Protection net and the canopy layer decouple gas exchanges, affecting carbon and water fluxes: the case of a kiwifruit orchard in a Mediterranean environment

Covers are increasingly used to protect crops from pests and extreme meteorological events. Their use affects plant microclimate and physiological responses as well, but this is only partly understood, particularly when considering the interaction among the cover and the training system. This study focuses on the microclimate effects of the concurrent use of a horizontal hail protection net and tree canopy. The system splits the orchard environment in three layers, giving a shaded understory, a volume comprised between the canopy and the net, and the atmosphere above the net. To accent the effects, we considered a high-water demanding crop – kiwifruit, in an environment characterized by high evaporative demand (Bernalda, southern Italy). Three full eddy covariance and radiative balance-equipments were used to synchronously monitor meteorological variables (air temperature and relative humidity, wind, upward and downward short and long wave radiation, carbon dioxide and water vapour concentrations) and infer fluxes (carbon dioxide, water vapour, and sensible heat) between the three layers. Data from a typical clear sky summer day, at different hours, are considered in this paper. While the net strongly reduced wind speed (-79%), it had a modest impact on all other variables. Conversely, the tick canopy layer had a major impact on all variables, determining a highly shaded, cooler and more humid understory, with very modest wind and higher carbon dioxide concentration. Most carbon assimilation and evapotranspiration were due to the tree canopy, while the understory played as a carbon source via respiration. The combination of the net and the pergola training system determined low energy understory conditions, which facilitate the buildup of water vapour and carbon dioxide and their recycling within the system. Given the large and increasing use of crop covers, a deeper understanding of their impacts on the drivers of climate change, such as the carbon, water and energy fluxes, is necessary.