The growth performance of poplar under different spacings after four years of plantation is presented in Table 1. The results clearly indicate that tree spacing had a considerable influence on the growth characteristics of poplar plantations. In the present investigation, one-year-old saplings were transplanted during January 30 to February 2, 2017 at six spacings (3×3 m, 4×3 m, 5×3 m, 6×3 m, 7×3 m and 8×3 m). At the time of transplantation, the average height, basal diameter and diameter/girth at breast height (DBH/GBH) of the saplings were 5.46 m, 3.51 cm and 2.83/8.9 cm, respectively. After four years of growth, noticeable differences in growth parameters were observed under different spacing treatments. The data presented in Table 1 revealed that tree height ranged from 14.3 m to 15.7 m under different spacings. The maximum height (15.7 m) was recorded under 5×3 m spacing, followed by 4×3 m spacing (15.0 m), whereas the minimum height (14.3 m) was observed under 6×3 m spacing. The relatively greater height under intermediate spacing could be attributed to moderate competition for light among trees, which promotes apical growth. Under closely spaced conditions, trees compete for light and tend to grow taller, while under wider spacing lateral crown expansion occurs rather than increased vertical growth.

The diameter at breast height (DBH) showed a clear and statistically significant response to spacing. The maximum DBH (22.6 cm) was recorded under the widest spacing of 8×3 m, followed by 7×3 m (21.2 cm) and 6×3 m (20.4 cm), whereas the minimum DBH (17.1 cm) was observed under the closest spacing of 3×3 m. However, the difference between 4×3 m spacing (18.7 cm) and 5×3 m spacing (19.1 cm) was statistically non-significant. The increase in DBH with increasing spacing may be attributed to reduced inter-tree competition for essential resources such as solar radiation, soil moisture, nutrients and rooting space. Wider spacing enables better crown development and root expansion, which enhances photosynthetic efficiency and accumulation of biomass in the stem. Conversely, closer spacing results in intense competition among trees, thereby limiting radial growth and reducing stem diameter.

The results of the present study are consistent with the findings of Dhillon et al. (2017) and Sirohi et al. (2022a) who reported that wider spacing in poplar plantations significantly increased tree diameter due to reduced competition for growth resources in semi-arid agroforestry systems. Similarly, Chauhan et al. (2012) observed that poplar trees planted at wider spacings showed higher DBH growth and improved canopy development compared with closely spaced plantations, primarily because of greater availability of light and soil nutrients. Further, Kumar et al. (2019) reported that wider spacing in poplar-based agroforestry systems promoted greater stem girth and biomass accumulation because trees were able to expand their crowns and root systems more effectively under reduced competition.

The availability of solar radiation under different spacings of poplar during the Rabi season (November 2020 to April 2021) was measured and expressed as percent reduction in available sunlight compared with open conditions (sole crop). The data presented in Table 2 clearly indicate that the presence of poplar trees significantly influenced the penetration of sunlight to the understory crop canopy and the magnitude of reduction varied according to tree spacing and seasonal phenology of poplar. During November, when poplar trees still retained a considerable amount of foliage, the reduction in light intensity was relatively high under closer spacings. The maximum reduction (72.01%) was recorded under 3×3 m spacing, followed by 4×3 m spacing (66.26%) and 5×3 m spacing (55.70%), whereas wider spacings such as 7×3 m (22.16%) and 8×3 m (17.47%) recorded comparatively lower reductions in sunlight. This pattern indicates that increased tree density resulted in greater canopy overlap and shading, thereby restricting solar radiation reaching the understory crops. During the winter months (December to February), the reduction in sunlight declined considerably across all spacing spacings. This reduction ranged from 70.87% to 7.74% in December, 53.58% to 4.59% in January, and 52.67% to 5.36% in February, from closest to widest spacing, respectively. The lower shading effect during these months can be attributed to the deciduous nature of poplar, which sheds its leaves during winter, allowing greater penetration of sunlight to the crop layer. Among all spacings, 8×3 m spacing consistently recorded the lowest reduction in light intensity, followed by 7×3 m spacing, indicating that wider spacing enhances light transmission even during periods of partial canopy presence.

