Adaptation in Wood Anatomical Traits to Temperature and Precipitation—A Common Garden Study

Indisputably, temperature and precipitation are key environmental variables driving plant trait variation and shaping plant ecological strategies. However, it is challenging to ascertain their relative influences because site temperature and precipitation are often correlated. Here, using Eucalyptus as a model system representing woody evergreen species more broadly, we sought to disentangle their influence on wood anatomical traits underpinning plant hydraulics. From a common garden we sampled 29 pairs of closely-related Eucalyptus species, each species-pair representing either a contrast in site temperature or precipitation, but never both. Very clearly, and both in phylogenetic and non-phylogenetic analyses, species from lower-rainfall and from colder regions had thicker vessel walls, likely an adaptation to drought and freezing, enabling water transport at more negative water potentials with reduced risk of cavitation or vessel implosion. On average, species from warmer regions had smaller vessels, but theoretical hydraulic conductivity remained stable across site temperatures due to increased vessel density compensating for reduced diameters. These trends being observed for adult plants grown under common conditions suggests that key hydraulic anatomy traits are "hard-wired", and gene × environment interactions are relatively weak. This is a key insight for understanding the trait-basis of plant ecological strategies related to site climate.