All Together Now: A Mixed-planting Experiment Reveals Adaptive Drought Tolerance in Seedlings of 10 Eucalyptus Species

The negative impacts of drought on plant productivity and survival in natural and crop systems are increasing with global heating, yet our capacity to identify species capable of surviving drought remains limited. Here, we tested the use of a mixed-planting approach for assessing differences in seedling drought tolerance. To homogenize dehydration rates, we grew seedlings of 10 species of Eucalyptus together in trays where roots of all individuals were overlapping in a common loam soil. These seedling combinations were dried down under cool and warm temperature conditions, and seedling responses were quantified from measurements of chlorophyll fluorescence (Fv/Fm). The day of drought (T) associated with an 88% decline in Fv/Fm (TF88) varied significantly among species and was unrelated to seedling size. No significant differences in water potentials were detected among seedlings dehydrated under warm conditions prior to leaf wilt. The rank-order of species TF88 was consistent under both temperature treatments. Under cool conditions, seedling TF88 increased with decreasing cavitation vulnerability measured on adult foliage. Under both treatments, a quadratic function best fit the relationship between seedling TF88 and sampling site mean annual precipitation. These results provide evidence for adaptive selection of seedling drought tolerance. Our findings highlight the use of mixed-planting experiments for comparing seedling drought tolerance with applications for improving plant breeding and conservation outcomes.