Introduction 

For many years “modern agriculture” was the synonym of large areas covered by resource-consuming, high-yields crop monocultures ([4]). Consequently, the demand of agricultural land led to a progressive decrease in biodiversity, landscape trivialization, and forest degradation ([10]). Recently, the European Regulation 1305/2013 in support of rural development strongly encourage the adoption of “greening” techniques, like the agroforestry system, to avoid loss of biodiversity and mitigate climate change ([23]). Agroforestry, like multifunctional agriculture, has the purpose of reaching productivity according to environmentally sustainable rural development ([25]). Studies showed that the so-called trees outside forest (TOF) could positively affect adjacent cultivation areas in terms of ecosystem services, such as biodiversity and landscape conservation, carbon sequestration in biomass and soil, pest and pathogen reduction, moderate yield, reduction of soil erosion and nutrient leaching ([16], [4]). Damianidis et al. ([7]) report that agroforestry system reduces wild-fires risk, as the integration of trees on culture crops determines a lower biomass accumulation in the ground. Moreover, agroforestry systems based on poplar are used as windbreak ([26]), decreasing the wind damage and limiting the soil evapotranspiration. Agroforestry combined with livestock farming is a common practice in arid environments, since conventional crop cultivation would not guarantee revenue in these fragile ecosystems ([24]). In temperate European agriculture systems, the monocropping management (both for food and timber production) is strongly subsidized compared to agroforestry. European and Italian policies have essentially ignored the agroforestry system in the last 30 years, and supports from the Common Agricultural Policy (CAP) and Rural Development Plants (RDPs) were tailored without intercropping between trees and crops ([10]).

Consequently, farmers used marginal agricultural land for tree plantations resulting in poor tree growth and relatively low gain ([14]). Moreover, farmers and landowners do not consider agroforestry as a profitable alternative to standard agricultural practices since they believe that crops compete with young trees for resources, probably due to a lack of information. Several studies show that agroforestry can lead to beneficial effects without decreasing gains from crop cultivation. Graves et al. ([12]) combined the intercropping walnut with fodder crops, and they reported that this could be one of the most profitable management systems in western Europe. Such silvoarable systems also present higher transpiration and generate microclimatic conditions with lower soil evaporation when compared with their equivalent monocropping cultivation ([18]). Beneficial effects of silvoarable management are related to tree parameters, such as tree basal area, canopy, and tree density. It becomes crucial for agroforestry a better planning based on multi-species cropping system modeling ([2]). Diversification of agroecosystems can cope with price and crop yield variability, thus increasing farmer’s income stability and decreasing economic risk ([22]).

Our work aims to revisit the dataset collected by Prevosto ([19]) to evaluate the actual feasibility of poplar in agroforestry combined with cash crops (wheat and rice) in the Po Valley (Northern Italy). For many years farmers adopted the tree farming of poplar in field borders for different reasons, like fuel source, windbreaks, and private property fencing. Updated estimates of the advantages of this management system could be helpful for farmers.

  Material and methods 

Former studies

Prevosto ([19]) carried out his surveys in ten farms located in the western-central part of the Po valley (Italy), in Spring-Summer months at the latitude 45° N. The poplar trees (Populus × euramericana Dode Guinier “I-214”) were planted in rows as field borders, spaced at a distance of 5 m. Surveys were carried out for 10 years, and the results showed that up to the fourth year of plantation poplar rows did not influence the yield with any orientation. From the fifth year a decrease of production was noticed within 25 m from the poplar row. The highest reduction of yields was noticed on fields North of the poplars, and the lowest one southward. Intermediate decrease was found on fields East and West of the row (Tab. 1). Prevosto observed a direct effect of poplar roots and shadow from North, while only of roots for South (Fig. 1, Fig. 2, Fig. 3, Fig. 4). In a previous work on the same fields, Castellani & Prevosto ([5]) showed that improved fertilization reduced the decrease in crop yield of the South field, so the decrease was correlated to the poplar roots presence. The average reduction of yields in six years was higher for rice (loss of 25% in the yield of North exposed fields and 8% of South fields) than for wheat (loss of 21% in fields at North exposition and 7% in those at South - Tab. 1)

Fig. 1 - Shadow lengths of 25 m high poplar row, oriented West to East, on 15 June, at 45 ° North latitude. The horizontal point labeled 1, 2, 3 are the three poplars that generate the shadows under/upper the poplar row at different hours of the day.

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Fig. 2 - Shadow lengths of 25 m high poplar row, oriented North to South, on 15 June, at 45 ° North latitude. The vertical points across the line labeled 0 m are the three poplars that generate the shadows right/left to the poplar row at different hours of the day.

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Fig. 3 - Shadow lengths of 25 m high poplar row, oriented West to East, on 15 August, at 45 ° North latitude. The horizontal point labeled 1, 2, 3 are the three poplars that generate the shadows under/upper the poplar row at different hours of the day.

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Fig. 4 - Shadow lengths of 25 m high poplar row, oriented North to South, on 15 August, at 45 ° North latitude. The vertical points across the line labeled 0 m are the three poplars that generate the shadows right/left to the poplar row at different hours of the day.

