Ecophysiology of Faidherbia Albida; Effect of Pruning on tree Water Relations, Cambium Dynamics and Understorey Wheat Productivity in Ejerssa Joro, Ethiopia

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Addis Ababa University


Faidherbia albida (Delile) A. Chev.; is one of the most compatible agroforestry tree species commonly found in agricultural fields in semi-arid region of Ethiopia in association with wheat (Triticum aestivum), teff (Eragrostis tef) and maize (Zea mays). Farmers heavily prune the tree for various purposes. The impact of pruning on tree water relations, cambium growth, and leafing phenology is not well understood. This study was therefore designed to investigate the impact of pruning F. albida on tree water use, cambium dynamics, leafing phenology and understory wheat productivity. A field experiment was conducted at Ejerssa Joro Kebele, Lome woreda Oromia Regional State between April 2015 and October 2017. Three of the trees were pruned mimicking the farmers practice while others three were left unpruned as a control. Sap flow rate and sap volume was determined using heat ratio method (HRM). Radial cambium growth was measured using micro dendrometer, leafing phenology was scored every month. Split plot design was employed with 1m2 areas under both tree managements at 1, 3 and 5m distances and from three directions. Soil moisture from different distances as well as under and outside of the tree canopy was measured. Photosynthetically Active Radiation (PAR) levels, air temperature, relative humidity (RH) were measured. Soil physical and chemical properties were analyzed from different distances of pruned and unpruned tress from inside and outside of the tree crown of pruned and unpruned trees. Wheat growth, leaf gas exchange and chlorophyll content were measured under each tree management. Wheat productivities (aboveground biomass and grain yield) were measured after the wheat was harvested in October 2015 and 2016. Results showed that both diurnal sap flow rate (78.9 ms-1) and sap volume (4590 L month-1) were significantly higher (p<0.05) in unpruned compared with pruned trees. Sap volume from unpruned trees were higher during the dry season as well as when the tree canopy was fully foliated from December to March each year. On the contrary, sap volume was very low when the trees were pruned and during defoliation in unpruned trees, mainly around the onset of the main rainy season. The highest daily sap volume being 110.7 L in unpruned tree compared to 53.5 L in pruned trees during the dry period, as the result pruning F. albida reduced sap volume up to 52%. There was a significant (p<0.05) difference in leaf water potential between midday and predawn in pruned and unpruned trees. However, the difference in midday and predawn leaf water potential in pruned and unpruned were statistically insignificant. A higher cambial growth was observed towards the onset of the dry period in both tree managements. A higher cambial growth was observed in unpruned both in dry and wet season compared with the pruned one. Both temporal and spatial variations of leaflet number per twig were scored. The highest mean leaflet remaining per twig (up to 23) was scored during the dry period (October–January) when the tree crown was fully foliated. However, number of leaflets declined towards March and May each year followed by complete defoliation from May to June each year. The leaf iv number/twig remaining in June and October was significant (p<0.05) in 2015. Higher rate of assimilation (A) of wheat leaves was recorded (4.8μmolm-2s-1) at 1 m distance under unpruned F. albida but declined with increasing distances away from the tree trunk. Rate of assimilation of wheat leaves showed significant (p<0.05) difference between 1 m and 5 m distance of same tree. Total chlorophyll content (based on SPAD readings) of wheat grown under unpruned F. albida was significantly (p<0.05) higher compared to under pruned trees. However, no significant difference was observed in RWC of wheat leaves under both tree managements and along the different distances. Soil organic carbon, nitrogen and available phosphorous under unpruned trees were significantly (p<0.05) higher compared with the soil under pruned tree of similar distances. Wheat grain yield under unpruned F. albida tree was significantly (p<0.05) higher compared with outside of same tree as well as under and outside of pruned tree. The grain yield found at 1, 3, and 5m distances from the trunk of pruned tree was lower by 26, 27 and 29%, respectively, than the grain yield of wheat under unpruned F. albida at similar distances. The highest biomass (3.29 t ha-1) was found under unpruned F. albida and the lowest (2.10 t ha-1) was found outside of pruned tree. Pruning reduced wheat biomass by 30 and the grain yield by 27%; however, the difference in aboveground biomass within unpruned tree under and outside the tree canopy and outside of pruned F. albida was insignificant. The results showed that pruning F. albida canopy reduces the complementarity benefits of wheat productivity and significantly reduces tree water uptake. In general, the crown of unpruned F. albida improved microclimate condition such as reduced PAR level significantly and reduced air temperature up to 6%. A higher soil moisture content as well as majority of soil macronutrients found under unpruned were higher which contributes to enhance understorey wheat productivity. Therefore, farmers should be encouraged to retain the branches of F. albida in order to optimize agricultural productivity and to increase yield where evapotranspiration is the liming factor in semi-arid Ethiopia.



Cambium Dynamics, Ejerassa Joro, Fabel, Faidherbia Albida, Heat Ration Methods, Par, Pruning, Reverse Phenology, Sap Volume, Spad, Water Use Efficiency