Fig. 8. The relationship between reproductive and vegetative biomass in wheat plants growing in 1 m
2
plots with green alive or brown dead maize hedges oriented E – W or N – S.
Regression models are: green hedges, E – W: y = 0.5918 + 65.705
R
2
= 0.82;
N – S: y = 0.6723x + 91.858
R
2
= 0.67;
brown hedges, E – W: y = 0.5398x + 146.88 R
2
= 0.42; N – S
y = 0.5996x + 67.746 R
2
= 0.69.
tively, in high and low-density plots Table 1 by the presence of barnyardgrass hedges.
Response to the barnyardgrass hedges was also evident in aboveground biomass production at the
end of the crop cycle. Overall the hedge signifi- cantly improved wheat vegetative and reproduc-
tive biomass, in both the high and low Italian ryegrass seed bank density treatments Table 2.
In all the treatments high and low Italian rye- grass seed bank density, pure and mixed stands
barnyardgrass hedges reduced Italian ryegrass vegetative yield compared to the non-hedge areas,
but the hedges did not significantly affect repro- ductive yield of Italian ryegrass Table 2.
Fig. 9. The relationship between reproductive and vegetative biomass in ryegrass plants growing in 1 m
2
plots with green alive or brown dead maize hedges oriented E – W or N – S.
Regression models are: green hedges, E – W: y = 0.3433x + 1.962 R
2
= 0.58; N – S: y = 0.1008x + 1.1774 R
2
= 0.76; brown
hedges, E – W: y = 0.2618x + 6.8231 R
2
= 0.59; N – S y =
0.1351x + 1.6479 R
2
= 0.87.
3. Results and discussion
3
.
1
. Experiment
1
In all of the mixed Italian ryegrass-wheat stands, total cover measured 140 days after wheat
planting was similar regardless of Italian ryegrass seed bank density or the presence or absence of
barnyardgrass hedges Table 1. However, relative Italian ryegrass cover in the mixed stands was
reduced 3- and 8-fold, respectively, in the high and low-density plots. In the pure Italian ryegrass
stands, cover was reduced 3- and 4-fold, respec-
Ghersa et al. 1994 documented the effect of reductions in radiation, R:FR light ratio and the
reduction in thermal amplitude on germination of Italian ryegrass seeds relative to wheat seed germi-
nation, and on crop yield. In their experiment, wheat yield was improved because of a lower
ryegrass-wheat seedling density. Reductions in light intensity and in the RFR ratio may also
increase shoot-root ratio Ballare´, 1993. Consid- ering that ryegrass is more responsive than wheat
to light environmental changes during early stages of development Hashem et al., 1998, barnyard-
grass interference may have played a major role in the observed lower cover of ryegrass in the pres-
ence of the hedge. On the other hand, it is well established that plant response to interference re-
sults from complex interactions. Ryegrass has been reported to have higher below ground com-
petitiveness relative to wheat Stone et al., 1998. It is possible that barnyardgrass reduced the com-
petitiveness of ryegrass in our experiment, or changed microclimatic factors during ryegrass es-
tablishment and tillering. In our experiment, ryegrass biomass was gener-
ally not as drastically reduced as cover by grow- ing hedges Table 2. Two factors are responsible
for the reduction in tillering rate, largely deter- mining cover in grass species: a decrease in light
interception per plant and lower RFR ratios at their base Deregibus et al., 1983. However, it
has been demonstrated that during ryegrass growth tillering might be reduced by a phy-
tochrome response triggered by low RFR before a serious scarcity in energy availability is pro-
duced by mutual shading Casal et al., 1986. These authors observed that lower densities af-
fected tillering without depressing dry matter accumulation.
3
.
2
. Experiment
2
In the first experiment, separate effect of hedges shading, changes in the light quality and soil
resource exploitation were not accounted for. Since it is known that hedge height, orientation
and distance between them will affect the spatial pattern of resource availability and light quality,
the second experiment was designed aiming to vary them differentially, in order to evaluate pos-
sible interactions. Therefore, the following aspects were considered to lay the different treatments 1
soil resource exploitation is greater in the first 30 cm of the soil profile and in the first 50 cm from
the row. It decreases exponentially with depth and distance
away from
the row
Plavychenko, 1937a,b; Davis et al., 1965; Ca´rcova et al., 1998;
2 a hedge oriented E – W, would shade the soil all day on the N side; and if it has green tissues it
should also reduce the R:FR ratio near the soil surface, up to a distance depending on its height
and the sun angle. On the S side of the hedge, no shading is expected but R:FR ratio is reduced by
the light reflected by the leaves, when they have live tissue Ballare´ et al., 1987; 3 a hedge ori-
ented N – S should shade, and if it is alive, it will reduce the R: FR ratio in a similar way in both E
and W sides, except for the fact that soil shading will vary along the day with the sun’s trajectory.
