The objective of most studies of tree ring widths is to deduce past climates. But for commercially valuable tree species, an equally important question is: How will future climate change affect tree growth? For the ever-economically-essential-to-the-Pacific-Northwest Douglas fir, the answer, writes Christina Restaina and colleagues in their recent tree ring analysis, is negatively.
Before we get to the study’s results, let’s consider its methodology.
While we usually think of climate as composed of quantities that we might measure with a home weather station—temperature and precipitation—thinking from the tree’s perspective, as the recent study’s authors did, requires us to consider more physiologically-relevant quantities. One such quantity at the heart of the recent analysis is vapor pressure deficit (VPD).
VPD is the difference between how much moisture is currently in the air versus how much that air could potentially hold. Think of VPD as an interaction between temperature and relative humidity. VPD increases with temperature but also decreases with relative humidity. From the tree’s perspective, high VPD periods are low growth periods because high VPD indicates times when the plant loses or can’t hold onto much needed water, either due to temperature increases or a loss of humidity or some combination of the two. Because VPD can be observed in the record of tree rings, it can provide important data for parsing out both past and potential climates, which is what Restaina and colleagues did in their recent paper.
The researchers analyzed VPD and tree ring data for Douglas fir across the species’ current US range, which runs from New Mexico to Washington state and everywhere in between. And from New Mexico to Washington state and everywhere in between, the researchers found that the year-to-year association of VPD and growth is such that high VPD corresponds to low Douglas fir growth.
Taking the results of their analysis of observed correspondences of VPD and Douglas fir growth, the researchers then threw into the mix future projections of climate across Douglas fir’s range to produce a series of projections for how climate change could affect Douglas fir growth. The answer—as our spoiler above indicates—was not good. The resulting projections showed large increases in VPD across the tree’s range, especially in California and the Southwest, suggesting Douglas fir growth will decrease in a warmer and drier future climate.
However, if there’s a silver lining to their results, it’s this: in the Northwest, where Douglas fir trees currently blanket the mountains in lucrative abundance, VPDs are relatively low. This means the projected decreases in growth, at least in our region, are less worrisome than elsewhere.
The study was published in the Proceedings of the National Academy of Sciences of the United States.
A Side Note: Two of the study’s authors David Peterson and Jeremy Littell have worked with CIRC co-lead Philip Mote. Peterson also worked closely with CIRC researchers on two climate assessment and adaptation plans: the Blue Mountain Adaptation Partnership and the Northern Rockies Adaptation Partnership. Peterson and colleagues have also developed the Climate Change Adaptation Library for the Western United States, an extensive online tool that covers forests and climate in the Western US. (How’s that for a plug, Dave?)
Citation: Restaino, Christina M., David L. Peterson, and Jeremy Littell. “Increased water deficit decreases Douglas fir growth throughout western US forests.” Proceedings of the National Academy of Sciences113, no. 34 (2016): 9557-9562.