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Improving the Traditional Acacia Senegal-Crop System in Sudan: The Effect of Tree Density on Water Use, Gum Production and Crop Yields


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The traditional Acacia senegal bush-fallow in North Kordofan, Sudan, was disrupted and the traditional rotational fallow cultivation cycle has been shortened or completely abandoned, causing decline in soil fertility and crop and gum yields. An agroforestry system may give reasonable crop and gum yields, and be more appealing to farmers. We studied the effect of tree density (266 or 433 treesha−1) on two traditional crops; sorghum (Sorghum bicolor) early maturing variety and karkadeh (Hibiscus sabdariffa), with regard to physiological interactions, yields and soil water depletion. There was little evidence of complementarity of resource sharing between trees and crops, since both trees and field crops competed for soil water from the same depth. Intercropping significantly affected the soil water status, photosynthesis and stomatal conductance in trees and crops. Gum production per unit area increased when sorghum was intercropped with trees in low or high density. However, karkadeh reduced the gum yield significantly at high tree density. Yields of sorghum and karkadeh planted within trees of high density diminished by 44 and 55% compared to sole crops, respectively. Intercropping increased the rain use efficiency significantly compared to trees and field crops grown solely. Karkadeh appears to be more appropriate for intercropping with A. senegal than sorghum and particularly recommendable in combination with low tree density. Modification of tree density can be used as a management tool to mitigate competitive interaction in the intercropping system.
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Improving the traditional Acacia senegal-crop system in Sudan: the effect
of tree density on water use, gum production and crop yields
A.M. Gaafar
*, A.A. Salih
, O. Luukkanen
, M.A. El Fadl
and V. Kaarakka
Viikki Tropical Resources Institute, University of Helsinki, P.O Box 27, FIN-00014, Finland;
Research Center, Agricultural Research Corporation, Khartoum, P.O. 7078, Sudan; *Author for correspon-
dence (e-mail:
Received 3 June 2004; accepted in revised form 24 February 2005
Key words: Intercropping, Karkadeh (Hibiscus sabdariffa), Sand soil, Soil water, Water use
The traditional Acacia senegal bush-fallow in North Kordofan, Sudan, was disrupted and the traditional
rotational fallow cultivation cycle has been shortened or completely abandoned, causing decline in soil
fertility and crop and gum yields. An agroforestry system may give reasonable crop and gum yields, and be
more appealing to farmers. We studied the effect of tree density (266 or 433 trees ha
) on two traditional
crops; sorghum (Sorghum bicolor) early maturing variety and karkadeh (Hibiscus sabdariffa), with regard to
physiological interactions, yields and soil water depletion. There was little evidence of complementarity of
resource sharing between trees and crops, since both trees and field crops competed for soil water from the
same depth. Intercropping significantly affected the soil water status, photosynthesis and stomatal con-
ductance in trees and crops. Gum production per unit area increased when sorghum was intercropped with
trees in low or high density. However, karkadeh reduced the gum yield significantly at high tree density.
Yields of sorghum and karkadeh planted within trees of high density diminished by 44 and 55% compared
to sole crops, respectively. Intercropping increased the rain use efficiency significantly compared to trees
and field crops grown solely. Karkadeh appears to be more appropriate for intercropping with A. senegal
than sorghum and particularly recommendable in combination with low tree density. Modification of tree
density can be used as a management tool to mitigate competitive interaction in the intercropping system.
Introduction and background
Traditionally, the local tree Acacia senegal (L.)
Willd. is managed in time sequence with agricul-
tural crops as a means of restoring soil fertility in
the North Kordofan area in the Sudan (Ballal
et al. 2005). The cycle consists of a relatively short
period of cultivation followed by a relatively long
period of fallow. The bush fallow cycle starts by
clearing an old gum garden for the cultivation of
crops. Trees are cut at low stumps that are left for
coppicing. The cleared area is cultivated for a
period of 4 – 6 years, during which time the cop-
pice re-growth is cut to enhance establishment of
agricultural crops (Ballal 2002). When the soil
fertility declines, the area is left as fallow under A.
senegal. Trees are tapped for gum arabic until the
age of 15–20 years when the area is cleared again
for crop cultivation. Farmers obtain reasonable
yields of crop and gum arabic.
With increasing human and animal populations,
however, severe pressure is put on the land. The
problem is aggravated by recurrent droughts. As a
result, the tree fallow has reduced (Ballal 2002).
Over-cultivation of the fallow has, in extreme
cases, resulted in complete eradication of the tree
Agroforestry Systems (2006) 66:1–11 Springer 2006
DOI 10.1007/s10457-005-2918-y
(Sief El din 1984). Gum yields were reduced by 30–
70% between 1973 and 1984 (Bayoumi 1996).
Signs of imbalance in the system started to appear
decades ago, and at present the region experiences
serious degradation of soil fertility, erosion and
desertification. The bush fallow system is no
longer viable under the present conditions; a more
sustainable one must be developed.
Sorghum bicolor (L.) Moench is one of the most
important cereals globally and is the principal
food crop in Sudan for the rural and urban pop-
ulations. In Kordofan, the largest group of sor-
ghum producers are the small-scale poor farmers
who have no access to production inputs such as
fertilizers or pesticides. Production trends for
sorghum and other cereals have been declining and
fluctuating in North Kordofan State, where the
yield is low compared with the productivity in the
whole Kordofan region and at the national level
(El-Dukheri 1997). The average sorghum produc-
tion of farmland in North Kordofan is estimated
as 131 kg ha
for grain and 931 kg ha
of total
above-ground biomass.
Among the main cash crops, Hibiscus sabdariffa
L., known as karkadeh in Sudan, is important in
Kordofan due to its steadily increased demand and
consistently high prices over many seasons.
Karkadeh is a suitable cash crop for North
Kordofan because it is drought-tolerant and can
early be grown as part of an intercropping system;
it also seems to have few pest problems (Eltohami
1997). Data collected North Kordofan over a
period of 10 years estimate the average farmland
production as 36 kg ha
for flower and
418 kg ha
for biomass in karkadeh, respectively.
The presence of trees diversifies the farm income
and provides alternative sources of revenue, but
trees also compete with crop plants for resources.
ICRAF (1998) reported that yields of crops plan-
ted with Cajanus trees were decreased significantly,
compared to crop yields in plots without trees. The
competition between trees and crops was most
serious below the ground, largely for water (Ong et
al. 1999). In Kenya, it was concluded that for each
mm of water consumed by trees, the maize yield
was reduced by about 10 kg ha
(ICRAF 1997).
