Investigating drivers of tree mortality in western forests

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Forests play a critical role in the Earth’s carbon cycle and climate system and provide us with other irreplaceable goods and services, such as clean water, hydrologic regulation, biodiversity, critical habitat, wood products, and recreational opportunities. Given their importance, understanding how forests might change in the future and what might drive those changes is becoming increasingly relevant.

In the western U.S., several large-scale tree mortality events have occurred during the last two decades. Some mortality has been driven by severe drought and bark beetle outbreaks. Additionally, background mortality rates -- the typical mortality in otherwise healthy systems – have nearly doubled in some forests in recent decades. Although less striking than the intense, localized diebacks, these small but widespread increases in background mortality rates can eventually lead to larger reductions in the amount of carbon stored in a forest.

An understanding of the causes of tree death is central to making projections of how forests might change in the future. It is a deceptively simple problem. We know some of the players: competition, bark beetles and other pathogens, among many others. However, we know surprisingly little about the relative importance of the factors that drive tree mortality or what environmental drivers cause them to come into play. Which tree-killing organisms should we focus the most attention on? Which are the ones that dominate mortality processes? Do they work in concert? Are there ‘sleeper’ organisms that might become more prominent as temperatures change or during a drought?   

These questions have persisted largely because we have lacked adequate long-term datasets. Trees have long lives, and, barring major events, they simply don’t die very often. Furthermore, information about what killed a tree gets obscured with time, so data collection soon after mortality death is critical. To address these questions, long-term research efforts are necessary, with scientists visiting trees on a regular basis.

The USGS has a long-term research effort in old-growth forests of the Sierra Nevada mountain range of California, and tens of thousands of trees have been tracked annually for decades. Researchers took advantage of this dataset to quantify the factors causing tree mortality. They found that biotic agents-- insects and pathogens-- appeared to dominate, with bark beetles in particular playing a key role. They also found that these organisms appear to act independently, rather than in unison, with the various agents largely acting opportunistically. Defoliators (insects that remove leaves from a tree or shrub) occurred infrequently, especially compared to bark beetles, suggesting that research that focuses on how a tree defends its stem, rather than the current focus on leaves, deserves more attention.

Understanding and quantifying the importance of biotic mortality agents are helping us improve models of forest change, which in turn will help managers strategically target their forest treatments. Most current models presume that tree mortality is driven primarily by competition, with other mortality being essentially random. Such models rarely make explicit attempts to characterize the impact of organisms such as bark beetles or the fungal pathogens that cause root rots. As the critical role of biotic agents is incorporated into models, managers will have more realistic tools to assess their landscapes and develop plans to adapt to an uncertain future.

In other words, when you think about the forest, you need to remember to see the insects for the trees.

The paper, “Why do trees die? Characterizing the drivers of background tree mortality”, was published in Ecology.

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