Drought-related mortality, growth and non-structural carbohydrate dynamics in two conifer species during early stages of development

Abstract

To examine the mechanisms associated with growth, survival, and mortality under drought conditions during early developmental stages, physiological and structural parameters were measured on Pseudotsuga menziesii (PSME) and Pinus ponderosa (PIPO) first-year seedlings grown under two moisture regimes (drought and control) in a common garden experiment. By the end of the 76-day experiment, PSME mortality in the drought and control treatments had reached 30.4% and 11.2%, respectively, while there was only 5.3% mortality in drought PIPO and virtually no mortality in control PIPO. Weekly predawn and mid-day leaf water potential differed significantly between treatments starting ~ mid-way through the study. Although we observed significant differences in photosynthetic gas exchange between species throughout most of the study, there were only minimal differences between treatments within species until the very end of the study. Root, stem, and leaf biomass were significantly greater in PIPO than in PSME throughout the study. PSME seedlings that were dying or recently dead showed a pronounced shift in non-structural carbohydrate (NSC) distribution, with root NSC levels strongly depleted and stem NSC levels increased relative to healthy seedlings. These results suggest a potential role for drought-related constraints on phloem transport in mortality.