For the first couple of years of tree planting, we adopted a pretty haphazard approach, planting a bit of everything everywhere, and waiting to see what would stick. It took only a couple of months to highlight which areas offered the conditions for revegetation at a respectable pace, and which did not. Some patches only appeared to support certain species, others seemed to support nothing at all. The challenge has been to work out why. For enthusiastic amateurs like us, the working out comes through plenty of observation, plenty of reading and plenty of research.
For some areas on our farm, vegetation is a valuable indicator, plants like sea barley grass (Hordeum marinum) suggests mild salinity, and, in our case, seasonal waterlogging. In others, aspect and soil type present challenges. Three years of tree planting and walks through local bushland have also given us a sense of which local species might suit which locations. We keep trying new configurations in the same places, and also try to mimic the natural process of succession by planting hardy pioneers first, then waiting for them to establish shelter and canopy before adding others. As the balance of the landscape has changed through clearing, cultivation and the associated effects of erosion, shifting water tables and changes in nutrients and soil biology, many of the species that may have dominated a couple of centuries ago no longer tolerate certain areas of the block. Likewise, some areas that may have been conducive to vegetation when woodland was already present, once cleared, they seem to be hostile to its re-establishment: for example, a north-facing corridor of grey, cracking clay that has resisted our affections for two years now.
When we first got the block, we conducted some basic tests for dispersive soils in some of the most active areas of the property. While we thought many of the actively erosive areas had settled with the removal of livestock and nascent revegetation efforts, with recent downpours, many of these areas have reawakened. Dispersive soils are soils that become mobile when saturated, and while they can be sodic, they’re not necessarily saline (this online guide from Landcare provides accessible information on dispersive soils). We utilised this guide from NSW to conduct our own assessments of identified soils dispersive or slaking nature, supported by further tests of soil texture and salinity, using this guide.
Despite the evident scars of erosion across the property, our tests show that only a two currently eroding areas have significantly dispersive subsoils, and both have stable topsoil. Another area slaked, but showed limited dispersive characteristics. The two dispersive areas were not especially saline, however the slaking area registered moderate salinity. In some of these areas, we’ve tried several waves of plantings, and sometimes with limited success. As a consequence, while we still try to match local plants, we’ve also begun exploring plants from beyond the Fleurieu. This also provides an opportunity to match plants that may continue to thrive in a hotter, drier, climate future. In the permaculture spirit of each element serving many functions, in addition to their hoped for ability to manage erosion and salinity, we also try to select plants that offer utility in the form of food, timber, or bee or livestock forage. There are four books we regularly refer to in matching species to our varying criteria, Economic Native Trees and Shrubs for South Australia, and What Seed is That?, both by Neville Bonney, From Mangroves to Mallee, by Todd Berkinshaw, and Farmtrees for the Mount Lofty Ranges, by Peter Bulman.
In one compacted, seasonally waterlogged area of cracking grey clay we’ve begun planting a small woodlot of Eucalyptus occidentalis, from southern Western Australia. Planted at the end of a drier-than-average winter in two drier-than-average years, these trees didn’t seem to notice, and stoically settled in, putting on significant new growth over the spring and summer. Given their success, we’re keeping these in reserve for other areas of the farm as well. We planted these not only to manage water and salinity in the soil, but also as a hard timber and bee forage.
With slaking, saline subsoils that collapse when saturated, another challenging area is slowly being bisected by tunneling. Our first year of planting locally indigenous species had two survivors. In the second year, seeking something with matting roots and suckering potential to bind the soil, we planted another timber species Acacia salicina, typically hailing from further north on the Adelaide plains and beyond. They all died. We then thought the West Australian Casuarina obesa might be a good fit, a species that also offers excellent firewood. However, in looking to source seedlings we were advised that its ability to tolerate salinity, drought, waterlogging and cracking clays has now qualified it as a declared weed in South Australia. So we’re now looking to Melaleuca halmaturorum. Naturally occurring on Kangaroo Island, the Adelaide Plains and the lower lakes of the eastern Fleurieu, M. halmaturorum offers a shallow, spreading root system and is recommended for both saline and erosive sites, and is a prized bee forage and habitat species. We’ll plant it, and see how we go.