Clark geographer in Australia to study climate change, drought and the death of trees

June 8, 2016
Christopher A. Williams in forest in Australia


Scientists predict that 2016 is on track to become the hottest year on record. They estimate that record-breaking temperatures will stem in part from the current El Niño event, which affects weather worldwide. But they say climate change is also playing a role this year, just as it has for “the previous 17 record-breaking hot years back to 1937,” according to The Conversation, an online, independent academic and research news site based in Australia.

Similar to New England’s record-breaking warm winter, Australia just posted its hottest autumn ever, an event that led to the bleaching of 93 percent of the Great Barrier Reef. The continent also suffered through the 2000-09 Millennium Drought, which has been blamed, in part, for the death of thousands of trees. This “death of a landscape” has led to “a permanent shift in species distributions,” The Conversation reports. Meanwhile, rising CO₂ levels are reportedly leading to faster tree growth across Australia, and may be reducing trees’ vulnerability to drought.

These sweeping environmental impacts drew Christopher A. Williams, associate professor in the Graduate School of Geography at Clark University (pictured), to Australia for the past nine months to explore “how drought impacts ecosystem productivity, growth and rates of tree mortality.” Williams is on sabbatical as a visiting fellow with the Hawkesbury Institute for the Environment (HIE) at Western Sydney University, working with Professor Belinda Medlyn, director of the Climate and Forest Ecosystem Modeling Group.

Williams’ work in Australia connects closely to the work of his lab at Clark, the Biogeosciences Research Group, which studies how global changes are affecting terrestrial ecosystems; for example, his lab has examined the ecological impacts of the drought in the American West. He and other lab researchers have identified “patterns in forest health, forest growth and carbon stocks, and the drivers of these patterns,” he says. “We have long been using remote sensing to detect disturbances of forests and associated tree dieback.”

Seven Continents, One Summer 2016

This story is part of our 7 Continents, 1 Summer series, which highlights the interesting work that Clark students, faculty, alumni and staff are doing all over the world. Have a great story of your own to share? Let us know and we’ll be in touch.

The Australia research “has given us an opportunity to apply some of our tools to diagnose how forested ecosystems respond to severe and lasting droughts in a different place that has very different vegetation types and is in a unique climate setting,” Williams adds.

We asked Williams to share more about his work in Australia:

Can you elaborate on drought-induced tree mortality in Australia?

Some important changes have been taking place across Australian landscapes over the past few decades. Studies show increased vegetation cover (areal extent and plant density) that might be related to rising atmospheric carbon dioxide concentrations, which can stimulate plant growth and enable vegetation to grow more with the same or less water.

Simultaneously, the continent has suffered some major droughts lately, with the early 2000s having had many years of below average rainfall that reportedly killed trees over large areas. For example, there are reports of mortality of river red gum across the Murray-Darling basin. River red gum is a riparian eucalypt, living along river banks where it can access groundwater. Low rainfall has stressed trees throughout much of southeastern Australia, and these river red gum trees likely faced a double whammy as the groundwater on which they rely saw declines because of pumping for human use.

Similar tree dieback has been reported for ribbon gum trees in the Monaro region of New South Wales, and for jarrah, banksia and other woodlands of southwestern Western Australia.

One of the things I have been working on is to reconcile these two general trends ‒ vegetation expansion and drought dieback. Part of what’s missing is a careful look at just how much plant dieback occurred because of the droughts. Field-based accounts are anecdotal and really need to be complemented with large-area assessment. We are working on exactly that with space-borne observations, studying how much vegetation decline was detected in satellite records, and assessing how that relates to the intensity and timing of drought episodes in different places and figuring out how all of that fits into longer term trends of vegetation expansion or decline.

Can you talk about the impact of the Millennium Drought in Australia?

The Millennium Drought spanned 2000 to 2009 and has been described as the worst drought on record for southeast Australia. This drought event stands out as a major outlier in century-scale records, even for a continent known to have huge boom and bust cycles with respect to its water resources.

Agriculture was particularly hard hit, with major reductions in the important growing region of the Murray-Darling Basin (across the states of New South Wales, Victoria and Australian Capital Territory). 

Water supply for major cities was under threat, and many irrigated agricultural fields were left fallow.  Governments imposed widespread water use restrictions, sparking vigorous debate about water resource allocation for agricultural, environmental and urban uses, and innovative approaches to water allocation including market-based solutions. 

Fire activity was strongly increased during the drought period, and insect outbreaks are likely to have been particularly extensive then as well.

Obvious parallels have been drawn to the droughts in the American West, including California, New Mexico, Arizona, Texas, Nevada, etc.

Seven Continents, One Summer: Researching Tree Damage in Australia

You’ve studied the connections between drought and fires in the American West. What have you learned about fires in Australia?

Fires are a very common part of the life cycle of Australian landscapes. This complicates our work on droughts in a few ways. First, it is a confounding factor because we are trying to study drought-induced decline of vegetation, but fire can cause abrupt vegetation decline as well. Fires that happen during or just after drought interrupt our ability to study the direct effects of drought on ecosystem and their growth dynamics. Fire can also be more common during droughts in some cases because droughts decrease fuel moisture, making forests more flammable, and increase the likelihood of weather conducive to fires (air is hot and of low humidity).  So part of the drought effect is a stimulation of fire activity.

