Understory evolution related to organic fertilization in a silvopastoral system established under Pinus radiata D. Don. in an acidic soil

Mixed pasture establishment is considered a key point to increase livestock production due to the higher quality it has compared with unmanaged shrubby species. They provide also a set of ecosystem services such as carbon sequestration and higher biodiversity. Moreover, herbaceous pasture establishment effectively contribute to biomass production while replacing shrubs therefore reducing forest fire risk. One of the main factors promoting the establishment of the herbaceous pasture are sowing and fertilisation. This study evaluates the understory composition evolution of a silvopastoral system established with cocksfoot and clover in an acidic forest soil under Pinus radiata D. Don with four treatments that consist of no fertilization, fertilization with mineral fertilizers (500 kg of mineral 8:24:16) and fertilization with three doses of sewage sludge (160, 320 and 480 kg N ha-1), in Galicia (NW Spain). Botanical composition data were analysed through species abundance diagrams in a time window of 20 years. Results shown clear evolution in understory composition from the initial situation. Organic fertilization was found to ease sown-mixture persistence on time as well as avoiding shrub colonisation with regard to mineral and no fertilization treatments. Nutritional and sun light high-demanding species were mostly replaced in favour of autochthonous ones after sewage sludge fertilization stopped and tree canopy intercepted light that reached the understory.