Comparative Ecology: Analysis of Two Plant Communities.


Abstract: The difference in the soils between wet and dry sclerophyll coupled with the aspect of the region creates two opposing ecologies, whereby vegetation and abundance is different. Eucalyptus distribution is

directly correlated with soil properties namely moisture retention which favours root penetration which flows on to affect diversity. Eucalytpus regnans is favoured by the moist deep soil at Sherbrooke forest and becomes the dominant species of the whole region across all stratum. The soil properties at Selby enable a much larger degree of diversity due to a spread of dominance throughout the stratum.

Introduction:
In ecological terms a community is defined as an “organic entity, and that like an organism; each part is interdependent on every other part” (F.E Clements (1916)). A plant community can be defined as an assemblage of plant species which interact among themselves and with their environment within a time-space boundary. Spatial boundaries that are in place enable differences in communities to occur, these boundaries are in the form of biomes, stratum, habitats or microhabitats. There is a mechanism in place which determines the type of ecology in these specific communities controlling the diversity and abundance of the area. Ecological filters work on the basis of a spatial hierarchy where by the various levels interacts and flow on to the next one. On the regional level, a broader scale there is a pool of species expressing multiple traits which is exposed to a particular climate which creates an overall limiting factor for organisms. Certain characteristics of the landscape such as geology and topography are then of focus for certain organisms inorder to facilitate or adjust the regional conditions. Within these landscape traits are even more specific limiting conditions which create habitats and then microhabitats which rely on a number of various interactions. At these three levels the interaction of the particulars within between the levels can influence the climate and further variation. Within the lowest level of microhabitats and habitats biotic interactions play a role in further filtering of communities. The combination of all the interactions involved can lead to an understanding to what are the factors that favour particular species and organize the arrangement of species on a spatial scale as well as on a time scale. In the areas of study whereby a comparison of a wet sclerophyll and dry sclerophyll forest is to be made, time scale is of relevance..
A sclerophyll forest consists of vegetation characterized by hard, leathery, evergreen foliage that is specially adapted to prevent moisture loss and fire. The wet sclerophyll forest is located in Sherbrooke and the dry sclerophyll forest in Selby both are in south eastern Melbourne within 20 kilometres of each other. Dry sclerophyll forests are 10 - 30 metres tall and have a hard-leaved understorey, whereas wet sclerophyll forests are taller than 30 metres and have a soft-leaved understorey, such as tree fern. Different conditions on all levels are clear to produce on observation very different types of vegetation and arrangement, utilization of stratum. The traits of these communities that when observed provide valuable data for comparison are growth form composition, species richness, structure and dominance. All these attributes are linked and flow on to each other, growth form composition is an examination of the stratum level layout which is a visual expression of the attributes. Stratum levels are clear set out boundraries in which different vegetation can be grouped together according to the limiting region of habitation. There are three main levels of significance , tree layer ,shrub (subcanopy) layer and ground layer all consisting of different exposure to the conditions of the region. Dominance is important to observe as it indicates that certain species exert powerful control over occurrence of other species. By analyzing the two communities with these four main attributes in mind and applying the ecological filters an understanding of the interactions that are occurring to separate these communities was derived.

Method:
At each of the sites, students were divided in to 12 groups of four or five members which then selected an area within the forest and marked a 10 metre by 10 metre quadrat out with measuring tape. Hence in total at each site there was a collection of 12 quadrat to analyze. Within this area the group would perform the following tasks: the soil moisture would be detected using an electronic device called a Theta probe. Where possible the height and cover abundance of each layer was recorded as well as three dominant species in each. Cover abundance was recorded in at the stratum level referring to the amount of the quadrat that the level occupied. Average height of each stratum level was also noted. A species list was determined for each layer by using information sheets coupled with pictures that were provided as well as consulting botanical experts on site. Furthermore from the centre of the quadrat, all the vegetation in the tree layer within a 15 metre radius would be measured in reference to their diameter at breast height. Other characteristics of the region were noted such as type of soil , parent rock, aspect and topography. All data was pooled and an excel spreadsheet was compiled from this averages ,standard errors and graphs were constructed.
Results:
Initially obvious differences in the vegetation at each site are noted, in respect to dominant species and diversity. At Selby the region appeared to be dry and draught affected, the quadrats observed were densely packed with vegetation on all three levels. The tree layer was dominated by a variety of Eucalyptus namely Eucalyptus obliqua , three other genus of eucalyptus appeared in significant numbers throughout the region.

