Disentangling the effects of ontogeny and environmental factors on xylem anatomy in a semiarid chamaephyte

• Ontogeny determined a shift towards a more efficient hydraulic configuration in the eighth year.
• Plants at south-facing slopes produced narrower rings, smaller vessels and higher vessel density.
• Safer anatomic features occurred in years with low rainfall at the beginning of the growing season.
• Combining quantitative xylem anatomy along growth rings and GAMs solves ecological questions.
• Ontogenetic and topographic effects cannot be neglected when dealing with plant distribution.

The hydraulic system of plants reflects a balance between efficiency and safety which is continuously adjusted along plant's life to guarantee survival under changing conditions. One of such adjustments is the formation of new conductive cells, so vessel traits along ring series reflect part of plant's hydraulic plasticity. In this work we disentangled the effects of ontogeny and environmental factors on wood anatomy of a semiarid chamaephyte, Linum suffruticosum. We sampled individuals on contrasting slope aspects in the Ebro valley (Spain), and measured ring width and vessel traits to obtain annually-resolved data series. General Additive Models (GAMs) were used to identify the variability associated to cambial age, slope aspect, and climatic factors: a shift towards a more efficient xylem configuration was observed at the eighth year of life; narrower rings characterized by higher vessel density and smaller vessel sizes were found at south-facing slopes; and low precipitation at the onset of the growing season resulted in safer vessel features. The combination of quantitative analysis of vessels along ring series and GAMs allowed a global understanding of xylem's functional plasticity; moreover, the need for adequately assessing ontogenetic and topographic effects when predicting plant distribution under global change scenarios was reinforced.

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