Intraspecific variation in seedling drought tolerance and associated traits in a critically endangered, endemic Hawaiian shrub

Background: Climates are changing at a rate that exceeds the adaptive capacity of species, especially endangered species. Genetic variation and phenotypic plasticity are important for population persistence, yet few studies have linked traits to seedling performance under drought in endangered species. 

Aims: We assessed intraspecific variation and trait plasticity under drought, to understand how an endangered species would cope with increasingly severe droughts. 

Methods: Using greenhouse experiments, we assessed drought tolerance of a critically endangered Hawaiian shrub, Schiedea obovata. Seedlings from five maternal families (three selfed and two interpopulation crosses) were subjected to daily watering or a simulated drought, and we measured growth, traits linked to drought tolerance, and days until death under terminal drought. 

Results: Drought reduced growth but not days until death. We detected genetic variation in growth, but no trait plasticity except in carbon:nitrogen, which decreased under drought. We did not detect traits that enhanced performance under drought but identified four physiological traits  whose effects on growth varied under control and drought. 

Conclusions: Our results indicate moderate drought tolerance of an endangered shrub, and low trait plasticity. Conservation of endangered species under shifting climates will benefit from studies of stress tolerance, particularly at the vulnerable seedling stage.