SELECTION OF ARTICLES APPEARING IN THE GUNDAROO GAZETTE IN 2018
February 2019 Worry-wort – On the comings and goings of weeds, featuring St John’s Wort.
May 2019 What next? – What are the implications for plants of electing a climate no-action government?
June 2019 Frozen – How plants survive freezing. Again and again.
Plant names that end in “wort” indicate a history of use in Europe as medicinal or food plants. For example, the well-known weed St John’s Wort (Hypericum perforatum) has been used for thousands of years as an antidepressant, antiseptic, anti-inflammatory, expectorant and tonic for the immune system. It has recently had a revival in modern human medicine as an anti-depressant, although not without some controversy.
Since its introduction to Australia, St John’s Wort has been such a problematic weed that scientists started biological control trials in the 1920’s. Of twelve agents introduced, six have established: three beetles, an aphid, a midge fly and a mite. This long scientific journey has demonstrated that while biological controls can be important agents of control, they are not magic bullets. The early work in controlling Prickly Pear with beetles was a huge success that is now permanently imprinted on the nation’s psyche. This is unfortunate as it was a complete fluke whose success has never been repeated.
In the case of St John’s Wort, all those predatory invertebrates have had only a modest impact, and in our district, it seems that they are having no effect this year. The amount of St John’s Wort growing could be viewed as something to get depressed about, which is somewhat ironic given its medicinal properties.
The plant is palatable to livestock, but poisonous when eaten in large quantities. Oddly, the effects of eating St John’s Wort include sensitivity to sunlight and … depression! The photo-sensitization results in serious skin damage, which in turn leads to weight loss, reduced productivity and, in extreme cases, death. Hence the early and prolonged search for biological control agents.
Its yellow-flowering presence has been conspicuous in December and January in paddocks, along roadsides, seemingly everywhere. I am guessing the combination of winter drought and warm season rainfall has played a role here. From February, the yellow will gradually turn to green then dark brown as the flowers finish and the seeds start developing.
At Gang Gang we were certainly caught out this season. Over the last 15 years we have been pulling out any plants that we have encountered, which seemed to be in just a couple of small patches. This year is different. It has appeared in many new spots. Possibly, powerline inspection vehicles have bought in seed. Not that our vehicles are squeaky clean, we just try hard to avoid driving over the place for this exact reason. Alternatively, the new patches could be small plants, previously unnoticed. Who knows.
The only practical response is to be less haphazard with our weeding, and be more vigilant and systematic. To this end, we have tagged the nine sites we know of with a GPS and put reminders in electronic diaries to go and check them next December. Because the worst part of this story is that the weed puts up suckers in a circle around the larger flowering plants. If you do not want to use herbicides you have to dig up a tracery of surface roots the radiate out from the established plants, or you will have many more new shoots coming up.
Competition from perennial pastures is considered essential for long-term control. This makes the use of herbicides tricky if the surrounding non-target pasture species are damaged by spraying. Grazing can also be used to manage St John’s Wort and may be the only practical measure on steep slopes. Don’t graze when it is flowering however, as it is most toxic at this stage of growth. Also, provide shade trees to reduce the effects of photosensitization.
If you are a long-term reader of this column, you may remember the near-panic over Fleabane (Conyza) which had a good season in 2012, at which time many people thought it was taking over the landscape. I soothingly suggested that this was not the case, and indeed, no one has mentioned Fleabane since that time. Hopefully, St John’s Wort is going to do a similar thing, and gradually subside in dominance in coming less-favourable seasons.
Do not confuse St John’s Wort with the innocuous native Hypericum gramineum. The native is smaller, the flowers are more orange than yellow, and it does not sucker aggressively. Unlike the European wort, the native wort is a responsible grassland citizen of modest demeanour.
The chips are down. The nation has sent a clear signal that policies to address climate change are not a priority. This mean that no significant action is going to be taken in the foreseeable future and the export of fossil fuels will continue unabated. What will this mean? The loss of homes and livelihoods to millions of people in low-lying and drought-prone areas is certain. This disruption will have serious consequences for the all of humankind, whether climate refugees or not. The specifics are hard to predict but is is difficult to see the future as anything but threatening. Which is perhaps why we seem psychologically unable to respond.
A more tractable question is to consider what is likely to happen at the local level in terms of ecosystem change. Though even that question can only be considered in a limited way. In my attempt to glimpse our ecological future, I’ve drawn from a CSIRO modelling tool  which provides projections for a range of emission scenarios and climate models. It can be used to identify the places in Australia whose current climate represents the likely future climate that we will experience around Gundaroo. The results are complex and depend on which time frame and emissions scenario you select. However, a reasonably consistent message comes through – it is going to be hotter here in the future and even if the rainfall does not decline dramatically, soil will be drier due to the warmer conditions.
In thirty years, the climate that we are most likely to experience around Gundaroo is that of current day Wagga Wagga. This town is located on the very western edge of the south-west slopes, where the Riverine Plain begins. It is about two degrees hotter than we are and has nearly 100 mm less rainfall. Of course the warmer temperatures will increase the evaporation, making the rainfall even less effective in terms of plant growth. If things get really extreme, we will experience a semi-arid climate like Griffith’s, but without the irrigation water to keep things green.
