Agroforestry Systems 40: 207–213, 1998. 1998 Kluwer Academic Publishers. Printed in the Netherlands.
Short communication
Effect of Poplar (Populus deltoides) shelterbelt on the growth and yield of wheat in Punjab, India H. P. SINGH, R. K. KOHLI* and D. R. BATISH Department of Botany, Panjab University, Chandigarh 160 014, India (* Author for correspondence) Key words: biomass, crop density, phenolics, root length Abstract. A study was conducted around Chandigarh (Punjab), India to assess the effect of Populus deltoides shelterbelt on wheat. Crop density, root and shoot length, biomass and grain yield of wheat at crop maturity were significantly lower, up to 12 ± 1 m in the fields sheltered by P. deltoides than in unsheltered plots. The reduction of biomass and grain yield of wheat were comparatively more than that of other parameters. The observed reduction in the growth and yield of wheat could be attributed to phytotoxic interference of the phenolics from the tree with the crop.
Introduction In India, during the past two decades, fast growing exotic trees have been introduced in order to obtain maximum monetary gains from a given land unit in a short span of time. Poplar (Populus deltoides), a winter deciduous tree, is one such tree that is being planted either in rows in crop fields or along field borders as boundary marker or shelterbelts. The present work was conducted to study the effect of P. deltoides shelterbelts on the growth and yield of wheat – an associated winter crop.
Materials and methods The study was conducted in the farmer’s fields situated on the outskirts of Chandigarh (30°42′ N, 76°54′ E, 333 m sl) in Punjab, India. The fields were sheltered from the North by a nearly 50 m long, double-rowed belt of Populus deltoides (six years old; height 16.2 ± 0.2 m; diameter at breast height 23.4 ± 1.2 cm) with an average inter-tree and intra-row distance of 5 m and 4 m, respectively. Wheat (Triticum aestivum var. HD-2329) was sown in the fields during the second week of November 1994 following the conventional methods of sowing, watering, caring and harvesting (Jaiswal, 1980). Wheat growth and yield were assessed at harvest time at the distances of 1, 2, 3, 5, 7, 9, 11, 12, 13, 15 and 20 m southwards from the tree belt. ADC (Adjustible, Decimal, Collapsible) quadrat of 1 m (Khan, 1974) was placed at each distance and the measurements were made. All plants with their stem in the quadrat were
208 recorded. At each distance 15 quadrats were laid. Similar observations were also made in unsheltered fields in continuation with sheltered ones but 50 m away. In both sheltered and unsheltered fields, the same cultural practices such as sowing, irrigation, fertilizer application, and weeding were followed. Besides, the yield was also estimated at the distances of 25, 30, 35, 40, 45, 50, 55 and 60 m from the shelterbelt. Soil samples were collected from depths of 0–10 cm, 10–20 cm and 20–30 cm from ten randomly selected points in sheltered as well as unsheltered fields. The pH and conductivity were measured from a soil paste in water (1:2, w/v). The content of organic carbon (Walkley and Black’s method), available nitrogen (by kjeldahl method using alkaline potassium permanganate), phosphorus (Olsen’s method) and potassium (by flame photometer after extraction with ammonium acetate) was determined by following standard procedures (Jackson, 1967; Piper, 1950). In addition, the soil samples were collected at the distances of 1, 2, 3, 6 and 12 m from the shelterbelt in the sheltered fields or in the unsheltered fields (for comparison). The phenolics were extracted from the soil following the method of Kaminsky and Muller (1977) using sodium salt of EDTA as an extractant. The study was repeated in three fields to serve as replication. The data were analysed using one-way anova, Duncan’s multiple range test and correlation coefficient (SPSSPC, 1990).
