Temporal Variations in the Carbon Budget of Forest Ecosystems in Spain

Temperate and boreal forests of the Northern Hemisphere have recently been identified as important carbon sinks. Accurate calculation of forest carbon budget and appraisal of the temporal variations of forest net carbon fluxes are important topics to elucidate the ‘‘missing sink’’ question and to follow up the changing carbon dynamics in forests. In this article, recent carbon budgets of the forests of a region in northern Spain have been calculated using data from forest inventories and a carbon cycle model for the human perturbations to the forests. Two methods are used, one that requires data from two forest inventories (growing stocks), and another that uses data from only one inventory (forest growth). The methods are complementary and give a picture of the temporal evolution of the forest net carbon flux. Forests in northern Spain are important carbon sinks, accumulating, on average, 2.91 6 0.47 Tg C/yr (1.46 Mg C·ha21·yr21) from 1972/1973 to 1986/1988 (these figures have been derived using data from two forest inventories). The big errors associated with the net fluxes of carbon make it difficult to draw out firm conclusions about the temporal dynamics of carbon budgets, but it can be concluded that the net carbon flux from the forests in the north of Spain has experienced drastic changes through the period 1968/1973 until 1982/1988: the sink strength has diminished in this period from 8.4 6 4.4 to 4.9 6 3.6 Tg C/yr (each figure derived from data from one inventory), as a result of the increase of emissions due to human perturbations of the forests (harvest and forest fires), which have not been compensated by an equivalent forest growth. Northern forests represent the largest proportion of the forest carbon sink in Spain. This is a consequence of the high productivity and areal coverage of the fast-growing tree species that dominate the landscape of the region. Growing stock (timber volume) in the forests of the region has increased by almost a third in ø15 yr. Temporal evolution of tree densities (stem diameter distributions) and timber volumes during those years indicate maturation of the forests on average. The growth in timber volume is shown to be due mainly to the increase of mean tree heights in the same diameter classes and not to a shift in the stem diameter distribution toward bigger diameter classes. Reasons for this growth of timber volume are discussed, and although climatic and CO2 fertilization effects may play a role, it is concluded that increase in the growing stock is also compatible with ‘‘normal’’ stand evolution.