The hydraulic architecture of Pinaceae - a review

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 (Pinol and Sala 2000; Martinez-Vilalta and Pinol 2002; Oliveras et al. 2003) was used to study the relationships among hydraulic properties of stems. These included: leaf-to-wood area ratio (A(L):A(W)), wood- and leaf-specific hydraulic conductivity (K-W and K-L, respectively), vulnerability to xylem embolism (Psi(50PLC)), carbon isotope composition of needles (delta(13)C) and minimum needle water potential (minimum Psi(L)). 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 Psi(L)). The only exception was an increase of delta(13)C with decreasing minimum Psi(L) and P/E. Overall, A(L):A(W) ratio decreased with increasing vulnerability to xylem embolism, and with increasing K-W and K-L (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.