Since European settlers left Australia in the late 18th century, the country has experienced extensive forest degradation and deforestation. About 75% of Australia is covered in arid lands and inhospitable deserts that do not support forest growth. As a result, forest quality and cover has significantly declined over the last six decades, threatening the biodiversity of native species. It is estimated that the country has lost 40% of its forests (Bradshaw, 2012). Victoria is termed as the state that has lost the largest percentage of its native vegetation since European colonization. The state has lost nearly 66% of its native vegetation since then (Turner, Pressey, & Levin, 2009). This has been contributed by various factors such as increased human population, industrialization, urban development, climate change and other human activities (Deo, Links between native forest and climate in Australia, 2011); (Deo, Syktus, & McAlpine, Impact of historical land cover change on daily indices of climate extremes including droughts in eastern Australia, 2009); (Meers, Enright, Bell, & Kasel, 2012). As a result, many indigenous plant species in Australia have been lost (Guerin & Lowe, 2012) and those remaining are under threat (Bennett, et al., 2013); (Tozer, Leishman, & Auld, 2014). But one of the endangered native species that have endured all these challenges is Cherry Ballart. This plant species is the focus of this study.
Cherry Ballart (Exocarpos cupressiformis) is one of the fascinating plant species in Australia. The plant resembles cypress tree but has small juicy and sweet fruits (cherries). It has inconspicuous flowers bunched on short thorns. A fruit is formed from one flower on every thorn (Skraskova, 2018). The plant is pyramidal in shape and usually 3 to 8 m tall, as shown in Figure 1 below. It usually grows in dry eucalypt woodland. Most of the Australian forests are classified as either low eucalypt forests or high eucalypt forests (Australian Bureau of Rural Sciences, 2010). This is one of factors that may be contributing to the existence of Cherry Ballart until today in Australia. Australians and early European settlers used the fruits as food by eating them raw or cooked. Other known uses of Cherry Ballart are: timber for making tool handles, gun-stocks and furniture; wood for making bull roarers and spear throwers; producing ornamental and decorative artworks due to its suitability for turning and curving; as Christmas tree; medicinal (for curing snake bites); smoking; insect repellant; etc. The plant’s fruits also have several nutrients and are still liked by people until today (Patykowski, Dell, Wevill, & Gibson, 2018), some of which can be used for production of seed oil rich in acetylenic fatty acids (Okada, et al., 2013).
Cherry Ballart is semi-parasitic (or hemi-parasitic) during its younger stages (Thomas, 2016). It typically grows while attached to the roots other plants particularly Eucalypts. The younger plant depends on the attached trees for water and food since it does not have a mature stem to produce food. Because of this parasitism, the plant grows in shallow soils. But as the plants become mature, photosynthesis gets established in their bright green stems and they become less dependent on parasitism. The leaves cannot perform photosynthesis because they are usually reduced to scales.
Fruits of Cherry Ballart are attached to colourful pedicels that attract birds. The greenish, hard fruit is inedible when unripe. When it matures, the fruit turns red or yellowish and becomes an edible cherry. The bird’s digestive juices weaken the hard nut thus enabling easy germination of the internal seed. The plant can also regenerate from damaged or cut stumps and scatters several suckers. Germination of the plant also takes place through propagation of the seed with Lucerne (Medicago sativa) and Kangaroo Grass (Themeda triandra) (Craigboase, 2016).
Even though Cherry Ballart may be seen as an undesirable shrub, it plays a critical role in native ecosystem particularly as a source of food mammals and birds and its nutrient-rich leaf litter is important in nutrient-recycling. Previous studies have shown that the number of birds in Australian woodlands has declined significantly due to deforestation and decline in forest cover (Paton & O’Connor, 2009); (Sodhi, Brook, Bradshaw, & Levin, 2009).
