0234-B1

Tree-Crop Interaction Studies in Natural Agroforestry System: A Case Study from Western Himalayas in India

R. Kaushal^[1] and K.S. Verma

ABSTRACT

Above and below-ground interactions of grewia (Grewia optiva) and wheat (Triticum aestivum) were investigated by laying out sample plots at 1 m, 2 m, 4 m, 6 m and 8 m distances towards the outer canopy of single scattered trees of grewia. To quantify the above and below-ground interactions, observations were recorded for soil moisture, soil temperature, soil nutrients, light and beneath-canopy temperature.

It was found that growth and yield of wheat crop was influenced negatively below the tree crown, while it increased with increase in distance from the tree trunk. Soil moisture, soil nutrients, soil temperature and beneath-canopy temperature were reduced considerably at 1 m and 2 m distance during all the growth stages of wheat. Further, grewia did not pose any competition to wheat for light at sowing and tillering. However, at panicle initiation, milking and harvesting of wheat, light was reduced considerably below grewia.

Introduction

Mountain farming systems are the typical agroforestry system developed and modified by peasants over the time in response to change in social, cultural, economical and biotechnological needs. Hill farmers, irrespective of size of landholdings retain few trees on farmlands. These trees not only provide fuel wood, fruits, fodder and timber but also are also helpful in bringing microclimatic changes by reducing soil and air temperature, irradiance and wind speed. However, retaining these trees normally requires a price in terms of competition for moisture, light and nutrients for the field crops. Understanding the degree of competition in these systems can provide the base line data for determining the optimum tree density, devising suitable management options and to maximize the resource use and productivity of the systems. Thus, the present study was undertaken to quantify the degree of above and below ground interactions in natural agroforestry system.

Materials and Methods

The study was carried out at experimental farm of Dr Y.S. Parmar University of Horticulture and Forestry, Nauni, of Himachal Pradesh in India. The study site is located in sub-tropical sub-temperate mid hills (1250 m asl) of western Himalayas. Single tree of Grewia optiva with average height of 8.2m, average girth of 105 cm and average crown spread of 2.8 x 3.1m was selected on the farm bund. Wheat crop was sown during the first week of January 2000. The effect of tree on crop production was estimated by recording observations on various crop parameters by laying out sample plots of one square meter in concentric circles on perpendicular lines starting from the base of each tree. The distance of sample plots on the line was spaced at 1m (D1), 2m (D2), 4m (D3), 6m (D4) and 8m (D5) towards the outer canopy region of the tree. Distance D5 was considered as open area in the experiment. To quantify the above and belowground interactions, observations were recorded for soil moisture, soil temperature, soil nutrients, light and beneath canopy temperature. The observations were recorded at the time of sowing (S1), tillering (S2), panicle initiation (S3), milking (S4) and harvesting (S5) of wheat. Phenophases/growth stages of Grewia, and wheat are presented in table 1.

Table 1. Phenophases/growth stages of Grewia optiva and wheat.

Soil moisture content at surface Dp₁(0-15 cm) and sub-surface Dp₂ (16-30 cm) soil depth was measured gravimetrically at different distances and different growth stages of wheat. Soil temperature was measured with the help of soil thermometer at 0-5 cm (Dp₁), 6-15 (Dp₂) and 16-30 cm (Dp₃) soil depth at each distance and growth stages of wheat. Soil nutrient contents viz. available N, P and K were also determined at the time of sowing and after wheat harvest.

Light intercepted by wheat below the tree canopy was measured with Lux meter at different distances and growth stages of wheat. Beneath canopy temperature was measured with the help of infrared thermometer at different growth stages.

Results

All the growth and yield attributes of wheat (Table 2) depicted significant reduction of varying magnitudes underneath Grewia. Plant height and number of tillers per plants decreased significantly at 1m distance as compared to other distances. Grain and straw yield also depicted the maximum reduction in the close proximity of the tree at 1m distance, where about 51.9 per cent reduction was recorded as compared to control/8m distance. Further, yield improved in a linear fashion with increase in distance from the tree base.

Table 2. Growth and yield attributes of wheat as influenced by distance underneath Grewia optiva

Soil moisture measured during the cropping stages of wheat at different growth stages (Table 3) showed significant variations in its level due to distance and depth of soil layers. At the time of sowing, distance in general did not influence the soil moisture. But at all other growth stages, moisture at surface layer of soil was reduced considerably at 1m and 2m distances. At 3m distance, moisture level rose considerably and remained almost constant or decreased slightly up to 8m distance. However, at the time of harvesting of wheat, soil moisture at surface layer increase considerably. It was also observed that surface soil moisture was maximum at the tillering stage which reduced to its lowest at the time of panicle initiation followed by milking, the most critical growth stages for grain formation and filling.

Table 3. Variations in soil moisture (%) content during wheat cropping period at varying distances and soil depths below Grewia optiva

Nearly similar pattern for moisture in sub-surface layer was evident as that for surface layer. Overall, soil moisture was more in this layer as compared to surface layer.

