How are trees under threat?
There are now considerable efforts in urban communities around the world to enhance tree canopies, increase the density of trees and improve access to “green spaces” for the community. This is highly desirable and needs to be strongly supported. For example, there are programs in Sydney to plant an additional 5 million trees in the period till 2030 with an ambitious aim to double the tree canopy of the Greater Sydney region (from approximately 20% to 40%).
But (and there is always a but) in order for this to be successful and sustainable there has to be considerable research, planning and preparation. Unfortunately the data from tree planting exercises around the world to date show that as much as half the trees planted will die prematurely, usually extremely prematurely (i.e. less than 5 years after planting), and that relatively few reach maturity.
The urban environment is an extremely stressful place – reduced run-off, poor water penetration, a soil/root environment that may contain relatively little soil and potentially large amount of toxins, exposure to toxins in the air and a vast array of physical impacts and damage from vehicles, people and sadly vandalism.
And this is even before we start to talk about the impacts of climate change with expectations of higher temperatures and reduced reliable rainfall and pressures from introduced pests, diseases and invasive plants as a result of global trade and travel.
How do we improve these statistics?
There are a number of key factors that need to be taken into account. Firstly species selection is critical – how resilient is the species, will it tolerate the current and future climatic environment, is it likely to be adversely affected by pests and diseases (those here now and those we know are likely to arrive at some time in the future) and will it support fauna?
We have recently commenced a pilot project called “Stop the Rot” to assess the extent of root rot pathogens in trees being used for urban greening projects. These pathogens, mostly species of Phytophthora (literally Latin for plant destroyer!), result in trees that struggle to grow and which will often die, and of course are a source of infection to existing neighbouring trees. Our preliminary data indicates that as many as one in five trees tested are infected and we are now looking to source support to expand the testing program and to work with suppliers to improve conditions in the supply chain.
We have tended to be extremely conservative with tree selection in urban communities and favour a relatively small number of species – the over planting of plane trees (Platanus species) is a good example of a tree that is over used and whose negative impact is exacerbated by its tendency to cause allergies in people.
Critically decisions about species selection need to take place well in advance of planting programs as it is not a trivial process to get the horticultural industry producing the number of quality trees needed in a short time frame. Fortunately research programs at Macquarie University and Western Sydney University have generated data to assist in this process and helping to provide direction for planners and for industry.
The other key factor is that soil preparation, planting and post-planting care be done effectively. Anyone who has planted and then cared for a tree will realise this is a very substantial and long-term commitment and for a successful outcome the tree needs to be at the right stage of growth when planted, soil prepared, water and nutrient programs in place and the growth of the tree monitored to ensure branches and stems are trained so that there are no structural weaknesses resulting as the tree matures (decisions not made in early growth can cause branch or tree failure a decade or two later).
These are all aspects of tree cultivation that botanic gardens excel in – we have experience in cultivating the broadest range of plant species possible, have monitored tree growth over decades and have expert arborists that understand the requirements of monitoring and modifying tree growth to prevent future failures. Our very long-term perspective of understanding tree growth in the different environments of our gardens is a huge resource and when combined with our scientific expertise in plant pathology, genetics and restoration is an unparalleled font of tree knowledge that has direct relevance to community needs.
Protecting our natural environment
Of course, the most critical thing we can do right now is protect existing trees in all our landscapes whether that be in urban environments, rural landscapes or in broad natural ecosystems. These trees have already trapped huge amounts of greenhouse gases and continue to do so on a continuing basis.
Every time a mature tree is removed there is a substantial environmental, social and cultural loss – and a very significant economic loss as the cost to replace a mature tree, if even possible, is extremely large. We all need to be advocates for the preservation of these amazing living organisms that contribute in so many ways to our sustained existence.
There are several ways to support our conservation and research through programs like Youth Community Greening or by booking a tour to learn more about these incredible plants or simply picking up a Wollemi Pine sapling from one of our three plant sale locations.