It’s a preoccupation of ours to develop our understanding of the historic ecology of the farm. We’ve pored over books such as Mangroves to Mallee (Berkinshaw 2009), and The Native Forest and Woodland Vegetation of South Australia (Boomsma & Lewis), but their listings of different plant associations were often bewildering as we tried to nut out which was the best fit for our patch of ground. We had some information about climate, and a basic knowledge of soils, but only limited local remnant vegetation to refer to for an idea of what might’ve been here before colonisation. Our property has a couple of old red gums, but little sense of what the landscape might have looked like two hundred years ago. Early advice we received suggested that our landscape was once “pink gum woodland”, yet most of the initial pink gums we planted died. Having a sense of how our landscape was in the past not only assists our success in habitat regeneration, but also offers insight into how our landscape works in general.
We’ve looked at historical photographs, and they seemed to confirm our sense that there were once more trees than there are now, but by the late 1800s they already depict a deforested landscape. Through early colonial accounts we’ve also pieced together a rough picture of what the landscape may have been like, with a particular focus on how water may have been managed in the vegetation and soil. We’ve even brainstormed a sequence of events for how the landscape may have changed between colonisation and now, but what we lacked was local ecological detail. It was time to go back to the library, this time to trawl through past ecological studies of the region. With many written in the early 20th century, the papers we found articulate connections between soils, rainfall and then-living memory of plant communities throughout the Mount Lofty Ranges.
In 1924, the Chair of Botany at the University of Adelaide T. G. B. Osborn, and “ecologist of world repute” R. S. Adamson published their pioneering study of the ecology of the eucalypt forests of the Adelaide region. Shaped by climate and soil, Osborn and Adamson identified three main forest types in the Adelaide region: stringybark forests on the high-rainfall (greater than about 750mm), sandy soils of the highlands; blue gum and peppermint gum savannah woodlands on lower-rainfall, finer-particle soils of the lowlands; and red gum woodlands tracing waterways through the region. Osborn and Adamson considered the stringybark forests and savannah woodlands ‘climax ecosystems’: relatively stable networks of ecological associations for a particular soil and climate. While these associations were identified by particular iconic species – Eucalyptus obliqua for the stringybark forest, Eucalyptus leucoxylon for moderate rainfall savannah woodlands, and Eucalyptus odorata for drier savannah environments – Osborn and Adamson also observed local variation within each ecosystem.
While Osborn and Adamson’s work focussed on a relatively discrete radius of Adelaide, subsequent studies, such as that by J. G. Wood in 1930, expanded their analysis to the Fleurieu Peninsula. Wood writes, “the nature of the vegetation of the [Fleurieu] can best be made clear by comparing the biological spectrum of this region with that of the Mount Lofty Ranges given by Adamson and Osborn … the spectra for the two regions agree substantially in every category; and, indeed, the same associations occur…” (p. 118). The broad associations of the Adelaide Plains and Fleurieu, articulated by Adamson, Osborn and others, are illustrated in the map above, from Specht’s Vegetation of South Australia (1972).
In describing the ecological progression for the southwestern Fleurieu, and the region of Yarnauwi farm between Normanville and Second Valley, Wood writes, “At Normanville a thin tongue of glacial sands lies between the dune area of the coast and calcareous and arenaceous beds of Pre-Cambrian age, which were originally covered with savannah forest of Eucalyptus odorata (peppermint gum), of which patches still remain. On the glacial sands themselves a remarkable community occurs, dominated by trees of Banksia marginata. These trees are on average 20-30 feet high, and specimens with trunk two feet in thickness are common…” Perhaps describing the foothills now occupied by the Links subdivision towards the cliffs of Lady Bay, Wood described how, “Banksia marginata is replaced by a savannah in which [Allocasurina verticillata] is the dominant tree, with Acacia pycnantha, Olearia ramulosa, Hibbertia stricta, and Xanthorrhoea semiplana as the most important undershrubs, and Stipa variabilis as the commonest native grass.” Moving more immediately into the region of the farm, Wood notes, “on the higher foothills to the south pure savannah forest of E. leucoxylon is developed” (1930, p. 119).
Throughout stringybark territory, the “whole area is naturally a forest region,” with the only break in the forest typically occurring in “flat-bottomed, swampy valleys” (Osborn and Adamson 1924, p. 97). These forests are dominated by Eucalyptus obliqua, the messmate stringybark, with its companion, Eucalyptus baxteri occupying “poorer and shallower soils”. The pink gum (E. fasciculosa) and cup/scrub gum (E. cosmophylla) are also associated with the stringybark forest, with manna gum (E. viminalis) and sheoak (Allocasuarina verticillata) also making occasional appearances, depending on moisture and exposure. Osborn and Adamson go on to describe in detail the composition of the understorey of the stringybark forest, while also alluding to the changes in structure according to landscape change.
