Sowing time and tree spacing as determinants of wheat growth and poplar performance in a poplar-based agroforestry system

Introduction

India's cultivated land base has been relatively static at approximately 143 million hectare in spite of the ever-increasing population and the Green Revolution's impact, has resulted in a legacy of degradation and decline in soil fertility and an increased vulnerability to climate change. This has triggered interest in climate-smart and resource-saving land-use systems such as agroforestry which increases the level of soil organic carbon and nutrient (Pardon et al., 2017) [1] and improves soil aggregation and structure in areas covered by trees (Gupta et al., 2009) [2] . Poplar (Populus deltoides) has been widely adopted agroforestry species for the Indo-Gangetic plains due to its fast-growing nature, deciduous behavior, its capacity to tolerate pruning, and its ability to intercrop with cereal crops offering significantly higher profits to farmers than sole-crop (Chavan et al., 2019) [3] . The poplar species have also shown significant contribution to carbon sequestration in the long-term (Chavan et al., 2023) [4] and optimize overall land-use return using boundary and block plantation methods (Chavan et al., 2022) [5] . However, the benefit of using poplar based agroforestry to wheat intercropping has a strong dependence on the age and spacing of the trees. A study carried out on wheat intercropped under Populus deltoides at Punjab reported that a 23.3% decrease in wheat yield under three-year-old plantation with a significant relationship between poplar girth increment and wheat yield reduction even though, the pruning of poplar following the third year allowed partial recovery (Ralhan et al., 1992) [6] . Tree spacing has been considered as a major factor for managing the competition between the crop and the tree as seen from a two-year's field trial in Haryana conducted to evaluate wheat grain yield and determine the effect on plant growth under various geometrical spacing of poplar ranging from 3 m × 3 m to 8 m × 3 m and as the distance between trees increases wheat grain yield increased progressively while the closest geometry recorded a 44-46% and 39% decrease in wheat grain and straw, respectively, compared to the sole-crop control but, the widest geometry reduced the difference considerably (Kumar et al., 2024) [7] . Sowing time is equally important as a management practice since the delayed sowing reduces the germination, tillering, plant population, and dry matter accumulation of wheat mainly due to unfavorable temperature conditions during the early growth stages. However, these effects are modified by the presence of poplar canopies particularly due to its deciduous characteristic; as the tree loses its leaves in the winter season and flushes them in the early spring season, early sown wheat can fully utilize the high-light period before the new canopy development while, the competition for late sown wheat intensifies with the active growth of the canopy further reducing the plant stand and plant growth. Despite individual evidence on spacing effects (Kumar et al., 2024) [7] and tree-age (Ralhan et al., 1992) [6] , the interactions between sowing time and poplar spacing along with the poplar's own growth behavior within the entire duration of the cropping cycle, were scarcely studied. Therefore, this study aimed to (i) evaluate poplar growth performance from pre-sowing (November 2023) to post-harvest (April 2024) of wheat and (ii) assess the effect of sowing time on wheat germination, plant population, and growth (plant height, dry matter accumulation) under varied spacing of poplar.

Materials and Methods

The present research was conducted in the Department of Forestry, CCS Haryana Agricultural University, Hisar (29°10′ N, 75°43′ E; 215.2 m amsl) located in the dry, subtropical monsoon zone of northwestern India, where approximately 90% of the cultivated area is irrigated with canal or tube-well water. The experiment was conducted in an existing plantation of poplar (Populus deltoides), established in February 2017, with wheat (Triticum aestivum) intercropped during the rabi season of 2023-24 under two poplar spacing geometries (4 m × 3 m and 6 m × 3 m) and a sole-crop (tree-free) control. Three wheat varieties (WH-1105, WH-1184, and DBW-187) were sown at four dates -late October (28th), mid-November (14th), late November (29th), and mid-December (13th) -in rows 20 cm apart at a seed rate of 100 kg/ha under sole-crop and 125 kg/ha under the agroforestry system. The experiment used a Factorial Randomized Block Design with three replications. Standard cultural practices were followed throughout the experiment. The seedbed was prepared by two ploughings with a disc harrow and one with a cultivator followed by planking after pre-sowing irrigation. A standard fertilizer dose of 150 kg N, 60 kg P 2 O 5 , 50 kg K 2 O, and 10 kg ZnSO 4 per hectare was applied, with half the nitrogen and the full phosphorus and potassium doses at sowing and the remaining ni-trogen top-dressed as urea after the first irrigation. Five irrigations were conducted at recommended critical growth stages. Crops were hand-harvested with a sickle at 19-20% grain moisture and harvested in the morning to limit shattering losses and sun-dried for 3-4 days before threshing. Poplar growth parameters, viz. girth at breast height (measured at 1.37 m by a measuring tape), tree height (measured with an altimeter), and canopy spread (mean of north-south and east-west crown measurements), were recorded before sowing and after wheat harvest. Wheat growth was evaluated by days to germination, plant population per square meter at 20 days after sowing (DAS) (recorded using a 1 × 1 m quadrant), plant height (measured on ten randomly tagged plants per plot at 30, 60, 90, and 120 DAS), days to 50% heading and days to maturity, and dry matter accumulation (sampled at 30-day intervals using a 1 × 1 m quadrant, oven-dried at 70 °C to constant weight, and weighed on a digital balance).

