For Day 9 of 12 Days Of Conifers, we feature two very different conifer species: incense cedar (Calocedrus decurrens) and sugar pine (Pinus lambertiana)--and the role of tree species in determining what trees die during drought.
12 Days of Conifers: A Tale of Two Conifers: Incense Cedar and Sugar Pine
Incense cedar and sugar pine are pretty different, despite both being conifers.
They are in totally different families (Cupressaceae and Pinaceae). Incense cedar cones are on the small end of the scale among conifers, less than 2 inches long. Sugar pine cones are the longest of any conifer, with the longest on record nearly 2 feet long (most are closer to a foot, though)! Incense cedars have scale-like leaves. Sugar pines have needles that are about 3 inches long.
So why feature these two together?
It turns that these two trees provide a great example of how different tree species respond differently to forest disturbance. A USGS and US Forest Service study published in 2019 looked at five tree species in Sequoia National Park during and just after the severe drought from 2012-2016. The researchers wanted to better understand how drought killed trees, and how the drought interacted with bark beetle attacks to drive mortality (drought stress can make trees more susceptible to bark beetle attacks). To study this, every living tree in the study plots greater than 1.37 m tall was mapped, tagged and assessed annually for mortality through 2017, with trunk diameters measured at 5‐year intervals.
The results showed that species mattered a lot in determining whether a tree would die from bark beetles during the drought. For incense cedars, small trees experiencing moderate or high drought stress were most at risk. In contrast, it was the largest sugar pines that were most likely to die from bark beetle attack, regardless of their level of drought stress. One reason for these differences may be that different bark beetle species attacked different tree species—cedar bark beetles in the genus Phloeosinus were found to be the culprits on incense cedars, while mountain pine beetle (Dendroctonus ponderosae) was the main killer of sugar pines.