Evaluating hydrogel efficacy on biomass and survival of Casuarina equisetifolia L. and Grevillea robusta A. Cunn. ex R. Br. seedlings.

Dryland afforestation in Ethiopia is frequently hindered by high seedling mortality during drought, leading to the proposal of hydrogels as a mitigation tool. However, the physiological trade-offs between biomass gain and survival longevity at low dosages remain poorly understood. This study evaluated the efficacy of a 3.5 g/pot application of Stockosorb® 400 K on the growth and survival of Casuarina equisetifolia and Grevillea robusta using a 2 × 2 factorial Randomized Complete Block Design. The experiment was conducted in a greenhouse using a sandy loam substrate (bulk density 1.31 g/cm³; organic matter 0.84%) across two phases: Phase 1 (well-watered) assessed growth, while Phase 2 (water stress) evaluated survival via Kaplan-Meier analysis. Results indicated that under well-watered conditions, hydrogel did not significantly impact height or root collar diameter but significantly enhanced total biomass (P = 0.02); mean biomass reached 114.4 g for G. robusta (Control: 99.2 g) and 106.5 g for C. equisetifolia (Control: 90.4 g). Under severe water stress, the 3.5 g dosage failed to significantly extend survival time (P = 0.959). However, treated seedlings maintained significantly higher terminal total biomass compared to controls (P < 0.001): 129.47 g vs. 106.33 g for G. robusta, and 106.33 g vs. 90.67 g for C. equisetifolia. These data indicate a "growth-over-endurance" strategy, where seedlings prioritize metabolic mass gain over drought survival. We conclude that while a 3.5 g application of Stockosorb® 400 K is effective for increasing biomass in both C. equisetifolia and G. robusta, it is insufficient to significantly extend seedling survival during prolonged drought.