Evergreen broadleaf greenness and its relationship with leaf flushing, aging, and water fluxes

Remote sensing capabilities to monitor evergreen broadleaved vegetation are limited by the low temporal variability in the greenness signal. With canopy greenness computed from digital repeat photography (PhenoCam), we investigated how canopy greenness related to seasonal changes in leaf age and traits as well as variation of trees' water fluxes (characterized by sap flow and canopy conductance). The results showed that sprouting leaves are mainly responsible for the rapid increase in canopy green chromatic coordinate (GCC) in spring. We found statistically significantly differences in leaf traits and spectral properties among leaves of different leaf ages. Specifically, mean GCC of young leaves was 0.385 +/- 0.010 (mean +/- SD), while for mature and old leaves was 0.369 +/- 0.003, and 0.376 +/- 0.004, respectively. Thus, the temporal dynamics of canopy GCC can be explained by changes in leaf spectral properties and leaf age. Sap flow and canopy conductance are both well explained by a combination of environmental drivers and greenness (96% and 87% of the variance explained, respectively). In particular, air temperature and vapor pressure deficit (VPD) explained most of sap flow and canopy conductance variance, respectively. Besides, GCC is an important explanatory variable for variation of canopy conductance may because GCC can represent the leaf ontogeny information. We conclude that PhenoCam GCC can be used to identify the leaf flushing for evergreen broadleaved trees, which carries important information about leaf ontogeny and traits. Thus, it can be helpful for better estimating canopy conductance which constraints water fluxes.

Automated summary

This study investigates the relationship between canopy greenness and seasonal changes in leaf age, leaf traits, and trees' water fluxes. It finds that sprouting leaves contribute to the rapid increase in canopy greenness in spring. The study also reveals differences in leaf traits and spectral properties based on leaf age. Sap flow and canopy conductance are well explained by environmental factors and greenness. The study concludes that PhenoCam green chromatic coordinate can be used to identify leaf flushing and provide important information about leaf ontogeny and traits, which can help estimate canopy conductance.