MARCH: WOODLAND AND FOREST HEALTH
Our speaker was Professor Giles Hardy, Professor in Forest Pathology at Murdoch University. His main research interests are the impact of diseases on the health and functioning of ecosystems, both natural and managed. Within this field he has focussed mainly on the biology and control of Phytophthora dieback disease, the health of eucalypt plantations and the decline of rural native trees.
“Trees don’t like to die,” Giles began by telling us, referring to the resilience of these large, long-lived plants, each with its group of symbiotic relationships. But there are some tree species in the South West that are in decline, such as Peppermint, Tuart, Wandoo, Flooded Gum and Marri. The causes of these declines are often complex—a combination of several factors which combine to overwhelm the tree’s defences.
Giles groups these factors into three categories. First, there are predisposing factors, such as climate, impoverished soils and disturbance. Then there are inciting factors such as frost, drought and infection. And then there are the contributing factors such as root pathogens, wood and bark borers and canker fungi.
Marri (Corymbia calophylla) is an important keystone species in the South West, ranging from Geraldton to east of Albany. Its reliable, prolific flowering makes it useful to apiarists. The flowers, seeds and other parts of the tree are the basis of many forest food chains. It is also an excellent shade tree, and of great aesthetic and ecological value in cities. But Marri and the Red Flowering Gum (Corymbia ficifolia) are subject to attack by the Marri Canker (Quambalaria coyrecup). This is a basidiomycete fungus, similar to a smut, which disseminates its spores via wind, rain or insect vectors. There is a long battle between tree and pathogen, with the tree healing its damage,
forming calluses, and the canker then causing more damage, until the tree finally succumbs. During that process, the canopy often appears to be in good health despite a developing canker. The tree shows similar symptoms to other causes such as attack by Longicorn Beetle, but for an unknown reason it never re-sprouts after a canker attack. It is puzzling as to why this pathogen is quickly spreading in recent times despite it being an endemic, having evolved along with Marri. Loss of Marri trees has a large impact, with loss of bird food, habitat and other ecosystem services as well as loss of honey production and the cost of removing dead trees in urban areas. Marri decline is a syndrome of environmental factors such as the stress caused by a drying climate together with biotic factors such as Marri Canker and another related pathogen, Marri Blight (Quambalaria pitereka), which attacks the shoots, buds and flowers.
Giles talked to us about research being done on Marri Canker. A series of transects, 100m apart, were done at each of 17 sites throughout the SW. A pattern emerged, where many more canker-affected trees occurred in transects close to a disturbed forest edge, such as alongside a road. Deeper into the forest there was very little of the disease. Giles suggested that predisposing factors are present in the disturbed or forest-edge sites, where pesticides, herbicides and fertilisers change the soil chemistry. The research shows, for example, that there is a much smaller diversity of mycorrhizae in the soil along roadsides and agricultural land than deeper in the forest. Mycorrhizae are symbiotic fungi known to improve soil structure and help their host to resist some pathogens as well as bringing mineral nutrients to the host in return for sugars. There is a question as to whether the dieback disease Phytophthora (there are six species found to be associated with Marri across the its range in the SW) predisposes Marri to the canker. Pot trials suggest that this may be so, because seedlings inoculated with Phytophthora lose fine roots, which are vital to mycorrhizae, and some of the Phytophthora species kill seedlings.
Giles’ talk brought home to us the fact that there are a great many possible contributors to the decline of trees in the South West. These include fragmentation of forests, climate change, changes in soil composition (chemistry, moisture and temperature and biota including the mycorrhizal diversity) as well as pathogens such as species of Quambalaria and Phytophthora. It is likely that a combination of these is causing the current wave of tree decline. The research raises many questions and there is much more research to be done.
Giles proposed the idea that the loss of native mammals may possibly be a contributing factor, since the soil-improving effects of e.g. bandicoot digging and the spread of mycorrhizal spores in the scats of the native animals has declined catastrophically. The spores of many mycorrhizal species germinate better after passage through an animal’s gut.
Genetic trials of Marri trees at Margaret River and the Porongorups may possibly result in the development of canker-resistant trees from different provenances. Giles explained his research and the complex problem of tree disease in a clear and interesting way.
Mike Gregson