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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.

Ten two hectare forest plots were established on two distinct reserves, one mainly on volcanic deposits and the other on sedimentary and alluvial soils. These were investigated for their site carrying capacity and species diversity in relation to edaphic factors, using accumulated basal area as the growth indicator. A soil survey was conducted in each reserve to classify the soil types at the series level. Five two hectare sites each reflecting different soil types were chosen from each reserve. All trees of 10 cm dbh and over were enumerated on each soil type and identified to species level. Fourteen tree species were selected for foliage sampling. A representative soil pit was dug for each soil type and the different horizons were sampled. In addition, ten composite samples were randomly selected to represent soil depths 0-15 cm and 15-30 cm. A complete physical and chemical analysis was carried out on the samples. The results clearly exhibited the influence of parent material on particle size distribution, soil colour, bulk density, porosity, moisture retention and soil chemical compositions. In addition the phosphate fixation problem was discussed. Soil fertility is governed by the parent material from which the soil is derived and is parallelled by the site carrying capacity of the tropical rain forest. Reserve of K was demonstrated to be the main growth limiting nutrient. This was well supported by both soil and foliage analyses data. The nutrients N and P also influence growth but in these cases consistent relationships with basal area were only established after removing some out-lying points. The available and exchangeable soil nutrients were found to be poor indicators of growth. There appears a possibility of using Shorea leprosula as indicator species for foliar analysis in order to assess site fertility. Certain physical properties such as bulk density, clay content, site gradient and available water also exerted some influence on the growth of the trees. Fertile sites contained less species than nutritionally poorer sites. The competition-domination-suppression phenomenon is put forward in explanation. The geological body from which soil is derived is more dominant in determining species diversity than is the soil series per se. On poor sites dipterocarp species are more diverse and they also grow better on lower gradients. Chance factor plays more of an indirect role in species diversity and is believed to be very critical (especially on fertile sites) in the regeneration of dipterocarp species.