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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Date
2020-04-04
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Addis Ababa University
Abstract
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
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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
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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.
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Keywords
Cambium Dynamics, Ejerassa Joro, Fabel, Faidherbia Albida, Heat, Ration Methods, PAR, Pruning, Reverse Phenology, Sap Volume, SPAD, Water Use Efficiency