Contrasting effects of two hydrogels on biomass allocation, needle loss, and root growth of Picea abies seedlings under drought

Abstract

Adapted management methodologies are required to alleviate increasing and/or more frequent water stress under climate change, enabling successful tree seedling establishment after planting. Hydrogel amendments have been suggested to increase soil water holding capacity, tree seedling growth and survival. However, the effect of a non-uniform addition of hydrogels to tree planting pits as compared to homogenously admixed hydrogels in agricultural soils, as well as effects of hydrogels on woody root system establishment, have been poorly studied.Here we studied the effect of two commercial hydrogels (Polyter and Stockosorb) on the above- and belowground growth of Picea abies seedlings during a ∼ 5-weeks-long drought period. Rhizoboxes with a non-uniform distribution of nutrients and hydrogels were used to monitor root system development in a stratified soil. Biomass allocation, root morphology and root elongation rates were related to evapotranspiration rates. Proximity effects of hydrogels on root traits were determined.Total plant biomass was significantly greater if plants were grown in Polyter-amended soil (+16, 17 %) compared to the Stockosorb and control treatments, respectively. Dead needle mass was significantly greater in Stockosorb than in Polyter (+719 %) and control (+274 %). Root elongation was faster in both hydrogel treatments compared to the control during the first part of the drought period, but subsequently ceased to the order Polyter > control > Stockosorb. Root morphological changes in relation to hydrogels were limited.The better performance of Polyter-amended seedlings was related to a slow water release from the hydrogel, while Stockosorb released the additional soil water content prematurely. The latter might have imposed a “false sense of security”, and the subsequent lack of acclimation responses early in the drought resulted in an even more severe drought stress and a higher needle loss of Stockosorb-amended seedlings as compared to control. However, both hydrogels did not result in preferential growth towards or away from the hydrogel, suggesting that roots did neither sense the hydrogel-bound water nor that secondary compounds hampered root growth. A deeper root system placement and thus potential advantages for drought events at later growth stages were not encountered, asPolyter also had a strong positive effect on root growth in the fertile top soil. As hydrogel amendments may influence root system establishment and seedling vitality differently during drought, further studies are urgently needed before large scale applications to mitigate climate change effects on forest regeneration / restoration can take place.