Implications of increased plant productivity on ecosystem hydrology: a case study on the 2018 heatwave in Europe.

In central and northern Europe, the year 2018 made headlines for being one of the hottest and driest on record. In this study, we investigated the overall ecohydrological impacts of the anomalous year using eddy covariance data collected at flux towers across the affected region . At site level, the year 2018 was generally characterized by a dry summer shouldered by a warm spring and fall. The warm shoulder seasons in 2018 led to higher CO₂ uptake through photosynthesis (gross primary productivity, GPP) compared to previous years on record, while dry conditions mostly lead to reductions in late summer GPP. This seasonal compensation resulted in little effect on annual productivity compared to the abnormal annual climate. In contrast, ecosystem evapotranspiration (ET) showed a slight increase at the annual scale. Interestingly, by partitioning ET into the plant associated transpiration (T) and evaporation (E), we found that the response of T generally followed a similar dynamic as GPP, with little overall increase in 2018. This finding indicates that the observed increase in annual ET was more due to abiotic evaporation. Due to the warm shoulder seasons in 2018, it seems like the growing season simply extended, resulting in a year with average gross productivity across the region despite the drought year, and ultimately more ecosystem water use. In fact, runoff simulations show that the larger than normal ET lead to an extended reduction in runoff throughout second half of 2018. In summary, we find that lack of responses of vegetation at the annual scale does not necessarily reflect the responses at the seasonal scale. These seasonal dynamics may have significant implications on availability of water resources.