"Green pointillism": detecting the within-population variability of budburst in temperate deciduous trees with phenological cameras.

Phenological cameras have been used over a decade for identifying plant phenological markers (budburst, leaf senescence) and more generally the greenness dynamics of forest canopies. The analysis is usually carried out over the full camera field of view, with no particular analysis of the variability of phenological markers among trees. Here we show that images produced by phenological cameras can be used to quantify the within-population variability of budburst (WPVbb) in temperate deciduous forests. Using seven site-years of image analyses, we report a strong correlation (r2 = 0.97) between the WPVbb determined with a phenological camera and its quantification through ground observation. We show that WPVbb varies strongly (by a factor of 4) from year to year in a given population and that those variations are linked with temperature conditions during the budburst period, with colder springs associated to a higher differentiation of budburst (higher WPVbb) among trees. Deploying our approach at the continental scale, i.e., throughout phenological cameras networks, would improve the understanding of the spatial (across populations) and temporal (across years) variations of WPVbb, which have strong implications on forest functioning, tree fitness and phenological modelling.

Environmental control of carbon allocation matters for modelling forest growth.

We aimed to evaluate the importance of modulations of within-tree carbon (C) allocation by water and low-temperature stress for the prediction of annual forest growth with a process-based model. A new C allocation scheme was implemented in the CASTANEA model that accounts for lagged and direct environmental controls of C allocation. Different approaches (static vs dynamic) to modelling C allocation were then compared in a model-data fusion procedure, using satellite-derived leaf production estimates and biometric measurements at c. 104 sites. The modelling of the environmental control of C allocation significantly improved the ability of CASTANEA to predict the spatial and year-to-year variability of aboveground forest growth along regional gradients. A significant effect of the previous year's water stress on the C allocation to leaves and wood was reported. Our results also are consistent with a prominent role of the environmental modulation of sink demand in the wood growth of the studied species. Data available at large scales can inform forest models about the processes driving annual and seasonal C allocation. Our results call for a greater consideration of C allocation drivers, especially sink-demand fluctuations, for the simulations of current and future forest productivity with process-based models.

Potential use of the PRI and active fluorescence for the diagnosis of the physiological state of plants under ozone exposure and high atmospheric vapor pressure deficit.

Assessing photosynthesis rates with remote sensing is important for tracking the physiological state of plants. The photochemical reflectance index (PRI) is a good estimator of short-term light-use efficiency (LUE) at the leaf scale but its responses to environmental factors are poorly understood. In this study, we assessed changes in the responses of the PRI to ozone exposure and to an increase in atmospheric drought (separately and combined) in oak (Quercus robur) and holm oak (Quercus ilex) that were planted in climatic cells under controlled conditions. The aim was to evaluate the ability of the PRI as a relevant indicator to assess the impact of abiotic factors on photosynthesis. Leaf-scale measurements of biochemical, physiological and spectral properties, including the PRI in dim light on dark-adapted leaves (PRI0), kinetics of PRI responses to PAR variations (photosynthetically active radiation), and leaf chlorophyll fluorescence parameters were performed. The results show that PRI0 is a good proxy of the leaf chlorophyll content, and is correlated to chlorophyll fluorescence parameters on dark adapted leaves (Fo, Fm). The correction of the PRI from the leaf chlorophyll content variations (PRIc) significantly improves correlations between the PRI and NPQ (non-photochemical quenching). The variability of PARsat (estimated PAR value at PRI saturation using PRI vs. PAR relationships) depends on ozone exposure and on the increase in atmospheric vapor pressure deficit. For Quercus robur, results highlight that PARsat is linked to abiotic stress indicating that the PRI may be used as a relevant indicator of abiotic factors limiting the photosynthesis. Quercus ilex did not show significant variability in PRI0 and PARsat, which suggest that it is a more drought resistant species than Q. robur.

Linking phytoplankton pigment composition and optical properties: A framework for developing remote-sensing metrics for monitoring cyanobacteria.

This study has been performed in the framework of a research program aiming to develop a low-cost aerial sensor for the monitoring of cyanobacteria in freshwater ecosystems that could be used for early detection. Several empirical and mechanistic remote-sensing tools have been already developed and tested at large scales and have proven useful in monitoring cyanobacterial blooms. However, the effectiveness of these tools for early detection is hard to assess because such work requires the detection of low concentrations of characteristic pigments amid complex ecosystems exhibiting several confounding factors (turbidity, blooms of other species, etc.). We developed a framework for performing high-throughput measurements of the absorbance and reflectance of small volumes (∼ = 20 mL) of controlled mixtures of phytoplankton species and studied the potential of this framework to validate remote-sensing proxies of cyanobacteria concentration. The absorption and reflectance spectra of single and multiple cultures carried a specific signal that allowed for the quantitative analysis of culture mixes. This specific signal was shown to be related to known pigment absorbance spectra. The concentrations of chlorophyll-a and -b, phycocyanin and phycoerythrin could be obtained from direct absorbance measurements and were correlated with the concentration obtained after pigment extraction (R2 ≥ 0.96 for all pigments). A systematic test of every possible two-band and three-band normalized difference between optical indices was then performed, and the coincidental correlation with chlorophyll-b (absent in cyanobacteria) was used as an indicator of non-specificity. Two-band indices were shown to suffer from non-specificity issues and could not yield strong and specific relationships with phycocyanin or phycoerythrin (maximum R2 < 0.5). On the other hand, the three-band modified normalized difference indices yielded strong specific relationships (R2 > 0.8).

Explaining the variability of the photochemical reflectance index (PRI) at the canopy-scale: Disentangling the effects of phenological and physiological changes.

Assessing photosynthesis rates at the ecosystem scale and over large regions is important for tracking the global carbon cycle and remote sensing has provided new and useful approaches for performing this assessment. The photochemical reflectance index (PRI) is a good estimator of short-term light-use efficiency (LUE) at the leaf scale; however, confounding factors appear at larger temporal and spatial scales. In this study, canopy-scale PRI variability was investigated for three species (Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) growing under contrasting soil moisture conditions. Throughout the growing season, no significant differences in chlorophyll content and in violaxanthin, antheraxanthin and zeaxanthin were found between species or treatments. The daily PRI vs PAR (photosynthetically active radiation) relationships were determined using continuous measurements obtained at high frequency throughout the entire growing season, from early spring budburst to later autumn senescence, and were used to deconvolute the physiological PRI variability related to LUE variations due to phenological variability and related to temporal changes in the biochemical and structural canopy attributes. The PRI vs PAR relationship is used to show that the canopy-scale PRI measured at low radiation depends on the chlorophyll content of the canopy. The range of PRI variations at an intra-daily scale and the dynamics of the xanthophyll pool do not vary between days, which suggests that the PRI responds to a xanthophyll ratio. The PAR values at PRI saturation are mainly related to the canopy chlorophyll content during budburst and senescence and to the soil moisture content when the chlorophyll content is no longer a limiting factor. This parameter is significantly lower in the oak species that experience less stress from variations in soil moisture and is species dependant. These results provide new insights regarding the analysis and the meaning of PRI variability as a proxy for LUE at the canopy scale.