## 3 Results and Discussion
### Productivity and NPP Estimates  
Using stand volume as a key productivity indicator provides an indication of the growth potential under the various climate projections. Results are masked by the distribution ranges predicted in the previous phase. The results indicate stand volume at 20 year ranges from 150 to 650 m\({}^{3}\)/ha. Highly productive stands were modeled to occur in the south-east coastal regions, east interior areas, and  
Figure 4: Climatic modifiers in a monthly time-step; (**a**) Soil water and temperature, (**b**) frost and VPD.  
central Taiwan; while stands with relatively poor productivity occurred in the northern part of the study area. It is these areas that are projected to become more suitable for Chinese fir into the future. Based on historic data [2,43], we assume that areas with 450 m\({}^{3}\)/ha or greater stand volume at age 20 are most suitable for productive Chinese fir plantations. Results suggest that in total, 4.3% (\(\sim\)12,000,000 ha; Figure 6a and 3.3% (\(\sim\)9,000,000 ha; Figure 6b) of the modeled species distributions have a stand volume of more than 450 m\({}^{3}\)/ha under projection A1B and A2, respectively. Both projections indicate that Taiwan and central China would potentially be the most suitable for Chinese fir plantation.  
Figure 5: Chinese fir distribution under different climate projections; **(a)** current; **(b)** A1B projection; **(c)** A2 projection.  
Existing stands, if not diminished by climate change, tend to gain more growth over the same growing period under the projected climate. In Figure 7b and c it is apparent that stands currently occupying areas in southern Sichuan and northern Guizhou provinces (center of the study area) are likely to develop into more productive forests based on these stand volume predictions.  
Distributions of the productivity regimes for stand volume are shown in Figure 8. Under the current climate conditions (Figure 8a), the distribution is more symmetrical than under the change projections where the shape is skewed towards lower productivity stands. It is also apparent that peaks of productivity are slightly shifted to lower values from the current climate to predicted future climate conditions. This change in productivity distribution results from the newly established distribution of the species in the northern region of the study area (Figure 7). The peak in distribution in the A1B projection is explained by the increase in area where the climate is projected to become more suitable for Chinese fir. Compared to A1B, which represents a moderate development and population growth, the A2 projection demonstrates a more severe decline in volume, as the A2 climate projection mimics greater population increase, economic development, and limited climate adaptation and mitigation activities.  
Figure 6: High productive sites (\(>\)450 m\({}^{3}\)/ha stand volume) within predicted present areas; (**a**) A1B projection; (**b**) A2 projection.  
Figure 7: Volume (m\({}^{3}\)/ha) at age of 20 years, predicted by 3-PG; (**a**) current climate; (**b**) A1B projection; (**c**) A2 projection.  
Net primary production was also calculated using the 3-PG model. Figure 9 shows the current annual carbon uptake of Chinese fir (baseline), averaging 18 Mg/Dry matter/ha/per year, or 930 g carbon/m\({}^{2}\)/year across the region. Figure 10 shows the percent change between the baseline and two future climate projections. Although changes in NPP are more profound in projection A1B (Figure 10a), both projections indicate a NPP decrease in southern China and a noticeable increase (\(>\)50%) in central China.  
Figure 8: Histograms of predicted volume (m\({}^{3}\)/ha) with areas (km\({}^{2}\)) under climate projections; (**a**) current (baseline); (**b**) A1B projection; (**c**) A2 projection.  
Figure 10: Percent changes in NPP between baseline and climate projection; (**a**) A1B projection; (**b**) A2 projection.  
Figure 9: Total NPP (MgC/year) of Chinese fir under current climate.