To manage forest response to drought, pay attention to "the little things that run the world"
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).
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.
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