Drivers of variability in transpiration and implications for stream flow in forests of western Oregon

by Moore, Georgianne W.

Abstract (Summary)
I compared transpiration among different types of forest stands in the

western Cascades of Oregon. The three major questions were: 1) How does

transpiration compare between a young and old stand and why? 2) Does diversity

of overstory trees affect transpiration? and 3) How is transpiration related to

stream flow? Transpiration was quantified using thermal dissipation sap flow

measurements scaled to a ground-area basis using sapwood surveys for periods

during the summer months of 2000, 2001, and 2002, and in a subset of plots, for a

full year. I found that a young, rapidly growing stand ([approximately] 40 years since

disturbance) used 3.3 times more water during the growing season than an old-growth

stand ([approximately] 450 years since disturbance) because the young stand had 2.3

times higher sap flow rates per unit sapwood in Douglas-fir, had a 21% greater

total sapwood basal area, and had a larger component of hardwoods that use 1.41

times more water than conifers per unit sapwood. In two-species mixtures of

Douglas-fir and red alder, I found evidence that mixtures are less productive and

have lower annual transpiration than monocultures of these two species. The

observed differences were probably due to altered biomass rather than diversity

itself, but diversity likely played a role in altering biomass.

Such stand age- and diversity-related differences in transpiration

potentially impact stream flow. In a small watershed with a 450-yr-old forest, I

examined the role of vegetation in stream flow patterns at hourly, daily, and storm

scales. Transpiration apparently controlled stream flow during the summer at

hourly scales with lags of at least five hours. In contrast, at daily and storm scales,

soil water apparently controlled both stream flow and transpiration during the dry

season, but there was no relationship during the wet season. These results indicate

that forest management practices that reduce stand age and decrease diversity may

lead to increased transpiration and consequently may reduce summer stream flow.

Bibliographical Information:

Advisor:Bond, Barbara J.; Jones, Julia A.; Brooks, Renee; Irvine, James

School:Oregon State University

School Location:USA - Oregon

Source Type:Master's Thesis

Keywords:hydrologic cycle oregon cascade range ecohydrology cascades plants transpiration forest ecology


Date of Publication:09/10/2003

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