Arginine phosphate (ArGrow©) treatment on Norway spruce and Scots pine seedlings at different planting times and under varying planting site conditions in boreal forests

Climate change increases the likelihood of extreme weather events and the risk of damage to seedlings. At the same time, forest growth and thus carbon sequestration should be increased to mitigate climate change. A decrease in the number of forest workers requires an extension of the planting season from the traditional spring to fall. These changes in the operating environment can also affect root growth and early performance of seedlings. Granular arginine phosphate has been developed to improve root growth of tree seedlings and may be a potential tool to improve field performance of seedlings after planting. The aim was to study the effects of arGrow (R) Granulat (ARG) on the field performance of Norway spruce (Picea abies (L.) H. Karst.) and Scots pine (Pinus sylvestris L.) seedlings planted at different planting dates and under different site conditions in boreal forests. Container seedlings were planted in mounds on the forest regeneration sites and under the harsh conditions in the former gravel pits in Central Finland. Pine seedlings were planted in May 2021 and spruce seedlings in May and September 2021 and in June 2022. ARG granules were added to the planting hole for half of the seedlings, while the other half remained the untreated control. In the former gravel pits, there was a lot of drought damage, more in spruce than in pine. Under these harsh conditions, ARG treatment increased root growth in Scots pine seedlings and height growth in both species when damage level was moderate. In the forest sites, the exceptionally warm and dry early summer of 2021 and severe night frosts in June 2023 damaged spruce seedlings in particular, and animals pine seedlings. The ARG treatment affected damage levels only in fall planted spruce, with slightly less severe damage in treated than in untreated seedlings. No other differences in damage levels between seedling treatments were found in forest sites. Damage found at most sites made it difficult to reliably compare seedling treatments, and differences in height between treatments varied. On sites with the least damage, ARG treatment increased seedling height of healthy seedlings in both pine and spruce, but the effects were clearer in pine. In conclusion, it is difficult to study true growth effects of ARG treatment under relevant forest conditions, but there was some evidence that arGrow (R) could increase seedling growth, more so in pine and under harsh planting conditions.