Soil temperature exhibited an overall increase as the season progressed towards the maturity at all the soil depths (Table 4). During the initial growth stages i.e. sowing and tillering, distance did not exhibit any much effect on the soil temperature in all the soil layers. In the later stages of wheat, however, soil temperature showed a significant increase with increase in distance from the tree base. The temperature was maximum at 8m and minimum at 1m distance at all the growth stages of wheat. Further, a gradual increase in the temperature was observed up to harvesting of wheat at all soil depths. During all the growth stages, soil temperature was maximum in 0-5 cm soil depth, which was followed by 6-15 cm, and 16-30 cm soil depth (Table 4)

Table 4. Variations in soil temperature (°C) during wheat cropping period at varying distances and soil depths below Grewia optiva

Available N, P and K in soil (Table 5) at sowing and harvesting under grewia showed significant variation due to distance from the tree base. At the time of sowing, plots under the Grewia at 1 and 2m had less available N as compared to 8m distance. At harvesting, similar trend was observed too. Removal of soil N as estimated by difference at sowing and harvesting was however greater at distance 1 and 2m as compared to other distances. Available P and K also followed the similar trends both at sowing and harvesting as depicted for soil N. The quantity of the nutrients removed by were however less to that of N.

Table 5. Pre-sowing and postharvest status of available NPK (kg/ha) in experimental soil below Grewia optiva

Relative illumination was influenced significantly due to sampling distances at various growth stages (Table 6). At sowing and tillering, no significant difference in relative illumination was observed at different distances. At panicle initiation, milking, and harvesting, minimum light was recorded in the close proximity of the tree at 1m and 2m distance. Light availability again increased with increase in distance from the tree base towards the outer side.

Table 6. Per cent relative illumination below Grewia optiva at different distances during the active growth stages of wheat

Beneath canopy temperature during wheat cropping season, was not influenced by Grewia at sowing and tillering (Table 7). At panicle initiation, milking and harvesting, beneath canopy temperature was however, reduced significantly at 1m, 2m and 4m distance from the tree. The respective reduction at these stages were 2.83°C, 2.12°C and 2.57°C over the open area i.e. 8 m distance. Again, an increasing temperature with increase in distance from tree base was recorded.

Table 7. Beneath canopy temperature (°C) below Grewia optiva at different distances during the active growth stages of field crops

Discussion

The influence of Grewia on the wheat produce varied according to the distance of the tree. Competition for growth and yield was more pronounced in close vicinity of the tree at 1 and 2 m. The more negative effect in close vicinity of trees can be ascribed to more competition for moisture, nutrients and light, which is also evident from the present study. Reductions in yield of wheat below the tree crown due to resource competition were also reported by Puri and Bangarwa (1992) and Dhillon et al. (1998).

An agrisilviculture system (wheat + grewia) was established in the same region and it was reported that integration of Grewia optiva (tree density 666 trees/ha) with wheat pollarded at 1m height reduces the wheat grain yield by 24 per cent only as compared to 50 per cent in natural agroforestry system in this paper. This indicates that reduction in the wheat yield due to natural growing Grewia trees on farmland can be reduced by about 50 percent of the yield obtained in present studies by adopting tree management practices of pollarding (Verma et al., 2002). The authors also reported an additional financial return of INR 2500 (US $ 55) from Grewia leaf fodder and fuel wood/ha.

At the time of sowing of wheat less soil moisture competition can be attributed to dormant period of Grewia at this stage. The significant lower moisture at 1 m and 2 m distance at all other growth stages of wheat can be attributed to the competition for moisture by the surfacial root systems of the tree; as most of the lateral root spread of Grewia is confined within 2 m distance from the trees base (Zegye, 1999). Competition for moisture in agroforestry systems is common occurring phenomenon, which can affect the system adversely (Ong et al., 1991; Rao et al, 1991). Further, active growth stages/phenophase of field crop/tree species coincided temporally, which might also have created competition for soil moisture at these stages. In addition heavy canopy load (phytomass) of grewia at these stages might, therefore, have extracted large volume of soil water.

Higher moisture in subsurface layer as compared to surface layer may be attributed to more sorption of moisture by crop and tree roots from upper layer since both the species are having shallow root system in general. Further, majority of roots of Grewia remains confined to 60 cm soil depth (Zegye, 1999). The occasional encounter of Grewia roots during sampling, often shallow and laterally extensive in upper soil layer also confirms the above findings.

Below Grewia canopy, lower soil temperature was maintained at 1m and 2m distances at the time of milking and harvesting. A similar effect of tree canopy on soil temperature was observed by Vandenbelt and Williams (1992). The influence of soil temperature is probably related to the shading effect of full crown of Grewia at these stages. At subsurface more reduction in soil temperature can be attributed to more soil moisture in this layer.

Lower content of nutrients in soil below trees is generally not acceptable hypothesis but, negligible addition of leaf litter to the soil (as Grewia leaves are heavily lopped by the farmers in winter season) seems the reason for low nutrient content of the soil below their crown. This is also evident from the present study, where removal of nutrients at 1 m and 2 m distance was more as compared to outer distances. Further, the majority of roots of Grewia generally remain in top 60 cm layer, which is also nutrient sorption zone of wheat and thus might have created competition for nutrients.

A less competition for light at sowing and tillering is a direct result of the dormant period of Grewia coinciding with these crop growth stages; while during other growth stages, reduction of light at 1 m, 2 m and 4 m distance from the tree base can be attributed to full crown of Grewia.

Beneath canopy temperature also showed more reduction at 1 and 2m, which again can be ascribed to direct shading effect of Grewia on wheat. Reduced temperature below trees has also been reported by Monteith et al. (1991), Hazra and Patil (1996), and Thakur and Kaur (2001).

Conclusion

Retaining large unmanaged trees on farmland for fuel, fodder and timber etc.form integral part of the rural farmers community. These systems offer great potential in terms of the farmers need but are often constrained by the competition for light, nutrients and moisture. Yield reduction below these high shading trees is a challenge and requires a serious consideration. A careful selection of these trees can thus be helpful in devising the new agroforestry systems and modifying and managing the existing agroforestry systems.

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