A major challenge for managing simultaneous
agroforestry in drylands is to retain the positive
effects of tree canopies and roots on soil physical
and chemical properties while reducing the nega-
tive effects of the below-ground competition for
limited resources (Ong and Leakey 1999). When
water is stored at deeper soil layers, crop selection
to more completely tap the soil water would assist
in optimizing the water use efficiency (WUE) in
cropping systems (Loyd et al. 2001).
Planting density has been shown to affect the
distribution and depth of tree roots (Boswell et al.
1975). Attempts to identify trees that have few
roots in shallow soil layers that are occupied with
crop roots have failed because the majority of both
tree and crop roots occupy the top soil layer
(Smith et al. 1997; Odhiambo et al. 1999).
There is strong evidence that agroforestry has
potential for improving the WUE by reducing the
unproductive water component. Manipulation of
tree density in agroforestry systems may modify
the biomass production of component species by
controlling the inter-specific competition for re-
sources (Eastham and Rose 1990). Benefits in
physical yields are to be expected only with com-
plementarities of resource capture by trees and
crops (Cannell et al. 1996). There is an increasing
tendency to study tree crop interaction under
varying spacing regimes in order to improve the
productivity of an agroforestry system (Gupta et
al. 1998).
The general objective of this research was to
provide management tools for natural A. senegal
forests that are being cleared for farming purposes
and for A. senegal plantations, in the fragile eco-
system of western Sudan. The specific objective
was to evaluate the interaction of A. senegal trees,
at a low and a high density, with two traditional
crops; sorghum and karkadeh, with regard to
WUE, crop yield, and gum production. Such
information is important also for designing a more
suitable A. senegal agroforestry system and for
restocking the gum belt.
Materials and methods
Study area
The study was conducted at the Domokeya forest
reserve (1316¢N; 3012¢E) near El Obeid town in
North Kordofan state, western Sudan, during July
1999–December 2000. The soils of the experi-
mental sites consisted of deep, highly uniform
sandy soils, classified as sandy, siliceous, Isohy-
perthermic Typic Torri psamments which are easy
to work but poor in organic matter and essential
nutrients. Such soils are non-saline and non-sodic
and slightly acidic, with a low water holding
capacity. The total available water (water retained
between field capacity and permanent wilting
point) was measured as about 16%. The water
infiltration rate as measured with double ring in-
filtrometer was high (6.0 mm min
), thus surface
runoff is not common on these sites. The long-term
average annual rainfall at this location is 300 mm.
The first rainy season during the study was char-
acterized by a markedly high rainfall 364 mm,
typical for a wet year, while the second season was
characterized by a rainfall typical for a dry year,
226 mm. The mean relative air humidity is 34%,
decreasing to 14% during the drier months and
increasing to 60% in the wet season. The mean
annual evaporation is 15.5 mm day
and in-
creases to 20 mm day
in the hot summer
months. The daily mean minimum and maximum
temperatures are 20 and 34 C, respectively.
Temperatures can be as high as 46 C during the
hot summer months. Existing trials on A. senegal
provenances, methods of stand establishment, tree
age groups and tree planting density, as well as the
occurrence of natural A. senegal stands made this
experimental forest an appropriate site for the
present research (cf. Ballal 1991). Most of the gum
produced in the Sudan is obtained from A. senegal
trees growing in this type of environment.
A field experiment of 1.9 ha was established in a
six-year-old A. senegal plantation (established in
1993) in the Domokeya research forest during the
rainy season of 1999. The experiment was ar-
ranged in a randomized complete block design
(RCBD) with three replications and 24 plots
(600 m
each). Two densities of A. senegal were
chosen, and thinning was performed, when
appropriate, to adjust the density. These densities
were 16 or 26 trees/plot, corresponding to 266 or
433 trees ha
. Two crops, sorghum, (early
maturing ‘Zinari’ variety) and karkadeh (local
variety), were sown during the two rainy seasons
(1999 and 2000) as soon as the rainfall was suffi-
cient to moisten the soil. These crops were estab-
lished solely and with the two selected tree
densities. Adjacent monocropping plots and weed-
free stands of trees with the two densities were
used as controls.
Rainfall was measured with of a rain gauge in-
stalled at the site of the experiment. The soil
moisture under trees and crops was measured with
a neutron probe in three replications. Two vertical
access tubes, one 3 m long and at 1 m distance
from a randomly selected tree in the middle of
each plot and another 1-m long and 4-m away
from the same tree, were installed in the soil in
each intercropped plot. In addition, an access tube
of 3 m was also installed in the middle of each pure
A. senegal plot (6 plots). An access tube 1 m long
was also installed in the middle of each plot of
pure crops (6 plots). Readings were taken at
depths of 25, 50, 75, 100, 150, 200 and 250 cm
from the 3-m access tubes and at 25, 50, and 75 cm
from the 1-m access tube. Total soil water to 75
and 250 cm depths was calculated for comparison
between different intercropping designs. Soil
moisture readings were taken from the time of
sowing of crops (July) up to crop harvest
A portable gas exchange system (Licor-6200,
USA) was used to measure the net photosynthetic
rate (A) and stomatal conductance (g
) in trees and
crops at regular intervals simultaneously with soil
moisture measurement. Three readings were taken
and averaged.
All measurements were carried out before mid-
day (from 8 to 10 h local time). Soil samples were
collected from experimental site at different depths
(0–25, 25–50, 50–75, 75–100, 100–125, 125–150,
150–175 cm) for chemical and physical analyses.
The roots of five trees from the experimental site
were excavated for ocular inspection of root dis-
tribution to a depth of 1.5 m.
The intrinsic water use efficiency (IWUE) for
both trees and crops was calculated by dividing the
average instantaneous net photosynthetic rate by
the average stomatal conductance (lmol of
of CO
). The rain use efficiency (RUE)
was calculated for different cropping designs as
economical yield (gum + grain, gum + flower,
and sole gum, grain or flower) per unit rainfall
(kg ha
rainfall). The air-dried crop yield
and above-ground air-dried biomass (total dry
matter) were weighed after harvest. Trees in all
replicates were tapped annually (five branches/
tree) from mid-October onwards at five gum
pickings, and the gum yield in each treatment
(intercropping design) was collected and weighed.
The harvest index was calculated for both crops as
the economic yield in relation to total above-
ground dry matter production.