Results and discussion The study revealed that the crop density, root and shoot length, biomass and grain yield of wheat were reduced in plants that were closer to the tree-line in the fields sheltered by P. deltoides, compared to the unsheltered ones. At one meter distance from the tree-line, only 15 plants per square meter were counted compared to 204 in the unsheltered fields (Figure 1a). However, with increasing distance from the tree, the crop density markedly improved and beyond 12 m it was similar to that in the unsheltered fields. Similarly, the root and shoot length of wheat were shorter in the sheltered plots than that of control up to 11 m (Figures 1b and 1c). Maximum reduction was observed in case of biomass and grain yield which also continued up to 12 m from the tree line (Figures 1d and 1e). As the distance from the tree line increases, the wheat growth and yield improve considerably. Similar observations in the case of Eucalyptus have also been made earlier under a similar set of conditions by Kohli et al. (1990). The yield of wheat was only 58.76% of the unsheltered control fields when measured up to 20 m from the shelter belt (Table 1). Beyond 20 m there was no significant change in the yield of wheat in the P. deltoides sheltered area and was similar to that in the unsheltered fields (Figure 2). In all there was a reduction of 346 kg ha–1 wheat grains in the sheltered fields compared to unsheltered ones (Table 1). The difference in growth and yield of wheat in the otherwise similar shel-
209
Similar letters along respective curves represent insignificant difference at 5% level applying DMRT. ‘r’ represents the value of correlation coefficient between respective parameters and different distances in unsheltered area (ru)or in P. deltoides sheltered area (rp). Figure 1. Mean values of (a) crop density, (b) root length, (c) shoot height, (d) biomass, and (e) grain yield of wheat at different distances from the shelterbelt of P. deltoides or in the unsheltered fields in Punjab, India.
tered or unsheltered fields (with respect to climatic, edaphic and topographic aspects) could be due to factors like shading and/or competition and phytotoxic interference with the tree. The shading effect could have been negligible as the study was conducted on the southern side of the shelterbelt which allows maximum sunlight to fall on the field throughout the day. Moreover, the tree, being winter deciduous, remains leafless during the active growing period of wheat. As per the conventional agricultural practice in North India, the wheat
210 Table 1. Grain yield of wheat in fields sheltered by P. deltoides trees in Punjab, India. Fields
In the 1 × 20 m block (from 1 m to 20 m)
In the 1 × 40 m block (from 20 m to 60 m)
Total yield (kg ha–1)
Reduction in yield over control Total (kg ha–1)
Per cent
Unsheltered (Control)
8.27 ± 0.84 (0.4136)
16.69 ± 0.24 (0.4172)
4165 ± 131.20
–
–
Sheltered by P. deltoides
4.86 ± 0.55 (0.2432)
16.66 ± 0.79 (0.4165)
3819 ± 133.01
346.0
8.31
Values in parenthesis represent grain yield in kg m–2. ± represents standard error of the mean.
‘r’ represents the value of correlation coefficient between grain yield and different distances in unsheltered area (ru)or in P. deltoides sheltered area (rp). Figure 2. Grain yield of wheat at distances beyond 20 m from the shelterbelt of P. deltoides or in the unsheltered fields in Punjab, India.
Conductivity (µS cm–1) 205.0 ± 55.7 (160 ± 22.4) 141.35 ± 62.8 (84 ± 21.3) 133.25 ± 46.6 (58.4 ± 12.4)
pH
7.35 ± 0.28 (6.65 ± 0.21) 7.38 ± 0.19 (6.98 ± 0.14) 7.37 ± 0.29 (6.89 ± 0.11)
Values in parentheses represents values in unsheltered fields. ± represents standard error of the mean.
20–30
10–20
0–10
Depth (cm)
0.43 ± 0.17 (0.42 ± 0.05)
0.36 ± 0.14 (0.39 ± 0.10)
0.59 ± 0.16 (0.65 ± 0.11)
Organic carbon (%)
172 ± 20.4 (163.5 ± 10.2)
179.5 ± 19.3 (172 ± 14.3)
202.5 ± 26.6 (167 ± 22.3)
Nitrogen
54.45 ± 7.1 (53 ± 6.2)
51.7 ± 10.8 (45 ± 8.5)
44.43 ± 5.6 (62 ± 10.2)
Phosphorus
Available nutrients (kg ha–1)
Table 2. Physico-chemical characteristics of the soil in the P. deltoides sheltered or unsheltered fields in Punjab, India.
