## 4 Discussion
### Dynamic of NPP Estimated with Biometric Method  
In his definition of ecological succession, [PERSON] [82], among others, stated that the succession \"culminates in a stabilized ecosystem in which maximum biomass (or high information content) and symbiotic function between organisms are maintained per unit of available energy flow\". Characteristic of _NEP_ dynamic throughout the succession, according to [PERSON] [82; 83], is a rapid increase during the early stage of succession that reaches maximum during the mid-succession, followed by decline, and eventually reaching zero _NEP_ at the late stage of succession. However, a recent publication by [PERSON] and [PERSON] [83] question [PERSON]'s hypothesis of steep decline and zero _NEP_ at late succession in the case of temperate broadleaf forests. It appears that the wood _NPP_ in some forests, transitioning from early to middle succession, exhibits resilience that is attributed to changes in species composition [84]. In temperate forests of Western and Central European lowlands, the pedunculate oak is a typical species of the final stage in forest succession [85]. Forest management in Croatia is considered to be close to nature, striving at securing environmental and economic functions of forests [34]. In other words, current, close-to-nature management of pedunculate oak forests (tending, thinning, and regeneration cuts) is aiming at continuously maintaining pedunculate oak as the main species. The absence of regeneration cuts, e.g., due to conservation of old pedunculate oak stands, may lead to ecosystem regression and can result in forest-like stand of _Cratageus monogyna_[86]. Jastrebarsko forest at the EC site is in the middle successful stage. Woody biomass _NPP_ (\(NPP_{\text{WBI}}\)) showed a negative trend during the investigated period and it could be an indicator of the declining _NPP_, which would be in line with the theoretical framework of [PERSON] [82]. However, the observed trend is also affected by the unfavorable meteorological conditions in 2017 (the last year of our study), which had a strong negative effect on _NPP_ and consequently impacted the trend. Looking into other possible reasons, we should consider reports pointing out that decreasing soil nutrient availability and increasing stomatal limitation, leading to reduced photosynthetic rates, are the two main causes of _NPP_ decline in forest stands [87]. On the other hand, a recent study in an oak forest [88] pointed out that tree mortality was the primary cause of the decline in aboveground biomass accumulation with stand age, and not a reduction in individual tree growth. Excluding data from 2017 and performing further analysis with respect to tree species (Section 3.4.2) revealed that only _Fraxinus angustifolia_ and _Alnus glutinosa_ have a statistically significant decrease in \(NPP_{\text{WBI}}\). In the last several years, the spread of _Hymenopteraosyclhus fraxineus_ fungi (more widely known by its older name _Chalara fraxinea_) played an important part in the decline of _Fraxinus angustifolia_[89]. On the other hand, the decline in _Alnus glutinosa_ is more likely age-related, due to its shorter maximum lifespan of 100-160 years [90]. For example, the typical period of rotation for stands dominated by _Alnus glutinosa_ in Croatia is 70 years, as compared to 140 years long rotation for _Quercus robur_ stands. Hence, the increased mortality of _Alnus glutinosa_ and _Fraxinus angustifolia_ probably contributed to the observed trend of \(NPP_{\text{BM}}\). This can be corroborated if we look at the evolution of species-specific densities at Jastrebarsko forest (Table 14). We can note that the _Alnus glutinosa_ had the lowest survival rate of 61.4%, when compared to the average survival rate of 81.5%. While a decreased share of _Fraximus angustifolia_ probably did not affect nutrient availability, the reduction in the share of _Alnus glutinosa_, which is a nitrogen-fixing species, could have had a negative effect on nutrient availability and \(NPP_{BM}\) trend.