How the use of our planet impacts our native trees

By Peter Geelan-Small

This month, Peter Geelan-Small shares with us summaries of two articles, both highlighting the human impact on our native plants. 

Rescuing Gondwanan rainforests

Before Gondwana began to break up over 100 million years ago, it was covered with rainforests similar to those now found between Newcastle and Brisbane. Due mainly to logging and land clearing for agriculture, only 1 % of the original rainforests in this area remains today, with much of this protected in national parks. Newer threats to these ecosystems include the 2019-20 bushfires and the myrtle rust fungus, which has put 16 Myrtaceae species on the verge of extinction.

Many of the mature trees in the Sydney Royal Botanic Garden come from these ecosystems and scientists at the Garden are now focussing on rescuing at-risk species and rehabilitating these northern NSW forests. To do this, they are investigating how to collect and store seed and cuttings and how to produce resilient plant populations from this propagation material. A challenge here is that seed of many rainforest species can only be used if freshly picked.

Many plants in these rainforests are culturally significant to Indigenous people and researchers are working with local Indigenous communities to map how these trees moved around the landscape. The black bean (Castanospermum australe) and bunya pine (Araucaria bidwillii) are important food sources and occur in larger numbers near important cultural meeting places, suggesting that Indigenous people actively cultivated them. Using genomic tools, researchers have found low genetic diversity among black bean tree populations growing from the coast to the uplands. As the tree’s genetic journey through the rainforests and Dreaming tracks of local Indigenous people are compatible, it appears Indigenous people dispersed the seed.

Another part of the rainforest species rescue project is working with groups such as Big Scrub Landcare to develop ways of involving the community in propagating at-risk rainforest species.

Using both genetic information and propagation technology, resilient patches of bush can hopefully be restored and seed production areas created with genetically diverse plants. This will allow interbreeding and the development of generations of plant populations with the resilience to resist future threats to their survival.

The author, Brett Summerell, is Chief Scientist & Director of Research, Australian Institute of Botanical Science.
Originally published by Foundation and Friends of the Botanic Gardens in “The Gardens” magazine (Autumn 2022 edition, issue 132)

Without urgent action, these street trees are unlikely to survive climate change

With climate change, urban cooling is a growing need. Governments are spending more on tree planting in public places but need to consider what species will thrive as global temperatures rise and whether adequate water is available to sustain plantings, points often omitted at the planning stage.

In January 2020, Penrith hit 48.9 degrees C, the hottest temperature ever in Greater Sydney. Out of 5,500 street trees assessed after this event, 10 % had canopy damage, particularly exotic deciduous species, with intense heat and drought together causing the damage.

Researchers at Western Sydney University ran a glasshouse experiment on 20 broadleaf evergreen tree species, typically growing in habitats from tropical rainforests to semi-arid woodlands, to study the effects on them of a five-week drought followed by a six-day heatwave.

The 20 species had widely varying responses. The species that performed best had dense wood and small, thick, dense leaves, which are water-efficient, including orange jasmine (Murraya paniculata), inland or western rosewood (Alectryon oleifolius) and Australian teak (Flindersia australis). Swamp banksia (Banksia robur) and powderpuff lilly pilly (Syzygium wilsonii) had much crown dieback even with adequate water, showing heatwaves may threaten trees even if water is available.

Common urban street trees in Australia, such as weeping fig (Ficus microcarpa) and London plane tree (Platanus x acerifolia) have soft, low-density wood and thin, large leaves, making them vulnerable to heat. Their large canopies effectively cool urban areas but they are only suitable where water is available.

For urban trees to survive hotter and drier summers, they need adequate water. To achieve this, planting programmes must include “blue” infrastructure, based on principles of “water-sensitive urban design” (using engineering methods to retain water in urban landscapes). Examples are passive irrigation systems, where trees draw water from stormwater storages, and raingardens.

Integrated planning can achieve the multiple benefits of healthy trees, urban flood mitigation and less demand on local water supplies, and will satisfy the dual need to both green and cool cities under increasingly harsh climatic conditions.

Original article published on The Conversation, 11 January, 2022 written by:

Renée M. Prokopavicius, Postdoctoral Researcher in Plant Ecophysiology, Western Sydney University

David S. Ellsworth, Professor, Western Sydney University

Sebastian Pfautsch, Research Theme Fellow – Environment and Sustainability, Western Sydney University