122.5 ± 29.5 (106.67 ± 18.6)
121.67 ± 26.9 (86.67 ± 22.2)
208 ± 76.4 (140 ± 24.5)
Potassium
211
212 crop is sown in November, by which time the leaf fall of P. deltoides has already started. The already fallen leaves are ploughed while preparing the soil for wheat sowing. The leaf fall continues till December and the leaves which fall after the sowing interfere in the emergence and/or seedling growth of wheat. This serves as one of the reasons of poor wheat performance near the P. deltoides shelterbelt. By the middle of March when the new leaves on the tree appear, the seed setting of wheat is already complete and near maturity. It is only at the time of harvest, i.e. the beginning of April that the tree is covered with the new leaves. Thus, during the active phase of wheat growth, i.e. from December to March the tree remains leafless and its leaf litter adversely affects the germination as well as initial growth of wheat. The chemical analysis of the soil revealed that soil pH, conductivity and organic carbon were nearly the same under both sets of conditions. However, the amount of available N, P and K was more in the sheltered fields than in the unsheltered fields although cultural practices of irrigation, fertilizer application, weeding, etc. were kept similar (Table 2) which therefore does not support the possibility of nutrient stress in the sheltered area. Since the study was conducted under irrigated conditions, the observed reduction in the growth and yield of wheat in the sheltered fields could not be ascribed entirely to moisture stress. However, the roots of P. deltoides mostly remain in the upper 0–30 cm soil profile (Toky and Bisht, 1992) and their competition with wheat roots for moisture, nutrients cannot be ignored. Contrarily, the increase in nutrients under sheltered fields could be due to their release from P. deltoides leaves which fall in plenty during this season. On the other hand, the soil closer to the tree line in the sheltered fields was found to be rich in phenolics, the content of which, estimated at different distances from the shelterbelt, showed a negative correlation with wheat performance (their amount decreased with increasing distance from the shelterbelt, Table 3). These phenolics may be responsible for the observed depressive trend of wheat, as the phenolics in general, are widely reported to deleteriously affect the germination and initial growth of a number of plants (Inderjit, 1996). Kuiters (1989) reported that litter from trees generally contains phenolics. The phenolics in the present study may have been released Table 3. Amount of phenolics (mg/100 g soil) extracted from the soil collected at different distances from the P. deltoides shelterbelt or in unsheltered fields in Punjab, India. Depth (cm)
0–10 10–20 20–30
Distance from the tree-line sheltered field (m)
Unsheltered field
0
1
2
3
6
12
212a,2 65c,4 78b,1
232a,1 107b,1 70c,2
137a,3 42c,5 55b,3
132a,3 72b,3 54c,3
89a,4 84a,2 40b,4
76a,5 69a,3,4 35b,5
20a,6 18a,6 17a,6
Similar letters within a column or similar numerals within a row represent insignificant difference in values at 0.05 level applying Duncan’s multiple range test.
213 from the leaf litter of P. deltoides as its leaves are already known to release phytotoxins identified as phenolics (Singh, 1996). Thus, the accumulation of phenolics in the sheltered fields may be due to their release from the tree ‘wheat being a common factor in both the fields’. The presence of more phenolics nearer the shelterbelt further strengthens this view, as the number of wheat plants and their growth was greatly reduced in that area. Upon their release they get bound to the soil particles reversibly or irreversibly. With the mass action of water (irrigation or rain) the reversibly bound phenolics get released in the soil, affecting the germination and growth of crop seeds (Kaminsky and Muller, 1977). Thus, the observed reduction in growth and yield of wheat in the present study can be ascribed to phytotoxic interference of the phenolics present in the soil of P. deltoides sheltered fields.
Acknowledgement H. P. Singh is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi for financial assistance in carrying out this work.
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