Productivity and Nutrient Cycling in Recently Harvested and Mature Bamboo Savannas in the Dry Tropics

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Abstract
1. 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 savannas, on the Vindhyan plateau in India. 2. The total biomass was 34 900 kg ha-1 at the harvested site and 47 260 kg ha-1 at the mature site. Harvesting increased the relative contribution of belowground biomass. 3. The mean total net productivity (TNP) for the two annual cycles was 15 830 kg ha-1 year-1 at harvested and 19 320 kg ha-1 year-1 at mature site. Nearly half (46-57% of the TNP was to be found in the belowground parts. Short-lived components (leaves and fine roots) contributed about 80% of net production of bamboo. At the harvested site, 79% of bamboo net production was allocated belowground (cf. 49% at the mature site). 4. Soil, litter and vegetation, respectively, accounted for 54-98%, 0.1-3% and 1-43% of the total nutrients in the system. 5. 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 belowground compared to 20-41% at the mature site. 6. 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. 7. Annual turnover rate of nutrients on the savanna floor ranged from 64 to 90%. 8. Each bamboo harvest represents aboveground biomass nutrient loss of 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, growing in oligotrophic conditions, makes efficient use of N and P through internal cycling, and conserves these nutrients by accumulation in belowground parts and immobilization in the decomposing leaf mass.

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