Soil–water content and air-filled porosity affect height growth of Scots pine in afforested arable land in Finland

We examined the soil–water content (SWC) and air-filled porosity (AFP) of afforested arable land in situ and related them with tree growth, which was expressed as the total length of 5-year-height growth above 2.5 m stem height. A total of 34 randomly selected sites in western Finland afforested with Scots pine (Pinus sylvestris L.) were sampled and SWC was measured using time domain reflectometry (TDR). Increasing AFP up to 30% and correspondingly decreasing SWC significantly increased tree growth while concentrations of foliar nutrients and contents of soil nutrients had no effect. Increasing organic matter content (OMC) and decreasing bulk density (BD) were accompanied by increasing SWC and decreasing AFP in the 0–10 cm soil layer. SWC values above 70% indicated critical AFP below 10%. It was found that 44% of the studied sites had mean AFP lower than 10%. The results obtained indicate that the afforested Finnish arable land with high soil OMC is commonly characterized by high SWC and low soil aeration, which can limit tree growth.