Biomass and macro-nutrients (above- and below-ground) in a New Zealand beech ( Nothofagus) forest ecosystem: implications for carbon storage and sustainable forest management

Abstract

Above- and below-ground biomass and nutrient pools were measured in an uneven-aged stand of predominantly hard beech ( Nothofagus truncata) in a warm-temperate climatic zone of New Zealand. The total amount of organic matter in the ecosystem was 680 t ha −1. The total live tree biomass (includes all above- and below-ground tree and shrub components) was 479 t ha −1 with hard beech contributing 89%. Of the hard beech, 32% was harvestable stemwood, 22% was roots and the root/shoot ratio was 0.28. Dead roots amounted to 17 t ha −1. Coarse woody debris (CWD) was the largest detritus component and represented 7% of the ecosystem organic matter. The ecosystem stored 344 t C ha −1, of which 62% was in hard beech and 16% was in the soil organic matter (0–60 cm depth). The amounts of N, P, K, Mg and Ca stored in trees (including shrubs) were 625, 219, 836, 128 and 1507 kg ha −1, respectively. Twenty-seven percent of the N in hard beech was in the leaves and fine roots (0–5 mm diameter). The equivalent percentages for P, K, Mg and Ca were 10, 12, 21 and 9%, respectively. Hard beech root/shoot ratios for N, P, K, Mg and Ca were 0.24, 0.40, 0.34, 0.47 and 0.18, respectively. Much larger quantities of Ca were present in the bark than in the wood, with this ratio as high as 5:1 in the non-harvestable stems and branches. In the detritus the amounts of N, P, K, Mg and Ca were 249, 23, 74, 35 and 238 kg ha −1, respectively. The pattern of ecosystem nutrient totals (K>Ca>Mg>N>P) reflected the relative total amounts in the soil. The amounts of exchangeable K, Mg and Ca were approximately 1% of the respective total soil values whereas soil organic P was 68% of the total soil P. Estimates of the amounts of C and nutrients directly removed in 15 stems ha −1 proposed under a “near-natural forest management” system would represent <3% of the C and nutrients stored in the combined tree, detritus and dead root pools before stem removal. Debarking the stem before removal would substantially reduce the estimated loss of N, P, Mg and Ca from the forest ecosystem.