Statistical analyses were performed with the
JMP (3.2.2) statistical software by SAS Institute
Inc. (JMP 1995). Simple regression techniques
were employed to determine the relationship be-
tween soil water status, gum yield, CO
and other physiological traits. Treatment effects
were considered significant if p<0.05. For yield
comparison among intercropping designs, analysis
of variance was used for all replications in a ran-
domized complete block design. When significant
difference was detected, a comparison of all mean
values was done by Tukey–Kramer HSD test at
a= 0.05.
Effects of intercropping on soil moisture
The different intercropping designs significantly
varied in terms of effect on the total soil water in
the 0–75 cm soil layer during the growing season
(Figure 1). The highest soil water content was
found when field crops were grown alone.
Karkadeh decreased the soil water more than did
sorghum. Intercropping and sole acacia trees re-
sulted in lower and rather similar soil water
Gum production
Intercropping affected the gum production per tree
significantly (Table 1). The highest gum produc-
tion was obtained by intercropping sorghum with
trees of high density (433 trees ha
), followed by
karkadeh under low tree density (266 trees ha
The lowest gum yield/tree was recorded when
karkadeh was intercropped with trees of high
density and when trees were planted alone at low
As was the case with per-tree gum yield, the
highest gum yield per hectare was also obtained
from trees at high density intercropped with sor-
ghum. It is noteworthy that this gum yield was
considerably much higher (115 vs. 81 kg ha
than that obtained from the same density of A.
senegal trees without intercropped sorghum. At
the lower tree density, intercropping with sorghum
seemed to slightly increase the gum yield per
hectare, but this effect (not confirmed statistically)
was not as conspicuous as in the case of the higher
tree density.
Intercropping of karkadeh with trees at both
densities also slightly seemed to increase the gum
yield per hectare compared with gum obtained
from sole tree stands at low density. When trees
were grown at the higher density, however, inter-
Figure 1. Effects of different intercropping designs on average soil water. Data were collected during two rainy seasons (July–
September) in Domokeya, western Sudan. LD = 266 trees ha
, HD = 433 trees ha
, S = sorghum, K = karkadeh.
cropping with karkadeh appeared to decrease the
per-tree gum production as compared with sole
trees. Again, these trends could not be statistically
Of the per-hectare yields, only that of the best
combination (trees at high density with sorghum)
statistically differed from the other intercropping
combinations. The obvious positive effect of sor-
ghum (and gently karkadeh) intercropping on gum
yield per hectare was a key finding of the present
experiments and has important management
There was significant correlation between the
gum production per tree and soil water in the soil
profile (0–250 cm) during the growing season,
when A. senegal was grown with field crops (Fig-
ure 2a, b). The soil water content in the 0–250 cm
soil layer explained about 66% of the observed
variation in gum yield when A. senegal was grown
Figure 2. Dependency of per-tree gum production on soil water in 0–75 and 0–250 cm soil layers for A. senegal, when intercropped
with (Hibiscus sabdariffa) or sorghum (a,b) and for sole A. senegal plots (c,d) corresponding to those shown in (a,b). Data were
collected during the preceding rainy season and up to the end of November. Data were collected during two consecutive years in
Domokeya western Sudan.
Table 1. Gum yield and intrinsic water use efficiency of A.
senegal under different cropping designs.
Treatments Gum yield IWUE (lmol mol
tree (g) Hectare (kg)
LD + K 224ab 59b 36.1a
HD + K 140b 61b 43.2a
LD + S 215ab 57b 46.0a
HD + S 265a 115a 57.6a
LD 185ab 49b 43.2a
HD 187ab 81b 37.0a
Mean values followed by the same letter are not statistically
different at p<0.05. Data were collected during two years (1999
and 2000) in Domokeya, western Sudan. LD and HD denotes
266 and 433 trees ha
, K, S, denotes karkadeh (Hibiscus sab-
dariffa) and sorghum crops, IWUE, denotes intrinsic water use
in combination with agricultural crops. In con-
trast, the soil water in the 0–75 cm soil layer af-
fected the gum production per tree, when trees
were grown without crops (Figure 2c, d).
Tree physiology
When trees were intercropped with field crops, the
photosynthesis rate (A) in trees was positively
correlated with the amount soil water in the 0–
250 cm soil stratum, while the correlation of A
with soil water in the 0–75 cm layer was low
(Figure 3a, b). The correlation of Awas signifi-
cantly higher with soil water in the 0–75 and 0–
250 cm soil layers (Figure 3c, d) when trees were
grown solely. Unlike the photosynthesis rate (A),
the stomatal conductance (g
) in trees seemed to be
more sensitive to moisture changes in the topsoil
(Figure 4) when A. senegal was grown with field
Crop yields
Yields of karkadeh and sorghum were significantly
lower under intercropping than as sole crop (Ta-
ble 2). Sorghum and karkadeh yields diminished
by 44 and 55 %, respectively, with high tree den-
sity (433 tree ha
) compared to monocropping.
The respective reductions were 24 and 26% at low
tree density. For biomass, the reductions were 23
and 61% for sorghum and 37 and 57% for kark-
adeh with low and high tree density, respectively,
compared to sole crops.
Crop physiology
The stomatal conductance (g
) of both crops was
affected in a similar manner by the soil water in the
Figure 3. Relationship between tree photosynthesis rate (A) and total soil water in the 0–75 and 0–250 cm soil layers during inter-
cropping (a,b) and in sole trees (c,d). Data were collected during two rainy seasons (July–September) in Domokeya, western Sudan;
field crops include both sorghum and karkadeh (Hibiscus sabdariffa).
0–75 cm soil stratum during the middle and final
stages of growth (Figure 5a, b). The stomatal
conductance was more responsive to water in the
0–75 soil layer in sorghum (R
= 0.70,
p= 0.0008) than that of karkadeh (R
= 0.60,
p= 0.006).
Table 2. Effect of A. senegal tree density on yields and (IWUE) of sorghum and karkadeh (Hibiscus sabdariffa).
Treatments Sorghum yield (kg ha
) IWUE (lmol mol
) Karkadeh yield (kg ha
) IWUE (lmol mol
Grain Biomass Flower Biomass
LD + crop 126b (24) 1030a (23) 80.5a 127ab (26) 622a (37) 36.1b
HD + crop 93a (44) 527a (61) 36.7b 77a (55) 422a (57) 87.6a
Mono crop 167b 1346b 59.4ab 172b 982b 45.7a
Means followed by the same letter are not statistically different at p<0.05. Data were collected during two rainy seasons in Domokeya,
western Sudan. Numbers in parentheses indicate % reduction in yield compared to mono crop. LD and HD denote 266 and 433
trees ha
, respectively. IWUE denotes intrinsic water use efficiency (A/g
Figure 5. Effect of soil water in the 0–75 cm soil layer on sorghum (a) and karkadeh (Hibiscus sabdariffa) (b) leaf stomatal. Each data
point represents the mean of two readings. Data were collected during two consecutive rainy seasons from intercropping plots in
Domokeya, Western Sudan.
