Beyond vegetative propagation of indigenous fruit trees: case of Dacryodes edulis (G. Don) H. J. Lam and Allanblackia floribunda Oliv.

The importance of domesticating and integrating high value fruit trees in agricultural landscapes by farmers is increasingly being recognized in the humid tropics. However in an agroforestry context, the growth attributes of intercropped trees above and belowground determine whether the association will be complementary or competitive. The focus of the present research was on two African indigenous fruit tree species (Dacryodes edulis G. Don) H. J. Lam (Burseraceae) and Allanblackia floribunda Oliv. (Clusiaceae), which are currently under domestication, and have high food, income and environmental security values. On D. edulis, the thrust was to assess and compare the structural and fine rooting systems together with the aboveground growth attributes of fruiting trees propagated sexually and vegetatively, while on A. floribunda, the focus was on how to reduce the long juvenile phase of about 10-12 years before first fruiting to less than 5 years through grafting techniques. The results of the investigations on D. edulis indicate that trees of seed origin were characterized by a tap root, reaching depths of about 1.20 m, whereas, trees of cutting origin showed three strong vertical roots (d > 5 mm) with the longest reaching depths of 1.31 m. Similarly, trees of marcot origin were observed to have thick, relatively short prominent vertical roots, reaching depths of 1.15 m. In addition, trees of seed origin had greater fine root density (RD) (P ≤ 0.001) than trees of vegetative origin (cuttings and marcots) in the upper soil stratum (0-30 cm). Trees of seed origin were also shown to have an exponential distribution pattern for fine root density and root length within depth to 80 cm. In contrast, the distribution pattern of fine roots of trees of vegetative origin (cuttings and marcots) were quadratic for the same variables which increased in the 20-30 cm soil depth stratum before declining steadily to a depth of 80 cm. Furthermore, shoot density, defined as, number of shoots per tree, and height differed significantly (p=0.004 and p=0.005, respectively) amongst tree origins. Trees of seed and cutting origins had single-stem shoots whereas marcots had 6 shoots per tree on average. Trees of cutting origin grew tallest, with mean height of 8.4 ± 2.2 m compared to 6.7 ± 0.9 m and 7.6 ± 1.9 m for trees of marcot or seed origins respectively. Mean carbon (C) sequestration estimated using allometric models differed significantly (p= 0.014) between trees of vegetative origin and those of seed origin, with 10 years old D. edulis trees of cutting and marcot origins sequestrating averagely 26.8 ± 19.1 Mg C ha-1 and 21.74 ± 12.8 Mg C ha-1 respectively over 10 years, compared to 13.10 ± 9.4 Mg C ha-1 for trees of seed origin. In summary, the results from the studies on D. edulis suggest that vegetative propagation on the species, besides reducing the long juvenile phase to fruiting and maintaining trueness in the transfer of desirable traits over generations, also results in trees that are apparently less competitive for belowground resources. In addition, vegetative propagated trees of D. edulis were shown to have apparently stable root system and the trees bigger in stature and higher in carbon sinks than trees of seed origin. In parallel, A. floribunda scions were taken from female trees, and grafted onto rootstocks using side tongue, top cleft, side veneer, and whip-and-tongue methods under nursery conditions. In a separate experiment, side tongue and inverted ‘T’ budding methods were also tested in situ on young A. floribunda wildings growing under semi-deciduous and evergreen tree covers. In addition, the effects of protecting side tongue grafts with non-perforated, translucent plastic, perforated translucent plastic and aluminium foil were assessed. A. floribunda was shown to be amenable to grafting both under nursery and field (in situ) conditions. Under nursery conditions, side tongue grafts were significantly more successful (80.0 ± 6.3 %), than grafts of side veneer (52.5 ± 7.9 %), top cleft (55.0 ± 7.9 %) and whip–and-tongue (50.0 ± 7.9 %). The success of side tongue grafts was further increased (86.7 ± 6.2 %) under evergreen shade when grafts were protected by non-perforated translucent plastic. A grafted A. floribunda tree transplanted in the field in 2007 flowered and carried a single fruit to maturity after 4 years, thereby reducing the long juvenile period to first fruit production of about 10-12 years reported in literature to less than 5 years. In conclusion, the effect of propagation methods on the belowground and aboveground growth attributes and their C storage potential studied in the present research on D. edulis, have not yet been studied and published anywhere according to the authors knowledge. Similarly, the reduced long juvenile period to first fruit production to less than 5 years in A. floribunda grafts reported in this research is the first published prove of concept on this species according to the author’s knowledge. Therefore, the research results presented in this thesis should be considered as pilot case studies aimed at obtaining insights into the effect of vegetative propagation methods on the above and belowground growth and development of improved planting materials of these two African indigenous fruit trees (Dacryodes edulis and Allanblackia floribunda) under domestication.