Yield forecasting and cutting policy in Pinus Patula stands at the Sao Hill Forest Project (Tanzania)

An sustained yield is emphasized in the Tanzania’s forest policy, it is important to find out how it can be implemented and its effect on industrial development. The objectives of the study were: -To investigate the possibility of forecasting increment in understocked areas using an empirical formula.- As an example of the technique, to work out the long term sustainable yield or balance quantity for part of the Sao Hill Block of exotic pines in southern Tanzania for a period of 30years. The forest inventory covered 1227 ha. Systematic sampling by rows with a sampling intensity of 1% was used in compartments older than 10 years. In compartments youngers than 10 years, only stocking (trees/ha) was estimated. The last 5 year volume increment was estimated from 95 randomly selected plots, with 41 more plots subjectively sited in understocked area to investigate the application of Gehuhardt’s empirical formula. Volume/ tree was estimate by the help of height-diameter curves and Tanzania standard volume table for Pinus Patula. The last 5 year volume increment was determined by a ratio estimate. The compartments were stratified into stands. In fully stocked stands, both the 5 years periodic volume increment and thinning’s were estimated directly from a normal yield table while in under-stocked stands Gehrhardt’s formula was used and the thinning estimate from the yield table was reduced proportionally by a formula. The stand forecast was carried out for 6 five year periods. “Balance quantity” defined as the maximum stable quantity that can be felled in a forest until an increase may take place on a permanent basis” (Nersten, 1969) was worked out following the guidelines listed below:-Thinning being a silvicultural operation received first priority before clear-felling -Non- declining yield in any period -Stand’s condition (age, site class, density) -Rotation age -Age class distribution Four cutting programmes were designed successively. The fourth programme which included 5 years period out- turns of 100,000m3 in period 1,90,000m3 in period 2, 105,00 m3 in period 3 and 120,000m3 for of the remaining periods broke the condition of non declining yield in period 2, but was accepted as a reasonable compromise solution between a balance quantity and improving towards normality. The constant c in Gehrhardt’s formula was determined as 0.81 with confidence limits of 0.776 to 0.844 at the 95% probability level. The sources of error for both Gehrhard’s formula and the balance quantity calculation are discussed. The usefulness of yield forecasts to the forest managers in making decisions and actions are also discussed. The following recommendations are made: -The calculation of the precision of yield forecasts -The application of the balance quantity calculation to other forest projects -The investigation on other cutting policies (e,g cutting at financial maturity) -The inclusion of diameter distribution projection in yield forecasts currently given in volume only -The construction of a yield table for Pines Elliottii Sao Hill