Petr Kupec and colleagues have published a new paper about possible future hydrological regime shifts in Central European forests. They used long-term data, detailed field measurements from an experimental forest catchent and PERSiST modelling, we show that there is a prolonged and persistent decline in annual runoff:precipitation ratios. This decline is most likely linked to the longer growing seasons associated with global warming. They performed a long term (1950–2018) water balance simulation for a Czech upland forest headwater catchment calibrated against measured streamflow and transpiration from deciduous and coniferous stands. Their simulations were corroborated by long-term (1965–2018) borehole measurements and historical drought reports. A regime shift from positive to negative catchment water balances likely occurred in the early part of this century. Since 2007, annual runoff:precipitation ratios have been below the long-term average. Notably, annual average temperatures have increased, but there have been no notable long term trends in precipitation. Since 1980, there has been a pronounced April warming, likely leading to earlier leaf out and higher annual transpiration, making water unavailable for runoff generation and/or soil moisture recharge. Their results suggest a regime shift due to second order effects of climate change where increased transpiration associated with a longer growing season leads to a shift from light to water limitation in central European forests. If their finding can be generalized, it will require new approaches to managing forests where water limitation has previously not been a problem.
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