Incident Light and Leaf Age Influence Leaflet Element Concentrations of Cycas micronesica Trees

The need for improved knowledge on conservation and management of cycad species has generated recent interest in compiling a database on leaf nutrient concentrations. However, the sampling protocols have not been consistent among reports and the influences of some plant and habitat traits on the plasticity of cycad leaf nutrient concentrations has not been adequately determined. We used Cycas micronesica K.D. Hill trees to determine the role of incident light level and leaf age on leaflet content of 11 essential elements. Shade leaves exhibited increased mass-based concentration for nitrogen, phosphorus, and potassium above that of sun leaves. Shade leaves exhibited decreased area-based concentration for all of the macro- and micronutrients below that of sun leaves. Mass-based concentration of nitrogen, phosphorus, and potassium decreased with leaf age, and that of calcium, magnesium, iron, manganese, and zinc increased with leaf age. These findings indicate the relative leaf age and the amount of shade or incident light at the leaf level must be recorded and reported for leaf tissue studies in cycads in order to reduce ambiguity and ensure repeatability.