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The growth of Sitka spruce was examined on a range of soil types: a flushed peaty gley, a podzolised brown earth, a surface water gley, a stagnant waterlogged peaty gley and a non-flushed peaty gley. There was a large difference in productivity between sites, local yield class (LYC) decreasing in the order; 26, 22, 20, 18 and 14 m3 ha-1 yr-1 respectively. The soil water regime was characterised at each site and measurements were made of tree roots, soil oxygen status and site nutrition. Conditions remained well oxygenated in the top 30 cm (Ah horizon) at the flushed peaty gley, despite saturation below 20 to 25 cm during autumn and winter. The lateral flow of oxygenated water through this horizon prevented the development of anaerobiosis and enabled roots to survive down to 40 cm. The freely draining podzolised brown earth dried out very quickly, tensiometers remaining failed for two months. Rooting occurred down to induration at 55 cm. The surface water gley had a freely draining topsoil which remained well aerated throughout the year. Rooting was extensive in the Ap horizon (3 cm to 28 cm depth) and occurred down to 80 cm. There was a constantly high, de-oxygenated water-table in the waterlogged peaty gley, which only dropped below 30 cm depth for 46 days during the summer. Rooting was restricted to a depth of 12 cm. The peaty gley had a de-oxygenated water-table which remained around 15 cm depth during autumn and winter. Roots that grew down the profile during the summer as the soil dried out to around -1 bar, were killed by subsequent waterlogging. The differences in LYC between sites are explained in terms of soil water regime. A significant reduction in yield occurred on all but the flushed peaty gley site as a result of an initial period of growth check. Drought stress had little effect on site productivity at the podzolised brown earth. Low yields on the peaty gley are attributed to the diversion of photosynthate into the replacement of roots killed by soil anaerobiosis.

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.