by Marie-Claude Boileau | 30 January 2019
Published in New Forests 46: 319–337. https://doi.org/10.1007/s11056-014-9463-3
Some site preparation is generally recommended to enhance the growth and survival of planted and naturally regenerated seedlings, but it must be justified both economically and environmentally. More severe preparation is thought to be necessary for intensive plantation silviculture, e.g., using fast-growing, ameliorated stocks, especially in boreal ecosystems. Although not justified scientifically, deep-planting of seedlings is often discouraged and may even be financially penalized in eastern Canada. We thus evaluated early seedling growth and survival of hybrid larch (Larix x marschlinsii Coaz) in an experiment including mechanical site preparation and planting depth treatments. Our results suggest that satisfactory early hybrid larch establishment and growth could be met using low environmental impact or low cost treatments (such as soil inversion using an excavator or single-pass disk trenching), and that deeper planting has no negative effect. Structural equation modelling (SEM) was used to explore causal relationships between factors influencing seedling performance at the local scale (planting microsites), including soil moisture, soil temperature, surrounding vegetation, and seedling nutrition. SEM analysis supported the absence of overall differences among treatments, while also highlighting the negative impact of increased soil water content where drainage was suboptimal, as well as the unexpected positive impact of increased competition on growth mostly through seedling nutrition, among others. These early observations will need to be confirmed over a longer period, as well as with a more comprehensive assessment of site environmental conditions and competition intensity.
by Claire Morin | 30 January 2019
Published in Remote Sensing of Environment 112: 3201-3211
We show that observed co-variations at sub-hourly time scales between the photochemical reflectance index (PRI) and canopy light use efficiency (LUE) over a Douglas-fir forest result directly from sub-hourly leaf reflectance changes in a 531 nm spectral window roughly 50 nm wide. We conclude then, that over a forest stand we are observing the direct effects of photosynthetic down-regulation on leaf-level reflectance at 531 nm. Key to our conclusion is our ability to simultaneously measure the LUE and reflectance of the Douglas-fir stand as a function of shadow fraction from the “hot spot” to the “dark spot” and a new finding herein, based on radiative transfer theory, that the magnitude of a normalized reflectance difference index (NDRI) such as PRI can vary with shadow fraction only in case the reflectance of the shaded and sunlit leaves differ in at least one of the NDRI bands.
Our spectrometer measurements over a nearly 6 month period show that at a forest stand scale, only two NDRIs (both containing a band near 570 nm) vary with shadow fraction and are correlated with LUE; an NDRI with a band centered at 531 nm roughly 50 nm wide, and another near 705 nm. Therefore, we are able to conclude that only these two bands’ reflectance differ between the sunlit and the shaded elements of the canopy. Their reflectance changes on time scales of a few minutes or less. Our observations also show that the reflectance changes at 531 nm are more highly correlated with variations in canopy light use efficiency when only sunlit canopy elements are viewed (the hot spot), than when only shaded elements (the dark spot) are viewed. Taken together then, these results demonstrate that the observed sub-hourly changes in foliage reflectance at 531 nm and 705 nm can only result from corresponding variations in photosynthetic rates. The importance of our results are as follows: (1) We show that variations in PRI with LUE are a direct result of rapid changes in foliage reflectance at 531 nm resulting from photosynthetic down-regulation, and can be observed at forest scales. (2) Our findings also suggest a new sensor and methodology for the direct retrieval from space of changes in forest LUE by measuring PRI as a function of shadow fraction using a multi-angle spectrometer simultaneously retrieving both shadow fraction and PRI.
by André Boily | 30 January 2019
Abstract of the paper presented to the IEEE International Geoscience and Remote Sensing Symposium (IGARSS). September 20-24, 2004. Anchorage, Alaska. 3: 2038-2041.
Understanding the dynamics of the global carbon cycle requires an accurate determination of the spatial and temporal distribution of photosynthetic CO2 uptake by terrestrial vegetation. Optimal photosynthetic function is negatively affected by stress factors that cause down-regulation (i.e., reduced rate of photosynthesis). Present approaches to determine ecosystem carbon exchange rely on meteorological data as inputs to models that predict the relative photosynthetic function in response to environmental conditions inducing stress (e.g., drought, high/low temperatures). (…)
by Claire Morin | 30 January 2019
Published in Remote Sensing of Environment 113: 2463-2475
Estimation of photosynthetic light use efficiency (ε) from satellite observations is an important component of climate change research. The photochemical reflectance index, a narrow waveband index based on the reflectance at 531 and 570 nm, allows sampling of the photosynthetic activity of leaves; upscaling of these measurements to landscape and global scales, however, remains challenging. Only a few studies have used spaceborne observations of PRI so far, and research has largely focused on the MODIS sensor. Its daily global coverage and the capacity to detect a narrow reflectance band at 531 nm make it the best available choice for sensing ε from space. Previous results however, have identified a number of key issues with MODIS-based observations of PRI. First, the differences between the footprint of eddy covariance (EC) measurements and the MODIS footprint, which is determined by the sensor’s observation geometry make a direct comparison between both data sources challenging and second, the PRI reflectance bands are affected by atmospheric scattering effects confounding the existing physiological signal. In this study we introduce a new approach for upscaling EC based ε measurements to MODIS. First, EC-measured ε values were “translated” into a tower-level optical PRI signal using AMSPEC, an automated multi-angular, tower-based spectroradiometer instrument. AMSPEC enabled us to adjust tower-measured PRI values to the individual viewing geometry of each MODIS overpass. Second, MODIS data were atmospherically corrected using a Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm, which uses a time series approach and an image-based rather than pixel-based processing for simultaneous retrievals of atmospheric aerosol and surface bidirectional reflectance (BRDF). Using this approach, we found a strong relationship between tower-based and spaceborne reflectance measurements (r2=0.74, p<0.01) throughout the vegetation period of 2006. Swath (non-gridded) observations yielded stronger correlations than gridded data (r2=0.58, p<0.01) both of which included forward and backscatter observations. Spaceborne PRI values were strongly related to canopy shadow fractions and varied with different levels of ε. We conclude that MAIAC-corrected MODIS observations were able to track the site-level physiological changes from space throughout the observation period.
