Transpiration decrease in shaded hazelnuts: a green light for experimenting new orchard structures

Global yields of cereals and fruit trees are projected to decrease as a consequence of the increasing air vapour pressure deficit (VPD) (Hsiao et al. 2019). VPD synthesize the effect of relative humidity and temperature of the air, which is unceasingly increasing due to anthropogenic global warming since 30 years of 0.2°C (IPCC 2018). Mediterranean regions are among the most affected areas by this change. At the same time, VPD is one of the most significant parameter affecting plant physiology because it controls stomata aperture and carbon inflow into leaves. Long term exposure to high VPD leads to reduced growth (Sanginés de Cárcer et al. 2018) and tree mortality even more than just high temperature rise (Eamus et al. 2013). Still, some food trees are more sensitive to high VPD than others are. Hazelnut (Corylus avellana L.) is one of these. Although it is a temperate forest species, it is cultivated since centuries for the nut production in the whole Europe. In recent years, its cultivated surface is rising significantly. Italy plans to reach 20.000 hectares of hazelnut orchards at national level. In the last years, the research on hazelnut eco-physiology highlighted the sensitivity of the species to VPD rise. Hazelnut resulted shade-tolerant, it barely stands wind (Baldwin et al. 2003), stomata closure occurs above 10hPa VPD thus reducing carbon assimilation. This leads to the hypothesis that decoupling hazelnut trees from the atmosphere prevents water loss in case of VPD rise. During summer 2019, we set an experimental study in the University of Padova (45°20’55’’N, 11°56’59’’E, 5 m a.s.l.) hazelnut orchard, located in Legnaro (PD), Italy. Three 12 years old hazelnuts were covered with a polypropylene net in mid-June, while three other net-free individuals served as a control. Trees were rain-fed for the entire growing season. In each treatment, we measured carbon assimilation (An), stomatal conductance (gs) and light saturation curve at leaf level with a porometer from DOY 203 to 262. In parallel, we installed in all trees TDP probes to measure the sap flow. Sap flow measurements, together with meteorological parameters and soil water content were recorded by a datalogger with a 15 minutes record frequency. Diurnal sap flow data and VPD were used to calculate the canopy conductance G in the two treatments with G = k*EL/VPD (mm s-1), where k=115.8+0.4226*T°C (m3kPa°C kg-1), EL= transpiration per unit of LAI (g m-2s-1) as described in Tang et al. (2006). At the leaf level, the light saturation curve showed that 48% of the CO2 assimilation was reached already at 100 μmol m-2s-1 of PAR and remained stable for PAR > 500 μmol m-2s-1, close to what found by Tombesi et al. (2015). The shading net did not change the response to light. Daily An resulted slightly higher in control respect to shaded treatment, especially between 7 and 8AM and between 3 and 5PM. The shading net provided 10% decrease in mean daily VPD compared to the control. This caused a decrease in the canopy transpiration per unit of leaf area (EL) of 13.9% (max 30.5%) in shaded compared to control treatment (0.024 and 0.028 g m-2s-1 respectively). The whole tree canopy conductance (i.e. gas exchange capacity) between the two treatments was 8% higher in control respect to shaded trees (0.16 2 and 0.14 mol m-2s-1 respectively). The cover net shifted the maximum G from 6 to 10hPa, but the response of G to VPD remained similar in the two treatments (Wilcoxon test, p>0.05). Even if the leaf carbon assimilation is negatively affected by shade only in the early morning and some afternoon hours, the relative gain in whole tree conductance remained lower in control respect to the amount of water saved by shading the trees. Indeed, it is possible to design agroforestry systems where hazelnut grows with other non-shade tolerant species (e.g. Populus spp.) as dominant trees. These partner trees can benefit from the water saved from the new hazelnut orchard layout, and on its turn, provide shelter from wind, heat waves or VPD increase in the long term. Still, more research is needed to include the effect of shade on nut yield, which is though difficult in hazelnut due to its alternate bearing of fruits. Still, this work suggests that hazelnut can tolerate the coexistence of dominant trees with water savings that might compensate carbon assimilation losses.