Soil water-holding capacity does not mediate aridity effects on plant functional traits in Iberian dune ecosystems

Aridity drives plant adaptations such as reduced stature, sclerophyllous leaves and increased phenolic production. While these patterns are well documented, the role of soil properties in modulating the impact of aridity remains understudied. Trait responses may also vary - converging, diverging, or remaining uncorrelated - across intraspecific and community levels, adding complexity to predictions of ecological responses to arid conditions.

We investigated how aridity influences six plant functional traits - lateral spread, maximum height, leaf area, specific leaf area (SLA), and the concentrations of total phenolics and flavonoids - at both the species level (focusing on Helichrysum italicum, the dominant species across the surveyed sites) and the community level across 24 dune ecosystems along the Atlantic-Mediterranean coastline of the Iberian Peninsula. We also collected soil samples and used piecewise structural equation modelling to assess whether physico-chemical soil variables - water-holding capacity, nutrient availability, pH and organic matter content - mediate the effects of aridity on plant functional traits.

We found a significant negative relationship between aridity and both plant height and lateral spread in H. italicum, while leaf area, SLA, total phenolics and flavonoids were not significantly affected. At the community level, aridity was also negatively associated with plant height and lateral spread, positively associated with SLA, and showed no significant relationship with the concentrations of phenolic compounds. Importantly, water-holding capacity was strongly correlated (positively) with aridity; however, the influence of aridity on plant functional traits was not mediated by variation in this factor.

This study demonstrates that aridity consistently influences structural plant traits across species and community levels in Iberian dune ecosystems, with largely convergent responses across organismal scales, and these patterns occur independently of key soil variables such as water-holding capacity.