Water and carbon fluxes in an apple orchard during heat waves

Prolonged hot periods known as heat waves (HW) are likely to increase in frequency and intensity due to climate change. Several studies analyzed the impact of recent heat waves on different un-managed terrestrial ecosystems, while little is known about the change they provoke in the carbon and water fluxes on irrigated agroecosystems. In this study we analyzed observations from 6 years continuous eddy covariance measurements (2013–2018) in an apple orchard located in South Tyrol (Northern Italy), regularly irrigated to prevent any limitations in soil water availability. The aim was to assess the impact of the heatwaves (at least 3 consecutive days of unusually high maximum temperature for the summer period) on the gross primary production (GPP), net ecosystem exchange (NEE), and actual evapotranspiration (ETa) fluxes. Out of the 13 heat waves that emerged from the temperature data analysis, five occurred in 2015, which together with 2013 was the hottest year in the considered period. In these two years, GPP and NEE patterns indicated a small but significant reduction in the assimilation capacity of the orchard with increasing Tmax, which was not present in the remaining years. ETa, instead, consistently increased across all the temperature range. During heatwaves, we observed an increase in the available energy, and a further reduction in the sensible heat flux in favour of latent heat, with a consequent increase in ETa. Additionally, during 9 out of the 13 heat waves, ETa values were among the 5% highest ever recorded in summer. Although heatwaves differed in length (max. 8 days) and magnitude (with peaks of 37 °C), ETa generally increased during heat waves by approximately 9% with respect to the week before. No similar consistent patterns were observed for GPP and NEE, which supports the hypothesis of decoupling between carbon and water fluxes during heat waves in an irrigated agroecosystem.