The study area in this report was La Trobe University Wildlife Sanctuary, located on Melbourne (Bundoora) Campus. The Sanctuary was created in 1967, the same year that La Trobe University was founded, as a project to restore and manage indigenous fauna and flora (La Trobe University, 2018a). The Sanctuary is now over 50 years old and provides an opportunity for the university community, local residents and visitors to learn about and experience biodiversity and natural history of a broad variety of plants and animal species (La Trobe University, 2018b). It is a remnant of River Red Gum Plains Grassy Woodland, a much depleted ecosystem in the north of Melbourne due to urbanization, agriculture and commercial developments. Even though the largest portion of the Sanctuary has been re-vegetated, there are some pre-European trees that still exist. Therefore the Sanctuary currently contains a combination of recovering and remnant natural vegetation. This Sanctuary plays a big role as far as regional conservation and biodiversity protection is concerned because it provides habitat, protects indigenous species, provides organisms with food, and it is used as a habitat passageway. Additionally, the Sanctuary offers ecosystem services including improving water quality, carbon capture and it is also a vital amenity for the University population.
This study was carried out bearing in mind that the Sanctuary has been largely cleared and is now set for conservation of different species of plants and animals, especially indigenous species. Therefore it was important to carry out a study so as to establish the recovery of native species in the Sanctuary. The species selected for the study was Cherry Ballart (Exocarpos cupressiformis). The study entailed establishing the landscape ecology of this species.
We visited the block we had been assigned on the site section of the Sanctuary where to assess the population of Cherry Ballart. Before commencing the actual count, we familiarized ourselves with the target species so as to establish if we could easily distinguish it from other similar species such as Black Sheoak (Allocasuarina littoralis). We started counting the number of Cherry Ballart plants (both juveniles and adults) in each sub-block. When we identified an individual plant, we measured its girth at breast height (GBH) and recorded the measurement in centimeters (cm). GBH was taken to represent age of the plant. When the plant was shorter than 150 cm, we recorded the measured GHB as 1 cm. The aim of this measurement was to establish the population of Cherry Ballart in the Wildlife Sanctuary and their age (which was represented by size).
For every Cherry Ballart located, we measured the distance from the tree to the trunk of the closest pre-European Red Gum tree and recorded the measurement in meters (m). It is assumed that trees with a GHB of 314 cm has a diameter at breast height (DBH) of 100 cm had been growing during or near the European settlement time. However, trees with GBH less than 314 cm are most likely to have recruited after the time of settlement, probably during the last 50 years when the La Trobe University Wildlife Sanctuary was created. When the distance from an individual Cherry Ballart to the nearest European Red Gum tree was greater 100 m, it was recorded as 120 m.
To establish if distribution of Cherry Ballart in the reserve is non-random (i.e. is influenced by the big trees), we chose several pre-determined random points from the GPS’s Find-GoTo function and measured the distance between individual point and the closest pre-European Red Gum tree and recorded it in meters (m).
We carried out this counting and measurement in different years. The first count was done on Wednesday 22nd August 2017 while the second count was done on Wednesday 21st August 2018. 84 and 120 trees were counted and measured in the 2017 and 2018 experiments respectively. The raw data of Cherry Ballart GBH (cm), DBH (cm), distance of observed Cherry Ballart to nearest pre-European tree (m), and distance of random point to pre-European Red Gum tree (m) was recorded and is provided in the Excel sheets.
The next step was to determine the distribution pattern of Cherry Ballart and test whether the species was more clustered to the pre-European Red Gum than random points. To do this, we created a histogram of the frequency of observed Cherry Ballart vs. distance from nearest pre-European Red Gum. The distance classes used had an interval of 10 m. We also created a second histogram of the frequency of random points vs. distance from nearest pre-European Red Gum. The distance classes used also had an interval of 10 m. After creating the histograms, we used two-sample Kolmogorov-Smirnov (K-S) test to analyze the frequency distribution of distances to the nearest pre-European Red Gum of the observed population of Cherry Ballart vs. random points. Two-sample K-S test is a Z nonparametric statistic that quantifies the empirical distribution functions of two samples (Antoneli, Passos, Lopes, & Briones, 2018). This is one of the most accurate and reliable non-parametric techniques of comparing distribution of two samples (Krzanowski & Hand, 2011). We ran the K-S test in Excel. K-S test is a statistical tool used for testing two samples to determine if they are of the same distribution (Zaiontz, Real Statistics Using Excel, 2014).
The distance classes of the histograms are expressed in exclusive form, i.e. 0 – 10, 10 – 20, 20 – 30, etc. For the first class interval (0 – 10), it means all values from 0 and less than 10 while the second class interval (10 – 20) means all values from 10 but less than 20.
