Kamel Soudani

Integrating clumping effects in forest canopy structure: an assessment through hemispherical photographs

Methods for analysing foliage nonrandomness in forest canopies by means of hemispherical photographs are assessed. These methods involve calculation of the canopy element clumping factor, at scales coarser than that of the shoot, to adjust for clumping effects on leaf area index (LAI) estimates derived from gap fraction measurements. Two approaches are presented. The first is based on a gap size accumulation method (the Chen and Cihlar clumping index), whereas the second relies on a gap size distribution method (the Pielou coefficient of segregation). Both methods take advantage of hemispherical photographs, allowing measurement of gap size and gap fraction from sequences of black (foliage) and white (sky) pixels along circular transects over the whole range of angles. Clumping factors generated by hemispherical photographs have been analysed and compared using (i) hemispherical photographs of simulated forest stands with fixed LAI (2, 4, and 6) and three-dimensional spatial distribution of foliage, ranging from complete randomness to full clumping; and (ii) in situ hemispherical photographs from forest canopies with known architecture. The following conclusions are drawn based on our experimental data set: (i) overall clumping factors can be obtained by integrating values over the same range of angles as that used to derive LAI, therefore ensuring consistency between estimations of LAI and clumping factors; (ii) the Chen and Cihlar clumping index tends to underestimate clumping in highly clumped canopies, whereas the Pielou coefficient of segregation tends to overestimate clumping in poorly clumped canopies; and (iii) hemispherical photographs provide an efficient tool for describing the degree of canopy nonrandomness in all directions of the hemisphere and to adjust clumping effects on LAI estimates derived from gap fraction analysis. However, clumping factors derived from hemispherical photographs need to be further tested in real canopies and compared with other methods to define their merits and limitations.

Indirect Measurement of Forest Canopy Structure from In Situ Optical Sensors

This Chapter covers the major requirements for the estimation of forest canopy structure from in situ remote sensing. In situ measurements of forest canopy structure provide the basis for remote sensing of forest attributes at all scales. Selection of a measurement strategy requires knowledge of available measurement methods and must address data set availability, constraints in forest applications and scale related considerations. The main objective of this Chapter is to describe methods that estimate parameters related to forest canopy structure. This will be accomplished through: (i) defining and describing the relevant elements of the canopy structure, (ii) providing background information on the theory and practice of in situ measurements of canopy structure, together with their limitations, and (iii) suggesting strategies for generalising ground measurements into spatially extended estimations.

Estimation of forest leaf area index from SPOT imagery using NDVI distribution over forest stands

Leaf area index (LAI) is a key parameter of atmosphere–vegetation exchanges, affecting the net ecosystem exchange and the productivity. At regional or continental scales, LAI can be estimated by remotely‐sensed spectral vegetation indices (SVI). Nevertheless, relationships between LAI and SVI show saturation for LAI values greater than 3–5. This is one of the principal limitations of remote sensing of LAI in forest canopies. In this article, a new approach is developed to determine LAI from the spatial variability of radiometric data. To test this method, in situ measurements for LAI of 40 stands, with three dominant species (European beech, oak and Scots pine) were available over 5 years in the Fontainebleau forest near Paris. If all years and all species are pooled, a good linear relationship without saturation is founded between average stand LAI measurements and a model combining the logarithm of the standard deviation and the skewness of the normalized difference vegetation index (NDVI) (R 2 = 0.73 rmse = 1.08). We demonstrate that this relation can be slightly improved by using different linear models for each year and each species (R 2 = 0.82 rmse = 0.86), but the standard deviation is less sensitive to the species and the year effects than the mean NDVI and is therefore a performing index.

Leaf area index from litter collection: impact of specific leaf area variability within a beech stand

Litter fall collection is a direct method widely used to estimate leaf area index (LAI) in broad-leaved forest stands. Indirect measurements using radiation transmittance and gap fraction theory are often compared and calibrated against litter fall, which is considered as a reference method, but few studies address the question of litter specific leaf area (SLA) measurement and variability. SLA (leaf area per unit of dry weight, m2·g‐1) is used to convert dry leaf litter biomass (g·m‐2) into leaf area per ground unit area (m2·m‐2). We paid special attention to this parameter in two young beech stands (dense and thinned) in northeastern France. The variability of both canopy (closure, LAI) and site conditions (soil properties, vegetation) was investigated as potential contributing factors to beech SLA variability. A systematic description of soil and floristic composition was performed and three types of soil were identified. Ellenbergs indicator values were averaged for each plot to assess nitrogen soil content. SLA of beech litter was measured three times during the fall in 23 plots in the stands (40 ha). Litter was collected bimonthly in square-shaped traps (0.5 m2) and dried. Before drying, 30 leaves per plot and for each date were sampled, and leaf length, width, and area were measured with the help of a LI-COR areameter. SLA was calculated as the ratio of cumulated leaf area to total dry weight of the 30 leaves. Leaves characteristics per plot were averaged for the three dates of litter collection. Plant area index (PAI), estimated using the LAI-2000 plant canopy analyser and considering only the upper three rings, ranged from 2.9 to 8.1. Specific leaf area of beech litter was also highly different from one plot to the other, ranging from 150 to 320 cm2·g‐1. Nevertheless, no relationship was found between SLA and stand canopy closure or PAI. On the contrary, a significant relationship between SLA and soil properties was observed. Both SLA and leaf area had the lowest values in the most hydromorphic soil with the highest nitrogen content. On the other hand, the highest values of SLA and leaf area were observed on the plots with the lowest nitrogen content. This spatial variability of SLA was taken into account to estimate LAI from litter collected at eight plots. For our study site, we conclude that neglecting SLA spatial variability is at the source of 8‐24% error in the calculation of LAI.

