Using Traits of Tropical Deciduous Tree Seedlings to Determine Drought Tolerance

Drought is one of the big challenges of forest restoration today. Drought stress affects tree growth and survival after planting. Functional traits play an important role in selecting suitable tree species for forest restoration under extreme environments. The objectives of this study was 1) to examine factors that affect the survival of seedlings in restoration plots, 2) to compare seedling performance during drought experiments, and 3) to explore relationships between functional traits and seedling survival in restoration plots. Nine studied species of deciduous trees were: Afzelia xylocarpa, Bombax ceiba, Careya arborea, Chukrasia tabularis, Gmelina arborea, Irvingia malayana, Oroxylum indicum, Phyllanthus emblica and Spondias pinnata. The survival of one-year old seedlings after planting out was influenced by planting sites and species. The species with top five highest survival percentages were C. arborea, S. pinnata, P. emblica, B. ceiba and G. arborea. All nine species, 42 seedings each, were studied during a simulated drought experiment. Half seedlings were assigned to a water treatment and the other half to no water treatment using a randomized complete block design. Wilting stages were evaluated using a scoring system of 0-5 , once every 2 days for the first 36 days, and once every 2 weeks till the end of the study (a total of 120 days). The above-ground parts of A. xylocarpa and B. ceiba remained alive (score number 4) at the end of the study. Their leaves were wilting only very gradually—a drought tolerant strategy. Thirteen functional traits including leaf, root, and stem traits were measured and calculated from seedlings of all studied species. When combined with survival data from both the nursery and the field, studied species could be classified into 4 groups based on their functional traits. The species that showed high survival percentages in the field tended to have high values of root traits; root:shoot ratio, root length, and specific root length. These traits contributed to seedling competitive ability to acquire water and essential resources. This study did not find any relationships between studied functional traits in the seedling stage and the survival in the plots. Root traits could be used as a criteria for selecting plant species for forest restoration in drought prone areas. However, drought tolerance alone may not be the only factor contributing to the success of forest restoration. Restoration practitioners should consider suitable seed sources and the overall quality of seedlings, seedling preparation and planting processes, maintenance after planting, and environmental conditions on the restoration sites.