The null hypothesis in this study was that distributions of Cherry Ballart were the same as those of Random Point distributions. The value of Dn (biggest value in column G from the K-S test in the Excel file) is 0.205924. The distribution of the data is said to be normal is the critical value Dn,α is greater than Dn (Zaiontz, Kolmogorov-Smirnov Test for Normality, 2018). From the K-S Table, the value of Dn,α is obtained as follows:
Dn,α = D120,0.05 =
Since Dn = 0.205924 > 0.12415 = Dn,α, it means that the pattern of the data is not normal distribution. Therefore the null hypothesis that the data for the Cherry Ballart are normally distributed is rejected.
From the 2017 histograms of the frequency of observed Cherry Ballart vs. distance from nearest pre-European Red Gum and the frequency of random points vs. distance from nearest pre-European Red Gum provided in Figure 2 and 3 above, the two distributions were different. Cherry Ballart was more clustered near pre-European Red Gums because most of the distance values were small implying that the Cherry Ballart trees were closer to the pre-European Red Gums. On the other hand, the distance values of random points were relatively greater thus distribution of the random points was sparser from the pre-European Red Gums. The null hypothesis of this study was that the distributions of Cherry Ballart and random points is the same. From the results obtained, the null hypothesis is rejected because the distribution of Cherry Ballart was different from that of random point.
On the other hand, the 2018 histograms of the frequency of observed Cherry Ballart vs. distance from nearest pre-European Red Gum and the frequency of random points vs. distance from nearest pre-European Red Gum provided in Figure 4 and 5 above, the two distributions were relatively the same. Both the Cherry Ballart and random points were more clustered near pre-European Red Gums as most of the distance values were closer to the pre-European Red Gums. This implies that the 2018 data supports the null hypothesis that the distribution of Cherry Ballart and random points was the same.
This basically means that the spatial patterning or distribution of Cherry Ballart was significantly contributed by the old (pre-European) trees. It attests to the fact that Cherry Ballart trees are hemi-parasitic during early stages of their life as they depend on other host trees for food and water uptake since they cannot support photosynthesis. The parasitic nature of Cherry Ballart makes it difficult to propagate simply because even if the seedling is dispersed either by wind, birds or mammals, it is likely to fail to germinate if it lands where there is no suitable host tree. It can be concluded from the host-parasite-disperser relationship that for the parasitic plant to grow, it has to be dispersed and propagated to a place where there is an appropriate host tree that will provide the necessary food and water until the tree develops its system to start photosynthesizing by itself.
The graphs in Figure 6 and 7 above show that the number of Cherry Ballart trees is concentrated on the left (at the smaller tree sizes) and declines as the tree size increases. This means that the number of individual Cherry Ballart trees declines with increasing size of trees implying that the distribution pattern of the trees in the site studied is reverse-J-shaped pattern. The graphs were similar in both 2017 and 2018. As a result of this, the Cherry Ballart trees can be said to be producing seedlings that are largely depended upon for dispersion and propagation. Unfortunately, these seedlings suffer mortality at varied phases before they reach maturity. The fact that majority of trees are of smaller size (less than 50 years) means that restoration efforts of Cherry Ballart in La Trobe University Wildlife Sanctuary are working. The trees had declined significantly due to various reasons, including climate change, which is largely caused by human activities (Keenan, 2017). Studies have shown that forest restoration has significant positive impacts on the ecosystem (Hobbs & Levin, 2009). Birds are among the species that benefit most because they can fly from one area of the forest to another and get food from different plants (Ford, 2011). Thus the ongoing efforts to restore Cherry Ballart will benefit the entire ecosystem.
Conclusion
Generally, the results obtained from this study show that distribution of Cherry Ballart in La Trobe University Wildlife Sanctuary is not random. This is because most of the Cherry Ballart trees were found to be closer to the pre-European Red Gum trees, which are believed to be hosts of the former during younger stages. Hence the current distribution of Cherry Ballart in the Sanctuary is still influenced by the distribution of pre-European River Red gums. In other words, the distribution of Cherry Ballart is not random, which proves the semi-parasitic nature of these trees. The K-S test also showed that the pattern of the data is not normal distribution hence the null hypothesis that the data for the Cherry Ballart are normally distributed is rejected.
References
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