Preparing for the Worst: Enhancing Seedling Traits to Reduce Transplant Shock in Semi-Arid Regions

The spatial extent of semi-arid hot regions is forecasted to grow through the twenty-first century, complicating restoration and reforestation plans. In arid and semi-arid climates, seedlings are more susceptible to transplant shock due to lower soil moisture throughout the year. Determining strategies to reduce seedling stress and improve survival post-planting will be paramount to continued reforestation efforts in a changing climate. We quantified seedling physiological, morphological, and field performance (mortality and growth) response for five species native to the semi-arid region of South Texas (Erythrina herbacea L., Celtis pallida Torr., Fraxinus berlandieriana DC, Malpighia glabra L., and Citharexylum berlandieri B.L Rob) to an antitranspirant (abscisic acid), drought, and elevated CO2. We examined post-treatment seedling gas exchange, non-structural carbohydrates, osmolality, root structure, and stomatal density and evaluated mortality and growth rate on a sample of the treatment population. For elevated CO2 and drought hardening treatments, seedling gas exchange, solute content, specific root length, and stomatal density varied by species, while abscisic acid strongly reduced transpiration and stomatal conductance in all species. However, these physiological and morphological differences did not translate to reduced mortality or improved growth rate due to high herbivory and above-normal precipitation after planting precluding seedlings from stress. We conclude that the simpler antitranspirant approach, rather than the more logistically challenging eCO2, has the potential to reduce drought-related transplant shock but requires more widespread testing.