The effect of N and P fertilizer application and botanical composition on the leaf/stem ratio patterns in spring in Pyrenean meadows

Abstract

The effect of botanical composition and nutrient availability on the relative allocation of biomass to stems and leaves in a permanent upland pasture in the central Pyrenees was assessed. Six short‐term and medium‐term fertilizer treatments (nitrogen and phosphorus) were applied to a meadow to create large differences in the proportions of the different species and a wide range of herbage nutrient status. The above‐ground herbage dry‐matter components (green leaves, sheaths and stems for grasses and dicotyledons, and senescent material) were measured for the first growth cycle.The leaf mass depended mainly on the herbage N status, whereas the stem mass depended both on the botanical composition and on the herbage N and P status. During spring growth, the proportion of leaves in the above‐ground dry matter decreased faster in plots that had the highest nutrient status or that were composed of species characteristic of nutrient‐rich (Festuca‐poor) habitats. Application of fertilizer decreased the proportion of leaves both for grasses and dicotyledons, but to a larger extent for grasses. There was a single relationship between the proportion of leaves in the above‐ground dry matter and the total mass of above‐ground dry matter, irrespective of the sampling date, the botanical composition or the herbage nutrient status. This statistical relationship resulted from (i) a faster increase in stem mass than leaf mass for a given botanical composition when the herbage nutritional status increased, (ii) a greater stem mass when the sward was composed of species usually found in nutrient‐rich habitats.