Shebrook region created a very different story, as it is rightfully named a wet sclerophyl forest it retains alot of water and moisture within the soil and its vegetation. It was observed in the quadrats that the vegetation was spread out with very minimal diversity and abundance of the species present. The most dominant species was the Eucalyptus regnans and no other eucalypts were found. Other vegetation found was mainly in the shrub layer and appeared to be ferns, due to the amount of moisture retained and the recent rainfall , fungi were found in the ground layer.

The aspect and parent rock coupled with soil characteristics of the two regions was also noted to be different and hence significant in highlighting the ecological filters of the two regions. Selby is situated on a north western aspect, the parent rock was found to be granodiorite and the soil consisted of a sandy loam of 1-2 metres deep. Whereas Sherbrooke lies on almost a mirror image of south eastern orientation with a parent rock of devonion dacite and a soil structure of clay loam of at least 3 metres deep.

The interpretation of the data that gave rise to the most insight in to the comparing the two different communities are presented below in graph format..

Figure 1.1 Plant species density in different strata of forest. Filled bars indicate data collected at Selby. Unfilled bars indicate data collected at Sherbrooke. Data presented as mean ± standard error (n=12 quadrats per site).
This graph indicates the average number of species per quadrat per stratum for each site. It highlights that the species density is greatest at the Selby site at each layer especially at the shrub layer.

Figure 1.2 Mean height in metres of different layers of Selby(filled bars) and Sherbrooke(unfilled bars) forests

This graph shows that the vegetation at the Sherbrooke site grows to a much greater height then that of Selby at both the tree and subcanopy layer ,basically neglecting the ground layer.

Figure 1.3 Average percent soil moisture at Selby and Sherbrooke with error bars

The graph indicates the average soil moisture readings taken from all 12 quadrats at each site.

Figure 1.4 – The diameter base height reading at Selby, where by unfilled bars indicate Eucalyptus raidata and filled bars indicate Eucalyptus obliqua.

Figure 1.5 – The diameter base height reading at Sherbrooke, where there is reference to only one species- Eucalyptus regnans.

Figure 1.4 and 1.5 show that the trees with the greatest DBH is at Sherbrooke which also exhibits a larger range of DBH compared to that of Selby which shows a very limited region DBH despite their being two species.

Discussion:

Soil properties can direct the type of vegetation of the region as each species requires specific amounts of nutrients and water. At Sherbrooke the soil is very fertile Loams and containts a dark loamy topsoil merging into clay loam which reaches to about 120 cm deep. This enables the soil to be of high porosity thus there is a high infiltration rate which enables free drainage to 1 metre deep. Whereas at Selby there is a layer of clay beneath the thin top soil which inhibits infiltration and seedling establishment. Water is trapped in the surface soils and eventually can become cemented which affects root depth. Taking this in to account coupled with figure 1.1 which shows that there is greater species diversity at Selby then Sherbrooke a limiting ecological filter is exposed. The distribution of eucalypts is directly attributed to soil moisture and root penetration which is connected to growth. Figure 1.2 and Figure 1.5 state that the trees at Sherbrooke were found to be a lot thicher and taller then that at Selby due to their roots being favoured. The dominant species at Sherbrooke of E.regnans is favoured by low altitudinal zone and large amounts of water supply whereas the dominant trees at selby E.obliqua and E.radiata rely on low moisture storages and excessively rocky soils. Another main drive between the differences in dominant tree species is the change in aspect between the regions , the South eastern aspect at Sherbrooke enables water to seep in to the soil with gravity and aids the deepening of the root system. Whereas at Selby the north western aspect assist in the flow of water through the area, thus maintaining the soil and unable to retain water.
Root penetration depth acts as an ecological filter that controls the diversity of the regions. At Selby the soil properties do not facilitate deep penetration of roots as indicated by the limited height of the trees highlighted in figure 1.4. The area in which the roots cover at Selby are also limited so the trees go straight up and do not sprawl out which allows a lot of shrub and ground layer diversity. Vegetation within these layers do not require intricate root systems so they are able to thrive within such a community as their resources are not limited. However at Sherbrooke , E. regnans has a deep intricate root system which stretches through out the moist soil. Within the other layers there is competition for nutrients in which the large E.regnans growth requirements win out acting as important controlling factor within the region.

Reference:

Ashton, D. (1981),"Ecology of the boundary between Eucalyptus regnans F.Muell and E. oblqua L'Herit. in Victoria"Proceedings of the Ecological Society of Australia v11

Eucalypt Ecology: Individuals to Ecosystems By John Woinarski, Jann Elizabeth Williams- published online

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