# Habitat Loss and Other Threats to the Survival of _Parnasius apollo_ ([PERSON], 1758) in Serbia
## 3 Results
### Geophysical Drivers of Gross Primary Productivity: A MODIS-Based Analysis  
A second-order polynomial regression revealed a significant non-linear relationship between daytime land surface temperature (LST) and gross primary productivity (GPP), with peak productivity occurring at intermediate surface temperatures (\(\approx\)25-28 \({}^{\circ}\)C), followed by a decline at higher values. This pattern is ecophysiologically consistent with established thresholds in photosynthetic temperature response. The fitted model (GPP = \(-0.194\cdot\)LST\({}^{2}\) + \(9.354\cdot\)LST \(-\) 43.02; R\({}^{2}\) = \(0.69\); \(p<0.001\)), based on MODIS data, captures this temperature-dependent response, suggesting an optimum range beyond which thermal inhibition reduces carbon assimilation efficiency (Figure 10). While temperature plays a central role, GPP is also influenced by the combined effects of moisture availability, vegetation type, and canopy structure. These relationships were examined through pairwise correlations and spatial patterns; however, no formal interaction terms were included in the models. The results should therefore be interpreted as statistical associations rather than causal mechanisms. Figure 9: Variety in vegetation on western side before (**a**) and shrubs and trees after fire (**b**,**c**) in August 2024. on Stol mountain. Descriptive statistics show that GPP values across the dataset range from near-zero productivity (<5 gC/m\({}^{2}\)/month) to peak values exceeding 120 gC/m\({}^{2}\)/month, with a mean of approximately 63 gC/m\({}^{2}\)/month. Daytime LST values span from \(-\)8 \({}^{\circ}\)C to over 35 \({}^{\circ}\)C, with a central tendency around 14 \({}^{\circ}\)C. At moderate temperatures (10-25 \({}^{\circ}\)C), enzymatic activity linked to Rubisco carboxylation and electron transport is maximized, leading to increased GPP. However, beyond higher thresholds (>30 \({}^{\circ}\)C), the relationship weakens or becomes asymptotic, indicating thermal inhibition or water stress--both known to reduce stomatal conductance and photosynthetic efficiency [85; 86]. Moreover, spatial stratification in the scatter plot indicates heterogeneity in site response. For instance, locations with high elevation or persistent canopy coverage maintain moderate GPP even at lower LSTs, likely due to the presence of evergreen species or favorable soil moisture conditions. Conversely, thermally exposed or water-limited areas show suppressed productivity despite elevated surface temperatures. These results highlight the non-linear, climate-sensitive nature of ecosystem carbon dynamics, where GPP is modulated not only by temperature but also by the combined influence of moisture availability, canopy structure, and vegetation type. These factors were analyzed separately in this study, and no statistical interaction terms were included. Satellite-derived metrics such as LST and GPP, when combined, serve as robust proxies for assessing the ecological response to climate variability at landscape scale [87; 88]. The correlation matrix (Figure 11) provides insight into the interdependence of environmental and productivity-related variables derived from MODIS datasets. A strong positive correlation is observed between daytime and nighttime land surface temperatures (\(r=0.92\), \(p<0.001\)), as expected due to their mutual climatic drivers. Daytime LST also correlates highly with potential evapotranspiration (PET; \(r=0.89\), \(p<0.001\)) and latent energy flux (LE; \(r=0.76\), \(p<0.001\)), indicating that surface heating significantly influences atmospheric water demand and energy partitioning. Figure 10: Non-linear relationship between daytime land surface temperature (LST) and gross primary productivity (GPP) across study sites (2010–2024), derived from MODIS data. Vegetation productivity metrics such as gross primary production (GPP) and net primary production (NPP) show robust mutual correlation (\(r\) = 0.89, \(p\) < 0.001), reflecting the physiological linkage between carbon assimilation and biomass accumulation.