Above and below-ground competition for solar radiation and soil moisture in a windbreak-soybean system

The main role of windbreaks in agricultural systems is to improve the microclimate and enhance the growth and yield of the protected crops. However, some competitive windbreak-crop relationships have been recognized. Windbreak-soybean above- and below-ground competition was studied in 1996 and 1997 at Mead, Nebraska (41$\sp\circ$ 29$\sp\prime$ N, 96$\sp\circ$ 30$\sp\prime$ W, and 354 m altitude). The factors in the study were: windbreak orientation (east windbreak, west exposed (EW), south windbreak, north exposed (SW), and west windbreak, east exposed (WW)), root pruning (pruned (P) and not pruned (NP)), and distance from the windbreak (0.75H, 1.5H, 2.25H, and 3.0H in 1996, and 0.5H, 0.75H, 1.5W, and 3.0H in 1997). Windbreaks were two rows of green ash (Fraxinus pennsylvanica L.) combined with eastern redcedar (Juniperus virginiana L.) or Austrian pine (Pinus nigra Arnold), 12-15 m height (H), and approximately 60% density. The most critical period of windbreak-crop competition for soil moisture occurred during soybean reproductive and grain filling stages. Windbreak-crop competition for soil moisture existed at EW and WW up to 0.75H, but it was negligible at SW. The differences in soybean yield between NP and P plots at 0.75H, were 192-601 kg ha$\sp{-1}$ (12.2-21.7%) at EW, 892-874 kg ha$\sp{-1}$ (32.1-40.8%) at WW, and 137-173 kg ha$\sp{-1}$ (5.4-6.1%) at SW in 1996 and 1997 respectively. Soybean canopy temperature (CT) was higher and soybean leaf water potential $(\Psi)$ was more negative at EW and WW compared with CT and $\Psi$ at SW, at NP compared with CT and $\Psi$ at P plots, and at 0.75H, compared with CT and $\Psi$ at other distances. Competition for solar radiation existed at the three windbreak orientations up to 0.75H, but it was significantly greater at SW when measured up to 0.5H. Incident photosynthetic photon flux density (PPFD) integrated over the crop season in 1997 was 45.6 and 85.2% at SW, 73.5 and 86.7% at EW and 62.3 and 82.7% at WW, at 0.5H and 0.75H, respectively, compared with PPFD at an exposed field. Shading effects reduced soybean yield by 2333 kg ha$\sp{-1}$ (68.6%) at SW 0.5H, and by 577 to 1712 kg ha$\sp{-1}$ (17 to 40%) at SW 0.75H, compared with yields at SW 3.0H. Fewer accumulated growing degree days (GDD) were observed at SW up to 0.75H. Correlation between GDD and soybean phenological development existed at SW, but not at EW and WW windbreaks. Soybean phenological development at EW and WW responded more to soil water availability than to GDD accumulation