Growth, Physiology, and Root Development in Seedlings of Woody Species Treated with a Seaweed Extract

The demand for saplings has risen in recent years as a consequence of massive planting campaigns targeted at increasing canopy cover. To test the hypothesis that seaweed extract can improve root biomass and length, an experiment was carried out at the ERSAF Regional Forest Nursery in Curno, Italy. The seeds of 5 woody species were planted in trays using a substrate amended with 0×, 1×, 2×, or 3× the label dose of a pure Ascophyllum nodosum extract. After germination, 6,400 seedlings were arranged according to a randomized complete block design with 10 blocks. After 1 growing season, plants were transplanted into 1.7-dm3 forest containers for 1 additional growing season. Root, stem, and leaf dry weights, total leaf area, total root length, and specific root length were measured over an 80-week period. Leaf gas exchange and greenness index were monitored for 78 weeks using an infrared gas analyser and a SPAD meter. Species differed for growth rate, biomass allocation to roots, and specific root length. The algal biostimulant increased stem and whole plant dry weights for 1 year only when applied at 3x the label dose. Significant effects on leaf gas exchange were found only at the highest dose and were mostly due to higher leaf greenness index than to lower diffusional limitations to photosynthesis. Results suggest that substrate amendment with Ascophyllum extracts may have short term positive effects on plant growth, likely due to a nutritional boost. However, they did not trigger structural changes in plant traits that can enhance transplant tolerance in the long run.