Physiological responses of tectona grandis (teak) and some food crops in mixed cropping

The taungya system was introduced in the West African sub—region with the aim of reducing pressure on forestlands For agricultural purposes. In Ghana teak (Tectona grandis) was adopted as the main tree species for the taungya system. However, there are speculations in Ghana that teak degrades the environment. The experiments were designed to investigate some problems encountered by teak growers. to determine the effect of the trees at different spacing on the yield of associated food crops and also to assess the influence of sclerotic leaves on the growth of teak. The work was carried out at Nkoaranza, in the Forest-Savannah Transitional zone and on an experimental field of the Forestry Research Institute of Ghana at Fumesua, in the Moist Semi-Deciduous forest Southeast subtype of Ghana. The highly used spacing by the farmers was 3.0 x 3.Om followed by 4.0 x 4.Om and 2.0 x 2.Om with percentages of 70, 26 and 4% respectively. The trees were interplanted with a mixture of food crops like plantain, maize, yam, tomatoes, cassava, and groundnuts. About 95.6% of the teak farms are privately owned, with the remaining 4.4% being owned by the community. The natives of Nkoranza owned all the private teak farms. Majority of the farmers had only basic education. There was decline in crop yield with stand development; and this can be attributed to both early canopy closure and soil fertility. Annual bush fires also contributed to decline in food crops. The application of either inorganic fertiliser or organic manure, as well as the use of mounds can ameliorate the decline in soil fertility. To prolong cropping of the stands for food crops, wider spacing like 6.0 x 2.0m and 4.0 x 4.0m, and introduction of leguminous trees are highly recommended. The trees recorded virtually no growth during the dry season. However, growth rate in the minor rainy season was greater than that in major rainy season. It could therefore be said that the least precipitation triggers growth in teak but too much rain restricts growth. Net assimilation rate of some sampled teak leaves was measured at weekly intervals and analysed in relation to Photosynthetic Active Radiation (PAR). Both Apparent quantum yield (α which is the rate of change in the net assimilation with light at low limiting irradiance) and Photosynthetic capacity (asymthotic value of photosynthetic rate at high saturation irradiance) increased with increasing leaf age up to a peak and thereafter declined to a minimum value and remained constant. Net assimilation rate was negative or very low in newly emerged leaves, peaked in fully expanded leaves and then declined to negative values, as the leaf became increasingly sclerotic. This is an indication that respiration exceeds net assimilation at this stage and the leaf therefore depends on the mother plant for survival. However, detachment of sclerotic leaves with the aim of enhancing growth was rather found to be detrimental to both tree height and especially stem volume growth, by 0.76 cm3 per tree. Nutrient analysis of young, sclerotic and chlorotic leaves indicated that potassium, nitrogen and sodium concentrations decreased with increasing leaf age. This suggests that these nutrients are withdrawn into the mother plant before abscission. On the other hand, while calcium and magnesium concentration increased with leaf age, phosphorus, showed no specific trend. Re-translocation of essential nutrients into the mother plant is more important to plant growth than the respiratory cost of maintaining the integrity of the sclerotic leaves.

Source: KNUST