While much is known about the relationship between water stress and the formation of emboli in tree xylem, little is known about the ability of naturally growing trees to recover hydraulic conductance (k) within a growing season by regrowth of xylem tissue or refilling of air filled conduits. The ability to recover k within a growing season has implications for estimating tree water use and photosynthesis following drought.
To test recovery ability we conducted a two year field experiment (2014-2015) on a cohort of 20 aspen ramets (Populus tremuloides) in Red Butte Canyon east of the University of Utah. Whole tree k was measured from mid-June through September from sapflow (Q) and pre-dawn and mid-day xylem pressure. For trees in the treatment group we induced embolism in the trunk using a split pressure chamber in early July in both years, targeting a percent loss of hydraulic conductance (PLC) between 50 and 80%. Treatment impact and recovery was noted by tracking k and Q through the growing season. To differentiate between recovery by growth vs. refilling we assessed trunk diameter at the embolism site. We also estimated the treatment impact on productivity from cumulative sapflow, basal area growth, and canopy area.
Out of the 23 non-lethal treatments applied over the study mean PLC was 66±0.04 (7 were lethal). Within season recovery of k via refilling (i.e in the absence of growth) was noted in 21 cases, with recovery to control values in 13. The magnitude of the refilling response varied between the two seasons, with a more consistent recovery response noted in 2015. Recovery was not correlated with the magnitude of the treatment or individual ramets.
Refilling success was correlated with a wetter growing season in 2015 (higher precipitation and lower mid-day vapor pressure deficit). Both of these factors contribute to higher xylem pressure and could promote passive refilling. Measured predawn xylem pressures in both years were similar to the threshold needed to collapse emboli. Trees with lower recovery ability in 2014 also showed lower basal area growth at the start of the 2015 growing season, suggesting a linkage between recovery and productivity. Because most trees recovered, the embolism treatment had a limited overall impact on cumulative Q, growth, and canopy area over the entire study period. This study provides evidence for the potential for naturally growing trees to recover xylem function by refilling during the growing season.