Results and Discussion

Growth performance of poplar under different spacing

It is observed from Table 1 that the Poplar height was slightly greater in 4 × 3 m spacing (18.8 m before sowing and 19.0 m after harvest) than in 6 × 3 m spacing (18.3 m and 18.5 m), but the difference was not significant. However, the GBH and crown spread were significantly higher for 6 × 3 m spacing treatment at both observation points (GBH: 66.8 cm vs. 62.9 cm before sowing, 67.4 cm vs. 63.3 cm after harvest; crown spread: 8.8 m vs. 7.6 m before sowing, 9.3 m vs. 8.1 m after harvest). The inverse relationship between the height and lateral growth suggests the resource-allocation response of the trees to the inter-tree competition. Under closer spacing, the neighboring canopies competed for the incident light and induced anisotropic tree growth where the vertical growth was prioritized over the radial and lateral growth. On the contrary, the wider spacing eliminated competition for light, resulting in higher allocation of photosynthetic products to the girth increment and crown spread. This is in line with findings that wider poplar geometries promote significantly better tree growth in agroforestry than closer configurations (Kumar et al., 2024) [7] .

Table 1. Growth performance of poplar before sowing (November, 2023) and after harvesting (April, 2024) of the wheat crop

2023-24

Spacing

Number of days to germination

Germination was considerably delayed under closer spacing, taking 7.5 days under 4 × 3 m spacing compared to 6.7 days under 6 × 3 m spacing and 6.5 days in the sole-crop control (CD = 0.92). The effect of sowing time was even more pronounced, with germination time rising steadily from 5.3 days (late October) to 9.0 days (mid-December) (CD = 0.1). The delay under closer spacing probably reflects modification of the microclimate under denser canopies, where reduced solar radiation reaching the surface lowers soil temperatures, slowing the metabolic processes governing seed imbibition and radicle emergence. The reduced light transmission under denser poplar geometries has been shown to modify the growing microclimate for intercropped wheat in agroforestry (Jia et al., 2021) [8] . The progressive delay with later sowing parallels the seasonal decline in ambient and soil temperature through November and December, which is a known constraint for wheat germination regardless of tree presence. ), significantly ahead of late-October (167.8) and mid-December (164.1) sowing (CD = 2.3). The reduced plant stand under closer spacing is consistent with intensified below-ground competition. Poplar develops a dense network of superficial fine roots concentrated in the upper 15-30 cm of soilthe same zone exploited by wheat roots -creating direct competition for moisture and nutrients that can suppress seedling establishment even when higher seed rates are used to compensate (Sharma et al., 2012) [9] . The peak population under mid-November sowing mirrors findings from a related poplar-wheat sowing-time study in northwestern India, where this window consistently produced the best crop performance across seasons, while both earlier and later sowings reduced establishment (Gill et al., 2009) [10]

Plant population at 20 DAS

Plant height

The investigation results revealed that poplar spacing, wheat variety, and their interactions had a significant effect on plant height at 30, 60, 90, and 120 days after sowing (DAS) as presented in Tables 4 to 7.

The sole wheat (control) treatment registered the maximum plant height (29.0, 54.3, 86.2, and 88.9 cm at 30, 60, 90, and 120 DAS) due to the absence of competition for sunlight and nutrients from poplar trees. On the other hand, increasing the tree spacing reduced the plant height as demonstrated by the control treatment registering the highest plant height, followed by wider spacing (6×4 m) and the nearest spacing (4×3 m) registering the minimum plant height (24. [8] . This competition for light explains why closer spacing recorded lower plant heights compared to wider spacing as the former intercepts more solar radiation than the latter.

Dry matter accumulation

Dry matter accumulation followed a similar trend to that of plant height, with differences between treatments increasing sharply over time. Dry matter increased from 72.26 g/m 2 at 30 DAS to 377.66 g/m 2 at 120 DAS under 4 × 3 m spacing compared to 189.17 g/m 2 and 1010.0 g/m 2 in the sole-crop control (CD = 1.60 to 7.07) -indicating that open-field wheat accumulated over 2.6 times more biomass than wheat under the closest poplar spacing by crop maturity.

While sowing time had no significant effect on dry matter at any stage, it had a clear influence on germination and plant population. The widening trend reflects the cumulation of reduced early photosynthesis and competition for water and nutrients between poplar and wheat roots throughout the season, since a plant that establishes with lower vigor falls progressively further behind under continued resource competition (Ralhan et al., 1992) [6] . The lack of sowing-time effect on final dry matter indicates that, once established, the intensity of season-long tree-crop competition becomes the dominant factor influencing biomass accumulation. ), while sowing in mid-November produced the best results in terms of establishment. Plant height and dry matter were highest consistently in the sole crop control and lowest under 4×3 m spacing at all stages of growth, with the yield gap widening progressively during the season (dry matter: 1010.0 vs. 377.66 g/m 2 at 120 DAS). A variety of WH 1105 was found to be best in terms of growth and shade tolerance, whereas, DBW 187 and WH 1184 were proved to be the most sensitive at early and later stages of growth, respectively, to closer spacing of poplar. The time of sowing significantly affected the germination and plant population but not the final dry matter accumulation.

Acknowledgement

AcknowledgmentThe authors express their sincere gratitude to the Department of Forestry, CCS Haryana Agricultural University, Hisar, for providing the experimental field, infrastructure, and necessary facilities to carry out this research. Sincere thanks are extended to [Major Advisor's Name], [Designation], Department of Forestry, for their invaluable guidance, constant encouragement, and critical suggestions throughout the course of this investigation.

References

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13. How to cite this article: Kamalkant, Poonia PK, Shallni, Gupta M, Singh D. Sowing time and tree spacing as determinants of wheat growth and poplar performance in a poplar-based agroforestry system. International Journal of Research in Agronomy. 2026;9(9):1037-1043. doi: https://www.doi.org/10.33545/261806 0X.2026.v9.i9n.6644