The LBG has recently fielded several enquires regarding canopy decline, with the concern that trees may be being affected by pest insects or disease. What has been found to be the primary cause of this phenomenon in our area, is acute water stress.
ACUTE WATER STRESS CAUSING CANOPY COLLAPSE:
When gazing out the kitchen window, your field of view focusses on the sharp contrast of browns rather than the verdant green canopy backdrop of the garden and bush beyond. Over the past couple of months, the Leschenault Biosecurity Group (LBG) has fielded several enquiries regarding canopy decline and the probable cause being insect and or disease related mortality. The group’s Weeds Management Project Officer offers the following related insights.
The South West does have numerous insects that cause significant morbidity in trees such as the Longicorn Beetle, Psyllid Lerp and recently the Polyphagous Shot Hole Borer (see related article in this newsletter) in Perth leading arborists to cull hundreds of iconic deciduous trees. Similarly, diseases such as Armillaria and Phytophthora root fungus lead do significant tree death. Infamously, the recent canopy collapse first coined by Dr Jo Fontaine in 2010/11 was a world first and is directly related to water stress.
A tree that dies with its foliage attached to the branches albeit brown is acute water stress and is relatively sudden. Tress and or shrubs with a shallow root profile such as those in the upper catchment in creek lines where soil has been washed down slope exposing granite outcrops or Jarrah colonising lateritic ridge lines are most susceptible. Further, members of the Proteaceae family such as Banksia’s with fibroid root systems which are fine and shallow, are dropping out of the landscape from Carnarvon to Esperance. To be clear this vegetation cohort are unlikely to survive period but some may coppice from the basal stem, creating a multi stem (Mallee Form) structure.
Insect and or disease is secondary to the primary cause of vegetation mortality. For example, in Stratham the LBG was called out to identify the causative agent in Marri decline which appeared to be insect related but was in fact, the Longicorn Beetle larvae posthumously boring into the heartwood of a tree suffering morbidity. Endangered black cockatoos then started stripping bark into bite size chunks to feed on the larvae to supplement their principal diet of seed. Marri has a defence when alive of exuding red gum caused by cambium bark damage both suffocating the borer and or plugging the wound from bacterial and or fungal entry.
It would be unwise to label the 2023/2024 summer as directly related to human induced climate change but rather diminishing rainfall in the Southwest has led to a lower resilience of the forest ecosystem over time. In the last 2 years our Shire has experienced a 600mm deficit in rainfall average (Lowden, author pers. com.) against a 30% decreasing trend since the 1970’s which has been linked to climate change.
Going forward beyond the Autumn season, the forecast looks bleak. Catch up rainfall for the year is unlikely, given the late break and absorption lag of moisture into the soil horizon will likely result in belated stream and surface flow. Canopy gap creation through tree death will increase soil desiccation, encourage ladder fuel mid storey species to proliferate and exposure to weed invasion. Relative humidity will decline through the warmer months increasing the likelihood of wildfires with an extended season.
We can all agree with the exception of water carters and the Longicorn Beetle that we need rain to fall steadily over time. The forest ecosystem is in trouble and we are currently mortgaging habitat for generations of endangered species into the future. The dispersal of forests South of Walpole is not an option!
Community members are encouraged to report suspected infestations of insects or disease that may be causing significant damage to trees in your area to Leschenault Biosecurity Group by phone at 0477 049 967 or e-mail at info@lbginc.org.au, or directly to the State’s Pest and Disease Information Services, PADIS by phone at 9368 3080 or e-mail at padis@dpird.wa.gov.au.