Oral Paper

         Ecophysiology

Life at the extreme: understanding how hydraulics constrain some of the longest living pines to unique elevational positions

Presenting Author
Steven Augustine
Description
Water moves through plant xylem conduits to replace the enormous volume of water transpired from leaves. This transport is driven by a negative pressure gradient and is inherently vulnerable because air entry into the system can render it dysfunctional. The air bubbles in the xylem that block water flow are most frequently caused by drought and freeze-thaw cycles. In response to freeze-thaw cycles, selection has favored narrower conduits. In tracheid-bearing conifers, drought tolerance is related to how much the torus overlaps the pit aperture, with drought tolerance increasing as torus overlap increases. Despite the importance of these traits to hydraulic functioning, there is a lack of information on how they vary within species. This study looked at how hydraulic traits, including both tracheid diameter and torus overlap, varied within three Pinus species (P. balfouriana, P. contorta, P. monticola) along an elevational gradient (2600 – 3300m). Within the three species, one spanned the entire gradient (P. contorta), while the others were excluded from either lower (P. balfouriana) or upper (P. monitcola) elevations. We found distinct species-specific trends, with each species having unique hydraulic syndromes. Species found at higher elevations had more conservative hydraulic traits (e.g., narrower conduits, lower hydraulic conductivity), while species at lower elevations were more hydraulically efficient, likely due to the underlying anatomy. While intraspecific variability was low, the large interspecific trends suggest that the underlying hydraulics of each species constrains it to distinct elevational positions.