Preparing seedlings for dry spells: Drought acclimation in the seedlings of two tree species of a seasonal tropical dry forest

The effect of two drought acclimation treatments—high (HS) or moderated (MS) drought stress—on seedling growth was assessed in the two tree species Caesalpinia platyloba S.Watson (Fabaceae) and Ipomoea wolcottiana Rose (Convolvulaceae) in order to prepare them for dry spells. These species are mainly distributed in mature and disturbed sites, respectively, in a seasonal tropical dry forest in Jalisco, Mexico. Without acclimation, the seedlings of C. platyloba showed low values in both Specific Leaf Area (SLA) and Net Assimilation Rate (NAR), suggesting a conservative biomass allocation, while in I. wolcottiana, the high Relative Growth Rate (RGR), Root:Shoot ratio (R:S), and low SLA suggested seedling drought evasion. After the acclimation period, the seedlings of both species showed reduced biomass allocation to leaves, as indicated by the decrease in Leaf Mass Ratio (LMR). Additionally, increases in R:S suggested greater allocation to water capture during the short growth season. After the dry spell simulation, the control seedlings of C. platyloba showed negative NAR, but the relatively high NAR observed in the seedlings subjected to HS and MS suggested that these treatments were sufficient to endure drought stress. In I. wolcottiana, a sigmoid relationship described that seedlings’ RGR rapidly increased when NAR was raised. Contrastingly, in C. platyloba seedlings, the constant RGR increase related to the raise in NAR suggested a conservative use of water. In the I. wolcottiana seedlings, the HS treatment induced leaf abscission. Both species were plastic enough to acclimate to short periods of drought, but different levels of drought stress were necessary to acclimate the seedlings of each species. Suggested drought-acclimation treatments are to water the C. platyloba seedlings at 25% (HS) and I. wolcottiana seedlings at 50% (MS) of field capacity.