Liuzzi, Greta (2026) Growth, hydraulics, and defence in forest trees: functional organization of xylem traits. [Tesi di dottorato]

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Abstract

In order to assess how trees respond to environmental and structural constraints, it is essential to understand resource allocation and xylem architecture. This thesis explores the phenotypic plasticity of wood traits at different scales, in different biomes and in different species. It shows that the functional relationships between growth, defence and hydraulics are context-dependent rather than fixed. The research is structured into three complementary studies, integrating dendrochronology, quantitative wood anatomy, and ecosystem data. The first study examines six conifer species and reveals that there is no direct trade-off between carbon availability, radial growth, and defence. Despite interannual productivity fluctuations, resin duct density remained stable, indicating a prioritised constitutive defence strategy. However, competition for resources was evident in the form of an inverse spatial relationship between conductive tissue area and resin duct density. The second study focuses on subalpine European beech populations at the treeline and demonstrates that climate attribution is proxy-specific; mean ring width is mainly associated with thermal regimes, whereas theoretical hydraulic conductivity reflects the structure of the growing season. Furthermore, temporal instability in these populations is structured by cambial age and elevation, indicating functional adjustments rather than an imminent critical transition. The third study investigates the plasticity of a Mediterranean species (Phillyrea), specifically contrasting the growth habits of a polycormic shrub and a monocormic tree. While negative axial vessel density gradients are conserved, hydraulic tapering and conduit packing vary plastically. The tree-like form exhibits more gradual conduit widening and stricter packing rules to compensate for a longer hydraulic pathway. Therefore, the thesis as a whole shows that xylem architecture can reorganise functional priorities in response to environmental stress, structural constraints and observational scale.

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