Figure 4. Relationship between A. senegal stomatal conductance and soil water in the 0–75 cm (a) and 0–250 cm (b) soil layers. Each
data point represents the mean of two readings. Data were collected during two consecutive rainy seasons from intercropping plots in
Domokeya, western Sudan.
Statistically significant differences among sor-
ghum harvest indices in different intercropping
designs were detected. In sorghum the harvest in-
dex seemed to increase as the tree density de-
creased (Figure 6a). The highest value was
obtained from sole crops. There was no clear
variation in the karkadeh harvest index among
different intercropping designs (Figure 6b).
Intrinsic water use efficiency
The intrinsic water use efficiency (IWUE) of trees
calculated as A/g
. The highest IWUE in trees
was observed when sorghum was grown with
trees at high density (Table 1). The seemingly
lowest IWUE values were observed, when kark-
adeh was grown with trees at low density or when
Figure 6. Comparison of harvest indices in karkadeh (Hibiscus sabdariffa) (a) and sorghum (b) under different cropping designs.
LD = 266 trees ha
, HD = 433 trees ha
, S = sorghum and K = karkadeh. Diamonds are the mean values. Standard er-
ror = 0.002 and 0.0013, respectively. Data were collected during two rainy seasons in Domokeya, western Sudan.
trees were grown solely at high density. However,
differences in tree IWUE were not statistically
The cropping design significantly affected the
IWUE in crops (table 2). The highest values of
IWUE in karkadeh were observed, when karkadeh
was grown with trees at high density. In sorghum
the highest IWUE was observed when the crop
was grown with trees at high density.
Rain use efficiency (RUE)
The rain use efficiency (RUE) of different cropping
systems was significantly affected by the cropping
design (Table 3). A high RUE was obtained when
sorghum was grown with trees at high density,
followed by either karkadeh or sorghum with trees
at low density. The lowest RUE values were ob-
tained from sole trees and sole crops.
Comparison between the amount of soil water
in an agroforestry system and in bare land at the
site studied Figure 7) indicated that there was no
significant difference in the soil water in the 0–
250 cm soil layer between these two situations
after the rainy season in November. However, for
most of the rainy season there was significantly less
soil water in all agroforestry plots compared with
bare land.
Discussion and conclusions
Water depletion was much higher for trees grown
at high density with agricultural crops than for
sole trees or sole crops. Root excavation (data not
Table 3. Comparison between rain use efficiency (RUE) in
different cropping designs.
Cropping design RUE (kg ha
LD + K 0.631ab
HD + K 0.468b
LD + S 0.616ab
HD + S 0.705a
LD 0.166c
HD 0.275c
K 0.583ab
S 0.563a
Means followed by the same letter are not statistically different
at p<0.05 as determined by Tukey–Kramer HSD-test. Data
were collected during two rainy seasons in Domokeya, western
Figure 7. Mean soil water content under agroforestry and bare land during and after the cropping season in July–November. Data
were collected during two years (1999 and 2000) in Domokeya, western Sudan.
shown) indicated that the concentration of lateral
and fine tree roots was maximal in the topsoil.
Consequently, trees and crops obviously competed
for water from the same soil layer. However, it is
possible that trees also partially utilized water
from below 75 cm soil depth, especially during the
dry season when the water in the topsoil was de-
The effect of different tree densities on the soil
water content was reflected in the yields of both
trees and crops and their physiological traits. The
gum yield increased as a result of higher soil
moisture at the end of the previous rainy season.
This relationship does not appear to fully account
for gum production, since the highest gum yield
did not correspond to the highest amount of soil
water. There may thus be a threshold soil moisture
for maximum gum production. Excessive soil
moisture promotes vegetative growth in A. senegal
and delays the tapping season, which is detrimen-
tal for gum production (Ballal et al. 2005). Kark-
adeh, with its deep taproot system, may have
depleted the topsoil moisture below an optimum
for gum production.
Intercropping with either one of the two agri-
cultural crops affected the gum production per
hectare positively compared with that (49 kg ha
obtained from a pure stand of low tree density
(266 trees ha
). The highest gum production per
unit area was obtained when sorghum was inter-
cropped with trees of high density and the lowest
when trees were planted at low density, with or
without crops. If karkadeh is to be used in this
agroforestry system, it should be grown with A.
senegal trees planted at low density, so as to
minimize competition for surface soil water.
To maximize the crop IWUE, intercropping of
sorghum with a high density of trees and inter-
cropping of karkadeh with low tree density is
recommended. Plants tend to use resources more
efficiently when these resources are scarce. Indices
of IWUE have been found to increase with
increasing aridity (Ares and Fownes 1999). How-
ever, we did not detect significant variation in the
tree IWUE among the different intercropping de-
sign treatments.
Tree photosynthesis seemed to be affected by the
presence of crops, as it was related to the topsoil
(0–75 cm) water when trees were grown solely and
to the soil water in the whole (0–250 cm) profile,
when trees were grown with agricultural crops.
This suggests that trees may be able to take up
some soil water from deeper layers, especially at
the time of gum formation and when crops are still
using water from the topsoil (October). At that
time, the topsoil is probably depleted and trees
have to rely on deep-soil water to maintain a
favourable CO
assimilation rate. Trees may have
a high concentration of roots in the topsoil and
still have some very deep roots that increase their
drought tolerance (Schroth 1999).
The effect of intercropping on tree g
seems to
depend on the influence of treatments on the soil
water status, since g
in A. senegal was highly
responsive to soil water changes in the topsoil. The
soil water content might control the stomata of A.
senegal especially when trees are competing with
agricultural crops for soil water. This result is
supported by findings in other studies, which
indicate that the soil water content may control
stomatal aperture (Masle and Passioura 1987;
Tardieu et al. 1991).
The physiological responses of A. senegal as
found in the present study are obviously coupled
with changes in soil water content in the topsoil.
This also suggests an adaptive mechanism in A.
senegal in response to drought.
It is evident that the amount of soil water in the
0–75 cm soil layer significantly affected the crop
yield. This result is in line with many previous
studies (Chaves 1991; Bisht et al. 2000). Yields of
the two crops were drastically diminished by in-
tercropping, and the reduction was more signifi-
cant with a high tree density. In both crops, g
apparently very sensitive to changes in soil water
and function as mechanisms for drought when
water loss can not be matched by equivalent water
Karkadeh shows more intensive root growth
into deep soil layers and thus occupy a larger soil
volume than sorghum, which is shallow-rooted.