This knowledge allows us to look at what plants are likely to survive here in the long-term. Broadly speaking, if a local species does not currently also occur on the western slopes of NSW it is less likely to persist here in the future. Note however, the soils do vary over these regions so was cannot assume that the species are absent for reasons that are entirely climate-related.
Even if a plant does have a current range extending westward, it is not a guarantee that is will survive in our district, as detailed modelling of the iconic Yellow Box (Eucalyptus melliodora) has revealed. The major eucalypt of grassy woodland on the Western Slopes is the White Box (Eucalyptus albens). If it can disperse, or more likely, be assisted to arrive our way, in plantings and restorations, it may be the better-adapted woodland eucalypt in the future.
I have been particularly bothered by the predictions of drier conditions in the cool season. This is the time of year that so many of our orchids and lilies are in leaf, gathering resources essential for spring flowering. While many of orchid distributions extend into the west, records are sparser there, suggesting the possibility that a drying trend will create marginal conditions for our orchids and lilies in the future.
The Rosy Hyacinth Orchid (Dipodium roseum). Will warmer and dryer conditions spell the end for floral beauties such as this in our district?
Having said that, these plants have been around for tens of millions of years and have likely seen many more droughts than we have. We know little about their potential to adapt to rapidly changing conditions. However with their fussy ways and delicate growth habits, as well as dependencies on specialized fungal associations and pollinators it is hard to image that orchids are going to thrive in this increasingly changeable and chaotic biosphere.
Just because the climate is heating overall, it does not mean that we are going to escape frosts, and certainly any lessening of frost does not seem to be happening this winter. Clear winter skies at higher altitudes play their part, now and in the future. Having tried to grow vegetables in a subtropical climate before moving to the Southern Tablelands, we have come to appreciate the value of a series of cleansing frosts. Many pests and diseases that happily live through a mild winter are purged by the icy conditions here. This makes growing vegetables in our temperate climate so much easier than in the sub-tropics. Such an unexpected outcome.
Winter frosts also enable us to grow fruit, such as cherries and plums which have a chilling requirement. So cold is not all bad, one tells oneself on a frozen winter morning.
You may have noticed the variability that plants display when they are frosted. Some do not seem to freeze at all, others melt into a brown mush at the first touch of winter – Basil being a notable example. Cell death occurs when ice crystals form in the plant’s living cells, damaging the cell membranes which collapse like a punctured balloon as the plant thaws in the warming morning that follows.
Some other species wake up crisp and icy in the morning but happily thaw out in the day, and even continue to grow slowly. The soft, apparently defenceless pansy first demonstrated this amazing resilience to us when we moved to Canberra. Day-after-day they got alternately frozen and thawed but continued to flower throughout the winter. Some lettuces, as well as spinach, have the same miraculous ability to freeze and thaw, continuing to provide fresh greens throughout winter.
The internal goings-on that prevent plants being damaged by frost are very complex, and they are not well understood either. Broadly speaking, if the live cells can be protected from ice damage, the plant will survive overall, even if the spaces between the cells freeze. Hence the miraculous recovery of an apparently frozen-solid plant as the day warms up. Such plants can protect their live cells by producing specialised proteins or increasing the concentration of some sugars in their cells, creating conditions that are like putting anti-freeze in a car radiator. Gradual seasonal cooling triggers these cell changes, preparing them for frost. Hence the concept of a plant acclimatizing to cold as the season changes, although a sudden icy night may catch them unprepared and leave them damaged.
Other ways plants protect themselves is through physical structures. Is it important to realize that all plants contain variable amounts of dead cells their cell walls, hardened with lignin and cellulose. These form the structures that hold a plant upright – stems, branches and trunks. The relative numbers of these dead cells are reflected in the woodiness of a plant. Even if a tree is burnt by frost, its woodiness stops it flopping into a heap the way a Basil plant might.
Apart from providing its own stable physical structure, plants can protect precious buds and leaves by forming a waxy coating or covering them with hairs or scales. Other protective design features are small leaves, or leaves that curl downwards on freezing, reducing the exposure of the leaf to the cold temperatures.
In addition to preventing freezing or protecting tissues from frost, a third approach to dealing with frosts is for a plant to retreat. The deciduous tree is the well-known example, dropping its thin vulnerable leaves and retreating to the buds along the fine branches, which re-shoot again in spring. Virtually no native trees have this strategy. Acacias and eucalypts sit put and battle out the cold, anti-freezing their precious cells and protecting their tough leaves with waxy coatings.
This frosty Silver Wattle (Acacia dealbata) will soon thaw out. Again, and again.
Native herbaceous (non-woody) plants commonly have a retreating strategy, storing their nutrients in roots, rhizomes or tubers, with the leaves dying back to perennial buds which are close to, or under, the ground. The summer-growing native grasses do this, which is why they look so dead and dull by later winter.
Seeds of course provide the most complete mechanism of retreat. The whole plant can die, and still leave a legacy in the soil in the form of dormant seeds.
So many different biological strategies. This is the gift of plant biodiversity – there is a plant for almost every challenge nature can provide.