# Environmental Effects on Normalized Gross Primary Productivity in Beech and Norway Spruce Forests
## 4 Discussion  
This study used two sets of regression models (SMLR and RF) to identify the main environmental variables influencing the ratio between actual and optimal gross primary productivity (termed as GPPnorm) across central European beech (at Stitna) and Norway spruce (at Blly Kriz and Rajec) species growing under contrasting climatic conditions. The presented results illustrate that the RF regression model outperformed SMLR and was highly effective for GPPnorm prediction at all the studied forest ecosystems (Table 2). Though these statistical analyses may not show the mechanistic cause of the observed patterns, the RF model also provided useful information about the variable importance (Figure 6) and the effect of their significant interactions (that are highly correlated; Figure 7) on GPPnorm. Similarly, the authors of [17] used a path analysis approach to evaluate the sensitivity of GPP to environmental drivers during the summer drought of 2018, but our study evaluated in broader terms the general limiting environmental conditions of GPP across each of the studied forest sites. As PAR is already a well-known primary driver of forest GPP, the results from our study revealed its overall significance in influencing GPPnorm across all the forest sites. However, aside from PAR, there were other site-specific environmental variables that also had significant impact on GPPnorm prediction. It is mostly hard to distinguish the direct impact and importance of Tair, VPD, and SVWC as they are mutually dependent [55; 56; 57]. However, our results show that the second most important environmental variable in GPPnorm prediction was VPD at the wet spruce forest site and SVWC at the beech forest site. Thus, indicating the contrasting water use strategies of both spruce and beech species. This is in line with earlier findings which show that the isohydric Norway spruce species has tighter stomatal control in response to increasing VPD than the anisohydric European beech species, which can tolerate drought stress better [29; 58; 59; 60]. Therefore, as more frequent heatwaves and drought spells characterize the proceeding climate change, an unavoidable shift of the Central Europe silviculture strategy is expected, providing likely higher opportunity for growing European beech while a higher risk for the coniferous Norway spruce [61; 62]. Additionally, GPPnorm reduction within the beech forest site was presumably mainly induced by edaphic drought stress (low soil moisture) conditions, further stressing the significant impact of SVWC on forest GPP, especially at the beech forest stand due to its different soil properties (more clayey soil) from the other forest sites [63; 64; 23; 65; 66; 23]. Moreover, at both spruce forest sites, a more detailed analysis of the variables splitting branches within RF (Figure 7) showed that Tair or Tair typically grouped subspaces with high and low (drought-reduced) GPPnorm values. Interestingly, at the wet spruce forest site in Bily Kriz, Tair split the subspace with lower values of GPPnorm at high Tair threshold values above 19 \({}^{\circ}\)C during days with lower SVWC values (<0.16 m\({}^{3}\) m\({}^{-3}\)). This suggests that high Tair values limit the forest productivity at the mountainous wet spruce forest site in Bily Kriz. Furthermore, at the dry spruce forest in Rajec, the three most important variable (VPD, Tair, and SVWC) had the highest scores among all the forest sites (scores > 0.1; Figure 6). This shows that the spruce forest productivity within a drier climate is often significantly impacted by both meteorological and hydrological drought events (decrease in atmospheric water content) [68; 69; 70; 71; 72]. Thus, under warmer climatic conditions, the impact of drought stress conditions on forest productivity will be more severe at spruce forests sites situated within drier climates [73]. Comparatively across all forest sites, the variable importance score of PAR was higher at the beech forest site in Stitna than at both spruce forest sites in Bily Kriz and Rajec (Figure 6).