Phosphorus concentrations in growth rings of trees as indicators of P availability and recycling efficiency in forest ecosystems

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Negative trends in foliar P concentrations at long-term monitoring sites have indicated that P tree nutrition in many European forests has declined over the last 2-3 decades. Possible reasons for this phenomenon comprise soil acidification, nutrient imbalances as a consequence of high N deposition and improved tree growth, soil liming, and increasing frequencies of droughts and mast (high seed production) years. Analysis of element concentrations in growth rings offer the opportunity to detect long-term trends in nutrition of P as well as other nutrients which may be related to soil acidification (Ca, Al, in tree rings), eutrophication (N), and other environmental fluctuations. In addition, tree ring profiles of P in stem cross-sections permit assessment of the recycling efficiency of P within the woody biomass and thus help to address one of the core questions of the DFG Priority Programme Ecosystem Nutrition. In the first phase of the Priority Programme, we have established that the Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry (LA-ICP-MS) approach is suited to measure P and other elements in growth rings. The method needs to be developed further, however, to capture the spatial heterogeneity of nutrients within growth rings. Preliminary results for tree cores from P. abies taken at 1.3 m height show that the P recycling efficiency is not higher in a P-recycling than a P-acquiring system, and that P content in hardwood at the poorest site is unrelated to ongoing soil acidification. Further analyses for more sites and F. sylvatica are currently underway. Here we propose to expand this analysis to include tree rings and bark from upper tree heights and thus stem regions with higher physiological activity. We hypothesize that within tree ring P concentrations in sapwood increase with height whereas those in heartwood remain constant. This will yield results that are more representative of entire tree stems. In addition, this approach allows us to track height-related changes in P concentrations within the same growth ring and thus offers a new method to calculate and model P stocks and uptake into woody tree biomass. In addition, the short-term immobilization potential for P from fertilizer will be assessed for F. sylvatica using the newly established fertilization trial of the Priority Programme at three sites differing in P availability. Here we hypothesize that immobilization of P into sapwood and bark is stronger in P-recycling than in P-acquiring systems. Our analyses are therefore essential for P budgets of the ecosystems analyzed in the priority program and to address one of its main hypotheses that recycling efficiency of P is higher in P-recycling than in P-acquiring systems.