In-situ grafting success and carbon sequestration potential of Vitellaria paradoxa in Northern Uganda

Abstract

A study to determine appropriate grafting technique and carbon sequestration potential of V. paradoxa was conducted in Uganda between 2013 and 2015. The specific objectives of the study were to: assess the effectiveness of splice and top wedge grafting techniques; establish the most appropriate rootstock height of wild juveniles for graft success on in-situ shea butter trees; and evaluate the amount of carbon sequestered by V. paradoxa trees on-farm in northern Uganda. Two grafting methods (splice and top wedge) and two varying heights (30-50cm and 51-100cm) were explored on in-situ juvenile rootstocks and monitored for 16 weeks. Data were collected on grafting success, rootstock height and the amount of carbon that can be sequestered by V. paradoxa. A non-destructive method of tree biomass estimation was used to assess the above ground biomass of shea trees. The amount of Carbon was then calculated based on generalized allometric equations stratified by diameter class. A regression model was used to determine below ground biomass from the knowledge of above ground biomass to assess the amount of carbon sequestered by V. paradoxa trees. Means and percentages were calculated, and one-way ANOVA carried out to test whether the grafting methods, different heights of in-situ juvenile rootstocks have significant effect on the graft success. The results showed that 43% of the grafts had slight union, 3% had full union with splice grafting technique up to 5 weeks. Up to 27% of the grafts had slight union and none of the grafts with top wedge grafting technique had full union even up to 5 weeks. Only 15% of grafts had slight union and 5% had full union with splice grafting technique up to 3 weeks. Fifteen percent of the grafts had slight union, and none had full union up to 3 weeks with top wedge grafting technique. There was significant difference between the grafting methods (F= 125.0, P= 0.008) and no significant difference between the rootstock heights (F= 0.03, P= 0.881). Lower rootstocks height (30-50 cm) had 25% splice graft success followed by 21.7% splice graft success with higher rootstock height (51-100cm) 5 months after grafting. A 3.3% top wedge graft success was registered with 30-50cm rootstock height followed by 1.7% top wedge graft success with rootstock of height 51-100cm 5 months after grafting. The amount of carbon sequestered by V. paradoxa by diameter classes of DBH < 20 cm, DBH>= 20cm. AND. < 60 cm and DBH>60cm were 4.92, 124.62 and 48.88 tCO2e/ha respectively; totalling to 178.42 tCO2e/ha. V. paradoxa with DBH < 20 cm sequestered as low as 4.92 tCO2e/ha, meanwhile V. paradoxa with DBH>60cm sequestered 48.88 tCO2e/ha higher than those with DBH < 20. V. paradoxa with DBH>= 20cm. AND. < 60 cm sequestered the highest amount of carbon (124.62 tCO2e/ha). On average V. paradoxa with DBH>60cm sequestered the highest average amount of (8.15 tCO2e/ha) compared to those individuals with DBH>= 20cm. AND. < 60 cm (1.81 tCO2e/ha). V. paradoxa with DBH < 20 cm sequestered the lowest average amount of 0.12 tCO2e/ha. In general, splice grafting technique had higher percentages of successful slight union and full union compared to top wedge grafting technique in both experiments I and II after five and three weeks respectively. Splice grafting technique was found to be better in ensuring survival and graft success compared to top wedge when grafting V. paradoxa using scions harvested with leaves to maintain its physiological activity. Variation in rootstock height did not have significant effect on graft success, suggesting that both heights used in this study (30-50cm and 51-100cm) can be used for field grafting of V. paradoxa since the age of field rootstocks is difficult to establish for wildings. On average V. paradoxa with DBH>60cm sequestered the highest average amount (8.15 tCO2e/ha) of carbon followed by V. paradoxa with DBH>= 20cm .and. < 60 cm (1.81 tCO2e/ha) of carbon and V. paradoxa with DBH < 20 cm sequestered the lowest average amount (0.12 tCO2e/ha) of carbon. Since V. paradoxa had the highest number of individuals and thus the most dominant indigenous tree species in the study site, it has the potential to store and sequester more carbon. Farmers should be trained on how to carryout splice grafting technique since it was found to be more effective in ensuring better graft success and higher full union in V. paradoxa. Further field study should be conducted to investigate the optimum grafting success for V. paradoxa under varying shade intensities and varying seasons of the year. While both rootstocks of 30-50cm and 51-100cm in height can be used for grafting of V. paradoxa under field conditions, rootstocks of higher heights could be tried as a way of minimizing possible attack of rot diseases at union point under field condition. A study involving long-term monitoring is recommended to ascertain the time taken for the successful grafts to fruit and resulting fruit characteristics compared to the parent tree. This is important for extended breeding work and establishment of field gene banks considering that V. paradoxa seeds are recalcitrant