In drier, environments in the stringybark zone, Osborn and Adamson observed the disappearance of plants such as Pultenaea daphnoides and Acacia myrtifolia, and its replacement by others, including Acacia pycnantha. On hot, north-facing slopes, pink gums (E. fasciculosa) become more dominant, together with yakkas and other heat and dry-tolerant species. As slopes increase in steepness and soils become shallower, Eucalypts may vanish altogether, replaced by sheoaks and yakkas. They observed that sheoaks (Allocasuarina verticillata) are “most abundant where solid rock outcrops, less so on broken rock. On cliffs with a southern exposure especially it becomes luxuriant, and may form closed communities, a condition that also occures on the steep sides of narrow gorges. [Allocasuarina verticillata] also replaces the Eucalypts where rock comes to the surface, not only on such slopes as described, but also on the edges of ridges and by waterfalls” (1924, p. 101).
On shallow, sandy soils, associated with coarse ironstone, Eucalyptus baxteri becomes most abundant, with E. fasciculosa as an “undertree”, together with E. cosmophylla. On steep, sunbaked slopes with these soils, the forest gives way to scattered, stunted pink gum scrub. In contrast, “when one passes to a region of the hard, fine-grained, ironstone a change in the vegetation is at once apparent. Forest ceases often quite abruptly, and its place is taken by a dense scrub that rises only 4-10 feet [1-3 metres]” (1924, p. 103). In these circumstances, the scrub can be dominated by thickets of E. cosmophylla, so dense that they suppress other plant growth. Typically, these communities occur on ridgetops and steep slopes. In contrast, in the wet gullies of the stringybark zone, manna gum (E. viminalis) begins to dominate, with Banksia marginata and blackwoods (Acacia melanoxylon) also appearing. Despite the obvious differences, including a complete lack of stringybarks in some of these communities, Osborn and Adamson insist that these communities are “very closely allied” with the stringybark forest, and represent “variations due to local changes”.
Blue gum woodland
According to Osborn and Adamson, the “forests of blue gum, E. leucoxylon, cover for the most part the lower hills, foothills, rolling country on either flank of the main range” (p. 110). Despite variations, the soils are typically “deep and fine-grained, not sandy” like those of the stringybark forest, and occupy a typical rainfall of about 500-750 mm. The landscape of blue gum woodland is also topographically different from the stringybark zone, “the slopes are much less steep and not rugged, the contours and general outlines are rounded” (p. 110). All fitting descriptions of our farm landscape.
In contrast to stringybark forests, the canopy of the blue gum woodland is open, “giving a park-like effect”, while the understorey is less shrubby than that of the stringybark, “essentially herbaceous and grass-like in general appearance” (1924, p. 110). While Osborn and Adamson note a relationship between stringybark, pink and cup gums, they assert that blue gums are more closely aligned with the manna gum (E. viminalis), especially in higher rainfall portions of the blue gum zone and damp gullies. River red gum, while lining rivers and valley bottoms, often spread into the blue gum woodland. Allocasuarina verticillata, “occurs scattered through the forests and becomes abundant or even locally dominant where the rock is near the surface. This tree is much more abundant in these forests than it is in those of stringybark” (p. 110).
With its “essentially herbaceous and grass-like” understorey, shrubs “practically never form a continuous layer”, although Acacia pycnantha is “prominent, and never absent except in the regions of lowest rainfall” and can become “very abundant” (p. 110). Other large shrubs and small trees that appear include daisy bushes (such as Olearia tubuliflora), the native cherry (Exocarpos cuppressiformis, yakkas (Xanthorrhoea sp.), sticky hop bush (Dodonaea viscosa), Bursaria spinosa, and others. An important characteristic of the blue gum woodland is the seasonal nature of the vegetation, where “with the exception of some of the shrubby species the whole become dried up” and become dormant during the hot, dry summers (p. 110). The authors go on to describe the many smaller bushes, shrub and understorey plants characteristic of this woodland.
Once again, there is noticeable variation in the composition of the blue gum woodland ecology depending on variations in topography and soil. On “lower slopes and on the northern exposures”, larger shrubs thin out, “the flora being composed of Hibbertia with herbaceous plants and grasses” (p. 111). Lilies, orchids, sundews (Drosera sp.) and yam daisies (Microseris sp.) become “abundant in sheltered situations which are moist in the Spring”. Cooler, moist, south-facing slopes “abound in shrubs”, particularly Acacia pycnantha, together with Cheilanthes tenufolia (Curly fern), and Pteridium aquilinum (bracken).
