The Phosphorus Economy of the Evergreen Tundra Plant, Lycopodium Annotinum

Abstract

The phosphorus economy of the sporophyte of Lycopodium annotinum L., a microphyllous Pteridophyte, was investigated at a site in the sub-arctic birch forest of Swedish Lapland. L. annotinum has a distinct structural pattern consisting of annually produced modules which can be aged up to 20 or more years. The dry weight, phosphorus concentration and content of modules of all parts of the plant were investigated. Rates of phosphate uptake in the field and in the laboratory, and the activity of the enzyme acid phosphatase were measured in relation to module age and type. The root:shoot biomass ratio is low (1:18.2) and rates of phosphate uptake were found to be low both in the field (53 nmol g-1 dry wt h-1) and the laboratory (5-40 nmol g-' dry wt h-'). Significant root growth and phosphate uptake are extended over 4-5 yr. Tissue phosphorus concentrations are low with an overall efficiency of phosphorus utilisation of 1.25 mg P g-1 dry wt. The recycling of phosphorus, estimated from loss of phosphorus from modules during their life-span, is high with 75% of the phosphorus recycled. The phosphorus requirement, estimated from module weights and tissue concentration, is approximately 670 lcg individual plant-' yr-'. This will make a demand of 0.69 mg P g-' root yr' after corrections for recycled phosphorus are made. The observed rates of phosphate uptake in the field are adequate to supply such an estimated demand within the short growing season. L. annotinum appears to be adapted to a low phosphorus supply by having an extended period of root growth and phosphate uptake, efficient utilisation of phosphorus in the tissues, efficient recycling of phosphorus from old tissues and a low demand for phosphorus.