Xylem hydraulic properties of roots and stems of nine Mediterranean woody species

Abstract

We studied the hydraulic architecture and water relations of nine co-occurring woody species in a Spanish evergreen oak forest over the course of a dry season. Our main objectives were to: (1) test the existence of a trade-off between hydraulic conductivity and security in the xylem, and (2) establish the safety margins at which the species operated in relation to hydraulic failure, and compare these safety margins between species and tissues (roots vs. stems). Our results showed that the relationship between specific hydraulic conductivity (K s) and resistance to cavitation followed a power function with exponent ≈-2, consistent with the existence of a trade-off between conductivity and security in the xylem, and also consistent with a linear relationship between vessel diameter and the size of inter-vessel pores. The diameter of xylem conduits, K s and vulnerability to xylem embolism were always higher in roots than in stems of the same species. Safety margins from hydraulic failure were narrower in roots than in stems. Among species, the water potential (Ψ) at which 50% of conductivity was lost due to embolism ranged between -0.9 and <-8 MPa for roots and between -2.0 and <-8 MPa for stems. Vulnerability to xylem embolism followed a pattern of: Quercus ilex=Acer monspessulanum=Arbutus unedo=Sorbus torminalis=Cistus laurifolius>Cistus albidus=Ilex aquifolium>Phillyrea latifolia>Juniperus oxycedrus. Gas exchange and seasonal Ψ minima were in general correlated with resistance to xylem embolism. Hydraulic safety margins differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. We hypothesize that this overly resistant xylem may be related to the shape of the relationship between K s and security we have found.

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…P50 and a values were obtained from vulnerability curves established on roots and branches using the dehydration (Q. ilex; [47]) and air-injection methods (P. sylvestris; [30]) for individuals from the same populations studied here. Values of P to estimate PLC were obtained from measured water potentials (ΨMD for branch PLC and ΨPD for root PLC).…”

“…Values of P to estimate PLC were obtained from measured water potentials (ΨMD for branch PLC and ΨPD for root PLC). All sampling methods and vulnerability curve measurements are detailed in Aguadé et al [30] and Martínez-Vilalta et al [47]. Since there is controversy regarding the best method for establishing vulnerability curves, particularly for species with long vessels such as Q. ilex, we also estimated PLC using the vulnerability curve coefficients (a and P50) estimated in another study comparing several methods to establish vulnerability curves in Q. ilex [48], which obtained higher embolism resistance than the study cited above [47] that was conducted at our study site.…”

Hydraulic properties and embolism in small-diameter roots of five temperate broad-leaved tree species with contrasting drought tolerance

“…Xylem properties can differ largely among different tree species and also between roots, stem and branches of the same tree (Martínez-Vilalta et al 2002). It is assumed that about half of a plant's total hydraulic resistance is located belowground, which emphasizes the crucial role played by the root system in water flux control (Passioura 1988;Tyree and Ewers 1991).…”

“…It is assumed that about half of a plant's total hydraulic resistance is located belowground, which emphasizes the crucial role played by the root system in water flux control (Passioura 1988;Tyree and Ewers 1991). Surface roots may function like valves in the soil-plant-atmosphere flow path by enabling rapid water transport when the soil is wet, but disconnecting the plant from the driest soil patches through the development of drought-induced embolism (Alder et al 1996;Martínez-Vilalta et al 2002). In agreement with this idea, Sperry and Saliendra (1994) argued that xylem embolism should be easier to reverse in roots than in stems because roots frequently experience positive or near-positive pressures.…”

“…Unfortunately, we know only very little about the hydraulic properties and anatomy of the xylem in the root systems of mature trees which hinders a better understanding of the functional role of roots in the flow path (Cinnirella et al 2002;Leuschner et al 2004). Studies analyzing the withincommunity variation in the hydraulic architecture of plants have mostly focused on the aboveground organs and only few dealt with roots (e.g., Hacke et al 2000;Martínez-Vilalta et al 2002;Domec et al 2004) which makes it difficult to characterize different functional types of roots with specific water absorption and water transport qualities.…”