As the dry season in Australia’s Top End draws to a close, the eucalyptus dominated savanna woodlands are readying themselves for a major change. Summer’s deluge is arriving, ending a prolonged and scorching winter where barely a drop of rain has fallen for the past five or six months and temperatures soared well beyond 30. The trees survived to earn a well-deserved drink, but just how did they do it? How did they manage the stress and what will happen if dry seasons get longer, drier and hotter due to a changing climate?
New research using TERN’s ecosystem observing infrastructure in the Northern Territory (NT) is helping answer these questions by investigating the seasonal water stress of our northern savanna forests. The research is also analysing the ecosystem’s vulnerability to collapse under a changing climate and water availability.
“I suspected that the water stress experienced by eucalyptus of northern NT would be highly seasonal, with the trees pressed to their limits by the end of the dry season,” says Jen Peters of Western Sydney University. “I also figured these trees would be well adapted to the harsh dry season and be resistant to hydraulic failure”
“However, I found just the opposite: The level of water stress at which significant hydraulic failure occurs for these trees is very low, on par with tropical rainforest species. This hydraulic vulnerability seemed ill suited to the environment. If the trees can’t handle dry conditions, how do they make it through the dry season?”
“The simple answer is that they don’t feel the stress,” adds Jen. “I found no change in the water stress of the trees between early and late dry season. Even with the massive seasonal fluctuations in environmental conditions, wet soils in the rainy season to chalky red dust in the dry, the trees maintain a sort of homeostasis at a surprisingly low level of water stress.”
Despite the counter-intuitive nature of Jen’s findings, they conform perfectly to previous research undertaken at Charles Darwin University (CDU). This work, also using TERN infrastructure and data, reported a remarkably stable rate of tree water use throughout the year at numerous sites across the savanna. This indicates that the trees are using the same amount of water each day, regardless of the surface soil water content or humidity.
“Deep soil moisture reserves play a critical role in maintaining the seasonal pattern of water use that both the CDU researchers and I observed,” says Jen. “This is quite different to other systems in which photosynthetic rates and tree transpiration more closely track rainfall and water availability, such as in seasonally dry rainforests of the NT and neighbouring Queensland.”
TERN's Litchfield Savanna SuperSite is located in the heart of Litchfield National Park and is operated in partnersips with Charles Darwin University, Bushfires NT, and NT Government’s Department of Natural Resources, Environment, The Arts and Sport (NRETAS) and The University of Western Australia
Jen’s research into the water use of Australia’s tropical savannas is part of a larger project looking at current levels of water stress in Australian forests across the whole continent. In addition to the TERN Litchfield Savanna SuperSite in the NT, Jen is undertaking research at six other TERN ecosystem observatories.
“The seven TERN research SuperSites span temperature and precipitation gradients that represent major Australian forest biomes and provided the perfect field infrastructure required to conduct my research,” says Jen.
“All across the nation I’m finding that tree species are fine-tuned to their environments and are operating close to their hydraulic limits. But, it’s also becoming apparent that species from dry habitats have greater safety margins than species from wet habitats. Even the slightest changes in water availability in rainforests place them at risk.”
To better understand these ecological tipping points, the data Jen’s collecting will be used in vegetation models that predict the drought risk to different forest biomes, under current and future climate conditions.
“Drought-induced tree mortality has the potential to impact species distributions, forest community structure, and associated ecosystem services with potential feedbacks to ecosystem hydrology.”
“In the face of climate change driven increased global temperatures and drought frequency, it is critically important that we understand and predict physiological thresholds affecting tree mortality.”
Jen’s work is definitely a significant step in obtaining this understanding and we look forward to reporting on the modelling that stems from it in future TERN eNewsletters.
|To measure water stress and the general health of trees at TERN’s Litchfield Savanna SuperSite in the NT, Jen uses a giant slingshot (called a Big Shot) at to launch a weighted rope that she uses to bring down a tree branch for measurement (above). She also collects tree core samples to measure internal tree structure and wood density (below) (photos courtesy of Jen Peters).|
Published in TERN newsletter November 2017