Evaluating drought impacts on ecosystem water use efficiency of three different boreal forest sites

European boreal forests are the biomes most affected by climate change. For example, extreme weather events such as droughts are expected to become more frequent and severe. Especially in the summer months, droughts can drastically impact the carbon and water exchanges of boreal forests. This study aimed to identify the drought response of the water use efficiency (WUE) in three contrasting boreal forests. WUE, is the link between the carbon- and water cycle of forests and therefore a strong indicator of their ability to withstand droughts. To assess the drought response, the inter-annual variations and between-site differences in WUE, net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration of three forest sites were compared. The sites included one mixed stand, one pine stand, and one pine stand with regular nitrogen addition. Drought stress was quantified using various drought indices (SPI, SPEI, and SMI) as well as changes in temperature, precipitation, vapor pressure deficit, and the difference between potential- and actual evapotranspiration. This study shows that GPP and NEE decreased in years with more drought stress. The results indicate that increased temperatures have the strongest negative correlation with WUE in all sites. It also found that under drought stress, the mixed stand had smaller losses of WUE compared to the two pine stands. Between the pine stands, WUE of the fertilized stand was more robust towards droughts. It was therefore concluded that nitrogen addition as well as mixing species can enhance forest resistance against droughts. These results provide information for practical management implications to make boreal forests more robust against droughts