Hot air or dry dirt: Investigating the greater drought risk to forests in the Pacific Northwest
| Authors: | Andrea Watts, Steve Wondzell, Karla Jarecke, Kevin Bladon |
| Year: | 2024 |
| Type: | Science Findings |
| Station: | Pacific Northwest Research Station |
| Source: | Science Findings 268. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6 p. |
Abstract
Elevation, soil composition, and depressions can influence the distribution of water belowground and its availability to tree roots. During the summer months when precipitation levels are at their lowest, this belowground soil moisture can stave off the effects of drought stress on trees. Although lack of soil moisture is the most well-known driver of drought stress, it may not be the only driver.
Working in the H.J. Andrews Experimental Forest in western Oregon, researchers investigated whether a lack of soil moisture or warm dry air is the primary cause of the drought stress exhibited by trees in the study area. Using soil moisture measurements, tree cores, and the Soil-Plant-Atmosphere model, the team explored how tree growth is affected by the interactions of soil moisture and vapor pressure deficit. Vapor pressure deficit (VPD) is the drying power of the atmosphere and results from the combination of air temperatures and the amount of water vapor in the air.
The researchers found that even when soil moisture is available, trees experienced drought stress when VPD was high; this stress resulted in measurable decreases of latewood (summer) growth. Additionally, increases in VPD decreased tree growth more than decreases in spring and summer precipitation. This finding has long-term implications for forest productivity in the Pacific Northwest, where climate change is expected to increase summertime VPD.