PRODUCTIVITY AND NUTRIENT CYCLING IN RECENTLY HARVESTED AND MATURE BAMBOO SAVANNAS IN THE DRY TROPICS

1. Data are presented for net productivity and nutrient (N, P, Ca, K a nd Na) dynamics in mature (5-year-old, harvested in 1982) and more rec ently harvested (in 1986) dry tropical bamboo savannas, on the Vindhya n plateau in India. 2. The total biomass was 34 900 kg ha-1 at the har vested site and 47 260 kg ha-1 at the mature site. Harvesting increase d the relative contribution of belowground biomass. 3. The mean total net productivity (TNP) for the two annual cycles was 15 830 kg ha-1 ye ar-1 at harvested and 19 320 kg ha-1 year-1 at mature site. Nearly hal f (46-57% of the TNP was to be found in the belowground parts. Short-l ived components (leaves and fine roots) contributed about 80% of net p roduction of bamboo. At the harvested site, 79% of bamboo net producti on was allocated belowground (cf. 49% at the mature site). 4. Soil, li tter and vegetation, respectively, accounted for 54-98%, 0.1-3% and 1- 43% of the total nutrients in the system. 5. Greater retranslocation f rom 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 belowground com pared to 20-41% at the mature site. 6. After adjustment for retransloc ation, the net uptake exceeded the amount of nutrients returned throug h litter fall and root mortality, indicating a tendency for nutrient a ggregation (13-29% of net annual uptake) in the vegetation. 7. Annual turnover rate of nutrients on the savanna floor ranged from 64 to 90%. 8. Each bamboo harvest represents aboveground biomass nutrient loss o f about 198 kg N, 11 kg P, 160 kg Ca, 157 kg K and 9 kg Na ha-1. 9. It is concluded that in the dry tropics the bamboo savanna vegetation, g rowing in oligotrophic conditions, makes efficient use of N and P thro ugh internal cycling, and conserves these nutrients by accumulation in belowground parts and immobilization in the decomposing leaf mass.