Unraveling the hydraulic vulnerability of tree seedlings using optical and acoustic techniques.

The seedling stage is critical for tree recruitment and forest regeneration, but faces high mortality rates, with drought being a major cause. However, knowledge of the hydraulic vulnerability of tree seedlings is scarce due to methodological difficulties related to their small size. We quantified the xylem vulnerability of the hypocotyl to drought-induced embolism using the ultrasonic acoustic emission and optical visualization techniques by performing simultaneous measurements on dehydrating 5- to 8-week-old seedlings of Acer pseudoplatanus, Sorbus aucuparia, Larix decidua, and Pinus cembra. Optical visualization was also used on the angiosperm leaves. Species-specific differences in hypocotyl and leaf vulnerability were observed. Acoustic emission data showed that the hypocotyl vulnerability of S. aucuparia, L. decidua, and P. cembra was similar to that reported for mature tree branches. Optical visualization was similar to acoustic emission vulnerability in A. pseudoplatanus and P. cembra, but higher in L. decidua and S. aucuparia (with differences of 0.83 and 2.50 MPa, respectively). The latter showed exceptional higher frequencies in small conduits, which may be difficult to observe with optical visualization. Both techniques can be used to provide new insights into tree seedling hydraulics, which will be crucial for better predicting forest regeneration in the face of climate change.