Unravelling Drought Responses of Ecosystems in the Netherlands

The droughts of 2018-2020 in the Netherlands created great social distress, induced huge economic damages in agricultural production, disrupted river navigation and damaged buildings and unique nature. The events exposed critical gaps in our knowledge about the behaviour of the Dutch water and soil systems during droughts and the vulnerability of our agriculture and nature systems. Better insight into the water-soil-plant-atmosphere system is urgently needed for developing climate change adaptation pathways for agricultural and water management.  Although there have been dedicated efforts to understand the causes and consequences of droughts in the Netherlands and to come to a more climate-resilient soil and water management strategy, most of these studies focused mainly on the hydrological cycle, i.e., precipitation deficit (‘neerslagtekort’), runoff, soil moisture, groundwater, with interventions aimed at minimizing the ‘harmful’ impacts (e.g., plant damages caused by either drought or excessive oxygen stress (wet damage)) and maximizing the ‘favorable’ impacts (e.g., via recharging soil and groundwater) of hydrological conditions. However, these studies overlooked an important aspect of the water-soil-plant system: the plant functioning (physiology) during a prolonged drought event. Based on data collected at the Speulderbos forest site, we found that the forest, which usually acts as a carbon sink, emitted CO2 between the 24th and 26th of July 2019 due to the heatwave and drought. This has not been observed since we started measurement in 2006. To understand such drought impact, it requires knowledge on leaf and canopy responses of photosynthetic CO2 assimilation to key environmental variables: light, temperature, CO2 concentration, vapor pressure deficit and soil water content.