Xylem sap composition: A tool for investigating mineral uptake and cycling in adult spruce

Xylem sap composition of spruce is influenced by several factors, such as the sampled organ, the sampling period, the availability of soil nutrients, and the soil water potential. Based on literature data and ongoing investigations carried out with adult trees, we present an overview on the main factors influencing xylem sap concentrations of Norway spruce. Direct measurements of nutrient fluxes in the xylem sap are then used to suggest a general scheme of mineral element cycling within adult trees. In Norway spruce (Picea abies Karst.), nutrient concentration in the xylem sap was higher in twigs and fine roots compared to the bottom of the trunk, the highest concentrations beeing observed in spring during the shoot elongation. Xylem sap concentrations were higher in spruce growing at nutrient rich sites than at poor sites. The combination of twig and trunk xylem sap analysis, together with xylem flow measurements in the trunk during the course of a vegetation period allowed the quantification of mineral fluxes via xylem sap flow in the trunk and twigs. These results were compared to gross mineral uptake measurements at the same site. Ca flux in the trunk xylem sap was lower than the gross uptake of Ca. Mg flux in trunk sap was approximately equivalent to Mg gross uptake whereas P and K fluxes in trunk sap were much higher than the gross uptake. Fluxes of Ca, Mg, K and P in the twig sap were much higher than that in trunk sap. Data suggest that internal cycling is responsible for a large part of the nutrient fluxes in the xylem sap of the crown. Xylem sap composition thus appears to be a tool which can complement other sources of information on mineral uptake and cycling in adult spruce