For the viewpoint of agricultural crop production,
karkadeh, therefore, has better characteristics as
an agroforestry crop than sorghum, as it causes
more competition with trees, increases the gum
yield per unit area and shows a smaller effect on
the harvest index.
When water is stored at deeper soil layers, crop
selection for more efficient utilization of soil water
would assist in optimizing the WUE of the crop-
ping system (Loyd et al. 2001). Recent reviews on
root research indicate that there appears to be
limited scope for spatial differentiation in rooting
between trees and crops (i.e. spatial complemen-
tarity) in water-limited environments, unless
ground water is accessible to tree roots (Ong et al.
2002). However, in the present study it was also
observed that the amount of water remaining in
the different soil layers does not differ substantially
between an agroforestry system and bare land,
probably due to a high loss of water through
evaporation from bare, unprotected sandy soil.
Moreover, A. senegal agroforestry evidently in-
creases the RUE of the whole cropping system by
creating a combination of species with different
marketable products.
Acacia senegal is always likely to compete with
agricultural crops for water; however, in the
present study some evidence of complementarity
was observed in the water use between A. senegal
trees at a low density and sorghum. It can be
concluded that, in A. senegal agroforestry, the tree
density affects the inter-specific and intra-specific
competition for soil water and thus can be used as
management tool to modify the competitive
interaction in intercropping systems.
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... Also, the traditional A. senegal based agroforestry was observed as sustainable in terms of its environmental, social and economic benefits (Deans et al., 1999;Nasreldin, 2004;Kissi, 2011;Isaac et al., 2011). In Northern Cameroon, between 1990and 2006, rural development agencies such as Rural Development and Land Management (DPGT) and ESA/SODECOTON have encouraged smallholders to establish and manage A. senegal plantations (Mallet et al., 2002;Palou Madi et al., 2010;Kissi, 2011). These established plantations were extended between 2007 and 2011 through Acacia gum project financed by the European Union (Palou Madi et al., 2010). ...
... The partial tree clear-felling seemed to be a better form of conversion because it recorded the higher sorghum (3.59 tha -1 ) and cowpea (2.4 tha -1 ) yields (Figure 3). Commonly, intercropping annual crops with tree species reduces crops yields because of the competition between trees and associated crops caused by tree density and size (Rao et al., 1998;Muthuri et al., 2005;Gaafar et al., 2006;Hadgu et al., 2009;Fadl, 2010). Furthermore, trees use the water in the topsoil where annual crops are grown rather than the water below (Raddad and Luukkanen, 2007). ...
... In addition, conversion of old plantations is associated with nutrients content decrease (El Tahir et al., 2009). These findings were benefit to farmers because intercropping sorghum with A. senegal enhances gum arabic production (Gaafar et al., 2006). ...
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Tree planted fallow is an agroforestry system that may restore degraded soils and protect them from erosion. In this study, sandy soils properties of Acacia senegal planted fallows (AF) were assessed and compared to those from the continuous cropped system (CC) in 3 sites from Northern Cameroon in order to determine its suitability to restore soil fertility and sustain crop productivity. Soil samples were collected from the topsoil (0 to 20 cm) and the subsoil (20 to 40 cm) and subjected to physicochemical analyses. The trials were established for 2 consecutive years, respectively with sorghum (Sorghum bicolor) and cowpea (Vigna unguiculata). Results confirmed the sandy (more than 80% of sand) and acidic (4.42 ≤ pH ≤ 6.59) soil characters. In every site, topsoil from AF was relatively more fertile than from CC. Globally, nutrients content were influenced by tree density and fallow duration. The more improved elements were organic matter, nitrogen and pH. Sorghum and cowpea yields were quite variable depending on fallow duration, tree density and conversion form. The highest crop yields (3.4 tha-1 for sorghum and 2.4 tha-1 for cowpea) were obtained in 19 years old AF converted by partial clear-felling. The intercropping process by partial clear-felling of trees was the best conversion form. Overall findings indicated that fallowing with A. senegal can reduce soil acidity, restore nutrients and therefore it constitutes a suitable agroforestry system that may sustain annual crops productivity. However, researches have to determine the best tree density for intercropping and the tools for their sustainable management.
... The traditional method of gum harvesting causes the death of many trees, especially in arid areas, and new methods need to be employed which result in minimal damage to trees (Gaafar et al., 2006;Saini et al., 2018). No research has been done on novel methods of exploiting Saqez gum and the effect of EI on increasing the oleo-gum-resin content of P. ...
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Pistacia atlantica one of the main wild species of Pistacia in Iran and one of its products is an oleo-gum-resin briefly called Saqez gum. The effect of Ethrel injection (EI) on Saqez gum production was studied during summer 2018 at three habitats (Kuh-e Birk, Goharkuh and Mirabad) inSistan and Baluchestan province, Iran. Four treatments, 0 (distilled water), 10, 20 and 30% concentration of Ethrel, were administered by injection into the tree trunk. The investigation was carried out in a randomized complete block design and 36 trees were injected. Saqez gum essential oil (EO) was extracted by Clevenger apparatus and identification and quantification of EO components were performed by GC-MS and GC-FID, respectively. The results showed that applying the EI treatment increased the gum yield of trees by an average of 4.4 times. The highest gum yield (29.33 g/tree) was obtained with EI at 10% in the Kuh-e Birk habitat, while the lowest yield (3.33 g/tree) was observed in the control treatment of the Mirabad habitat. In total, 39 compounds were identified in the EOs and α-pinene was found to be the predominant compound in all samples (66 to 91.8%). Other compounds such as β-pinene, camphene, sabinene, cis-limonene oxide, neo-iso-verbanol, ρ-cymene and ρ-cymenene were identified in the EOs. EI treatments did not have a significant effect on EO components and the amount of α-pinene and β-pinene in EI 30% treatment increased slightly. Due to the desired quantity and quality of gum in the EI 10%, this method of exploitation can replace the traditional method.
... Besides the crop yield, agroforestry increases the yield of wood and non-wood tree products. Gaafar et al., (2006) found increased gum production per unit area when sorghum was intercropped by trees in low or high density. Jama & Amare, (1991) reported that growth performance, such as height and diameter at breast height (DBH) of F. albida rotationally intercropped with Maize and Phaseolus aureus, found the higher mean height (140%) and DBH (24%) than tree alone. ...