How does it compare to what you’ve observed in North America?

Many of the same processes are active across forests of North America, particularly forests of the American West. One of the most striking trends in U.S. forests is a recent shift toward natural disturbances such as fires and beetle attacks and stress from drought, heat and dry air. In fact, perhaps oddly enough, one of the things I’ve worked on most intensively while visiting Australia has been a review of the current state of the carbon cycle in U.S. forests.

How is your research tied to climate change?

This work connects to climate change in two major ways. 

First, projections suggest not only a long-term trend toward warmer conditions over the next century but also an increase in extreme events including droughts. Droughts are expected to be more severe and longer lived in the future because of human driven changes in the climate system. We are trying to figure out what that might mean for the health of the world’s forests and the resources they provide. 

Second, forests typically offset a meaningful portion of the greenhouse gases humans put into the atmosphere each year, but droughts can diminish or even reverse this behavior, potentially adding to the problem of global warming. We are trying to assess just how close forests are to the threat of collapse, and how much extra greenhouse gas pollution their collapse could potentially produce. There is also hope of turning that narrative toward solutions if we can identify ways of bolstering forest resilience to the threat of a changing climate.

Can you elaborate more on your research in Australia?

One of the things I have set out to do during my sabbatical stay is to synthesize results from experimental research on how drought impacts ecosystem productivity, growth and rates of tree mortality. This makes use of the most recent findings from the Hawkesbury Institute for the Environment’s world-class plant experimental facilities and others around the world. Facilities manipulate water, temperature, humidity and carbon dioxide concentrations to see how plant growth responds to different conditions, with particular emphasis on the interactive effects such as drought plus heat plus more CO₂. One practical impact of this line of work is to aid diagnosis of the limits and vulnerability of forest resources and their resilience in the face of droughts to help guide predictions of where forests may undergo sizeable changes and perhaps even be lost in the future.

I have also been working with data from a global network of meteorological stations to examine whether forests really use more water than grasslands. The accepted wisdom is that forests evaporate more than grasslands, but some of my recent work reported on observations that seem to suggest otherwise. I am trying to figure out why. This is important because some land and water managers consider trees to be thirsty users of precious water resources, leading some managers to promote tree removal even though the evidence supporting that is not entirely clear.

All of this work connects to models of ecosystems and the global climate system in important ways as well. We are trying to make use of the latest findings, from remote sensing and manipulative experiments, to improve the models that are being used to predict how tree cover and land carbon sequestration rates will respond to the changing climate and potentially exacerbate climate change by releasing additional carbon to the atmosphere, or, alternatively, to continue to soak up the carbon we emit helping to mitigate human-driven climate change.

In addition, a PhD student of mine in geography, Tong Jiao, is working on large-area mapping and assessment of tree canopy dieback and recovery in the face of drought. Field reports of tree dieback and woody cover expansion are a backbone of the work. We are also bringing in lots of data from earth-observing satellites, including optical data from Landsat, AVHRR and MODIS satellite sensors that can be used to characterize vegetation cover and density. We are also working with passive microwave data from a range of satellites describing “vegetation optical depth,” which represents canopy water content and amount of live tree canopy, and starting to look at Quick Scatterometer (QuickSCAT) data that shows something similar but at a finer spatial resolution.

Where do you see your Australia research headed once you return to Clark?

Being in Australia has given me the chance to experience a whole range of different forest environments, with completely different species from what we are used to in North America, and are in fact globally unique. One thing I am thinking about launching is a comparative analysis of drivers and trends of forest change between Australia and North America to see if rates of growth and carbon uptake differ and why, and to examine if vulnerability to drought episodes and a warming world might be fundamentally different across these disparate settings.

Given my recent focus on the American West, our research group is in a good position to compare the ecological impacts of droughts in California, for example, to those in southeastern Australia, in terms of forest growth and dieback, and maybe also agricultural productivity and water resources, though those aspects have already received a good bit of attention. Another direction we may head would be to compare the rate of carbon sequestration in forests of Australia to that in the U.S., and to seek to attribute those changes to drivers. 

Work of this kind is needed now more than ever, not only because of the rapid climate changes taking place, but also because governments are looking to forests to help mitigate their greenhouse gas emissions and slow the pace of climate change. These trends make the recent dismantling of one of Australia’s premier, government-supported research institutions, the Commonwealth Scientific and Industrial Research Organization (CSIRO), all the more shocking. Massive job cuts here are crippling their flagship land, water, climate and oceanic science branches, which have been world leaders in those fields for decades. CSIRO has also provided a critical research pole in the southern hemisphere, playing a key role in assessing how lands and oceans in the region fit into the global picture of a fast-changing planet.

— At top, photo of damaged trees in the Yarra Ranges near Melbourne, Australia; all photos courtesy of Christopher A. Williams.