Individual study: Open pit-planting enhances growth of Acacia mellifera seedlings, ICRAF Field Station, Kenya
Von Carlowitz P.G. & Wolf G.V. (1991) Open-pit sunken planting: a tree establishment technique for dry environments. Agroforestry Systems, 15, 17-29
In arid and semi-arid areas, water stress is often a major cause of tree seedling mortality and poor growth during establishment. In this East African study, the commonly practiced method of ground-level tree seedling planting was compared with an open-pit sunken planting technique to assess if the latter would enhance tree seedling survival and early growth in an arid environment. The native Acacia mellifera, adapted to the environmental conditions of the trial site, was used in the trials. Although primarily viewing from an agroforestry perspective, this study has relevance for restoration of degraded dry woodland habitats.
Study site: The experiment was conducted at the ICRAF Field Station, located 70 km south-east of Nairobi, Kenya, (1º33'S, 37º14'E).
Experimental design and planting: On 26 April 1984, a few days after the onset of wet season rains, two types of planting holes (each replicated eight times) were prepared:
i) conventional pit (40 x 40 x 40 cm); the soil was removed, seedlings positioned with the root collar at ground level and the original soil mixed with one shovel of manure was used to refill the planting hole, thus establishing the seedlings at ground level.
ii) open pit (60 cm deep, with a top opening of 60 x 60 cm gradually tapering towards the bottom (c.20 x 20 cm at base area after seedling planting) to form a flat-bottomed cone-shaped pit; the top soil was separated from the sub-soil. The seedlings were planted at the bottom of the open pit such that the root collar was about 45 cm below the surround natural ground level; the topsoil was again mixed with a shovel of manure and used to fill around the roots.
Measurements and seedling health observations were made at monthly intervals. Biomass assessment was carried out at the end of the 21 month experiment in January 1986.
All seedlings in the trial survived. After 21 months, height growth of seedlings planted in open pits was almost three times that of seedlings planted at ground level: average height in January 1986 of the ground level-planted seedlings was 42.1 cm, with a cumulative growth increment of 25.5 cm over the 21 months; average height of the pit-panted seedlings was 77.6 cm, with a cumulative mean growth increment of 63.2 cm over the 21 months.
Total biomass production of open pit seedlings (average 179.6 g dry weight) was 78% greater than that of those planted at ground level (average 100.9 g).
Based on observations of wilting and top die-back, 88% of seedlings in the open pits were free of signs of water stress, compared to 25% of those planted at ground level.
Conclusions: Conditions were better throughout the dry season for Acacia mellifera seedlings planted in the pits, probably due to higher water infiltration and retention. Other reasons might include reduced evapotranspiration and radiation levels due to the sheltering and shading effect of the pit, and a fertilizing effect of litter that was trapped within them.
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