Impact of mechanized logging operations on wet and dry soils of Sao Hill Forest Plantations, Tanzania

Abstract

Mechanization of timber harvesting operations in Tanzania involves use of machinery such as feller bunchers, skidders and tractors which are generally heavy in weight ranging from 12 to 16 tones in unloaded state. The movements of these machines induce soil compaction owing to the exerted normal pressure, vibrations and shear stress. But little literature
has quantified such phenomena in Tanzania. This paper reports results of soil disturbance caused by timber harvesting machinery in Sao Hill Forest Plantation in wet and dry seasons. Soil characteristics were recorded two years after the plots were harvested using both visual classification and soil strength measurements. The results indicate that bulk densities of the upper 20 cm of soil on a plot logged in wet season increased by an  average of 60.5% to 1.61 g/cm3 while for a plot harvested in dry season the increase was 28.7% to 1.28 g/cm3 compared to those of adjacent undisturbed soils. Porosity of the soil reduced by 31.5% and 14.3% for the area harvested in wet and dry seasons respectively. In the top 35 cm of soil depth, the soil penetration resistance increased by 192% and 112% for the area harvested in wet and dry season respectively. The penetration
resistances for both areas exceeded the USDA allowable limits. In addition, the results indicate that logging in wet season can lead to restricted root growth (1.61g/cm3) while logging in dry season may only affect root growth (1.21 g/cm3). Compaction is a concern on Sao Hill forest soils especially  where fully mechanized logging occurs during moist antecedent soil  conditions. Compaction can be minimized by logging during dry soil
conditions.

Key words: Soil penetration resistance; USDA forest soil compaction standard; growth limiting bulky density