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This paper examines the role of farmland trees for the improvement of soil fertilities, enhancement of crop yield, soil and water conservation and Carbon sequestration. Natural resource degradation was one of the major problems that have been affecting social, ecological and economic situation all over the world in general and the highlands of Ethiopia in particular, where there is high population pressure, land susceptible to degradation, climate change, an exploitative farming style which further aggravated land degradation. Hence, different conservation strategies such as soil and water conservation, tree planting on degraded areas and area enclosures were launched at different times. Whereas, researchers have been recommended different ecologically sound agroforestry practices for Ethiopia especially in sloppy and areas susceptible to degradation. Some tree species were identified, and farmers conserved and maintained them on their farmlands traditionally. Hence, well-designed agroforestry practices have significantly reduced soil erosion and surface runoff due to interception of rainfall by the tree canopy, soil surface cover by litter, and formation of erosion-resistant soil structure. Studies indicated that agroforestry is efficient in insuring agricultural sustainability through enhancement of soil fertility by the addition of plant nutrients (N, P, K, Mg, Ca) because most of the agroforestry trees are leguminous that can fix nutrients that are essential for plant growth and development and helped in increasing crop yield. Besides, non-leguminous trees also add organic matter (OM) though the value varies with crop type, tree species, and agro-ecology. Therefore in introducing agroforestry practices, one has to consider crop type, tree species, and agro-ecology under which the practices will be implemented. Besides, agroforestry has the potential to sequester carbon at a higher rate than annual crops because annual crops can only accumulate through their roots and retention of crop residue, whereas the tree accumulated Carbon through roots, litter, and aboveground biomass and reduce global warming. Hence, integrating niche compatible trees into agroforestry practices has a higher potential for production and protection services mainly in sloppy areas. Keywords: Agroforestry, climate change, natural resources, nutrients, parkland, soil, trees.
... In arid and semiarid areas, the soils under the Acacia canopy are usually improved and developed more than those outside the canopy, having higher nitrogen and water contents, Waldon et al. (1989) and Kassa et al. (2017), mentioned the role of agroforestry in maintaining soil fertility, and storing higher soil organic carbon and nitrogen. Trees increased soil organic carbon contents from pruning and litter (Gaafar et al. 2006). The highest gas exchange (P n and g s ) recorded in sorghum plants grown in strips between Acacia trees, can be attributed to the higher content of nitrogen in the soil, which positively affects the leaves nitrogen content. ...
Taking the importance of agricultural production sustainability with limited resources to use efficiency in an arid area, afield experiment was designed to investigate the effect of three, Acacia trees (Acacia nilotica, A. seyal, and A. tortilis) planting combination on soil fertility and Sorghum bicolor L. growth and physiological performance. The sorghum planted in 7 strips between 14 rows of Acacia trees planting combinations and one treeless strip as control. Acacia species plantations significantly increase soil fertility in terms of available nitrogen (N), phosphorus (P), potassium (K) and organic carbon (OC) contents as compared to control, highest level of N and P content (59.01 ± 1.45 and 58.77 ± 1.10 mg/kg) was reported in strip between rows of A. tortilis. Although the highest net photosynthesis rate (P<sub>n</sub>) and stomatal conductance (g<sub>s</sub>) recorded in plants grown between rows of pure A. torilis, and rows of A. torilis–A. seyal, but different Acacia significantly enhanced sorghum growth and physiology with reference to net photosynthesis rate, stomatal conductance and chlorophyll fluorescence (chlF). The results stated linear relation between soil nutrients (N, P, K), P<sub>n</sub>, and chlF increasing soil fertility improve physiological performance of sorghum. In conclusion, the three Acacia improve soil fertility and sorghum growth. Generally, this plantation trial can be environment-friendly alternative agricultural practices in Saudi Arabia or any area with a similar ecological condition to amend the soil and improve crop performance.
... Agroforestry systems based on S. senegal and a variety of crops produce combined yields that are larger than when the trees or agricultural crops are grown separately. It accumulates a large biomass when given sufficient water (Gaafar et al., 2006). In Sudan, a traditional rotation with agricultural crops and S. senegal was shown to maintain soil fertility (Ballal et al., 2005;Barbier, 1992) and has a low input production cycle . ...
Changing environmental conditions in dryland areas exacerbate land degradation and food insecurity in many sub-Saharan African nations. Multipurpose tree species such as Senegalia senegal (L.) Britton, are favored for reforestation and land reclamation as compared to single-use species. A great deal of research has also focused on this tree species due to its ability to fix atmospheric nitrogen into ammonia, which is returned over time to the soil via the recycling of N-rich plant tissue. We review the recent literature on how S. senegal contributes to soil fertility and crop production especially in the context of sustainable and ecological agriculture. We also review the current literature on this legume species with regard to its microsymbionts, with the goal of further maximizing the potential of S. senegal for agriculture in sub-Saharan Africa. Senegalia senegal, which has the potential to restore degraded soils and to be used for agroforestry, is both economically and ecologically important for the dry areas of sub-Sa-haran Africa because it produces gum arabic, an important commodity crop for smallholder farmers; it succeeds where other crops fail. This tree species also can correct soil fertility loss caused by continuous agriculture and worsened by a reduced or non-existent fallow period. Senegalia senegal and its soil microbes are positively associated with this species' ability to survive in harsh conditions. This tree is an important candidate for restoring soil fertility and providing commercial products especially in countries with arid environments.
... As an N 2 -fixing species, A. senegal improves degraded lands and nutrient deficient soils (Isaac et al., 2011). This species is particularly used in tree improved fallows to replenish soil fertility (Deans et al., 1999;Raddad et al., 2005), and intercrops well with sorghum and other grasses (Gaafar et al., 2006). Its ability to produce arabic gum (the true food-grade gum as it contains no toxins) is of particular economic interest which justifies a rural forestry based on plantations, following the example of the Sudanese agroforestry farming system (Palou Madi et al., 2010). ...
