Vineyard water and carbon dioxide exchange during a heat wave

Heat waves are predicted to increase in frequency and intensity, endangering the productivity of agroecosystems including grapevines. In this work, we analyzed the effects of the long and intense heat wave (HW) occurring in July 2022 on the eco-physiological performance of a vineyard located in northern Italy (Caldaro, Province of Bolzano). The vineyard hosts two white grape cultivars (Sauvignon Blanc and Chardonnay on SO4 rootstock) with an average planting density of 6,500 vines ha^-1 and is equipped with a drip irrigation system. The interrow alleys are covered by grasses or cover crops. Summer 2022 showed a continuous daily temperature increase from the second week of July and peaked in a heat wave lasting 8 days (DOYs 196-203) with maximal Tmax= 38 and average Tmax= 36°C. The HW period was then interrupted by summer storms and rain. 

The methodology included continuous monitoring of NEE (-NEP), Reco, GPP, ET and energy fluxes by eddy covariance method. Continuous measurements at the plant scale included sun-induced chlorophyll fluorescence, active chlorophyll fluorescence, and sap-flow rates. Periodically, leaf gas exchange, leaf water potential and chlorophyll content were also recorded. The period considered for the analysis stretched from July 1 to August 11.

There was a general mild decreasing trend of GPP when the air temperature increased in July, reaching the minimum values during the HW (< 10 g C m^-2d^-1). Leaf photosynthesis also slightly decreased during the HW in both varieties down to values of 6-7 µmol CO₂ m^-2s^-1. Interestingly, even during the HW, GPP always markedly increased the day after irrigation water was supplied (5 irrigation events were performed in July with a total of approximately 45 L/vine).

Reco decreased when the air temperature increased, but a few days before the end of the HW it increased again, a process that was even more pronounced following the rainy period after the HW when negative values of NEP were recorded.

Vineyard ET and vine transpiration did not show a clear response to the HW, while they both increased each time vines were irrigated, suggesting that most ET in that period derived from vine transpiration and not from the vineyard alleys. Interestingly, after the end of the HW, vineyard ET did not increase despite the increased soil moisture occurring also in the alleys. The energy partitioning was unaffected by the HW, with the Bowen ratio dropping below 0.4 after the irrigations. Continuous monitoring of active chlorophyll fluorescence showed a slight increase in NPQ parameter (measured only on Chardonnay leaves) in the last and hotter days of the HW, while Fv/Fm was not affected by the higher temperatures.

In conclusion, despite the exceptional (for the cultivation area) intensity of the 2022  HW, the vines showed good tolerance to heat stress. We speculate that irrigation played an important role in the vines’ performance. The NEP, however, decreased during the HW and especially after its end due to soil-moisture triggered increasing Reco, which shifted the vineyard from sink to source of atmospheric CO₂.