## Results
### Impacts of Droughts and Heatwaves on the Carbon Use Efficiency  
The drought is a multi-scalar phenomenon and depends on the time scale over which water deficits accumulated, and on the period of the year when the droughts appear. To determine the timescale suitable for exploring the impact of drought on plant productivity, we have studied the dependencies between NPP, GPP, CUE, and SPEI at time scales of 1 to 24 months. It was found that CUE, NPP, and GPP depend strongest by five months SPEI (SPEI_5), the correlation coefficients between CUE, NPP, GPP and SPEI_5 being 0.51 (CUE), 0.69 (NPP), and 0.75 (GPP). NPP and GPP correlate strongly and positively with SPEY_5\({}_{\text{J}\alpha}\) (_r_ = 0.818 and 0.830 respectively). SPEY_5\({}_{\text{J}\alpha}\) has also a strong positive effect on CUE (_r_ = 0.684). The drought period that most affects plant respiration was displaced one month earlier, i.e., the respiration depends strongest on SPEI_5\({}_{\text{J}\alpha}\).  
Figure 3: Carbon use efficiency in Rila–Rhodope region during 2000, 2003, 2007, and 2012 droughts.  
Figure 2: (**a**) NPP (in kgC m-2 year-1) and CUE anomalies; (**b**) Regression of NPP to GPP forced through the origin. The slope of the relationship is 0.5118 with a SE = 0.008.  
In 2000 SPEI_5 reached the highest value of (~1.75) in August. During the growing season April-August, the region was hit by a severe drought. The respiration and biomass accumulation processes compete at the same rate and NPP = 0.439 kgC m-2 year-1 and R = 0.446 kgC m-2 year-1. Despite extreme conditions and significant reductions in productivity (NPP and GPP), the region is not a carbon source as CUE remains at 0.497. Similar was the situation in 2012, when the NPP = 0.484 kgC m-2 year-1 was of the same order of respiration R = 0.499 kgC m-2 year-1 and CUE was 0.487, i.e., close to 0.5. However, in this case the drought started from July and was unusually strong, with SPEI_5 Noverbase reaching extreme values of ~2.4. Although in both extreme years 2000 and 2012, CUE was about 0.5, the nature of droughs was different. In 2000, drought began as early as the beginning of February, with moderate dryness and at the beginning of March transformed into severe dryness, which continued to the end of the year with SPEI_5 between ~1.5 and ~1.7. While in 2012, we had extreme dryness conditions (SPEI_5 < -2) from June to the end of the year.  
In 2003 and 2007, there was a strong reduction in CUE, CUE2003 = 0.453 and CUE2007 = 0.443 i.e., the efficiency was about 10% lower than in the other extreme years 2000 and 2012, although the drought was not that strong. In 2003, SPEI_5 reached a maximum negative value of ~1.31 in September i.e., in the period of active growth May-September there were moderate dryness conditions. The better conditions during the growing season accelerated growth, NPP = 0.482 kgC m-2 year-1, which led to an increase in growth respiration and hence to respiration as a whole, R = 0.510 kgC m-2 year-1 and a corresponding decrease in CUE. In the winter of 2007, the drought was moderate, near extreme, SPEI_5 NPI = -1.46, which gradually weakened in spring and early June when SPEI_5 = -1.20. Low soil water content before the start of the active growing season and in the period of accelerated plant growth significantly reduced the NPP to 0.42 and increased the respiration to 0.52. This means that plants fail to acclimatize to the combination of rainfall and temperatures, resulting in a strong CUE reduction of up to 0.455.