Seasonal variation in native hydraulic conductivity between two deciduous oak species

Aims Mechanisms of plant drought resistance include both tolerance and avoidance. Xylem vulnerability to embolism and turgor loss point are considered traits that confer tolerance, while leaf abscission and deciduousness characterizes the avoidance strategy. While these mechanisms are thought to trade-off expressing a continuum among species, little is known on how variation in the timing and duration of leaf shedding in response to drought affect the relationship between xylem and leaf tolerance. In the present study, we explored the extent to which drought tolerance differs between two oak (Quercus) species that exhibit different leaf shedding behaviours. Particularly, we predicted that Q. deserticola Trel., which loses leaves at the end of the dry season (late-deciduous) and is thus exposed to a greater risk of cavitation, would be more drought tolerant and more conservative in its water use than Q. laeta Liebm., which loses its leaves for only a short period of time in the middle of the dry season (brevideciduous). Methods The study was conducted in central Mexico in a single population of each of the two oak species, separated from each other by a distance of 1.58 km, and by an altitudinal difference of 191 m. Quercus deserticola (late deciduous) is more frequent down slope, while Q. laeta (brevideciduous) tends to occur at higher elevations along the gradient. We assessed seasonal differences (rainy versus dry season) in native stem hydraulic conductivity, and tested for variation in xylem vulnerability to cavitation, leaf water use and leaf turgor loss point between the two species. Important Findings The two oak species did not differ in traits conferring drought tolerance, including xylem vulnerability to embolism, leaf turgor loss point, or stomatal conductance. However, both species had different performance during the dry season; the brevideciduous species had lower negative impact in the xylem function than the late-deciduous species. Overall, seasonal changes in plant physiological performance between the two oak species were determined by a reduction in the canopy leaf area.