firebreak and Natural area has obviously changed due the difference in burning, the density in the Firebreak is remarkably (70%) lower 1998, than 1999, and the opposite is true about the Natural area were the density was about 23% higher.

The 1998 study did not show the density of the Lawn and the Protected area, but the numbers for 1999 are put in the Appendix 1, Table 3.

The distribution may seam a lot different between the sites, but if some modifications are made, e.g. don’t count the saplings for the firebreak, the numbers are very alike those for 1998.

The Protected area had about 11% lower density than the Firebreak this is due to the high number of saplings in the Firebreak that increased the density. But the Firebreak had in return about 46% lower than the Natural areas’ density was.

This is most likely due to the competition about nutrients, and in the Protected area there have not been any fires for over 30 years, and that means, no nutrients from fires. And also in the protected area there was found the highest numbers of species (25 spp.) that compete about the same nutrients. While in the Natural were there is 10 species, the area has a fire every 5-10 years that gives the site nutrients and burn of species that can’t cope with the fires.

The Lawn is different to all the other sites, though it is artificially managed. The growing condition is different in the way that the Lawn is mowed, watered and fertilized.

The low numbers of saplings in the Lawn site is due to regular mowing, this has the same effect as fires. The Lawn has the highest number (190) and percentage (82.5%) of middle-aged and old trees of all sites, this suggest that the nutrients and disturbance frequency is not as frequent or severe as the other sites. In fact the Lawn is looked after both with nutrients and water, and the disturbance is minimum. Even though mowing can be equalized with fire, though it both removes the saplings.

Because of the “care” the Lawn gets through the ground staff, with mowing, watering and addition of nutrients, the growth of the trees tend to be “sped up”. So the girth of the trees in the Lawn site, tend to be bigger in general than the other sites, but this does not mean that the trees in the Lawn site is older than it’s “brothers” in the other sites, even though the girth differs.

There is a couple of ways to find out if this is true, one way is chop down a tree and count the annual rings, and compare those with another tree. Another way, and maybe less drastic, is to drill a core sample, and count the annual rings that way.

Due to that to ANOVA test showed that there was a significant difference between mean girths, we have to reject the null hypothesis H0.

The test does not, however, indicate which of the populations that are different, and to determine this we used the Tukey- HSD test.

The Tukey test shows that the mean tree girths in the Firebreak and Protected areas are statistically similar and the Protected and Natural populations are also statistically similar.

But the mean girths for the Firebreak and Natural area are significantly different. The mean girth for the Lawn site is significantly different from the mean girths for all the other sites.

Due to the major differences in fire frequency, the species quantity and differences are noticeable. In the Firebreak, which is burned by ground staff annually, only 5 different species were found, while in the Natural site, 10 different species were found. In the Protected area of the campus, on the other hand, the number of species is remarkably high, (25) that’s due to the extremely low fire frequency (not been burned for more than 30 years). In general the species, Eucalyptus platyphylla was dominant on all of the sites.

Conclusion:

The percentage of young and middle-aged trees combined for the different sites are, with the saplings from the Firebreak removed from the calculations show that, the Natural area had a percentage of 88%, the Firebreak 86.7%, Protected area 91% and the Lawn 81%. This shows that the distribution of trees with a girth between 10-110cm is the majority of the trees, so the big difference between the study of 1998 and 1999 is the role of the saplings.

The populations of E. platyphylla is very likely to decline with time eighter due to fire frequency or absence of fire germination effecting recruitment. If no artificial planting or drastic change is made to the fire frequency or absence of fire germination effecting the recruitment of the species is made at the Lawn, Protected and Firebreak site, the E. platyphylla is very likely to die out.

6. References:

Attiwill, P.M. and Adams, M.A (eds) (1996) Nutrition of eucalypts. CSIRO, Australia.

Goldammer, J.G. (1990) Fire in the tropical biota. Ecosystem Processes and Global Challenges. Springer-Verlag, Berlin.

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

Appendix A

Table: 2. Girth Class data for populations of E. platyphylla (Natural and Firebreak) on the JCU campus, 1998 data

1998 Firebreak Natural
Saplings 0.0% 30.2%
Young tree 43.0% 30.9%
Middle aged 54.2% 36.6%
Old trees 2.8% 2.3%

Table 3. Density data for populations of E. platyphylla on the JCU campus, 1998 and 1999 data

Density trees/ha Lawn Protected Firebreak Natural
1999 123.4 74.8 83.9 155.7
1998 25.1 191.9

Appendix B

Tree and shrub species recorded in three sites on the JCU campus

——————————————————————————————–

1. FIREBREAK – burned annually Total No. Species: 5

Eucalyptus platyphylla (dominant)

Acacia bidwillii

Zizyphus mauritania

Melaleuca nervosa

Corymbia tessellaris

———————————————————————————————

2. NATURAL burned “naturally” about every 5-10 years Total No. species: 10

Eucalyptus platyphylla (dominant)

Zizyphus mauritania (dominant)

Acacia bidwillii

Corymbia tessellaris

Ficus opposita

Cochlospermum gillivraei

Alphitonia excelsa

Dodonaea sp.

Planchonia careya

—————————————————————————————–

3. PROTECTED not burned for more than 30 years Total No. species: 25

Zyzyphus mauritania (dominant)

Eucalyptus platyphylla (dominant)

Eucalyptus drepanophylla (dominant)

Melaleuca viridiflora (dominant)

Acacia bidwillii

Melaleuca nervosa

Corymbia tessellaris

Ficus opposita

Cochlospermum gillivraei

Alphitonia excelsa

Planchonia careya

Larsenaikia sp.

Wrightia sp.

Acacia leptostachya

Bursaris sp.

Capparis sp.

Corymbia maritima

Corymbia tessellaris var dallachyana

Grevillea paral

Grevillea striatalela

Grewia latifolia

Grewia retusifolia

Maytenus sp.

Dodonaea sp.

Petalostigma sp.

Leave a Reply