Have you ever sat down to a cold drink on a hot day and sucked down most of the glass in the first sip? Increasing thirst with increasing temperature also applies to Earth’s atmosphere – as air warms, it can ‘hold’ more water. The difference between the air’s level of moisture and its moisture capacity is dubbed the ‘vapour pressure deficit’ or ‘VPD’, which is essentially a measure of the thirst of the atmosphere.

The relationship between VPD and temperature is non-linear, so as the planet warms it takes exponentially more water to maintain the same level of atmospheric moisture. A glass of water that is increasing in size but staying half full provides a good atmospheric analogy – to stay at the same proportion of the glass’ total volume (relative humidity), the amount of water in the glass (specific humidity) has increased at a global scale. The bottom line is that today’s atmosphere contains more water than a century ago. Crucially, since there is a fixed volume of water on Earth, more water in the atmosphere means less water on and in the ground. This shift in the global water balance is already producing epic droughts and water supply shortfalls are expected to intensify in many parts of the globe. 

For vegetation, this ‘suck factor’ extends from their environment into their tissues; the climbing VPD means that plants require more water as temperature increases so they can’t be sustained by the same amount of rainfall. The exponential relationship between temperature and atmospheric thirst means that it really sucks to be a tree. Unprecedented global tree mortality was documented a decade ago with significant mortality events on every wooded continent across diverse forest types. Mortality has continued and even accelerated since with millions of trees having died in California and recent reports of widespread tree mortality in the Northern Territory of Australia.

Stressed trees are dying and burning – but not necessarily in that order. Dead trees are generally no more flammable than live trees with the exception of fresh corpses that are still cloaked in a kindling shroud of needles or leaves. Whether dead or alive, tree flammability is directly related to moisture content so, as the VPD is sucking many forests dry, it’s also intensifying fire behaviour. 

The ‘warming stripes’ graphics you can see below represent the change in temperature over the past century. Each distinct stripe represents a new year. When I look at the stripes from California, USA and New South Wales, Australia, I see a visual representation of tree comfort (or rather, rising discomfort) over the last century. With VPD dialling up the temperature effect, the stripes paint a vivid picture of today’s crispy, brittle ecosystems. In the Sequoia National Forest where the Castle Fire torched ancient, towering giant sequoias this year, late-summer live fuel moisture in ponderosa pines has sat squarely in the single digits in each of the last three years. With live trees as dry as kiln-dried lumber and temperatures soaring over 100°F (37°C), explosive fire behaviour and spread is inevitable. Although fire is an essential ecosystem element in many forests, these supercharged blazes are decimating old trees and likely driving comprehensive shifts in forest structure and species composition.

Reduced emissions are vital to slowing the VPD train long-term but in the near-term, moisture management offers some local hope. However, appropriate forest management techniques to conserve moisture are complex and nuanced. Water is ‘lost’ from ecosystems through a series of processes that includes both evaporation and transpiration, or tree breathing. In the southwestern United States, experiments in the 1960s and 70s attempted to increase streamflow by removing thirsty trees yet the efforts failed in nearly every case, as the water gained from the loss of the transpiring trees was quickly lost through increased evaporation from the previously shaded areas. In hot, arid regions, trees perform a critical role as parasols that shelter the lower forest from the sun and trap moisture. 

In his book ‘Fire Country’, Victor Steffensen describes how Poppy, his Aboriginal Elder and mentor, would tell him that, “the canopy was sacred, another world that is above us and we must respect.” Steffensen explains, “Shade is as important as food and water, and it only makes sense to protect and respect the shade trees offer in every way possible. If the seasons get hotter then we need to ensure the trees are protected so that they can protect everything else.” This Traditional Ecological Knowledge provides a more sophisticated and holistic understanding of forest ecosystem dynamics and moisture conservation than that provided by contemporary forest management. Specialised, localised knowledge about best practices to conserve moisture within ecosystems should be driving and leading management actions. There is no one-size-fits-all prescription for maintaining hydrated landscapes but cool, understory burns are perhaps the most promising tool for reducing the suck factor and making trees a bit more comfortable, despite the heat. If we can’t turn down the thermostat or make the atmosphere less thirsty, we need to protect the parasols while culling some of their competition.

For a comprehensive discussion of VPD and fire dynamics, check out this excellent seminar from Park Williams at Columbia University’s Earth Institute.

Krista Bonfantine is a PhD student at Deakin University studying the effectiveness of environmental flows using DNA metabarcoding and citizen science. Her background includes water and fire management and her passion is connecting science and society for a better, wetter world. Learn more on her website or follow her on Twitter.

Title Image Credit: Krista Bonfantine, CC BY-SA 2.0, Image Cropped