Trees’ hydraulic limitations are tested as we face hotter and dryer summers worldwide. The hydraulic system threshold in a specific tissue is typically described by the xylem vulnerability curve (VC). In deciduous trees, the leaf is among the most vulnerable organs, and its vulnerability is assumed to be static through time. We hypothesized that the VC of a mature leaf is, in fact, dynamic along the season.
We measured the leaf VC in 10 deciduous tree species three times during the season (from the same leaf position). To understand if xylem embolism had taken place, we measured the stem water potential (Ψx) every 1-2 weeks and compared it with the VC.
All five Mediterranean/temperate species exhibited a shift of their VC to Ψx along the dry season, in addition to two of the five tropical/sub-tropical species. The seasonal VC adjustment was over two times higher as compared with the seasonal adjustment of the turgor loss point, resulting in improved hydraulic safety as the season progressed. In four of the species that exhibited VC seasonality, it was critical for avoiding embolism in the leaf.
The results suggest that leaves’ VC seasonality is common in species originating from habitats with large seasonal environmental changes. The results also highlight that measuring the minimal Ψx and the VC at different times can lead to erroneous estimation of the hydraulic safety margins. Changing the current hydraulic dogma into a seasonally dynamic VC should enable physiologists to better understand plants’ responses to their environment.
