## 4 Discussion
### Unresolved Questions  
The correlation between PRI and LUE has been tested over a wide range of ecosystems. A universal relationship of PRI with LUE has not been found yet due to the complex response of plant photosynthesis to radiation under various environmental conditions [44]. The relationship between PRI and LUE and the sensitivity of this relationship to bioclimatic factors vary in different types of ecosystems. For boreal forests, previous studies mainly investigated the effects of non-physiological factors (e.g., sun-view geometry) on the relationship between PRI and LUE, as the distinct canopy structure makes the observed PRI strongly dependent on the sun-view geometry [3,14,40,41,53,54,78]. With considering or eliminating these factors, PRI is strongly correlated with LUE and can be used to monitor regional productivity over long time periods [3,24,25,79]. Even though PRI is useful for tracking seasonal changes in LUE as affected by temperature and nutrient variations in evergreen forests, the influences of the seasonal Car/Chl ratio and temperature variations via the expansion of leaves and aging on PRI signals cannot be neglected [46,47,66,77,80]. These factors need to be further investigated. In temperate ecosystems, PRI is also useful for detecting water stress [81] and estimating LUE or carbon uptake [82,83,84,85] in conifer forests. [PERSON] _et al._[32] found different factors driving PRI variations and the relationship between PRI and LUE under various environmental conditions for temperate deciduous and evergreen broadleaf forests. Additionally, the highest correlations between PRI and LUE were mostly observed under clear or slightly overcast sky conditions. However, they reported an insignificant correlation of PRI with APAR or LUE during a drought period in 2010, due to an insufficient signal-to-noise ratio for the PRI measurement. Besides, according to a significant correlation between PRI and GPP found in their study, we infer that a long-term drought might induce changes in the pigment pool size and leaf morphology, which further confound the PRI signals [77,86]. Variations of solar radiation and diffuse radiation also affect the relationship between PRI and LUE [27,42]. [PERSON] _et al._[87] found that PRI could track seasonal (dry/rainy) variations of LUE in Botswana. However, [PERSON] _et al._[30] exhibited limited sensitivity of the PRI to LUE, while a model with both PRI and VPD as predictors is able to significantly improve LUE estimation for a tropical evergreen rainforest, as there is no clearly defined dry/rainy season cycle. In Mediterranean forests, which often suffer heat and drought stress in summer, PRI is successfully used to detect water stress on LUE and assess LUE across different seasons [88; 89; 90; 31; 91]. Studies of PRI in sub-tropical forests are very limited. To our knowledge, only one study in a _Pinus taiwanensis_ forest pointed out that PRI is more sensitive than photosystem II efficiency to water deficits, and is closely related to photosynthetic capacity at both high- and low-elevation sites in different seasons [92]. In our study, the strongest correlation between PRI and LUE was found during the dry season (Figures 8 and 9 and Table 1), which confirms the sensitivity of PRI to water and heat stress previously reported for tropical and Mediterranean forests [89; 90; 91; 93; 94; 95; 96; 97; 98; 99; 87; 93; 95] and its ability to detect variations of LUE under stress conditions similar to that in other forests. The distinction of illumination between the rainy and dry seasons and different proportions of observed sunlit and shaded leaves caused by the diurnal variation of sun-view geometry confounded this finding that PRI works better in tracking LUE under dry and heat stresses. Moreover, whether canopy pigment ratios changed under continuous heat and drought stress could further change the PRI signal is still unclear.