Xylem adjusts to maintain efficiency across a steep precipitation gradient in two coexisting generalist species

Trees adjust the configuration of their conductive system in response to changes in water availability, maximizing efficiency in wet environments and increasing safety in dry habitats. However, evidence of this general trend is not conclusive. Generalist species growing across broad climatic gradients provide an ideal framework to assess intra-specific xylem adjustments under contrasting environmental conditions. Our aims were to compare the response of xylem traits to variations in precipitation of two co-occurring generalist tree species, and to assess climate control on xylem trait variability and co-ordination.

We evaluated xylem traits of Embothrium coccineum (Proteaceae, evergreen) and Nothofagus antarctica (Nothofagaceae, deciduous) in three areas across an abrupt precipitation gradient, from 500 to 2500 mm, in southern Chile. We measured wood density, vessel lumen area and density, percentage of conductive area and vessel grouping, and estimated the hydraulic function from anatomical measurements in 60 individuals per species.

Both species shared a common pattern of response along the precipitation gradient, with an increase in vessel density with dryness, but without changes in estimated hydraulic conductivity. Xylem traits in E. coccineum were more variable and more responsive to the climate gradient, decreasing vessel lumen area and increasing wood density, whereas vessel grouping showed contrasting patterns between species. Additionally, the analysis of trait co-ordination at the individual level revealed a tighter co-ordination among xylem traits in E. coccineum.

Estimated xylem efficiency was maintained in combination with different levels of expected xylem safety within species. Reduction in vessel lumen area was compensated through large increases in vessel density, thus breaking the trade-off between xylem efficiency and safety. Otherwise, the existence of alternative internal adjustments in coexisting species to face similar climatic constraints might increase resilience of temperate forests against unpredictable changes in climatic conditions.