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Economic evaluation

We simulated different economic scenarios of rice and wheat gains combined with two standing poplar wood prices for the veneer using data by Prevosto ([19]) with updated cultivation techniques, yields, and market values. The management costs of the two cereal crops and those related to the poplar plantations during ten-year rotation were considered. The poplar plantation model is in single row with a 5 m spacing between trees in a field of 100 × 100 m (1 hectare). For each cash crop, we assumed four different positions of the poplar row orientation for each cardinal point and the relative shadows, according to original dataset ([19]) recorded in June and August. Tab. 2 reports all the agronomic management practices used for the economic calculation ([11], [17]). Costs and earnings were estimated for a ten-year poplar cultivation cycle according to current prices (Tab. 3) and yield losses up to 25 meters away from the poplar row, as assessed by Prevosto ([19]). Moreover, we also considered earnings from the sale of loose straw of wheat. For long-term investment, it is helpful to assess the Equivalent Annual Value (EAV - [13], [21], [20]). First, it is necessary to calculate the Net Present Value (NPV), according to eqn. 1. NPV (in € ha^-1) is the difference between revenues and costs during the complete production cycle, discounted to year zero ([3] - eqn. 1):

where R_t is the revenue from the agrosystem in the t-th year (€ ha^-1), C_t are the costs in the t-th year (€ ha^-1), i is the discount rate in the time horizon (n year). Then the EAV, i.e., the infinitive net present value converted to an annual payment (in € ha^-1 yr^-1), was calculated ([3]). We chose a discount rate of 3.0% for ten years (eqn. 2), according to Pra et al. ([20]).

No other specific risks such as disease, drought, phytosanitary problems, windblown, etc. have been considered in this study.

Two different scenarios were simulated with the current market values of the poplar stands of 50 (scenario 1) and 80 € t^-1 (scenario 2), according to Pra & Pettenella ([21]). The “Goal Seek” function in Microsoft Excel® was used to calculate the threshold area convenience, that is the minimum area above which economic convenience becomes positive.

  Results and discussion 

Two different scenarios were assessed using Prevosto ([19]) surveys with updated agronomic costs and adding the value of poplar wood after ten years. The economic analysis showed that the profitability of an agroforestry system (poplar with rice or wheat) is related to poplar row orientation and poplar’s market price. The competition for light between trees and crops is the factor that most discourages farmers from undertaking agroforestry, especially for Northern latitude farmers. It is a common belief that the lower elevation of the sun in the sky at high latitude (above 45°) leads to competition for light between the crops and trees due to a lower total annual radiation ([9]). Crop yield under shade depends on multiple factors, not only from the average global radiation during the growing season. The photosynthetic capacities are related to instantaneous microenvironmental conditions, as the path of the sun, tree planting density, silvicultural practices and tree phenological stage ([1]). Our simulations showed that wheat or rice in combination with poplar could be a good choice for farmers depending on row orientation and price of poplar wood (Tab. 4). We estimated the highest yield reduction in the scenario of wheat and poplar cultivation with poplar value of 50 € t^-1 (scenario 1) and North exposure of the field, where the loss reaches 10.34 € yr^-1; while in the southward one there was an increase in the EAV equal to 14.47 € yr^-1. In all cases of the 80 € t^-1 scenario (current market price updated to 2020), the agroforestry system provides an economic advantage that ranges from 51.11 € yr^-1 for the Southern exposure to 26.29 € yr^-1 for the Northern exposure. Concerning rice, with a wood value of 50 € ton (scenario 1), the loss ranges from 36.88 € yr^-1 in the North facing field to 5.99 € yr^-1 for the eastward one; the South row leads to an increase of 26.49 € yr^-1. In the scenario 2 (poplar wood value of 80 € per ton) the estimated loss is only 0.24 € yr^-1 in the fields with Northern exposure, while earnings ranges from 14.73 € yr^-1 in West exposures to 63.13 € yr^-1 in Southern exposures. The threshold area convenience is reported in Tab. 5. For wheat, in the first scenario the southward field reaches the threshold convenience with an area below 1 ha (0.98). In the second scenario, all the row orientation reaches the threshold area convenience below 1 ha. For rice cultivation, in the first scenario only the South field reached the threshold area convenience below 1 ha; the other orientations need an area up to 1 ha for the same earning of the crop alone. In the second scenario, all the field reach the threshold area convenience below 1 ha.

The differences between the gains from the two cash crops are mainly due to the life cycle of the crops: rice is spring cultivation and carries out a large part of its photosynthetic activities starting from the end of April/beginning of May, a period in which the poplars foliage is already developed ([15]). Moreover, water temperature in rice paddies is one of the most critical factors affecting the growth and yield of rice ([15]); in summer, the release of poplar’s cottonseed could decrease the water temperature of rice paddies due to a more reflection of the incident light ([8]). On the contrary, wheat is an autumn-winter crop and consolidates its development before the full development of the poplar foliage, with a lower impact on yield. Finally, this work shows that profit does not depend on supporting funds. The absence of subsidies and the markets’ insecurity are some aspects that strongly discourage investing in agroforestry, in addition to the long crop cycle. To encourage these agroforestry practices, national and international support to farmers is mandatory ([3]).

  Conclusion 

The new social, economic, and biological challenges and the progressive decrease of natural resources combined with climate change require new approaches to knowledge and innovation, and above all to political choices. The concept of “bioeconomy” or “bio-based society” has become an important component of national, EU and global policies, in which a transition from fossil sources to a more efficient bio-based economy based on renewable sources takes place through sustainable processes which also provide ecosystem service. In the above context, agroforestry based on poplar could be a good choice for farmers. Future investigations will be focused on the effects of shade West and East exposition, and on cultivation in rows of a group of poplars clones named “MSA” (in Italian “Maggior Sostenibilità Ambientale” = greater environmental sustainability), which have been selected for their lower cultivation needs in terms of treatments and management ([6]).

  References