Trees and Fire

Trees are a valuable asset, but when drought comes, they can pose a serious fire risk. 

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In good seasons when water is plentiful trees, hydrate the environment by releasing water vapour from their leaves and shading and cooling the soil surface, which reduces evaporation. New seasonal growth creates litter as the older leaves, stems and bark are dropped. During wet seasons this damp litter is broken down by fungi and soil organisms, ultimately creating humus that improves the soil. This tree growth also draws down carbon and provides valuable ecosystem services such as water filtration and habitat for nature. 

However, during dry seasons, the growth put on in wet periods becomes a fire risk.

As the climate warms, higher temperatures are leading to more water evaporation from the land and from trees. More extreme droughts associated with warming also bring more frequent and volatile fires.

All trees have a water budget. Some are naturally thirsty. Others can get by with small amounts of water. Drought-tolerance in plants is the result of evolutionary adaptation, allowing plants to adjust to changing environmental conditions over time. However, when pushed to the limit, all trees display the same efforts to save themselves from drought.

Water stressed trees survive drought stress by reducing their growth rates and dropping some of their leaves to reduce evaporative demand. This produces thinning canopies and an increase dry leaf litter levels on the ground. As drought continues, stems and branches dry out, ultimately becoming dead wood held on the tree or fallen to the ground. At the extreme end of drought, susceptible trees die altogether, producing dead stags or fallen timber. Water stressed trees are vulnerable to insect attack such wood borers which weaken trunks and stems, and tree death is often the result of combined stressors.

During high fire danger periods, carbon-rich litter and timber become extremely dry and highly flammable. Living plants with high levels of oils are also a serious fire hazard. Fire behaviour is becoming more volatile, extreme and dangerous with large, uncontrollable fires making fire and tree management more complex. 

Ultimately, when the megadroughts come, everything in the landscape dries up and becomes flammable, even the carbon in the soil. Under catastrophic, megafire conditions most trees will be extremely dry and burn without intense firefighting intervention.