From March onwards, the reduction in available solar radiation began to increase again due to the emergence of new foliage and canopy development in poplar trees. During March, the reduction ranged from 58.05% under 3×3 m spacing to 11.00% under 8×3 m spacing, while in April the reduction increased further, reaching 74.72% under the closest spacing (3×3 m) and 20.84% under the widest spacing (8×3 m). This increase in shading during late Rabi season corresponds with the active leaf development stage of poplar, which increases canopy density and consequently reduces the amount of sunlight reaching the crop canopy. Overall, the results clearly demonstrate that tree spacing plays a significant role in regulating light availability in poplar-based agroforestry systems. Closer spacing treatments (3×3 m and 4×3 m) resulted in substantially higher shading, whereas wider spacings (7×3 m and 8 3 m) allowed greater light penetration, thereby creating more favorable conditions for the growth of understory crops such as wheat. The observed trend is consistent with the findings of Chauhan et al. (2011), who reported that poplar based agroforestry systems allow winter crops to receive sufficient sunlight during the leafless period of poplar, which minimizes competition for solar radiation during early crop growth stages. Swieter et al. (2021) also reported that light availability in agroforestry systems decreases with increasing tree density and canopy size, and wider row spacing significantly improves photosynthetically active radiation available to crops.

Further, Dupraz et al. (2019) explained that seasonal variation in canopy development strongly affects the distribution of solar radiation in agroforestry systems. During the leafless period, solar radiation reaching crops increases considerably, while during the active canopy period shading becomes more pronounced.

Plant height

The data presented in Tables 3, 4 revealed that the plant height of wheat varieties was significantly influenced by poplar spacing as well as varietal differences at all growth stages (30, 60, 90 and 120 DAS). However, the interaction between variety and spacing was non-significant, indicating that all varieties responded similarly to variation in tree spacing. At 30 DAS, plant height ranged from 25.4 to 33.5 cm across the spacings. The maximum plant height (33.9 cm) was recorded under the control (sole crop) condition, whereas the minimum height (26.6 cm) was observed under 3×3 m spacing. Among the varieties, HD-2967 recorded significantly higher plant height (30.7 cm) than WH-711 (29.6 cm) and WH-1105 (29.4 cm). At 60 DAS, similar trends were observed. The highest mean plant height (62.9 cm) was recorded under the control, whereas the lowest (50.3 cm) occurred at 3×3 m spacing. Among varieties, HD-2967 (62.5 cm) exhibited significantly higher plant height than WH-711 (57.0 cm) and WH-1105 (49.4 cm). At 90 DAS, plant height further increased and ranged from 73.5 cm to 89.1 cm. The control treatment recorded the maximum mean plant height (84.5 cm) followed by 8×3 m spacing (83.1 cm), whereas the minimum height (76.3 cm) occurred at 3×3 m spacing. Among the varieties, HD-2967 (84.3 cm) performed significantly better than WH-711 (78.3 cm) and WH-1105 (77.2 cm). Similar results were recorded at 120 DAS, where the maximum plant height (86.4 cm) occurred in control and the minimum (77.7 cm) under 3×3 m spacing.

The reduction in plant height under closer spacing of poplar can be attributed to competition for solar radiation, soil moisture and nutrients between trees and crops, resulting in suppressed crop growth. Poplar canopy reduces photosynthetically active radiation reaching the understory crops, which adversely affects crop growth parameters. Similar findings were reported by Dhillon et al. (2017) and Sirohi et al. (2022b) who observed significant reduction in plant height of wheat under closer poplar spacing due to shading and root competition. Previous studies have also reported that growth of wheat is directly related to available light intensity under agroforestry systems, and wider tree spacing allows better light penetration and reduces competition, thereby improving crop growth (Chauhan et al., 2012).

The yield attributing parameters of wheat varieties such as tillers m^-2, ear heads m^-2, grains per ear head and test weight were significantly affected by poplar spacing (Tables 5, 6). The number of tillers m^-2 varied significantly among spacings, ranging from 248.0 under 3×3 m spacing to 344.5 under the control. Among the varieties, HD-2967 recorded the highest mean number of tillers (285.3 m^-2) followed by WH-711 (282.1 m^-2) and WH-1105 (281.5 m^-2). A similar pattern was observed for ear heads m^-2, which ranged from 239.9 under 3×3 m spacing to 337.1 under control conditions. Wider spacings such as 7×3 m and 8×3 m significantly improved ear head formation, which may be attributed to reduced competition for growth resources and improved light availability.

