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

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.