Factors Contributing to Forest Mortality in North America

Interview conducted by Alessandro PiroliniJun 17 2015

Dr. Christina​ Tague, Associate Professor at the Bren School of Environmental Science and Management at University of California, Santa Barbara, speaks to AZoCleantech about how forest die-offs have affected North America in recent years, and how preventing it is important for both economic and social reasons.

How has the rate of forest die-offs increased in recent years?

Millions of hectares of forests within North America have experienced mortality events in the past decades. This does not mean that ALL of the trees died in these forests, but rather that mortality events where a significant number of trees died were observed.

Other studies have found examples of forest stand mortality throughout the globe. It is challenging to quantify trends in the rates of forest die-off, but it is very clear that warm temperatures and drought lead to increased die-off in many, though not all, regions.

Could you briefly outline the study you carried out into the factors contributing to forest die-offs?

We are trying to improve our understanding of the factors that contribute to when and why forest die-offs occur. In a way this is similar to trying to understand why a person gets sick. One can attribute their illness to a virus, but one can also look at their basic heath and immune system.

For forests, we can look at insects that kill trees and how climate influences the populations of the insects. Warmer temperatures, for example, have allowed bark beetles to more readily expand into northern Canada. On the other hand we can look at the health of the tree and try to understand that drought might make a tree more susceptible to insects, or perhaps in some cases lead to mortality even without insects/pathogens being present.

Our paper is a review paper and grew out of a working group at NCEAS – the National Center for Ecological Analysis and Synthesis. NCEAS provided a great opportunity to bring together experts who look at tree mortality using different methods – from post mortem analysis of dead trees to using satellite remote sensing data. Our paper presents a review and begins to synthesize these different science-based methods.

Are there specific examples of forests around the World which you would consider to be “at risk” of dying soon?

California has recently been experiencing a significant, multi-year drought. We are already seeing increased evidence that forests are experiencing water stress such as needle drop and die-off of trees throughout the state.

Why is the Western USA considered to be such a hotspot for forest die-offs?

The Western USA still has a lot of forest cover and much of the Western USA is also water-limited. Water-limited means that forest growth is often limited by how much water is available, thus forests in this region can be sensitive to even relatively small declines in precipitation.

We see drought related forest die-off in other locations as well – such as bark beetle outbreaks in British Columbia.

What impact does forest mortality have on society in general?

Forest mortality impacts society in many ways. When insect mortality occurs in forests used for timber, there are direct economic consequences. Forests also provide many other services for people.

Millions of people come to national parks each year – and many of these people look forward to spending time in the forests. Proximity to healthy forests may increase land and home values. Forests also provide habitats for many species that we care about.

Another issue with forest mortality is that it may have a link with fire. Dead trees may contribute to fire severity, although research has found that the impact is a complicated one. Changes in forests can also impact water quality.

Do you think severe forest death will have a major impact upon nature-based tourism in the Western USA?

I personally think this is true. Forests are iconic for many people. People are attracted to forests – the majesty of a giant sequoia, the peace one feels when walking through a forest in the evening light. Forests are a place people go for renewal, for recreation, for exploration and play. It is hard to imagine how severe forest death would not impact nature-based tourism.

Sequoia National Park spans over 400,000 acres and attracted over 1,100,000 visitors in 2012. Image Credits: Richart777/shutterstock.com

How does the combination of insects (such as pine beetles) and drought affect forest mortality?

This is the interaction we are trying to better understand. In some cases, forest mortality may result simply from a thriving insect population, but in other cases drought has weakened the trees and this makes them more vulnerable to insects. In these situations drought contributes to the scale of forest mortality.

Do you believe that the combination of climate change and forest mortality affect each other, if so how?

Climate change is likely to increase forest mortality for two reasons. A warmer drought often has greater impacts on forest health than a cooler drought. When a drought occurs under higher temperatures, the vulnerability of trees increases.

Warmer temperature can also, in some situations, contribute to the expansion of particular insect populations into regions where cooler temperature formerly limited their populations.

How do you feel forest mortality could be reduced in years to come?

There are no easy solutions to forest mortality. In a warming climate, in many regions the area impacted by forest die-off during droughts will likely increase. It is hard to control a drought! Nonetheless, there are things that can be done.

One important action may be to expand the area of forests that we maintain through conservation. Even under the severe drought that California has recently been experiencing, there are still many healthy forest stands.

We need to preserve those. If we want to maintain forests for tourism – then we need to account for the fact that forest die off will likely occur in some of those areas – thus we need to preserve a larger area. In the same way that we don’t invest all of our money in a single stock, we cannot rely on small areas of forest to support the recreational activities that we desire because that area may be vulnerable to forest die-off.

There are also some forest management practices that can help. In some regions – but not all – years of fire suppression have lead to dense forest canopies. In these areas, thinning and controlled burns that reduce the likelihood of more severe fires may also reduce water stress in the forest and reduce vulnerability to drought.

About Dr. Christina Tague

Dr. Christina (Naomi) Tague’s (Associate Professor, Bren School of Environmental Science and Management at University of California, Santa Barbara) research is about integration and relationships: how different components of landscapes influence one another to create a dynamic system.

Current projects include modeling climate change impacts on snowpacks, forest growth and mortality, summer streamflow patterns in mountain environments in the Western US and Europe, and examining how urbanization alters drainage patterns and associated biogeochemical cycling in urban systems.

Dr. Tague received her Ph.D. from the Department of Geography at the University of Toronto, Canada and has an undergraduate degree from the Department of Systems Design Engineering at the University of Waterloo, Canada.

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