While generalising about savannah woodlands across South Australia in Vegetation of South Australia (1972), Specht asserts Themeda triandra (kangaroo grass) as the dominant grass of the savannah woodlands. However, its role in the ecology changed rapidly as kangaroo grass was highly palatable to introduced livestock, and the composition shifted first towards favouring the native Stipa and Danthonia grasses, ultimately introduced pasture plants.
On ridges running east-west, the authors observed marked differences in the vegetation on the north- and south-facing slopes. As noted, the north-facing slopes had significant spacing between the trees, limited shrub cover and herbaceous groundcover that dried out in summer, while south facing slopes offered trees at closer spacings, manna gums joined the blue gums, and an understorey dominated by Acacia pycnantha, Exocarpos cuppressiformis, Bursaria spinosa and the ferns mentioned above.
On the tops of broad ridges and level ground Acacia pycnantha “becomes especially abundant”. Like in the stringybark forest, on steep rocky slopes, Allocasuarina verticillata can assume dominance, together with yakkas, and may also be accompanied by pockets of Callitris pine. In wetter gullies, the woodland shifts towards red gum woodland, and also sees an increased presence of manna gums. Sheltered locations see abundant shrubs such as those mentioned above, joined by Banksia marginata.
The blue gum woodland transitions towards peppermint gum (Eucalyptus odorata) as the rainfall drops off, although their composition and characteristics are similar. In particular, they share similar soil dynamics, occurring on “relatively deep basic soils which have the general feature of being very retentive of water in winter, becoming in many cases more or less waterlogged, while in summer they become baked hard and dry” (1924, p. 116).
Red gum woodlands
Osborn and Adamson noted strong similarities between the composition of red gum (Eucalyptus camaldulensis) and blue gum woodlands. The red gum occurs primarily beside rivers and at the base of damp gullies, but expands wherever soil is sufficiently moist. Again soil types can vary, but with red gum’s they are “typically deep and with a fair proportion of fine-grained constituents”. Red gums can regenerate and self-seed enthusiastically resulting in dense stands of young trees. Blackwoods (Acacia melanoxylon) and silver banksias (Banksia marginata) also appear in this landscape. Shrubs may be consistent with those found in blue gum woodlands, with addition of moisture-loving Leptospermum sp., Acacia retinodes and others.
The readiness with which red gum regenerates is noted by Osborn and Adamson, describing it as “especially active in the recapture of its habitats” following clearing or other disturbance. “Then thick groves of seedling trees appear, and in their early years grow with rapidity, whether in a once pastured forest, a cleared area, or even on derelict arable land” (1924, p. 127). Because blue gum regeneration is “quite slow”, they note “where blue gum and red gum habitats adjoin, the latter on recolonisation appears to spread out and occupy part of the habitat of the former”, with the spread of the red gum “beyond the valley limits in the savannah country” attributable to its vigorous ability to regenerate from profuse seed production (1924, p. 127). This may be an explanation for the widespread predominance of red gums throughout the surrounding landscape of the farm: their spread has occurred as a consequence of the clearing of a former blue gum woodland.
It seems a subtle shift to go from imagining our block as pink to blue gum woodland, yet each brings with it distinctive ecological associations, formed in particular places for particular reasons. And while the land has been cleared too long to know what was there for certain, these studies highlight patterns that we can recognise: where our pink gums withered, blue and red gums thrived. A neighbour discovered a remnant blue gum a couple of hundred metres from one of our boundaries, while another noted manna gums, blue gum associates, in the damp gullies of their property. Our rainfall and soil type also seems to match that of blue gum woodland. When we came to audit the survivors of our revegetation efforts, those with the greatest survival rate reflect the lists of species, described by those scientists almost a century ago, as associates of the blue gum. Given the chance, it almost seemed that the landscape was trying to tell us what it wanted to be.
Adamson and Osborn, Wood and Berkinshaw below all contain detailed plant lists for local plant associations.
Adamson, R. S. and Osborne, T. G. B., 1924, The ecology of the eucalyptus forests of the Mount Lofty Ranges (Adelaide District), South Australia, Transactions of the Royal Society of South Australia, v. 48, pp. 87-144
Berkinshaw, T., 2009, Mangroves to Mallee: The Complete Guide to the Vegetation of Temperate South Australia, Greening Australia (South Australia), Pasadena
Boomsma, C. D. and Lewis, N. B., The Native Forests and Woodland Vegetation of South Australia, Woods and Forests Department, South Australia
Specht, R. L., 1972, The Vegetation of South Australia, Government Printer, South Australia
Wood, J. G., 1930, An analysis of the vegetation of Kangaroo Island and the adjacent Peninsulas, Transactions of the Royal Society of South Australia, v. 54, pp. 105-139