Contrasting Hydraulic Architectures of Scots Pine and Sessile Oak at Their Southernmost Distribution Limits.

Uwe G. Hacke,Hannes Seidel,Elisabet Martínez-Sancho,Annette Menzel,Isabel Dorado-Liñán

doi:10.3389/fpls.2017.00598

Abstract

Many temperate European tree species have their southernmost distribution limits in the Mediterranean Basin. The projected climatic conditions, particularly an increase in dryness, might induce an altitudinal and latitudinal retreat at their southernmost distribution limit. Therefore, characterizing the morphological and physiological variability of temperate tree species under dry conditions is essential to understand species’ responses to expected climate change. In this study, we compared branch-level hydraulic traits of four Scots pine and four sessile oak natural stands located at the western and central Mediterranean Basin to assess their adjustment to water limiting conditions. Hydraulic traits such as xylem- and leaf-specific maximum hydraulic conductivity (KS-MAX and KL-MAX), leaf-to-xylem area ratio (AL:AX) and functional xylem fraction (FX) were measured in July 2015 during a long and exceptionally dry summer. Additionally, xylem-specific native hydraulic conductivity (KS-N) and native percentage of loss of hydraulic conductivity (PLC) were measured for Scots pine. Interspecific differences in these hydraulic traits as well as intraspecific variability between sites were assessed. The influence of annual, summer and growing season site climatic aridity (P/PET) on intraspecific variability was investigated. Sessile oak displayed higher values of KS-MAX, KL-MAX, AL:AX but a smaller percentage of FX than Scots pines. Scots pine did not vary in any of the measured hydraulic traits across the sites, and PLC values were low for all sites, even during one of the warmest summers in the region. In contrast, sessile oak showed significant differences in KS-MAX, KL-MAX, and FX across sites, which were significantly related to site aridity. The striking similarity in the hydraulic traits across Scots pine sites suggests that no adjustment in hydraulic architecture was needed, likely as a consequence of a drought-avoidance strategy. In contrast, sessile oak displayed adjustments in the hydraulic architecture along an aridity gradient, pointing to a drought-tolerance strategy.

Highlights

Drought is a key factor of tree species’ distribution in the Mediterranean Basin (Mitrakos, 1980; Cherubini et al, 2003), and climate projections for the decades point to an increase in dryness in this region (Kirtman et al, 2013)
Our objectives were (1) to characterize the interspecific differences due to the diverging physiological strategies; (2) to assess intraspecific variability across the limit of the species’ distributions; and (3) to evaluate whether inter-site variability of hydraulic traits is linked to site aridity
Sessile oak is a deciduous temperate tree species predominantly distributed in central Europe

Summary

Introduction

Drought is a key factor of tree species’ distribution in the Mediterranean Basin (Mitrakos, 1980; Cherubini et al, 2003), and climate projections for the decades point to an increase in dryness in this region (Kirtman et al, 2013). In droughtprone areas such as the Mediterranean Basin where water availability is already limited (Giorgi and Lionello, 2008) and land-use changes are increasing the competition for this scarce resource (Ruiz-Benito et al, 2013), the above mentioned potential benefits have to be balanced against projected water limitations and their consequences on tree growth (Andreu-Hayles et al, 2011; Peñuelas et al, 2011)

Objectives

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Results

Conclusion