, Volume 171, Issue 1-2, pp 3-13

The hydraulic architecture of Pinaceae – a review

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We reviewed the literature to examine the vulnerability to water stress-induced embolism of Pinaceae relative to other conifers and to study the inter-relationships among the main traits involved in the hydraulic function within the Pinaceae. Results showed that Pinaceae (particularly the genus Pinus) are more vulnerable to xylem embolism, and show less variability in this character, than other conifers. Detailed data from 12 populations of Pinaceae (11 species) from three different areas (Piñol and Sala 2000; Martínez-Vilalta and Piñol 2002; Oliveras et al. 2003) was used to study the relationships among hydraulic properties of stems. These included: leaf-to-wood area ratio (AL:A W), wood- and leaf-specific hydraulic conductivity (KW and KL, respectively), vulnerability to xylem embolism (Ψ50PLC), carbon isotope composition of needles (δ13C) and minimum needle water potential (minimum ΨL). Results showed that hydraulic properties tended to be more correlated among each other than with indicators of environmental (precipitation to potential evapotranspiration ratio, P/E) or physiological water stress (minimum ΨL). The only exception was an increase of δ13C with decreasing minimum ΨL and P/E. Overall, AL:A W ratio decreased with increasing vulnerability to xylem embolism, and with increasing KW and KL (P<0.05). We found a strong positive relationship between carbon isotope composition and the estimated maximum loss of conductivity due to xylem embolism under field conditions, suggesting stronger stomatal control in more vulnerable species with higher levels of native embolism. Overall, results are consistent with a range of drought-avoidance strategies to minimise the gradient of water potential through the xylem, and show that different relationships among traits are possible depending on the scale of study (individual vs. species or populations). The strong interdependence among hydraulic traits implies that no single trait is a sufficient predictor of drought-resistance in Pinaceae. Finally, it is hypothesised that the intrinsically vulnerable xylem of pines may limit their survival under extremely dry conditions.