Within- and across-genus scaling of vessel diameter reveals consistent hydraulic responses to rainfall

Elijah Sabila Magistrado, Isaac R. Towers, Jugo Ilic, Zoe Ball, Daniel S. Falster

·      Xylem vessel diameter and lumen fraction are expected to track water availability via the hydraulic safety–efficiency trade-off yet observed trait–rainfall relationships are weak and inconsistent. This discrepancy may partly reflect the conflation of within- and across-lineage effects in comparative datasets.

·      We tested this possibility by analysing hydraulically weighted vessel diameter (Dh) and vessel lumen fraction (VLF) across 480 Australian native species from 129 genera. Using linear mixed-effects models with genus-level centering, we partitioned trait–climate relationships into within- and across-genus components along a precipitation gradient, comparing results to a conventional linear model.

·      Dh scaled positively with mean annual precipitation (MAP) at both within- and across-genus levels, with similar slopes but differing intercepts among lineages. The centering model explained 11.0% of variance from MAP alone and 78.6% including taxonomic effects, versus 2.8% for the conventional model. VLF showed no significant climate relationship, with variation associated mainly with phylogenetic grouping.

·      Separating within- and across-group responses offers a possible explanation for inconsistencies reported in vessel size–climate scaling: vessel size tends to increase with rainfall, while much of the remaining variance may relate to differing baseline hydraulic strategies across lineages, reflected as intercept differences. Within-subject centering is underused in plant ecology and could help reduce ecological fallacies in phylogenetically diverse studies.