Spatial and temporal variation of seepage water chemistry after femel and small scale clear-cutting in a N-saturated Norway spruce stand

The chemistry of seepage water was studied before and after small scale clear-cutting and femel cutting (removing 20% of the trees) between 1999 and 2002 at the Höglwald site in southern Bavaria. The interventions were performed in February 2000 on mature, N-saturated Norway spruce (Picea abies (L.) Karst.) stands with high NO3− concentrations before felling. Seepage water was collected with suction cups at 40 cm soil depth in the following treatments: (I) a mature stand (control), (II) a femel-cut, and (III) a clear-cut. In the femel cut subvariants were created with suction cups (plots) at varying distances from pre-selected spruce, which were later removed. The femel treatment was replanted with beech (Fagus sylvatica L.) saplings. On the clear-cut, subvariants of planted beech (close to the stem, interstem area), planted spruce (interstem), or natural spruce regeneration were investigated. Clear-cutting caused high NO3− peaks (average values up to 2750 µM) during 2000 and 2001 in all planted tree subvariants during times of comparatively low water fluxes. In contrast to peak concentrations, flux weighted yearly average concentrations showed different trends. In 2000, flux weighted yearly average NO3− concentrations were significantly elevated, but only on the subvariants of the interstem area, which covered in the clear-cut plot ca. 62% of the area. However, the subvariant close to the stem (31% of clear-cut area), or the natural spruce regeneration subvariant (6% of clear-cut area) exhibited no significant felling effect. With respect to the whole treatment area, this resulted in no significant felling effect as compared with the control. In the next year (2001), flux weighted yearly average NO3− concentrations were not significantly affected by clear-cutting, while the concentrations were even reduced for all of the clear-cut subvariants in 2002. On the subvariant natural spruce regeneration, NO3− concentrations remained below the European limit of drinking water (806 µM) during almost the whole investigation period. Selective cutting resulted in slightly reduced NO3− concentrations in 2000 and 2001 on the femel treatment. However, no significant effect could be detected for any subvariant in the femel-cut, even not for the subvariant with suction cups closest to the felled spruce. In contrast to many other investigations, clear-cutting did not increase the NO3− problem on the treatment to a relevant extend. Quite contrary, a decline in NO3− concentrations to values below the EU level for drinking water and levels below the control and femel treatment just 2 years after cutting were observed. Al3+ concentrations showed nearly the same trend as NO3−, while Ca2+, Mg2+, and K+ concentrations were affected to a lesser degree. Only in 2002 was Ca2+ significantly lower on the clear-cut as compared to the femel treatment, but not compared to the control. Mg2+ increased in 2000 on the clear-cut subvariants in the interstem area, but decreased in the years 2001 and 2002. Changes could be observed for K+ only periodically on some subvariants, but not for the whole treatment area. Concentrations of SO42−, Na+, and Cl− were reduced after clear-cutting and remained nearly unchanged after femel cutting.