Image_1_Minimum Leaf Conductance (gmin) Is Higher in the Treeline of Pinus uncinata Ram. in the Pyrenees: Michaelis’ Hypothesis Revisited.JPEG

• Main Authors: Amauri Bueno (11904332), David Alonso-Forn (11990558), José Javier Peguero-Pina (4405222), Aline Xavier de Souza (11990561), Juan Pedro Ferrio (9625388), Domingo Sancho-Knapik (4405225), Eustaquio Gil-Pelegrín (4405219)
• Format: Still Image
• Language: unknown
• Published: 2022
• Subjects: Botany; Plant Biology; Plant Systematics and Taxonomy; Plant Cell and Molecular Biology; Plant Developmental and Reproductive Biology; Plant Pathology; Plant Physiology; Plant Biology not elsewhere classified; cuticular waxes; minimum leaf conductance; Michaelis’ hypothesis; treeline; Pinus uncinata; Tundra
• Online Access: https://doi.org/10.3389/fpls.2021.786933.s001

The search for a universal explanation of the altitudinal limit determined by the alpine treeline has given rise to different hypotheses. In this study, we revisited Michaelis’ hypothesis which proposed that an inadequate “ripening” of the cuticle caused a greater transpiration rate during winter in the treeline. However, few studies with different explanations have investigated the role of passive mechanisms of needles for protecting against water loss during winter in conifers at the treeline. To shed light on this, the cuticular transpiration barrier was studied in the transition from subalpine Pinus uncinata forests to alpine tundra at the upper limit of the species in the Pyrenees. This upper limit of P. uncinata was selected here as an example of the ecotones formed by conifers in the temperate mountains of the northern hemisphere. Our study showed that minimum leaf conductance in needles from upper limit specimens was higher than those measured in specimens living in the lower levels of the sub-alpine forest and also displayed lower cuticle thickness values, which should reinforce the seminal hypothesis by Michaelis. Our study showed clear evidence that supports the inadequate development of needle cuticles as one of the factors that lead to increased transpirational water losses during winter and, consequently, a higher risk of suffering frost drought.