Water Retention by Superabsorbent Polymers Enhances Tree Seedling Establishment but Is Insufficient for Survival Under Drought Conditions

ABSTRACT Amid worsening drought conditions driven by climate change, water‐retaining soil amendments are increasingly needed. This study aimed to examine the characteristics of plant‐available water in soil amended with a superabsorbent polymer (SAP) under two distinct treatments: (1) seedling growth with intermittent watering after planting (watering period) and (2) time to wilting and death following the cessation of watering (drought period). Japanese cedar ( Cryptomeria japonica ), a water‐demanding species, and drought‐tolerant Japanese cypress ( Chamaecyparis obtusa) were planted in pots containing sandy soil mixed with four levels of SAP. During the watering period, higher SAP addition rates led to increased soil moisture content and water potential. Seedlings exhibited elevated evapotranspiration rates and enhanced growth, particularly in fine root development. In the drought treatment, soil moisture and water potential declined rapidly. The median time to death was extended by up to 12 days for cedar and 3 days for cypress compared to the control. SAP effects were more pronounced in cedar than in cypress. Most of the water retained in SAP‐amended soils was categorised as high‐potential, immediately available water (> −38 kPa). Conversely, the amount of readily available water (−38 to −98 kPa) decreased, whereas hardly available water (< −98 kPa) remained unchanged. In conclusion, SAP amendments promoted fine root development and improved survival in water‐demanding cedar but had limited effects on the drought‐tolerant cypress. To better address drought risks in seedling planting, the development of SAPs with enhanced water retention, especially at lower water potentials, is necessary to meet practical expectations.