Short-term and long-term variation in P-concentrations in growth rings of trees as indicators of P availability in forest ecosystems

P nutrition of trees in many European forest regions has declined over the last 2-3 decades as indicated by negative trends in foliar P concentrations at long-term monitoring sites. Soil acidification, improved tree growth and nutrient imbalances as a consequence of high N deposition as well as liming and climate change have been discussed as possible explanations. In this project we will examine whether P contents in tree rings of Picea abies and Fagus sylvatica, as determined by LA-ICP-MS (Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry), can be used to assess long-term trends and historical variation in P availability. So far there have been very few studies on P in tree rings and it has not been established, whether inter-annual variations of P in tree rings are related to fluctuations in P availability.

First, relationships between P in individual tree rings and P availability will be assessed using two approaches: a) on the basis of P-fertilizer trials with known times and rates of P application, and b) for the long-term monitoring sites of the DFG priority programme, for which long time series of P concentrations in foliage and litterfall are available. Secondly, we will analyze for the heartwood from long-term monitoring sites whether inter-annual variations in tree ring P content are higher in “P recycling” than in “P acquiring forest ecosystems”. This hypothesis is closely related to the main hypothesis of the DFG priority programme stating that forest ecosystems in which P fluxes are dominated by recycling processes are more susceptible to human impact and hence are more strongly affected by fluctuations in environmental conditions. Thirdly, we will assess whether P recycling within stemwood is more efficient in P recycling than in P acquiring systems. Accordingly, retranslocation of P from senescing to juvenile wood should be stronger in trees from P recycling than from P acquiring systems. Fourth, we will investigate whether P acquiring and P recycling systems have different long-term trends in P availability as documented in the heartwood of beech and spruce. For this purpose, not only P but also other elements (N, S, K, Ca, Mg, Al) will be analysed within sections of tree cores comprising 5 consecutive annual growth rings using conventional digestion and ICP-OES. For example, if P availability was reduced as a consequence of soil acidification, a reduction of P in tree rings should be accompanied by declining Ca and Mg contents and an increase in Al.

This study will provide long-term trends in P nutrition of the two dominant tree species at sites of the DFG priority programme as well as data on P stored in woody biomass and P immobilization rates, which will be required for nutrient budgets and biogeochemical models.