Journal Article

Hydraulic time constants for transpiration of loblolly pine at a free-air carbon dioxide enrichment site

Eric J. Ward, David M. Bell, James S. Clark and Ram Oren

Edited by Frederick Meinzer

in Tree Physiology

Volume 33, issue 2, pages 123-134
Published in print February 2013 | ISSN: 0829-318X
Published online November 2012 | e-ISSN: 1758-4469 | DOI:
Hydraulic time constants for transpiration of loblolly pine at a free-air carbon dioxide enrichment site

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The impact of stored water on estimates of transpiration from scaled sap flux measurements was assessed in mature Pinus taeda (L.) at the Duke Free-Air CO2 Enrichment (FACE) site. We used a simple hydraulic model with measurements of sap flux (J) at breast height and the base of the live crown for 26 trees over 6 months to examine the effects of elevated CO2 (eCO2) and fertilization (NF) treatments, as well as temporal variation in soil moisture (M(t)), on estimates of the hydraulic time constant (κ). At low M(t), there was little (<12%) difference in κ of different treatments. At high M(t), differences were much greater, with κ reductions of 27, 52 and 34% in eCO2, NF and eCO2 × NF respective to the control. Incorporating κ with these effects into the analysis of a larger data set of previous J measurements at this site (1998–2008) improved agreement between modeled and measured values in 92% of cases. However, a simplified calibration of κ that neglected treatment and soil moisture effects performed more dependably, improving agreement in 98% of cases. Incorporating κ had the effect of increasing estimates of reference stomatal conductance at 1 kPa vapor pressure deficit (VPD) and saturating photosynthetic active radiation (PAR) an average of 12–14%, while increasing estimated sensitivities to VPD and PAR. A computationally efficient hydraulic model, such as the one presented here, incorporated into a hierarchical model of stomatal conductance presents a novel approach to including hydraulic time constants in estimates of stomatal responses from long-term sap flux data sets.

Keywords: capacitance; elevated CO2; fertilization; free-air CO2 enrichment; Granier sensors; hierarchical Bayes; hydraulic resistance; Pinus taeda; RC models; sap flux; soil moisture; stored water use; tree hydraulics

Journal Article.  8949 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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