Mountain grassland restoration: Effects of sowing rate, climate and soil on plant density and cover.

Semi-natural grasslands are of great importance to European biodiversity. Their restoration in mountain regions often presents specific problems of raw soil stabilisation and difficult climatic conditions. Methods used to tackle these problems include increasing sowing density and fertilisation. However, past studies were performed at only one site or for a short time and do not clearly show how site traits and time affect restoration results. In six field experiments undertaken on raw soils in the Italian Alps, the effects of sowing density, fertilisation, soil, and climate on the development over time of plant density and cover were analysed over three to six years. The seed was collected from nearby grasslands and sown at rates of 335-21,410 seeds m. During the first one-two years, the percentage of seeds producing established plants at low seed rates was higher in stonier surfaces; however, at high rates a larger absolute plant number was recorded in finer soils. Two to five years after sowing, climatic conditions became more important. Full vegetation cover was achieved in all experiments. However, at high altitudes, individual plant cover became greater (and plant density decreased), probably because the grasses needed to accumulate more non-structural carbohydrates necessary to survive during the longer winter. Fertilisation accelerated the establishment of full vegetation cover by 1 to 3 years as it increased the mean cover per plant without affecting plant density. Grassland restoration is possible in mountain raw soils by using the seeds collected from nearby donor sites. High sowing densities are not required even in coarse or higher altitude soils because, in the short term, stones on the ground surface increase the percentage of seed producing established plants and, over the long term, plant density is not dependent on the sowing density. In low-fertile soils, fertilisation accelerates the establishment of full vegetation cover.