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La présente étude a été menée sur des plantations d'Acacia senegal (L.) Willd. installées en zone soudanienne du Cameroun entre les isohyètes 650 mm et 1 250 mm. L'étude concerne la croissance de l'espèce, le choix de la date d'incision et l'influence des facteurs climatiques et édaphiques sur la production de gomme arabique. Les plantations ont été réalisées de 1985 à 1989 et les essais de saignée de 1993 à 1998. L'espèce a présenté généralement une bonne adaptation et une bonne croissance dans les différentes conditions de site de la zone d'étude. Les observations montrent une meilleure production de gomme quand la saignée a été réalisée en début de saison sèche, lorsque l'humidité relative diminuait. Selon l'isohyète, la date optimale de saignée s'est étalée du 10 octobre (650 mm) au 25 novembre (1 250 mm). Entre 650 mm et 800 mm de pluviosité annuelle, la production moyenne annuelle de gomme dans chaque site a été de 100 g à 500 g par arbre saigné, correspondant à un rendement à la parcelle de 50 à 250 kg/ha/an pour une densité de 500 arbres/ha. Au-dessus de l'isohyète 1 000 mm, la production s'est montrée plus aléatoire. Bien qu'à l'échelle pluriannuelle la production de gomme ait été similaire dans les différents types de sols, le niveau de production annuelle a été plus variable sur sol sableux que sur sol argileux. La provenance soudanienne locale a été en général plus productive que toutes les provenances introduites d'origine sahélienne (Sénégal, Soudan) ou d'Inde. En plus de leur clarté remarquable, les quelques échantillons de gomme analysés de la provenance locale ont montré des propriétés classiques typiques des exsudats d'A. senegal de la ceinture sahélienne.
... In the less populated areas of Darfur, the old fallow system may still allow the trees to grow to maturity, but in Kordofan where pressures on the land are great; such a succession tends to face constraints. Development into more or less permanent agroforestry production systems is favoured by the farmers; in these, crops, trees and animals coexisting simultaneously may solve the problem of sustainability and a sufficient economic return; practical solutions, for instance, on optimal tree densities, have already been suggested (Gaafar et al. 2006). The people living off the land in the gum area in Kordofan belong either to the settled agriculturists or the nomads. ...
... In earlier studies conducted by Gaafar et al. (2006) on sandy soils of North Kordofan state in central Sudan, sorghum intercropped with 6-year-old planted A. senegal trees at a density of 266 trees ha -1 showed a decrease in grain yield of 19%, as compared to pure crop cultivation. At a density of 433 trees ha -1 , the yield decrease compared to monocropping was 44%. ...
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National food security has been a major policy goal in Sudan since the country gained its independence in 1956. One of the fundamental reasons is to ensure the social welfare for people living in rural areas. In this study we aimed to analyse how farmers secure their food and generate income in the semi-arid Sennar state in Sudan, using two selected sites, El Dali and El Mazmum, as examples. We interviewed 281 randomly sampled household heads, of which 145 at El Dali and 136 at El Mazmum, between July and November 2011. We identified four distinct land use systems, of which three consist of monocropping and one of cultivation in agroforestry parklands. Several statistical techniques and economic analysis were applied on the study data. Our results show that, in the two areas, the highest average yields over a 10-year period for the three crops studied, sorghum, pearl millet and sesame, were achieved in agroforestry system, except for the case of sesame at El Mazmum. Economic returns for the farmers, as indicated by net present value or benefit/cost ratio, followed the same pattern. The study concludes that farmers should rely more on agroforestry to improve their food security and cash income generation. Land use and land right policies, which currently discourage farmers from growing trees on their lands, should be revised, so as to give more incentive to them to adopt ecologically and economically more sustainable land use practices.
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Agricultural landscapes are increasingly being managed with the aim of enhancing the provisioning of multiple ecosystem services and sustainability of production systems. However, agricultural management that maximizes provisioning ecosystem services can often reduce both regulating and maintenance services. We hypothesized that agroforestry reduces trade-offs between provisioning and regulating/maintenance services. We conducted a quantitative synthesis of studies carried out in sub-Saharan Africa focusing on crop yield (as an indicator of provisioning services), soil fertility, erosion control, and water regulation (as indicators of regulating/maintenance services). A total of 1106 observations were extracted from 126 peer-reviewed publications that fulfilled the selection criteria for meta-analysis of studies comparing agroforestry and non-agroforestry practices (hereafter control) in sub-Saharan Africa. Across ecological conditions, agroforestry significantly increased crop yield, total soil nitrogen, soil organic carbon, and available phosphorus compared to the control. Agroforestry practices also reduced runoff and soil loss and improved infiltration rates and soil moisture content. No significant differences were detected between the different ecological conditions, management regimes, and types of woody perennials for any of the ecosystem services. Main trade-offs included low available phosphorus and low soil moisture against higher crop yield. This is the first meta-analysis that shows that, on average, agroforestry systems in sub-Saharan Africa increase crop yield while maintaining delivery of regulating/maintenance ecosystem services. We also demonstrate how woody perennials have been managed in agricultural landscapes to provide multiple ecosystem services without sacrificing crop productivity. This is important in rural livelihoods where the range of ecosystem services conveys benefits in terms of food security and resilience to environmental shocks.
The effects of Acacia senegal trees on soil moisture (SM) and hydraulic properties in relation to plantations age and associated changes in soil organic carbon (SOC) were investigated and compared to grasslands at two sites in Sudan. Soil hydraulic properties were computed using pedotransfer functions based on texture and SOC, and SM measured using TDR. The measured SM data was used to parametrize a simple daily water balance model in which Hortonian runoff was calculated using the SCS runoff curve number (CN) method and evapotranspiration calculated using crop coefficients, K c, adjusted to seasonal values using NDVI. Measured SM was higher in the grasslands than plantations, but increased with plantation age, reflecting a similar trend in plantation SOC and plant available water capacities. The modelling resulted in lower runoff from the plantations, increased infiltration, evapotranspiration, reduced drainage and lower SM, as shown by measurements. Greater SM contents in the grasslands were attributed to lower evapotranspiration and resulted in greater drainage fluxes compared to the plantations. The study highlighted the need for more empirical studies on the effect of tree density and cover on rainfall-runoff relationships, infiltration, evapotranspiration and drainage in drylands, especially those of the drier parts of semi-arid Africa.
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High density planting of tree crops has the potential of increasing yield and income during the early years of an orchard’s life. Eleven different planting densities of citrus were studied over a 10-year period to determine the effect of tree spacing on yield, tree growth, root distribution, nutrition, and economic factors involved. Extremely close plantings soon crowded to the point where they were unmanageable and tree removal became necessary. Pruning was needed to maintain a workable orchard in more moderatley spaced plots.
