Productivity and nutrient cycling in recently harvested and mature bamboo savannas in the dry tropics.
Data are presented for net productivity and nutrient (N, P, Ca, K and Na) dynamics in mature (5-year-old, harvested in 1982) and more recently harvested (in 1986) dry tropical bamboo (Dendrocalamus strictus) savannas, on the Vindhyan plateau, East Mirzapur Forest Division, Uttar Pradesh, N. India. The total biomass was 34 900 kg/ha at the harvested site and 47 260 kg/ha at the mature site. Harvesting increased the relative contribution of below-ground biomass. The mean annual total net productivity (TNP) for the two annual cycles was 15 830 kg/ha at the harvested and 19 320 kg/ha at the mature site. Nearly half (46-57%) of the TNP was in the below-ground parts. Short-lived components (leaves and fine roots) contributed about 80% of net bamboo production. At the harvested site, 79% of bamboo net production was allocated below ground (cf. 49% at the mature site). Soil, litter and vegetation, respectively, accounted for 54-98%, 0.1-3% and 1-43% of the total nutrients in the system. Greater retranslocation from senescing leaves (54-69% N, P and K) occurred in bamboo and other woody species than in herbs (25-44%). At the harvested site, 59-71% of the gross uptake of nutrients in bamboo was allocated below ground compared with 20-41% at the mature site. After adjustment for retranslocation, the net uptake exceeded the amount of nutrients returned through litter fall and root mortality, indicating a tendency for nutrient aggregation (13-29% of net annual uptake) in the vegetation. Annual turnover rate of nutrients on the savanna floor ranged from 64 to 90%. Estimates showed that each bamboo harvest represents above-ground biomass nutrient loss of about 198 kg N, 11 kg P, 160 kg Ca, 157 kg K and 9 kg Na/ha. It is concluded that in the dry tropics the bamboo savanna vegetation, growing in oligotrophic conditions, makes efficient use of N and P through internal cycling, and conserves these nutrients by accumulation in below-ground parts and immobilization in the decomposing leaf mass.