Effects of stand conversion by thinning and underplanting on water and element fluxes of a pine ecosystem ( P. sylvestris L.) on lignite mine spoil

Abstract

Not site conform and overstocked Scots pine monocultures ( Pinus sylvestris L.) on mine spoils in the Lusatian Lignite District (Germany) should be transformed into mixed deciduous forests by thinning and subsequent underplanting. But the effects of stand conversion on ecosystem dynamics continue to be poorly understood. Therefore, in a case study of a 66-year-old Scots pine stand ( Pinus sylvestris L.) on Quaternary sand, the ecosystem water and element fluxes of a conversion and a moderately thinned reference plot were measured. Reduction of stand basal area from 34 to 20 m 2 ha −1 decreased interception of annual bulk precipitation (729 mm) from 38 to 30%. In contrast, evapotranspiration remained almost constant, due to additional transpiration of the underplanted trees and ground vegetation. Thus, deep percolation increased from 121 to 164 mm year −1. Both measurement plots were characterized by considerable Ca and SO 4–S leaching (up to 1.5 and 2.0 kmol ha −1 year −1, respectively). However, stand conversion had no major effect on the element budgets; despite higher soil water fluxes, element output by seepage water was quite unchanged. The stand still acted as almost complete sink for growth-limiting nutrients such as N and P with an annual storage of 1.7–2.1 and 0.023–0.026 kmol ha −1, respectively. Nutrient cycling was not decoupled as the minimal N fluxes in 20 cm soil depth indicate.