## 4 Discussion  
The CUE exhibited a pattern depending on the main climatic characteristics such as temperature and precipitation and geographical factors such as latitude and altitude and PFTs. The regional CUE is not constant and varies from 0.444 to 0.552. CUE in 2000 and 2003 was lower than the global NPP/GPP ratio. In 2000 and 2003, CUE was 0.497 and 0.453, whereas then global CUE was 0.518 and 0.510, respectively [8]. However, the CUE of 0.61 for terrestrial ecosystems between 30\({}^{\circ}\) and 60\({}^{\circ}\) in the Northern Hemisphere was significantly higher than that for the same period in the studied region, located between 40.5 and 42.5\({}^{\circ}\) N.  
It was shown in [9] that gross ecosystem productivity was ~50% more sensitive to a drought event than ecosystem respiration. The same result is confirmed in the Rila-Rhodope region, with the relationship between GPP and NPP, and SPEY_5 was significantly stronger than the impact of drought on respiration. So, droughts induced decreases in NPP and respiration to an average of -0.20 and -0.07 kgC m\({}^{-2}\) year\({}^{-1}\) respectively, estimates being close to the global assessments [9], according to which the reduction of NPP and respiration is 16.6 and 9.3 gC m\({}^{-2}\) month\({}^{-1}\). The NPP/GPP ratio showed a significant variation depending on PFTs. In the Rila-Rhodope region, the mean CUE variation, depending on the type of biome, was 0.37 and was about two times higher than the estimate given in [10], according to which the variation of CUE between the different PFTs is about 0.19. Most sensitive to anomalies were the forest biomes, with the lowest efficiency and the highest CUE variance. The CUE variation in deciduous forests was strongest and its range was 0.080. This confirms the global estimate [8] that areas with a low CUE were largely occupied by forest ecosystems such as broadleaved evergreen or broadleaved deciduous forests. Heat waves and droughts lead to a reduction in CUE, which is strongest in deciduous broadleaf forests dominated by beech and significantly weaker in shrubs and savannas biomes, grasslands, and croplands. On a global scale [8], evergreen forests have a lower NPP/GPP ratio than deciduous forests. While the CUE of evergreen needleleaf forests in the Rila-Rhodope region was greater by 0.17 than the CUE of deciduous broadleaf forest, whereas in the normal 2014 this difference was minimal (0.11) and in the extreme 2007 it was twice as big. [PERSON] and [PERSON] [11] explored variability in carbon exchange, NPP, and light-use efficiency for 63 boreal forest stands in northeastern Minnesota using an ecophysiological model. The model was initialized with extensive field measurements of _Populus tremuloides Midx._ and _Picea mariana_ (Mill.). They found that conifers seem to have substantially smaller CUEs than broadleaved tree species, perhaps owing to their larger foliage biomass [4]. However, in the Rila Rhodope region, the CUE of deciduous forests of oak and beech is lower than the CUE of coniferous forests dominated by _Pinus nigra, Pinus sylvestris, Picea abies,_ and _Abies alba._ This distinction in CUE of deciduous and coniferous forests located in boreal and temperate climatic zones deserves to be studied separately, but we suppose it is due to different species composition and environmental conditions. CUE depends both on the strength of drought and on its duration and time in the year when it occurs (perhaps phenophase stage). The most significant impact on CUE are five-month-long droughts, regardless of the time of the year when they occur. The present study shows that SPEI cannot fully explain changes in CUE. It turns out that in some cases the mechanism of drought formation is important. For example, in 2007 moderate dryness conditions (SPEI_5) are formed as a consequence of the sequential alternation of short periods of precipitation and drought, and the magnitudes in GPP and respiration reduction are comparable.