The variability of stomatal sensitivity to leaf water potential across tree species indicates a continuum between isohydric and anisohydric behaviours

The relationship between stomatal conductance (g(s)) and leaf water potential (Psi(l)) is key to the understanding of plant function under changing climate. The variability among tree species gave rise to selection towards either of two contrasting water management types: isohydric or anisohydric. This study explores the variability of g(s) to Psi(l) across tree species. Curves of g(s)(Psi(l)) were collected from the scientific literature for 70 woody plant species. The data set is comprised of angiosperm and gymnosperm species from all major forest biomes. The hypothesis that curves from different tree species diverge between isohydric and anisohydric behaviours was tested. Species-specific curves formed a continuum, rather than dichotomy between isohydric and anisohydric, as confirmed by distribution models. Alternatively, the water potential at 50% of the maximum g(s) (Psi g(s)50) was used to quantitatively compare between species. A major difference emerged among xylem anatomy classes whereby ring-porous species had higher absolute g(s) at Psi(l) < -2 MPa than diffuse-porous and coniferous species. A positive, linear correlation was shown between Psi g(s)50 and Psi(l) at 50% loss of xylem conductivity. The results suggest that stomatal sensitivity to leaf water potential strongly relates to xylem characteristics. The use of Psi g(s)50 offers a quantitative alternative to the current, yet biased, distinction between isohydric and anisohydric species.