## 4 Discussion
### Response of PRI and LUE to Environmental Factors  
The growth environment (light intensity, temperature, and moisture) in which the plant is located affects the physiological state of the plant [38]. For subtropical evergreen forests, PRI and LUE exhibit very complex changes due to the changing weather environment. During the whole study period, PRI showed a significant correlation with LUE (Figure 4). Significant positive correlations between PRI and LUE were found on some days (Figure 5), months (Table 1), as well as throughout the study period (Figure 5). These results are consistent with previous studies conducted on canopy scales and different temporal scales [10; 11; 25; 43; 47]. PRI can be used to track LUE in subtropical evergreen coniferous and broad-leaved mixed forest. PRI can be used to track LUE in subtropical evergreen coniferous and broad-leaved mixed forests. A better correlation between PRI and LUE requires seasonal and monthly scale analysis (Figure 6 and Table 1). Obviously, the correlation between PRI and LUE in the dry season is higher than that in the wet season. From April to March of the next year, the correlation between PRI and LUE shows a trend of decreasing first and then increasing. The relationship between the PRI and LUE is mainly affected by environmental factors. Among them, radiation is the main driver of plant photosynthesis, and vegetation has higher photosynthesis under high light and optimal temperature conditions (Table 2). This is consistent with previous results [11]. However, under low radiation conditions, high LUE in plants was observed, which represented the best physiological growth state of vegetation light energy utilization. In contrast, LUE was lower under higher radiation conditions. At the same time, when the vegetation is in the optimal temperature condition, the radiation intensity increases, while the LUE decreases. Therefore, during the study period, the half-hour scale observation of radiation intensity plays a dominant role, in comparison to the temperature. This conclusion is consistent with the results reported by [PERSON] et al. (2008). While illumination is the determinant of the correlation between PRI and LUE, in the dry season, PRI and LUE are also sensitive to VPD. For example, as VPD increases, LUE also increases, indicating that LUE is limited to a certain extent to a high VPD environment. This is consistent with the results of [PERSON] et al. (2015) [10]. The difference is that PRI and LUE are not affected by temperature during the dry season, indicating that the temperature does not interfere with the characterization of LUE by PRI. Therefore, in the subtropical evergreen coniferous and broad-leaved mixed forest, both radiation and VPD will become important factors affecting the relationship between PRI and LUE. The different meteorological conditions corresponding to different months alter the relationship between PRI and LUE. For subtropical forests, finding an empirical equation for the relationship between PRI and LUE for a full growing season is still a problem.  
In our study, compared to temperate deciduous forests, the LAI\({}_{\text{e}}\) changes in the subtropical mixed forests in the Dinghushan area are small [40], the phenological changes are not significant, and the changes in pigment content in the leaves of subtropical coniferous and broad-leaved mixed forests are small. This is the reason why there is a weak dynamic change of LUE.