As expected, the dead maize plants brown hedges shaded the soil without altering the R:FR
Table 6 The relationship between number of tillers and individual
plant biomass in plots with maize hedges differing in color and orientation
Species R
2
Hedge Regression equation
Hedge color
orientation Wheat
Green N–S
Tillers = 0.2975 0.82
× biom−8.4575
0.81 E–W
Tillers = 0.3102 ×
biom−13.541 N–S
Tillers = 0.3236 Brown
0.71 ×
biom−40.258 E–W
Tillers = 0.2378 0.76
× biom+13.593
Ryegrass N–S
Green Tillers = 0.3625
0.73 ×
biom+8.5438 E–W
Tillers = 0.1892 0.74
× biom+15.284
Tillers = 0.3508 N–S
0.90 Brown
× biom+4.185
Tillers = 0.2828 E–W
0.85 ×
biom+8.3429
ratio which was around 1.0 at all the measured distances Table 3. Live maize plants green
hedges in the other half of each main plot both shaded the soil and reduced the R:FR ratio until
the second and third week of October when the first killing frosts occurred. Visual observations
revealed that at the crop seedling stage, the E – W hedges shaded up to 2.4 m towards N direction.
Therefore the maize plants shaded the soil throughout the day in the 1.2 and 2.4 m spacing,
but only to some extent, in the wider hedge spacings 3.6, 4.8 and 6.0 m. During the early
morning and late afternoon, when the inclination of the sun was low, the soil was fully illuminated.
Shading by maize plants increased as the sun rose until reaching its maximum at noon. Moreover,
variable proportions in these plots near the S side of a hedge were never shaded, but radiation
reflected from the live hedges still reduced the R:FR ratio. In the plots with live hedges, the
R:FR ratio was highest at noon, but never ex- ceeded 0.4 Fig. 3. Maize plants oriented N – S
shaded the ground nearly all the day except for 2 h around noon. The R:FR ratio in the plots with
N – S live hedge orientation varied with distance away from the hedge to a greater extent than in
those plots with live E – W hedges. The maximum values observed at midday in the center of the
plots with N – S hedge increased progressively with hedge spacing: from 0.4 in the narrowest to 1
in the widest. Therefore during the day the ratio changed from around 0.2 when the soil was
shaded to the corresponding midday maximum. By the end of November the maize plants in all
the hedges had lost a high proportion of their leaves, and by the end of December there were no
standing maize plants.
The principal components analysis for 1990 and 1991 data provided results with regard to the
most important agronomic manipulations hedge color, orientation and spacing affecting biomass
production of wheat and ryegrass. The first two axis of the ordination diagram explained nearly
90 of the data variance Table 4. Pearson and Kendal correlations with ordination axes indi-
cated that wheat yield was little affected by envi- ronmental manipulations. We found a strong
negative correlation of wheat vegetative and re- productive yield with the first ordination axis, but
this correlation is not associated to any of the environmental conditions generated with the
hedges. On the other hand, part of the ryegrass vegetative and reproductive biomass variability
24 is associated to the environmental condi- tions generated by the experiment, in particular,
orientation of the maize hedges Table 4.
Despite slight differences in weather conditions Table 5, interactions between hedge treatments
and harvest year were not statistically significant on wheat and ryegrass yield, and therefore, only
average figures are presented. Spacing and orien- tation of maize hedges did not affect wheat yield
P B 0.05, which averaged 2500 kg ha
− 1
. More- over, wheat yield was independent of sample dis-
tance from maize hedge Fig. 4. However, at the narrower hedge spacing, average vegetative and
reproductive biomass for wheat was significantly greater in the dead-hedge treatments than in the
live-hedge treatments Fig. 4. In the treatments with live hedges, competition for both, above and
belowground resources may have occurred as a consequence of the 2-week longer growth of the
maize plants in the non-sprayed hedges than the sprayed.
