Evaluating the effect of the 2018 drought on the NEE in the Sorø beech forest by means of trend analysis and mechanistic canopy modelling of GPP.

Denmark experienced a severe drought in 2018 lasting from the beginning of May to the end of August with very little rain during this period. The influence of drought on the net ecosystem CO₂ exchange (NEE) was analysed at the Danish ICOS DK-Soroe site (a mature beech forest). The site has a very long continuous flux data set starting in June 1996. The annual NEE of the site has been increasing over the years, mainly due to a prolonged growing season in the autumn and CO₂ fertilisation (Pilegaard et al., 2011).

The effect of the summer drought in 2018 was analysed by means of linear trend estimation based on monthly trends during 1996-2017. The observed monthly NEE in 2018 was compared to the predicted values from the monthly time series.

The analysis showed an increased NEE in May and June and a strongly reduced NEE in July and August. Overall, the NEE was reduced 25% compared to the predicted value.

The increased NEE in May and June can be explained by the benefit for the photosynthesis of the trees of the increased light and temperature, while there was still a sufficient water content in the soil. By the end of June, the low water content in the soil affected the NEE, and despite some heavy rain in the beginning of August, the NEE only recovered by September.

We used the flux data set together with a mechanistic canopy model to examine the tree physiological nature of the photosynthesis limitation. The results showed that stomatal limitation alone was not able to explain the large reduction of GPP during the drought. Based on these findings, we extended the approach and show the seasonal development of drought induced GPP limitation contrasting stomatal and biochemical photosynthesis limitations.

The effects on NEE and energy partitioning during the 2018 summer drought are compared to previous years with (less severe) summer drought. 

Reference:

Kim Pilegaard, Andreas Ibrom, Michael S. Courtney, Poul Hummelshøj, Niels Otto Jensen. Increasing net CO2 uptake by a Danish beech forest during the period from 1996 to 2009. Agricultural and Forest Meteorology 151 (2011) 934–946.

Acknowledgement:

The study was based on data from ICOS/DK.