## 4 Discussion
### The Temporal Dynamics of SIF and Tr in Different Species  
During the canopy closure period, when the canopy structure changes slightly, the canopy SIF and Tr exhibit similar seasonal dynamic characteristics. A similar trend has been found in other forest ecosystems, which indicates that SIF can provide information about vegetation growth and condition [18]. The SIF and Tr of the two deciduous plantations showed a downward trend in September, with poplar exhibiting a stronger decline than Chinese cork oak, while no such trend was observed in the evergreen species, arborvitae. This difference is related to variations in phenology and biological rhythms among different species [44]. A series of physiological changes occur to protect the photosynthetic structures of leaves in response to declining temperatures in autumn [45]. In deciduous tree species, chlorophyll decomposition reduces light absorption, and the extensive synthesis of lutein and abscisic acid induces stomatal closure [46]. Concurrently, the stomatal control ability of aging leaves decreases, potentially leading to reduced photosynthesis and transpiration [47]. The NDVI and MTCI of arborvitae showed slight seasonal variation, consistent with the study in a subalpine conifer forest [14]. Research found that the chlorophyll content of coniferous forests remained stable throughout the year, with APAR sustaining high levels even during winter. Therefore, it mainly relied on up-regulating non-photochemical quenching (NPQ) to dissipate excessive energy and protect photosynthetic structures.  
SIF and Tr are mainly driven by PAR during the day, showing an overall trend of increasing first and then decreasing, and similar trends have been observed in croplands and grasslands [32]. The diurnal dynamics of SIF and Tr exhibit differences, with SIF tending to peak around midday, while Tr saturates at midday. Chinese cork oak and arborvitae plantations exhibit midday saturation in Tr, while poplar Tr shows a distinct peak in Tr at midday on sunny days. Plants control transpiration by regulating stomatal aperture. In poplar, midday depression of Tr is absent, which may be influenced by two factors. On one hand, the VPD at the poplar's site is relatively low, so the trees do not experience atmospheric moisture stress. On the other hand, compared to Chinese cork oak and arborvitae, poplar exhibits a more anisohydric behavior water-regulation strategy, meaning it maintains stomatal opening even under water stress to ensure carbon fixation. [PERSON] et al. [48] found that poplar GPP exhibits strong resistance to micrometeorological stress, which may support our view that poplar exhibits stronger anisohydric behavior. The canopy SIF of Chinese cork oak and poplar exhibits a distinct daily peak, with the rate of change being higher in the afternoon than in the morning (Figure S1). This asymmetry in daily dynamics may be attributed to the geometry of sunlight observation [49]. Differences in leaf cluster size and the average fluorescence path length can lead to variations in the intensity of SIF emitted by sunlit and shaded leaves [50]. The canopy SIF of arborvitae does not exhibit a clear midday peak, which may be related to the strong clustering of branches and foliage in coniferous species [51]. When the solar zenith angle is small, the intense reabsorption of fluorescence within the canopy reduces the ratio of fluorescence escaping from the canopy (fesc) [52], especially on sunny days (Figure S2), causing SIF to saturate as midday radiation increases [53].