Using tree rings to reconstruct changes in soil P availability – Results from forest fertilization trials

Hitherto, there are only few studies that have analysed the variation of P contents in individual tree rings to reconstruct fluctuations in soil P availability. Therefore, this pilot study aimed to assess the relationship between changes in P content in tree rings and known changes in soil P availability resulting from fertilization of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) in fertilization trials at two different sites. We compared P contents in single tree rings from fertilized and unfertilized plots formed before and after P fertilization and assessed (1) whether fertilization leads to an immediate increase in P uptake and higher P contents in tree rings formed after fertilization, and (2) whether P is translocated to older tree rings that were formed before fertilization. After application of 70 kg P ha ⁻¹ , a prompt and extended increase in relative wood P contents could be observed in both Norway spruce and Scots pine. However, only at the Norway Spruce site, this increase could be properly assigned to a P fertilization signal in heartwood rings formed after fertilization. In sapwood rings, however, P fertilization signals were masked by the inherent increase in P content from older towards younger sapwood rings, which was at least one order of magnitude higher than the increase from fertilization. We could not observe a P translocation into older tree rings, which existed as sapwood rings at the time of fertilization. This pilot study underlines the potential of dendrochemistry for reconstructing changes in soil P availability and improves the conceptual basis for further dendrochemical research, not only in fertilized but also in unfertilized forest ecosystems.