How do different functional groups of crop perform in temperate silvoarable agroforestry systems? A case study

Agroforestry systems provide a number of ecosystem services and are frequently considered as a promising diversification strategy for more sustainable and climate resilient primary production. Still, less than 1% of the agricultural land in the European Union is silvoarable agroforestry. Most agroforestry field trials compare one crop type with a control in open field with no additional environmental treatments such as nutrient and water availability, thereby limiting our understanding of the ecological processes underlying the potential benefits of agroforestry for food production. The present experimental study addresses three factors (shade, fertilisation, irrigation) on three functionally different crop species (field bean, summer barley, summer rapeseed) and a C4-grass (Echinochloa crus-galli) in a Swiss agroforestry system. The objective of this study was to assess if and how crop performance (physiological traits, yield) between functional groups varies and if and how shade-induced crop yield reductions diverge between treatment combinations, aiming to provide general functional crop species and management recommendations as a guideline for a successful agroforestry practice in temperate Europe. Summer barley (−44%) and field bean (−38%) showed significant yield declines, similar to summer rapeseed with a significant biomass decline (−35%). Shade significantly increased the occurrence of lodging in barley. Rapeseed in particular performed better when fertilised (+40% biomass). Our results enable to estimate the range of potential yield losses in the competitive zone near mature trees for functionally different crop types and serve as a decision-support for species selection in temperate European agroforestry systems.


Introduction
Agriculture and climate are interdependent. Both climate change adaptation and mitigation exert mounting on 27 and 28 March 2021 (SI 1 Fig. 1). The shade plots were fabricated by means of artificial shade nets (R.G.

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Vertrieb, Austria) with 40% opacity to target the desired shading value (i.e. 60% of incident photosynthetically 148 active radiation). This threshold was chosen based on previous experimental studies and models which suggest 149 moderate shade conditions in modern AFS (e.g., Dupraz et al., 2018b). Control treatments had no shade net. The

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Irrigation was provided by drip irrigation connected to a total of four 1000 litre water tanks positioned in the tree 176 row. The amount of water was adapted to current weather conditions (SI 2 Fig. 3) and varied between 60-120 177 litres per plot (i.e. between 10-19 l m -²) and watering event (SI 2 Tab. 3). Throughout the growing season only 178 approximately 60 l m -² were irrigated due to the very wet growing season.

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To obtain specific leaf area (cm²/g), single leaves of four individuals per subplot were scanned 27 June and then 182 dried for two days at 80°C. Image classification and segmentation was carried out with ilastik (version 1.3.3), 183 pixel count performed with ImageJ (version 1.53n). Subsequently, leaf area was calculated in R (version 3.6.1) 184 by resolution-based conversion of pixel in area. Leaf area (cm²) was then divided by leaf dry weight (g).

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Leaf chlorophyll content was assessed indirectly by usage of a chlorophyll meter (SPAD-502Plus Konica 186 Minolta®) on 2 and 9-11 July within the four-hour period around noon. The measurement device determines the  plots. Likewise, irrigation decreased bean number in shaded plots but increased bean number in unshaded plots, 282 though this was not significant. Shade decreased total bean weight from on average 20 (± 8) to 14 (± 5) g per 283 individual (-30%, p < 0.001). In addition, in the post-hoc test fertilisation significantly reduced total bean weight 284 from 18 (± 8) to 15 (± 6) g per individual (-17%).

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The acute and severe pest damage, the germination failure for common millet and the unforeseen rainfall 371 quantities impaired the initial expected range of contrasts in functional groups and environmental conditions to 372 some extent. However, the presented results still hold explanatory power. The interaction of species and shade 373 significantly influenced crop yield reduction, i.e. field bean showed a more consistent yield reduction than 374 summer barley at the subplot level, though barley apparently had a stronger decrease (-44% compared to -38% in 375 field bean). Shade-induced yield reduction in barley was in the range predicted by Laub et al. (2022) in their 376 meta-regressions for C3 cereals, which amounted to -38% under 40% shade. In our study, yield variability was 377 high and standard deviation amounted to 67% and 45% in non-shaded and shaded plots, respectively. Possibly 378 bird grain foraging was reduced under the shade constructions. All yield-related traits (total seed number, seeds 379 per individual, total seed weight, seed mass, number of tillers) and biomass of barley were significantly reduced 380 under shade. Lodging is known to severely decrease cereal yields and is most often attributed to an increase in 381 plant height (Shashidharaiah, 2008). In the AFS in Windlach, shade increased plant height and lodging occurred 382 in 50% of the shaded plots (8 out of 16). Decreased biomass and increased plant height and SLA are classical In their study, the area of shelter protection reached from 3 to 24 m from the hedgerow with increased crop 432 yields. Our findings represent potential yield decreases in the immediate vicinity and Northern side of tree rows 433 and do only deliver an estimate of the range of maximum yield declines in the competition zone.

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In this experimental study combinations of shade, nutrient and water availability were tested on four crop species 437 of different functional groups in a temperate silvoarable AFS. The grain legume (field bean) showed the