Extreme droughts' impact on Scots pine net primary productivity in Poland 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 and dominant tree species in Poland – Scots pine ( Pinus sylvestris L. ). We identified four months with most severe drought conditions that occurred in Poland during 2002–2023: July 2006, April 2009, August 2015 and June 2019. To estimate the net primary productivity (NPP) of pine forests, we used the Moderate Resolution Imaging Spectroradiometer (MODIS) NPP product for homogeneous pixels for this tree species. The obtained values were further correlated with temperature, precipitation, evapotranspiration and climatic water balance. The identified summer droughts had considerable effect on pine forest productivity. August 2015 exhibited the lowest NPP among all Augusts within the study period, representing a decrease of 57.80% relative to the long-term August mean. Similarly, July 2006 and June 2019 recorded pronounced reductions in NPP, declining by 40.81% and 20.59%, respectively, representing the lowest NPP values for these months across the entire period analysed. Our study demonstrates that the 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. Pine forest productivity showed a prolonged decrease during summer droughts (e.g., August 2015, June 2019), with a negative relationship between NPP and temperature. Such long response of MODIS-derived NPP of pine is not clearly visible for drought occurring on the beginning of the growing season (April 2009). The results indicate that drought events significantly reduce pine NPP in Poland, with both the magnitude and duration of their impacts being strongly modulated by the timing of drought occurrence.