Citation:

Beedlow, P., Ron Waschmann, EHenry Lee, AND D. Tingey. Seasonal patterns of bole water content in old growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). AGRICULTURAL AND FOREST METEOROLOGY. Elsevier Science Ltd, New York, NY, 242:109-119, (2017).

Impact/Purpose:

Pacific Northwest forests are unique among forested ecosystems in the US in that they experience an annual summer drought, and the dominant trees can photosynthesize during the winter. Climate change is expected to bring hotter, drier summers and warmer, wetter winters to these economically and ecologically important forests. Changes in seasonal climate could have dramatic effects on the structure and functioning of these ecosystems and on the ecosystem services they provide—most importantly clean water, carbon sequestration and timber. Our ability to evaluate the vulnerability of important tree species to changing climate is vital to successful adaptation. A long standing paradigm is that the reason PNW forests are dominated by coniferous trees species, most notably Douglas-fir, is that they can photosynthesize throughout the winter and are able to withstand the pronounced summer drought by storing water in the boles (trunks) for use during the dry season. This concept is fundamental to many forest ecosystem models, which are used to evaluate management options for climate change adaptation. The research presented here suggests that this is not the case for perhaps the most important timber species in the region and worldwide—Douglas-fir. Bole water content was measured continually over ten years using dielectric devices. Concurrently, weather and soil moisture data were collected. The lowest bole water occurred during winter when precipitation and soil water were at their peak, and was greatest during the dry season—just the opposite of what was expected. Bole water content corresponded with growth patterns and environmental factors associated with sap flow, and not with the pattern of soil water depletion. These findings enhance our understanding of drought resistance in large old growth forests of the Pacific Northwest. This paper contributes to an FY 16 deliverable under ACE CIVE-2.4.

Description:

Large, old conifer trees in the Pacific Northwest (PNW), USA purportedly ameliorate the effects of seasonal summer drought by drawing down the water content of bole tissues over the summer months and refilling during the winter. Continuous monitoring of bole relative water content (RWC) in three 400+ and three ~150 year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees using permanently mounted dielectric devices for 10 years showed that seasonal changes in RWC closely followed growth and was positively correlated with temperature, vapor pressure deficit (VPD) and plant available radiation (PAR), but was inversely related to plant available soil water (ASW). RWC in the combined inner-cortex, phloem and sapwood increased during the late spring and early summer to maximum values in August as ASW decreased an average of 81% at the older trees and 67% at the younger trees. The difference between minimum RWC in the winter and maximum in mid-summer averaged only 4.5 and 2.3% for the older and younger trees, respectively, across all years. Growth peaked in July followed about a month later by peak RWC. During late summer and fall, RWC decreased, even as ASW increased with the onset of fall rains, to minimum levels during winter. These data do not support the hypothesis that large old trees in the PNW use stored bole water during the summer drought and recharge during winter. Rather, we hypothesize that RWC is a function of sap flow and growth, which respond to seasonal changes in factors driving water uptake and transpiration.

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