A whole-canopy approach to assess varying effects of cover crop vs grapevine competition and recovery

This work aims at quantifying dynamics and impact of increasing and decreasing cover crop to vine competition. A two-year study was conducted on well-watered potted 'Barbera'/'SO4' grapevines with the root system split into two 40 L pots. In the spring of 2020, four grassed (G) treatments were established, using Festuca arundinacea (FA) and Festuca ovina (FO) sown in either one or two pots (GG-FA, GT-FA, GG-FO, and GT-FO, respectively), and compared with a tilled control (TT). These treatments were maintained for the following season, with all grass covers terminated on June 30 using herbicide. In 2021, the whole-canopy gas exchange of three canopies per treatment was continuously monitored from Day of Year (DOY) 155-175 and DOY 179-249. Grass sowing in spring 2020 had a significant impact on final pruning weight, decreasing by 70 % in GG-FA compared to TT. The treatments imposed in 2020 severely limited canopy development in spring 2021, with the net carbon exchange rate (NCER) decreasing between 27 % (GT-FO) and 73 % (GG-FA) vs. TT, and transpiration (T) per vine registering a decrease between 16 % (GT-FO) and 43 % (GG-FA). Moreover, final yield recorded in 2021 was very severely limited in GG-FA (0.4 kg/vine) and, albeit to a lesser extent, in GG-FO. Terminating all grass covers on June 30, 2021 triggered a prompt vine recovery, with NCER and T rates catching up to TT levels in 54 days and canopy WUE showing a rapid resumption. This is the first study providing assessment of cover crop to vine competition encompassing seasonal gas exchange effects at the whole canopy level. It also shows that canopy water use efficiency decreases with increasing competition and vice versa.