Understorey vegetation shifts following the conversion of temperate deciduous forest to spruce plantation

The identity and composition of the overstorey trees can have large impacts on the understorey vegetation in temperate forests. Here, we assess the effects of the conversion of mixed deciduous forests to Norway spruce (Picea abies) plantations on the understorey vegetation composition and diversity. We also investigated whether the effects of this conversion depend on the soil fertility and if they affect the species pool at the landscape scale. The study area contained ancient deciduous woodlands mainly consisting of Quercus spp., Fagus sylvatica and Carpinus betulus in the Gaume, an area with a mild temperate climate in southern Belgium. Spruce plantations were established 30-50 years ago in this matrix of deciduous woodlands. Differences in overstorey and understorey vegetation, forest floor and soil characteristics between 40 paired plots in spruce plantations and adjacent deciduous forest were compared along a soil fertility gradient. The forest floor mass was higher and the soil pH was lower in spruce plantations. The composition of the understorey vegetation differed between deciduous forest and spruce plantations: the spruce stands contained more light-demanding and acid-tolerant understorey species than the adjacent deciduous stands. The mean understorey species diversity, i.e., the local species diversity in a plot (a-diversity) as well as the total number of species (gamma-diversity), did not differ significantly between the deciduous and spruce stands. Hence, the differing species composition of the spruce plantations contributes to a larger overall species pool and higher across site beta-diversity. Although the differences in the environmental variables within a plot pair increased along the fertility gradient, the differences in understorey vegetation diversity between deciduous and spruce stands showed no relationship with the gradient. Only the difference in Ellenberg indicator value for soil reaction (R) within a pair increased with increasing soil fertility. In line with the abiotic changes, the understorey vegetation composition had shifted towards a more acid-tolerant vegetation type 30-50 years after conversion to spruce plantations. The increase in light-demanding species was probably due to a more intensive thinning management in the spruce stands. (C) 2012 Elsevier B.V. All rights reserved.