Impact of spacing and rotation length on nutrient budgets of poplar plantations for pulpwood

摘要

The above-ground biomass and nutrient accumulation by poplar plantations were evaluated for pulpwood production in China. Experimental treatments applied in a split-plot design included four planting densities (1111, 833, 625 and 500 stems·hm-2), three rotation lengths (4a, 5a and 6a) and three poplar clones (I-69,NL-80351 and 1-72). The highest biomass was achieved in the highest stocked stand (1111 stem·hm-2) at 6 of rotation age for both clone 1-69 and clone 1-72, which is about two times that in the stands of 500 stems·hm-2 at 4 of rotation age. However, the highest occurred in the stand of 833 stems·hm-2 at 6-year rotation for NL-80351. Ranking of the plantation biomass production by component was stem ＞ branches ＞ foliage ＞ stem-bark and the production of the support components of the plantation was 10-fold that of the productive component, i.e., foliage. The pattern of accumulation of nutrients by the plantations was similar to the biomass. Nutrient accumulation in the plantations was in the order of Ca ＞ N ＞ K ＞ Mg ＞ P, but some differences existed in annual nutrient accumulation rates for four planting densities and three poplar clones. The mean annual accumulation of N and P in the plantations was 13.2 and 2.8 kg·hm-2 in stem, 12.1 and 1.9 kg·hm-2 in branch, and 98.5 and 9.5 kg·hm-2 in foliage. The mean Ca, K and Mg accumulations were 28.2, 18.5 and 2.9 kg·hm-2· a-1, 26.9, 11.0 and 2.3 kg·hm-2·a-1 in branch,and 116.5, 81.3 and 16.1 kg·hm-2· a-1 in foliage, respectively. Biomass utilization standards markedly affected the export of nutrients from the site. Whole tree utilization yields the most biomass and removes the most nutrients.Removal of stem with ≥ 10-cm diameter exports about half of the biomass, but N and nutrients removals are only 23% and 28% of the total, respectively. Removal of the entire stem provides about two-thirds of the total biomass and removes 31.1% total N and 37.5 % total nutrients respectively. Including the branches in the removal increases biomass yield to 92% of the total, and nutrient removal is about 68% of the total.