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This article is a part of the Fall 2019 issue of the Earth Science Matters Newsletter.

It seems remarkable that, in the 21st Century, we still don’t fully understand why trees die during drought. Developing this understanding is key if we are to maintain healthy forests, along with their invaluable goods and services. This is especially true as droughts are expected to become more frequent and severe. Recent research in this area has mostly focused on two proposed mechanisms of drought-induced tree death: hydraulic failure (dehydration) and carbon starvation (metabolic collapse). Both mechanisms can kill trees directly or contribute to tree death indirectly by making trees more vulnerable to natural enemies like insects. Regardless, it is widely accepted that the most physiologically stressed trees will be the ones that die during drought. That said, recent research indicates a weak relationship between metrics related to physiological stress and tree death during drought. Something else must also be going on…but what?

Part of the answer to this question is being unraveled in the forests of California’s Sierra Nevada where USGS research scientists have tracked the fates of tens of thousands of trees annually for 37 years. Every year, newly-dead trees get “autopsied” to delineate cause of death. One such example is bark removal that reveals characteristic galleries (tunnels) left by tree-killing bark beetles (see Figure 1 below).

Characteristic bark beetle galleries on white fir
Characteristic bark beetle galleries reveal that fir engraver beetles – responding to California’s extreme drought – killed this white fir in a long-term USGS monitoring plot.  Photo taken in Sequoia National Park, California, on 5 June 2018.  (USGS photo by N. Stephenson)
graphic depicting host-tree selection by bark beetles
Figure 2. Idiosyncratic host-tree selection by different bark beetle species – each of them associated with a particular tree species – indicates tree stress was only weakly related to which trees died during drought.  Shades of green indicate magnitude of tree stress (estimated from growth rates), ranging from low (dark green) to high (light green); large and small tree symbols correspond to trees with trunk diameters ≥20 cm and <20 cm, respectively. Trees with and without red Xs indicate bark beetle death rate with respect to tree stress and size class; trees with a red X demonstrate high mortality rate (25-50%), whereas no red X indicates low mortality rate (0-15%). (from Figure 7 in Stephenson et al., 2019)

During California’s historically unprecedented 2012-2016 drought, some 2,000 monitored trees died and received autopsies, providing a unique window into mechanisms of drought-related tree death. The majority of the dead trees were killed by native bark beetles however, the size and stress level of trees that were killed depended heavily on the particular tree preferences of different bark beetle species as demonstrated in Figure 2. Thus, even during such an extreme drought, substantial proportions of stressed trees survived because their size was one that mostly avoided fatal beetle attack. Conversely, substantial proportions of comparatively unstressed trees died because they were of a size selectively killed by outbreaking beetles. That is, idiosyncratic tree selection by bark beetles meant that tree stress was only weakly related to tree death.

These findings shine a spotlight on what scientist and author E.O. Wilson calls “the little things that run the world”, in that the small size of insects and other invertebrates belies their overwhelming importance in shaping our world (Figure 3). The findings further suggest that, even during extreme droughts formerly thought to kill trees directly (by hydraulic failure or carbon starvation), tree survival in selected areas might be substantially enhanced by controlling bark beetle populations. Targeted control methods have already been developed for a few particularly damaging bark beetle species, providing “proof of concept” which may help future efforts to maintain healthy forests. Furthermore, this research highlights the need to find targeted control methods for each species in a highly diverse array of bark beetles.

The paper, “Which trees die during drought? The key role of insect host-tree selection” was published in the Journal of Ecology.


image of mountain pine beetle
Figure 3. One of the “little things that run the world:” a mountain pine beetle (Dendroctonus ponderosae), no bigger than a grain of rice.  During drought, mountain pine beetles killed 70% of large sugar pines (Pinus lambertiana) – high-value trees for both humans and wildlife – that were growing at low elevations in the southern Sierra Nevada. (Credit: Joshua Dunlap, Oregon Department of Agriculture)

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