An interview with dendrochronologist and fire historian Tom Swetnam

An emeritus professor and former director of the Laboratory of Tree-Ring Research, University of Arizona, Tom Swetnam has studied fire, climate, and human history of forests across western North America, southern South America and central Siberia. He uses tree rings, fire scars, documentary history, and archaeology to reconstruct spatial and temporal patterns of fires and learn how climate and people shaped these patterns over past centuries.  Here Tom discusses how denchronology can shed light on our possible fire futures…

MS: Having studied the relationship between humanity and wildfire at both the local and global scale and across millennia, what have you found to be the most concerning trends of the Industrial age?

 TS: As a scientist who studies trees, and as a person who simply loves trees, I am deeply concerned about increasing tree mortality around the planet, especially ancient ones. This trend, due largely to climate change, is well-documented now. What is especially concerning is that the mortality is disproportionately the oldest and largest trees. The causes related to warming are drought stress, bark beetle attacks, and wildfires, among others.  Just in the past two years we have lost 15 to 20 percent of the largest and oldest giant sequoias due to high severity wildfires. 

MS: Indigenous societies dating all the way back to the Bronze age and likely before found ingenious ways to live with wildfire. Having studied fire cultures in indigenous communities, which do you think have most relevance to non-indigenous communities living at the wildland-urban-interface and why?

TS:  Our recent studies of fire and people have centred in the Jemez Mountains of New Mexico, where the Hemish (Jemez) people lived for centuries within ponderosa pine forests. Before the Spanish arrival in the late 1500s, the Hemish population density in these forests was equivalent to today’s definition of the “wildland urban interface” (WUI). Then, by the mid-1600s, disease and warfare killed off 80-90% of the people. The survivors were forcibly resettled down in lowlands, far from the forests. From study of tree rings, charcoal in soils, archaeology, and interviews with elders, we learned that fire was ubiquitous during the centuries of people living within forests. However, the fires were small in extent, and of low intensity. There is no evidence of extensive, high severity fires sweeping through forests and burning down villages. Then, over succeeding centuries many low severity fires burned freely and widely, until about 1900, when huge numbers of sheep were brought in. The grazing, and then active fire suppression by government agencies stopped the widespread burning. Now, 120+ years later, high severity fires are burning across these same ponderosa pine landscapes where Hemish people once lived sustainably for centuries. The causes are a combination of: 1) lack of low severity surface fire, 2) extensive logging followed by massive in-growth of dense stands of small diameter, stunted trees, 3) accumulated dead fuels, and 4) increasing warming and drought.

So, what are the relevant lessons for today?  First and foremost, it is possible for significant numbers of people to live within ponderosa pine forests without risks of high severity fire. The key is living with fire. That is, allowing lightning set fires to burn in places, and by setting fires frequently and judiciously to burn accumulation of fine fuels, to clear brush and thin dense stands of regenerating trees, to promote desired plants and animals, etc.  Also, utilisation of small diameter trees for building and for firewood helps maintain open forests, low connectivity of fuels, and reduced risks of high severity fire. All of these practices are possible to replicate today, even in the face of global warming. But it will require a very different mindset about fire and smoke than currently exists among most homeowners now living in the modern WUI.

MS: ‘Life finds a way’, as a certain fictional Chaos Theory expert once said. Life having found many ways to coexist with wildfire, of the pyrophyte species you’ve studied, which have inspired you most and why?

TS: I’m a dendrochronologist, so of course, trees inspire me! There are three species that are most awesome to me:  First, there is ponderosa pine. It is a sentimental favourite, in part because I grew up in forests of these trees in northern New Mexico, and that is where I now live in retirement. In addition to being beautiful in form, colour, and even in smell(!), it is a fire-loving tree that also keeps faithful track of a lifetime of fires in its annual rings. I am speaking of the fire scars that form on the lower boles of many trees, from which we can recover a chronology of the years and seasons when fires burned the understory of these forests. Ponderosa is a champion among trees in recording past, frequent fires. One tree we studied had 42 fires recorded as scars over a three-century lifespan. That included long periods with fires occurring every-other year, and one instance of fires occurring on consecutive years…at the base of one tree! 

Another species that is truly amazing to me is the Bigcone Douglas-fir. This is a close relative of the ordinary Douglas-fir which is widespread in western North America. Bigcone is found only in small, isolated stands on steep slopes in the coastal ranges of California. It grows in special refugial “islands” of the landscape within a “sea” of highly flammable chaparral. High severity fires sweep through the chaparral every 30 to 60 years or so, and also through the islands of Bigcone. Their crowns are scorched off, but then in subsequent years new branches and branchlets resprout from the mainstem and larger branches. They have very thick bark, so open basal fire scars are uncommon, but still there are old wounds deeply buried in their stems. From these scars in Bigcone survivors we can read the chronology of fires in adjoining chaparral over the past four centuries. This magnificent tree is surely one of the most fire-resistant and fire-adapted conifers in the world.

Last, there is the giant sequoia. Walking in these groves is like visiting a natural cathedral, and the feeling is one of awe, with a sense you are surrounded by ancient beings. For a dendrochronologist it is heaven! We were so fortunate to have an opportunity to take tree-ring samples from stumps, logs, and snags in six different groves along the west slopes of the Sierra Nevada.  We developed three thousand years of fire and climate history from massive table-sized slabs of wood we brought back to our laboratory in Tucson, Arizona. Like ponderosa pine, sequoias are surface fire-loving trees, and they are superbly adapted to surviving very frequent fires while also recording the timing of those fires in their rings. In addition to insights about the close coupling of climate and fire, there are hints of the human role in adding fire to sequoia groves.  Also, sequoia wood itself is gorgeous and inspiring to look at.