High and variable hydraulic resistance to embolism among three Asian juniper species

Abstract

Over a background of a warming planet, with increasing occurrence of extreme climate events, junipers stand out as tree species thriving in inhospitable environments, enduring the cold, hot, and dry. Being part of the drought-adapted Cupressaceae Gray, some juniper species have been identified having among the most negative P50 values (vulnerability to xylem embolism) in the plant kingdom. Still, previous analyses skipped many Asian juniper species, some of which growing in arid conditions. Here we measured xylem vulnerability to embolism in three, yet unexplored, Asian juniper species: Juniperus drupacea growing in Israel, and J. pseudosabina and J. seravschanica growing in Uzbekistan. Branches were sampled from five trees per species and vulnerability curves were constructed using the Cavitron method, from which P50 was calculated. Xylem anatomy measurements were used to define torus overlap and maximum xylem specific hydraulic conductivity. These hydraulic traits were tested against species and site climate and in the context of species phylogeny. P50 of the Eastern Mediterranean J. drupacea was -11.8±0.3 MPa, much more negative than that of the two Asian junipers measured here, but within the same range of values measured in European and North American junipers. P50 was more negative with mean annual temperature and with aridity index of the driest quarter at both species (climatic conditions at the entire distribution range) and site levels. We did not find the relationships generally found in conifers between the properties of xylem pit membrane and vulnerability to embolism, nor with the overall aridity of the environment. To the best of our knowledge, our observations identify J. drupacea as the most drought-tolerant tree species in Asia.