Variability in light associated traits under different agroforestry systems
Agroforestry includes the incorporation of trees with livestock and/or crops in different spatial arrangements. Growing crops in the interspaces of tree rows is attractive since it permits harvesting of intermediary products which fetches short-term profitability while maintaining the planted trees and enables efficient utilization of the land. Agroforestry is the important land use system which integrates trees, crops that is scientifically sound, ecologically desirable, practically feasible and socially acceptable to farmers. The choice of crops mainly depends on light availability below the canopy of trees which in turn depends leaf area, and leaf angle or the canopy architecture. The light requirement varied with crops depends on many light responsive photochemical and biochemical reactions and processes that would reflect on the growth and yield of crops. Therefore, the studly aimed to quantify the light available to crops grown under different agroforestry system to understand whether light penetrated per se is sufficient to meet the crop requirements in terms of maintaining photosynthetic rate, growth and yield traits. We have quantified the light availability to crops using light integrated meters equipped with three sensors for measuring photosynthetically active radiation, total radiation and photometric measurements and compared with light availability under open condition. As expected, open space showed higher light associated traits viz., photosynthetically active radiation, total radiation and photometric measurements which were measured by about 1648 µmole m− 2 sec− 1, 937 Watts m− 2 and 83838 lux respectively.
A significant (p = 0.05) difference was observed between different agroforestry systems, among which Dalbergia sissoo recorded maximum photosynthetically active radiation (681 µmole m− 2 sec− 1), total radiation (260 watts m− 2) and photometric measurements (35576 lux), whereas minimum photosynthetically active radiation of 347 µmole m− 2 sec− 1 was observed in Khaya senegalenesis. In case of Casuarina equisetifolia, a minimum total radiation of 119 watts m− 2 and photometric measurements of 2317 lux were also observed. The result indicated that the availability of light including PAR was higher under Dalbergia sissoo followed by Melia dubia, which is three times less than the open condition and further this has been validated with cowpea and fodder sorghum to understand whether available light is sufficient to trigger the physiological process including photosynthesis and yield components.
Measurement of gas exchange parameters and light interception of cowpea and fodder sorghum under different agroforestry systems
When annual and perennial crops are grown on the same unit of land, they will influence the growth and development of each other and affects the overall performance than when these are grown as sole crops. Understanding the interactions between different components of these agroforestry practices is necessary to increase productivity, minimize the negative interactions and develop successful agroforestry practices specific to the respective agro-climatic region. Developing suitable tree-crop combinations and assessing productivity is the major field to be considered in agroforestry (Jincy et al., 2022).
One of major interactions is the light interceptions by the crops grown under tree crops. The availability of light varied from different agroforestry system depending on leaf area index and canopy architecture and therefore, light interception by the canopy of cowpea and fodder sorghum was measured to understand how intercepted light influence the gas exchange parameters and yield components. Light interception ratio of Dalbergia sissoo were also observed maximum with 73% and 76% and minimum light interception ratio of 53% and 58% were noted in Khaya senegalenesis @ 40 and 60 DAS, respectively. In addition, it was found that cowpea and fodder sorghum grown in open intercepted higher light than crops grown as intercrop. Kermah et al., (2017) reported that sole legumes intercepted more PAR than intercrops whilst intercrops intercepted more PAR than sole maize as this was more evident after silking when maize leaves started senescing. As both the crops are not of senescing type, we could not observe differential response in our study.
In case of gas exchange parameters, photosynthetic rate and stomatal conductance of cowpea were found significantly higher under Dalbergia sissoo with 19.43 µmole CO2 m− 2sec− 1 and 0.09 cm2sec− 1 respectively compared to agroforestry systems. It is established fact that C3 crops requires less light than C4 crops. In this study, both the crops responded linearly with light availability for photosynthetic rate indicating that crops have not reached the light saturation and available light may be just sufficient to drive the photosynthesis. The cowpea showed 9.21% increase in photosynthetic rate under Dalbergia sissoo based agroforestry system that received higher light whereas the fodder sorghum showed 14.28% increase over the Dalbergia sissoo indicating the light requirement for the fodder sorghum has not attained the peak may be because of fact that C4 crops require more sunlight than C3 crops (Li et al., 2021). Singh et al. (2019) observed that with the advancement in height and crown expansion of trees, the shading effects on the crop had increased, which in turn affected the photosynthetic efficiency of the crop resulting in lower vegetative growth and lesser grain yield as compared to the sole crop.
Influence of different agroforestry systems on yield of cowpea and fodder sorghum
The different agroforestry system allows varying quantity of light intensity and impact physiological and biochemical process that would alter the yield parameters. Among different agroforestry systems, yield parameters like number of pods per plant (30.10) and yield (1150 kg/ha) were recorded higher in open space followed by Dalbergia sissoo with number of pods per plant by 29.33 and 923.33 kg/ha of yield. The minimum number of pods per plant (21.90) and yield (776.67 kg/ha) were noticed in Casuarina equisetifolia while compared to other agroforestry systems. Similarly, a different agroforestry system showed significant impact on yield of fodder sorghum. The highest yield was recorded in open space were 28.5 t/ha and 124.6 t/ha and lowest yield was observed in Khaya senegalenesis (5.6 t/ha and 35.5 t/ha), during single cut and four cuts respectively. The yield reduction in cowpea and fodder sorghum under intercrop system compared to sole crop or open system is evident that ranges from 13 to 25 percent which was mainly due to light interception is more in early growing season (Gebru et al., 2015). Similar reduction in yield was observed in cowpea which might be due to competition of crops under the tree canopy for light, water and nutrients (Ajaykumar et al., 2021)
Several studies indicate a higher water use from the tree layer compared to monocropping systems thus future provision of water or susceptibility to drought stress must be considered prior to promoting tree species diversity (Yang et al., 2021; Zhang et al., 2016). For example, conversion of forest land to silvipasture through tree thinning will be affected differently than the conversion of monoculture to multiple cropping. Coble et al. (2020) found that conversion of forest to silvopasture in north-eastern USA led to a 35% reduction in transpiration rates, which accounted for a greater overall water saving despite an increase in soil evaporation.