Nutrient cycling in a natural beech forest and adjacent planted pine in northern Spain

Abstract

Above-ground leaf biomass, leaf litterfall, leaf weight loss due to decomposition, redistribution of leaf litter and C and N return to the soil, and seasonal leaf nutrient dynamics have been studied in the Sierra de la Demanda, Spain, a Mediterranean climatic zone. The tree species considered were a climax beechwood (Fagus sylvatica L.) at 'Tres aguas' and planted Scots pine (Pinus sylvestris L.) at 'La Rasada'. The above-ground biomass was estimated by cutting and weighing seven trees from each site according to their diameter classes, recording the categories of trunk, branches and leaves. The carbon and nitrogen contents in the different fractions were also analysed. The calculated total biomass ranged from 132.7 Mg ha-1 in the beech stand to 152.1 Mg ha-1 in the pine stand, and leaf biomass from 3.4 Mg ha-1 to 7.0 Mg ha-1 in the beech and pine, respectively. The C: N (carbon : nitrogen) ratio was greater in the pine forest leaves. The total litterfall was 5791 kg ha-1 per year in the pine forest and 4682 kg ha-1 per year and the leaf litterfall was 2917 kg ha-1 per year and 2897 kg ha-1 per year in the pine and beech sites, respectively. The annual mean amount of N returning to the soil substrate was 29.9 kg ha-1 per year on the beech and 23.3 kg ha-1 per year on the pine. Weight loss of leaf litter due to decomposition was monitored for 2 years in the two forest ecosystems studied. The results indicated that weight loss was similar at the end of the experimental period in the two forest stands: 40 per cent in beech and 43 per cent in pine. Jenny's decomposition index (K) and Olson's decomposition index (Ko) were higher for the pine stand than for the beech stand. The dynamics of total C and N throughout the study period was similar on both experimental plots. The evolution of N in green leaves follows the same pattern in beech and pine stands. The highest leaf N concentration was observed in the spring and summer months, during initial leaf growth. Thereafter, it decreased due to retranslocation during the period of leaf-fall in autumn.