Relationship between photochemical reflectance index and leaf ecophysiological and biochemical parameters under two different water statuses: towards a rapid and efficient correction method using real-time measurements

The use of the photochemical reflectance index (PRI) as a promising proxy of light use efficiency (LUE) has been extensively studied, and some issues have been identified, notably the sensitivity of PRI to leaf pigment composition and the variability in PRI response to LUE because of stress. In this study, we introduce a method that enables us to track the short-term PRI response to LUE changes because of photosynthetically active radiation (PAR) changes. The analysis of these short-term relationships between PRI and LUE throughout the growing season in two species (Quercus robur L. and Fagus sylvatica L.) under two different soil water statuses showed a clear change in PRI response to LUE, which is related to leaf pigment content. The use of an estimated or approximated PRI0, defined as the PRI of perfectly dark-adapted leaves, allowed us to separate the PRI variability due to leaf pigment content changes and the physiologically related PRI variability over both daily (PAR-related) and seasonal (soil water content-related) scales. The corrected PRI obtained by subtracting PRI0 from the PRI measurements showed a good correlation with the LUE over both of the species, soil water statuses and over the entire growing season.

Deconvolution of pigment and physiologically related photochemical reflectance index variability at the canopy scale over an entire growing season

The sensitivity of the photochemical reflectance index (PRI) to leaf pigmentation and its impacts on its potential as a proxy for light-use efficiency (LUE) have recently been shown to be problematic at the leaf scale. Most leaf-to-leaf and seasonal variability can be explained by such a confounding effect. This study relies on the analysis of PRI light curves that were generated at the canopy scale under natural conditions to derive a precise deconvolution of pigment-related and physiologically related variability in the PRI. These sources of variability were explained by measured or estimated physiologically relevant variables, such as soil water content, that can be used as indicators of water availability and canopy chlorophyll content. The PRI mainly reflected the variability in the pigment content of the canopy. However, the corrected PRI, which was obtained by subtracting the pigment-related seasonal variability from the PRI measurement, was highly correlated with the upscaled LUE measurements. Moreover, the sensitivity of the PRI to the leaf pigment content may mask the PRI versus LUE relationship or result in an artificial relationship that reflects the relationship of chlorophyll versus LUE, depending on the species phenology.

Root exclusion through trenching does not affect the isotopic composition of soil CO2 efflux

Disentangling the autotrophic and heterotrophic components of soil CO2 efflux is critical to understanding the role of soil system in terrestrial carbon (C) cycling. In this study, we combined a stable C-isotope natural abundance approach with the trenched plot method to determine if root exclusion significantly affected the isotopic composition (δ13C) of soil CO2 efflux (RS). This study was performed in different forest ecosystems: a tropical rainforest and two temperate broadleaved forests, where trenched plots had previously been installed. At each site, RS and its δ13C (δ13CRs) tended to be lower in trenched plots than in control plots. Contrary to RS, δ13CRs differences were not significant. This observation is consistent with the small differences in δ13C measured on organic matter from root, litter and soil. The lack of an effect on δ13CRs by root exclusion could be from the small difference in δ13C between autotrophic and heterotrophic soil respirations, but further investigations are needed because of potential artefacts associated with the root exclusion technique.

Leaf area index and canopy stratification in Scots pine ( Pinus sylvestris L.) stands

In order to assess the contribution of each layer to effective leaf area index (LAI) in stratified forest canopies, two Scots pine stands were selected in Haguenau forest, Alsace, France. One stand, 80 years old, had a sparse tree layer and a dense herb layer. The other stand, 50 years old, had a closed tree layer and dense herb and shrub layers. Hemispherical photographs, taken at 10-metre intervals along transects beneath each layer, were digitized and analysed. In the two-layered stand, mean LAI was 0.9 for herbs and 1.5 for trees, contributing 37% and 63%, respectively, to total LAI. A positive and significant correlation was found between layers, suggesting that the herb layer had not yet adjusted to the recent thinning of the stand. In the three-layered stand, mean LAI was 1.1 for herbs, 0.4 for shrubs and 2.1 for trees, a contribution of 31%, 11% and 58%, respectively, to total LAI. The positive and significant correlation for LAI among layers may be explained by the presence of a mixture of light demanding and shade-tolerant species in the undergrowth. Horizontal spatial auto-covariance of estimates was not significant, denoting adequate sampling interval for statistically independent measurements. Such in situ assessments of LAI, differentiating vegetation strata, are useful from the perspective of remote sensing of forests, since undergrowth plays an important role in the integrated spectral response of whole canopies.

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.

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 years 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.