Extreme droughts' impact on Scots pine net primary productivity in the European temperate zone in the period 2002–2023

Severe drought episodes significantly influence the productivity of trees in many parts of Europe. This paper shows in detail the influence of the most severe drought events on the productivity of the most important forest-forming tree species in European temperate zone – Scots pine (Pinus sylvestris L.). We identified four months with most severe drought conditions that occurred in Poland in 2002–2023: July 2006, April 2009, August 2015 and June 2019. To quantify trees’ net primary productivity (NPP) we used Moderate Resolution Imaging Spectroradiometer (MODIS) NPP, which was further correlated with temperature, precipitation, evapotranspiration and climatic water balance. The identified summer droughts had considerable effect on pine forest productivity: August 2015 had the lowest NPP of all Augusts in the study period (0.033 kgC∙m-2∙month-1), similarly July 2006 (0.055 kgC∙m-2∙month-1) and June 2019 (0.096 kgC∙m-2∙month-1). Relationship between drought severity and pine’s productivity depends on the time during the year, when the drought occurs. Summer droughts, with their peak in June, July or August, resulted in significantly decreased productivity of trees, while spring droughts, tend to have an initial positive impact on pine’s condition. For summer droughts cases, weather conditions influence the decreased productivity of pine forest for a long time, e.g. a prolonged negative relationship between NPP and temperature for drought cases in June 2019 and July 2006. Such long response of spectral indicator’s value of pine is not clearly visible for droughts occurring either on the beginning (April 2009) or second half (August 2015) of the growing season.