The number of grains per ear head also increased with wider poplar spacing. The maximum grains per ear head (48.1) were recorded under control, whereas the minimum (35.6) occurred under 3×3 m spacing. Among varieties, HD-2967 recorded significantly higher grains per ear head (41.4) compared with WH-711 (40.1) and WH-1105 (39.5). Similarly, test weight ranged from 38.2 g under 3×3 m spacing to 47.9 g under control conditions, indicating significant improvement with wider tree spacing. Kaur and Singh (2022) also observed that wheat growth and yield attributes were significantly affected by poplar age. They reported reductions of 4.9-13.8% in tillers m⁻¹, 9.1-22.7% in ear length, 5.5-16.6% in grains per ear, and 7.8-19.6% in 1000-grain weight compared with the control as poplar age increased from 2 to 6 years. The reduction in yield attributes under closer spacing may be due to reduced availability of photosynthetically active radiation and increased competition for soil resources, which ultimately affects tiller formation and grain development. These findings are consistent with the results of earlier studies where wheat grown under poplar plantations exhibited lower yield attributes under closer spacings but improved performance under wider spacings (Sirohi et al., 2020). Agroforestry studies have also reported that moderate shade can alter crop physiological processes such as photosynthesis and biomass accumulation, which directly affects yield components (Neto et al., 2024).

The grain and straw yield of wheat varieties were significantly influenced by both variety and poplar spacing (Table 7). However, the interaction effect between variety and spacing was significant only for grain yield. The grain yield ranged from 2.39 t ha^-1 under 3×3 m spacing to 4.41 t ha^-1 under control conditions. Among the spacings under agroforestry, 8×3 m recorded the highest grain yield (3.83 t ha^-1) followed by 7×3 m (3.29 t ha^-1) and 6 3 m (3.11 t ha^-1). Among the wheat varieties, HD-2967 produced the highest grain yield (3.41 t ha^-1) followed by WH-1105 (3.16 t ha^-1) and WH-711 (3.14 t ha^-1). The maximum grain yield (4.10 t ha^-1) was recorded in HD-2967 under 8×3 m spacing. A similar trend was observed for straw yield, which ranged from 2.87 t ha^-1 under 3×3 m spacing to 5.75 t ha^-1 under control. The highest straw yield among agroforestry treatments was recorded under 8×3 m spacing (4.22 t ha^-1). The percentage reduction in grain yield over control was highest under 3×3 m spacing, where reductions of 50.21%, 45.04% and 41.64% were observed in WH-1105, WH-711 and HD-2967, respectively. The reduction in yield under closer spacing may be attributed to higher competition for light, moisture and nutrients between poplar trees and wheat crop. Xu et al. (2016) reported that delayed canopy senescence positively influences grain filling and wheat productivity, especially under moderate light reduction (approximately 12% PAR) occurring after the anthesis stage. Similarly, previous studies have indicated that when photosynthetically active radiation (PAR) is reduced by 40-60%, wheat yield may decline by 25-80% compared with full sunlight conditions (Artru et al., 2017; Lakshmanakumar et al., 2013). Tree-crop competition in agroforestry systems significantly affects the productivity of intercropped winter wheat, with yield reductions ranging from 15 to 65% relative to control conditions, depending on tree age (Chauhan et al., 2012). Overall, the results of the present investigation indicate that wider poplar spacing (8×3 m) is more suitable for wheat cultivation, particularly for the variety HD-2967, which exhibited superior growth and yield performance under agroforestry conditions. The results of the present study are in agreement with several studies conducted in silvoarable systems, which reported that yield reductions were primarily due to decreased light availability near tree rows, while water competition was not considered a major limiting factor (Reynolds et al., 2007; Carrier et al., 2019; Mantino et al., 2019).

The authors gratefully acknowledge the Indian Council of Agricultural Research (ICAR), New Delhi, and the ICAR-Central Agroforestry Research Institute, Jhansi, for providing financial support through the All India Coordinated Research Project on Agroforestry (AICRP-AF) for conducting this study.