A growth and yield model for Cupressus lusitanica in Munessa Shashemene State Forest Project (MSSFP), southwest Ethiopia
Mathematical growth and yield models were developed for the major plantation timber species, Cupressus lusitanica, growing in Munessa Shashemene State Forest Project (MSSFP), in Southwest Ethiopia, based on data from 106 temporary sample plots. Stand dominant height development was predicted by Schumacher's growth equation as a function of age and site index, defined as dominant height at reference age of 15 years. Initial diameter distributions of trees were generated using the Weibull probability density function with scale and shape parameters predicted as a function of stand parameters. Stand basal area for unthinned stands was predicted as a function of dominant height, stand age and number of stems using a modified Weibull growth equation. The basal area after thinning was simulated using a single tree diameter increment equation. The mid-diameters of each diameter class were incremented and added to the initial diameter values of each class for subsequent ages which when multiplied by class frequencies and summed gave the stand basal area. Thinnings were simulated using a power function. A compatible tree volume/taper estimation system which determined tree volume and taper was developed. The growth functions were integrated in a logical sequence, using the readily available and user friendly simple spreadsheet package, EXCEL, to produce a whole-stand distance simulation model (SIMCUP). This model also predicted final maincrop yield by size classes. Validation of the volume and diameter distribution models showed that the model (SIMUL) worked satisfactorily. Using SIMUL the effects of different levels of spacing, thinning frequency and thinning intensities on cumulative volume and sawlog volume were investigated. Experimental runs with the most frequently used thinning intensity (37.5% by stem number) at MSSFP showed that the current thinning intensity was too heavy and resulted not only in loss of cumulative volume but also merchantable volume. The cumulative volume of once, twice and three times thinned stands at the age of 25 years could be decreased by between 12.3%, 19.7% and 23% of the unthinned stand respectively. The highest merchantable sawlog volume was with no or one thinning at the age of 7 years with a rotation age of 25 years. In contrast by adopting a 25% thinning intensity the merchantable volume could be increased by between 20.4% to 10.7% relative to current yields for site index classes 27 to 12 and a stand with 1588 stems per hectare respectively. The highest merchantable sawlog volume per hectare to 15 cm top diameter was obtained at the spacing of 2.1 x 2. lm at the rotation age of 25 years The model is intended to promote silvicultural management of C. lusitanica stands in MSSFP, Southwest Ethiopia. Using SIMCUP, different levels of spacing, time of first thinning and alternative thinning intensities have been investigated and appropriate silvicultural measures suggested to achieve higher merchantable volume per hectare.