High-frequency field measurements of diurnal carbon isotope discrimination and internal conductance in a semi-arid species, Juniperus monosperma

Abstract

We present field observations of carbon isotope discrimination (δ) and internal conductance of CO 2 (g i) collected using tunable diode laser spectroscopy (TDL). δ ranged from 12.0 to 27.4‰ over diurnal periods with daily means from 16.3 ± 0.2‰ during drought to 19.0 ± 0.5‰ during monsoon conditions. We observed a large range in g i, with most estimates between 0.04 and 4.0 μmol m -2 s -1 Pa -1. We tested the comprehensive Farquhar, O'Leary and Berry model of δ (δ comp), a simplified form of δ comp (δ simple) and a recently suggested amendment (δ revised). Sensitivity analyses demonstrated that varying g i had a substantial effect on δ comp, resulting in mean differences between observed δ (δ obs) and δ comp ranging from 0.04 to 9.6‰. First-order regressions adequately described the relationship between δ and the ratio of substomatal to atmospheric CO 2 partial pressure (p i/p a) on all 3 d, but second-order models better described the relationship in July and August. The three tested models each best predicted δ obs on different days. In June, δ simple outperformed δ comp and δ revised, but incorporating g i and all non-photosynthetic fractionations improved model predictions in July and August. © 2009 Blackwell Publishing Ltd.