Lignotubers in Mediterranean basin plants

Abstract

Lignotubers are swollen woody structures located at the root-shoot transition zone and contain numerous dormant buds and starch reserves. This structure enables the plant to resprout prolifically after severe disturbances that remove the aboveground biomass. These are considered adaptive traits in ecosystems with highly frequent and severe disturbances—such as fire-prone ecosystems. In this paper, we aim to contribute to the knowledge of lignotubers in the Mediterranean basin and highlight the evolutionary implications. We first summarise existing knowledge on lignotuber species in the Mediterranean basin. We then provide a detailed morpho-anatomical description of early lignotubers in two common woody species (Arbutus unedo L. and Phillyrea angustifolia L.). Finally, we compare our anatomical results with those obtained in studies conducted with other lignotuberous species from different Mediterranean regions. Lignotubers were verified in 14 species in the Mediterranean basin; all being from lineages with origins dating to the Tertiary and thus pre-dating the Mediterranean climate. In A. unedo and P. angustifolia, lignotubers are macroscopically discernible in 4- and 2-year-old saplings, respectively. In these two species, the lignotubers have numerous buds protected by hypertrophied scales, and have a contorted xylem containing abundant starch. Our results challenge the traditional idea that pre-Mediterranean lineages suffered evolutionary inertia; instead, lignotuberous species may be considered examples of plants that adapted to the increased fire activity that occurred throughout the Tertiary and Quaternary. We also highlight the use of morpho-anatomical traits to unambiguously distinguish between lignotuberous and non-lignotuberous resprouting species.