Increases in the duration and severity of droughts caused by climate change may result in the widespread dieback of forests globally due to water stress.
An international research team involving 24 institutions from Australia, including the University of Tasmania, Europe, the UK, the USA and Canada, has collected data on 478 woody plant species from 185 sites around the world. Its findings were published today in the journal Nature.
The study reveals that it’s not only arid forested areas that are at risk, but also temperate and tropical forests not normally considered to be in danger.
Shifts in rainfall and temperature rises caused by climate change appear to increase the chance of hydraulic failure of a plant's water transport system. Air embolisms in the plant’s water vascular system, caused by prolonged reductions of soil moisture, reduce its ability to supply water to its leaves. The result can be plants drying out and dying.
Recent evidence indicates that the ability of woody plants to survive and recover from periods of sustained drought is strongly related to their resistance to these air embolisms.
Although the drought tolerance of woody plants varies between species, ecosystems and regions around the world, it appears flowering plants (angiosperms) fare worse than gymnosperms such as pines and conifers. This may be due to the fundamental taxonomic differences in the water vascular systems of these two plant groups.
However the genetic resistance to air embolism seems to be limited in most woody plant species, as plants fail to adapt hydraulically to the rapid change in climate. The evolutionary process of natural selection over many generations may not be able to keep up with climate change, resulting in a change in the composition of woodland areas with a potential catastrophic loss of biodiversity.
"The results are comprehensive, with data sourced from a great variety of forest and types and climates across the globe," says one of the core investigators involved, UTAS botanist Dr Tim Brodribb. "In Australia the drying trend is most evident in SW Western Australia, where large-scale dieback is in full swing in the tall Eucalypt forests."
Although there are a number of different mechanisms that effect drought-induced mortality, they are all interdependent and embolism is a key factor as it affects photosynthesis, heat damage and burning up of a plant’s energy reserves.
This study highlights the importance of understanding plant embolism resistance and the effect it will have on the structure and distribution of forest habitats as different regions around the world face increasing aridity. It puts a dynamic global vegetation model that incorporates drought safety margins within reach by using the broad range of the data collected.
"We need to get more information about how plants are feeling in the forests across the globe, how often are they getting close to death and where is this occurring. If we can predict regional-scale tree mortality in advance then it may be possible to attenuate the process pre-emptively," Dr Brodribb says.
The raw data for this study came from published research by Dr Brodribb and other authors over the last 15 years. The process of putting it all together was started at an Australian Research Council network meeting in 2010.