Gap formation in Danish beech (Fagus sylvatica) forests of low management intensity: Soil moisture and nitrate in soil solution

Soil moisture content (0–90 cm depth) and nitrate-nitrogen (NO3-N) concentrations in soil solution (90 cm depth) were monitored after gap formation (diameter 15–18 m) in three Danish beech-dominated forests on nutrient-rich till soils. NO3-N drainage losses were estimated by the water balance model WATBAL for one of the sites. Two forests were non-intervention forests (semi-natural and unmanaged), the third was subject to nature-based management. The study was intended to assess the range of effects of gap formation in forests of low management intensity. In the unmanaged and the nature-based managed forest, soil solution was collected for 5 years and soil moisture measured in the fourth year after gap formation. Average NO3-N concentrations were significantly higher in the gaps (9.9 and 8.1 mg NO3-N l1,respectively) than under closed canopy (0.2 mg l1). In the semi-natural forest, measurements were carried out up to 29 months after gap formation. Average NO3-N concentrations in the gap were 19.3 mg NO3-N l1.
Gap formation alone did not account for this high level, as concentrations were high also under closed canopy (average 12.4 mg NO3-N l1). However, the gap had significantly higher N concentrations when trees were in full leaf, and NO3-N drainage losses were significantly increased in the gap. No losses occurred under closed canopy in growing seasons. Soil moisture was close to field capacity in all three gaps, but decreased under closed canopy in growing seasons. In the semi-natural forest, advanced regeneration and lateral closure of the gap affected soil moisture levels in the gap in the last year of the study.