Organic matter and nitrogen dynamics in a mature forest of common beech in the Sierra de la Demanda, Spain

Abstract

Aboveground biomass, litterfall, leaf weight loss due to decomposition, N return and seasonal leaf N dynamics were studied in the Sierra de la Demanda, Spain, a Mediterranean climatic zone. The forest ecosystem considered was a climax beech (Fagus sylvatica L.) forest. Aboveground biomass was estimated by cutting and weighing seven trees from a beech stand according to diameter classes, recording the categories of trunk, branches and leaves. The results indicate a total biomass of 132.7 Mg ha-1. The litterfall was 4682 kg ha-1 yr-1, although variations from year to year were observed, mostly due to water stress in summer. Greater K (Jenny's constant) and Ko (Olson's constant) values were obtained for total litter than for leaves alone. It is possible that the mean soil humidity might not be a limiting factor in the decomposition process and that this effect would be due to the distribution of rainfall rather than to the total amount of precipitation together with elevated temperature and airing of the holorganic soil horizon. The leaf N contents of beeches growing in the Sierra de la Demanda were studied, relating the contents to other structural stand characteristics. For this, N was analysed in shed leaves, in leaf biomass and in leaves decomposing on a test plot in the "Tres Aguas" beech forest. Leaf nitrogen contents were measured over a vegetative cycle in the above beech forest. Finally, leaf nitrogen contents were studied in thirty beech stands in the Sierra de la Demanda located at different altitudes. Annual nitrogen accumulation in leaf biomass was 79.4 kg ha-1 yr-1, of which 22.9 kg ha-1 yr-1 were returned to the soil substrate through shedding and 2.1 kg ha-1 yr-1 were actually incorporated into the soil. Nitrogen cannot be a limiting factor for the development of the beech stands studied because all of them surpassed the leaf deficiency threshold. Only other factors such as soil texture and structure or silvicultural treatment have a decisive effect on the production of the stand studied. The correlations for leaf nitrogen contents and the structural stand characteristics explored revealed that leaf nitrogen was only slightly correlated with the mean height of the trees at the plot.