Significant interactions were detected between treatments
and hedge
spacing on
ryegrass biomass. Spacing of maize hedges affected Italian
ryegrass biomass production Fig. 5. Live hedges increased ryegrass vegetative biomass by about
25, compared with dead hedges. This increase was particularly large at 1.2 and 2.4 m hedge
spacing Fig. 5. The low R:FR ratio experienced by ryegrass leaves growing in plots with live
hedges may have affected canopy structure im- proving ryegrass light interception Ballare´, 1993;
Morgan and Smith, 1981. Differences between live and dead hedges were largely insignificant in
terms of Italian ryegrass reproductive biomass production Fig. 5.
Indirect effects of resource availability on the biotic interactions between crops and weeds have
been previously reported Shainsky and Rado- sevich, 1992. Hedge orientation and color pro-
duced changes in the competitive relationship between wheat and ryegrass Figs. 6 and 7. In the
plots with live green maize hedges oriented N – S, total biomass production of wheat was not
modified by the presence of ryegrass, and vice- versa. In the rest of the plots, total biomass of
each species decreased as the biomass production of the other species increased. The slope of the
linear model provides an estimate of the competi- tive effect of one species on the other Radosevich
et al., 1997. In these plots, biomass decrease of ryegrass almost doubled that of wheat Figs. 6
and 7. These findings are in accordance with the results of Hashem et al. 1998 who showed that
wheat was less affected than ryegrass as a result of competition in a wide range of plant spatial
arrangements.
No significant differences among treatments were observed in the proportion of biomass allo-
cated to reproduction by wheat Fig. 8. The relation between reproductive and vegetative pro-
duction has agronomic significance because it gives an indication of how much of the crop
biomass was allocated to the grain, which is har- vested, and how much of the weed biomass went
to seed production, which feeds the soil weed seed bank Weiner, 1988. In all the environmental
conditions, spikes dry weight was around 60 of wheat vegetative biomass. Instead, allocation to
reproduction was greatly modified in ryegrass by the orientation of the maize hedges Fig. 9. In the
plots with hedges oriented E – W a higher propor- tion of the biomass allocated by the ryegrass to
reproduction was observed than in plots with hedges oriented N – S, irrespectively of the hedge
color. Annual plants draw the carbohydrate com- pounds
for their
reproductive requirements
mainly from their current dry matter production Chapin et al., 1990. Because vegetative growth is
already coming to an end when seed development begins, the quantity and quality of the seeds are
influenced by the environmental factors prevailing before, as well as during, reproductive phase
Larcher, 1995. It is possible that in the plots with hedges oriented N – S, longer periods of
shading, poor supply of nutrients to ryegrass plants, or both, might have given priority to
vegetative development.
Live or dead biomass and orientation of plant hedges were important factors regulating tiller
number per unit of total biomass production of Italian ryegrass Table 6. In the plots with live
E – W hedges where the maximum level of the R:FR ratio never exceeded 0.4 Fig. 3 ryegrass
produced half of the tillers per unit of biomass than those produced in plots with N – S hedges
R:FR maximum ratio = 1. In the plots with brown hedges R:FR ratio = 1 the tiller produc-
tion per unit of biomass was the same for both orientations and similar to the one for the N – S
live hedges. As with the other measured variables, wheat tiller production per unit of biomass was
insensitive to hedge treatments. The effect of low R:FR ratio on the reduction of tillering has been
shown for ryegrass and other grass species Casal et al., 1985. This process is so sensitive to R:FR
that Paspalum dilatatum grown in pots placed at low densities B 7 plants m
2
do not shade each other but show a reduction of tillering of 40
compared to isolated controls Casal et al., 1986. None of the above results were modified when
we included in the analysis only the subplots that were further away than 1.2 m from a hedge not
shown. This strongly suggests that root competi- tion from the maize plants was not a factor
affecting wheat and Italian ryegrass competitive interactions during the second experiment. In con-
trast, the different responses of wheat and rye- grass to the environmental modifications can be
interpreted on the basis of differences in photo- morphogenic sensitivity between the species. Our
results suggest that the reductions in solar radia- tion and the R:FR ratio near the soil surface were
important factors that influenced the hierarchical organization of the studied species during seedling
emergence and establishment. This effect of early environment on later biomass production was evi-
dent in both, the first and the second experiment. Similar results obtained by Dunnett and Grime
2000 have provided strong evidence of an am- plification mechanism mediated by competition of
plant responses to controlled environmental ma- nipulations applied early in the growing season.
Photomorphogenic responses in field crops which can regulate to a great extent the outcome of
interactions, can be manipulated by changing the light environment or by altering the sensitivity of
the crop plants to photomorphogenic light signals Smith, 1992; Ballare and Casal, 2000.
4. Conclusions