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This paper describes recent advances in below-ground research in dryland areas, focusing mainly on studies in dryland Kenya, in which novel sap flow and allometric techniques for determining root structure and function were compared with simple, but labor-intensive methods involving soil coring and trenching. Studies using these approaches indicated that tree (Gliricidia sepium and Grevillea robusta) and crop (Zea mays) roots predominantly occupied the same soil volume, implying there was little spatial complementarity, and highlighted the existence of considerable variation in tree root density between seasons. The less labor-intensive approach of determining the "competitivity index", based on measurements of main roots, was not a reliable indicator of competition unless it was adjusted for tree size. Fractal relationships provided reasonable estimates of coarse root length, but seriously underestimated the length of fine roots. Sap flow studies showed the marked capacity of tree root activity to switch from one part of the root system to others in response to changes in soil moisture availability, and highlighted the importance of root function as opposed to structure in determining the severity of below-ground competition; such effects may provide both benefits and costs to neighboring plants. Thus, there is an urgent need to develop tree management regimes which promote effective sharing of below-ground resources.
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water depletion was greater in sunflower plots than in sorghum plots tively use stored soil water are questions of rooting (by 103 mm in 1985 and 112 mm in 1986). Sunflower roots advanced depth, advance rate of the rooting front, and prolifera- downward from 0 to 60 DAE at a rate and depth 46 and 35% greater, tion of roots within a soil layer. Because of its deep and respectively, than that of sorghum. The faster advance rate and deeper extensive root system, sunflower often is grown under depth of rooting aid sunflower in drought avoidance and use of water rainfed conditions (Blamey et al., 1997). Grain sorghum from deeper soil depths. occupies the most area of any dryland row crop in Kan- sas (Kansas Agric. Stat., 1999). Therefore, we had inter- est in comparing root systems of field-grown grain sor-
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This paper describes recent research findings on resource sharing between trees and crops in the semiarid tropics and attempts to reconcile this information with current knowledge of the interactions between savannah trees and understorey vegetation by examining agroforestry systems from the perspective of succession. In general, productivity of natural vegetation under savannah trees increases as rainfall decreases, while the opposite occurs in agroforestry. One explanation is that in the savannah, the beneficial effects of microclimatic improvements (e.g. lower temperatures and evaporation losses) are greater in more xeric environments. Mature savannah trees have a high proportion of woody above-ground structure compared to foliage, so that the amount of water 'saved' (largely by reduction in soil evaporation) is greater than water 'lost' through transpiration by trees. By contrast, in agroforestry practices such as alley cropping where tree density is high, any beneficial effects of the trees on microclimate are negated by reductions in soil moisture due to increasing interception losses and tree transpiration. While investment in woody structure can improve the water economy beneath agroforestry trees, it inevitably reduces the growth rate of the trees and thus increases the time required for improved understorey productivity. Therefore, agroforesters prefer trees with more direct and immediate benefits to farmers. The greatest opportunity for simultaneous agroforestry practices is therefore to fill niches within the landscape where resources are currently under-utilised by crops. In this way, agroforestry can mimic the large scale patch dynamics and successional progression of a natural ecosystem.
A field experiment was laid out during 1994-96 to assess the performance of ginger (Zingiber officinale Roscoe) and turmeric (Curcuma longa L.) under 10-years-old fodder trees, namely Grewia optiva Drummond (bhimal), Celtis austrelis (kharik), Quercus leucotrichophora A. Camusex (banj) and Bauhinia veriegata (kachnar). The results showed that yield of turmeric and ginger was affected significantly with different fodder trees. Both turmeric (12.04 tonnes/ha) and ginger (7.98 tonnes/ha) gave the highest yield with Quercus leucotrichophora. However, the highest green forage of trees was harvested from Bauhinia vereigata (7.7 kg/tree). Negative correlation (r= -0.77) was obtained between light intercepted by the trees and yield of under grown ginger and turmeric. Association of Quercus leucotricophora with turmeric and ginger was found to be the most suitable and remunerative silvi-horti combination.
Wheat seedlings were grown in soil of various strengths, obtained by changing the bulk density or the water content of the soil. Leaf expansion and transpiration rate were monitored from emergence until the main stem had 5-7 leaves. Leaf area, and shoot and root dry weights, were negatively correlated with soil strength as measured by penetrometer resistance. The growth of roots was less affected than that of shoots. Leaf expansion was reduced before the first leaf was fully expanded. Relative rates of leaf expansion thereafter were consistently lower at high soil strength, although not always significantly. High soil strength also produced substantially smaller stomatal conductances. All effects were the same whether variations of soil strength were brought about by changes in water content or in bulk density. Three possible causes of reduced shoot growth were examined: (1) a limiting supply of nutrients; or (2) of water, because of a restricted root system; or (3) a reduced carbon supply because of a higher carbon demand from the roots, or because of the low stomatal conductance. We conclude that these are all unlikely explanations for the onset of the effects of soil strength, which were independent of soil phosphorus content, of leaf water potential, and of the amount of carbon reserves in the seed. We suggest that growth of the shoot is primarily reduced in response to some hormonal message induced in the roots when they experience high soil strength.
The effects of tree density on the distribution of tree and pasture roots under an agroforestry experiment were investigated. Trees were planted in a Nelder fan design, and three planting densities of 2150,304 and 82 stems per hectare were chosen for this study. Proximity to trees and increase in tree density reduced pasture root growth, with lowest concentrations of pasture roots occurring under the highest tree density. Tree root systems were deeper and denser at high tree densities, although total length and mass of roots produced per tree decreased with increasing tree density. Tree root:shoot ratios increased as tree density decreased owing to greater root production at low tree densities.
Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots is unlikely.
Indices of shallow rootedness and fractal methods of root system study were combined with sapflow monitoring to determine whether these ‘short-cut’ methods could be used to predict tree competition with crops and complementarity of below ground resource use in an agroforestry trial in semi-arid Kenya. These methods were applied to Grevillea robusta Cunn., Gliricidia sepium (Jacq.) Walp., Melia volkensii Gürke and Senna spectabilis syn. Cassia spectabilis aged two and four years which were grown in simultaneous linear agroforestry plots with maize as the crop species. Indices of competition (shallow rootedness) differed substantially according to tree age and did not accurately predict tree:crop competition in plots containing trees aged four years. Predicted competition by trees on crops was improved by multiplying the sum of proximal diameters squared for shallow roots by diameter at breast height2, thus taking tree size into account. Fractal methods for the quantification of total length of tree root systems worked well with the permanent structural root system of trees but seriously underestimated the length of fine roots (less than 2 mm diameter). Sap flow measurements of individual roots showed that as expected, deep tap roots provided most of the water used by the trees during the dry season. Following rainfall, substantial water uptake by shallow lateral roots occurred more or less immediately, suggesting that existing roots were functioning in the recently wetted soil and that there was no need for new fine roots to be produced to enable water uptake following rainfall.