Spatial and temporal observations of in-canopy and leaf level chlorophyll fluorescence from the boreal forest, Finland

Chlorophyll fluorescence could become the remote sensing signal of choice for estimating global terrestrial primary productivity. However there are mechanistic ambiguities requiring investigation before chlorophyll fluorescence can be used to accurately infer productivity from space. For example, little is known of either the spatial (across species and light environment) or temporal (seasonal) variability of the leaf level fluorescence emission in the boreal forest. We present 8 yearsof in-canopy and leaf level measurements aimed at resolving these ambiguities which were collected at Hyytiälä forestry field station, Finland. First, we show several years of results from our continuous Monitoring PAM (MONIPAM) fluorometer. In addition, we present results from two measurement campaigns. The first campaign was conducted during peak growing season conditions, to answer the question "what controls the spatial distribution of spectral chlorophyll fluorescence and other photosynthetically-dependant leaf optical properties?". In this study we measured fluorescence and reflectance spectra in addition to morphological, biochemical andfunctional leaf traits. In the second campaign we collected similar measurements to the first (spatial) campaign, but conducted our observations over several months during the spring growth recovery period, with the objective of disentangling the temporal variability of the fluorescence spectrum. From the MONIPAM data we found a consistent linear relationship between the photochemical yield of energy use (linked to photosynthesis) and the yield of fluorescence across seasons. From the spatial data-set we found only limited spatial variability in spectral fluorescence across evergreen and broad-leaf species. Results from the final campaign showed temporal trends in spectral fluorescence during the progression of the spring recovery period. Taken collectively our results indicate that chlorophyll] fluorescence is a robust and dynamic indicator of (boreal forest) photosynthetic activity across space and time.