2018 ESA Annual Meeting (August 5 -- 10)

PS 46-123 - Programmed for growth, not for plasticity: Carbon thresholds and hydraulic dysfunction in 1st-year conifer seedlings

Thursday, August 9, 2018
ESA Exhibit Hall, New Orleans Ernest N. Morial Convention Center
Steven P. Augustine and Keith Reinhardt, Biological Sciences, Idaho State University, Pocatello, ID
Background/Question/Methods

Germinant (1st-year) seedlings represent a critical life stage in conifers, with mortality often greater than 90% in typical environmental conditions. Few studies have investigated the effect of drought on 1st-year seedling physiology, and none of these studies have quantified these responses throughout the first growing season. Two pine species (Pinus ponderosa and Pinus contorta) found at the lower treeline were used in a greenhouse study to research these responses. P. ponderosa is found at lower elevations and is generally considered to be more drought tolerant and P. contorta is found at higher elevations and is considered to be more cold tolerant. Germinated seedlings were exposed to a gradual dry-down for 12 weeks until mortality was reached. Whole seedling carbon and water relations were assessed throughout the drought to determine the interrelationships of the two. It was hypothesized that 1st-year conifer seedlings exposed to drought would (1) maintain a positive carbon balance despite experiencing hydraulic dysfunction; (2) show little plasticity in their hydraulic vulnerability when exposed to drought; and (3) have mortality more closely associated with hydraulic dysfunction than with dysfunction in carbon-relations.

Results/Conclusions

Initial patterns in gas exchange and mortality rates were informative on species-specific responses to drought. P. ponderosa decreased transpiration and photosynthesis a week into the drought treatment, while P. contorta maintained gas exchange for several weeks. Both species lost the majority of their leaf conductance daily, but failed to reach their turgor loss point (TLP) until mortality. TLP and the osmotic water potential decreased throughout development, with drought further decreasing the values. Seedlings maintained narrower hydraulic safety margins while cotyledonous, relying on hydraulic capacitance to buffer against changes in xylem pressure. P. ponderosa had a greater initial relative root and shoot growth rate, most likely due to high seed reserves. This spurt of growth lead to increased gas exchange in the cotyledonous stage, which allowed P. ponderosa to maintain a positive carbon balance 11 weeks into the drought. Comparatively, P. contorta was only able to maintain a positive carbon balance for 9 weeks even through gas exchange persisted during the drought, leaving it vulnerable to both depleted carbon reserves and hydraulic dysfunction. Mortality in P. contorta was persistent throughout the drought, while P. ponderosa had no mortality until 12 weeks of drought, experiencing 100% mortality over a few days.