Leaf area index (LAI) is the total one-sided area of leaf tissue per unit ground surface area. It is a key parameter in ecophysiology, especially for scaling up the gas exchange from leaf to canopy level. It characterizes the canopy‐atmosphere interface, where most of the energy fluxes exchange. It is also one of the most difficult to quantify properly, owing to large spatial and temporal variability. Many methods have been developed to quantify LAI from the ground and some of them are also suitable for describing other structural parameters of the canopy. This paper reviews the direct and indirect methods, the required instruments, their advantages, disadvantages and accuracy of the results. Analysis of the literature shows that most cross-validations between direct and indirect methods have pointed to a significant underestimation of LAI with the latter techniques, especially in forest stands. The two main causes for the discrepancy, clumping and contribution of stem and branches, are discussed and some recent theoretical or technical solutions are presented as potential improvements to reduce bias or discrepancies. The accuracy, sampling strategy and spatial validity of the LAI measurements have to be assessed for quality assurance of both the measurement and the modelling purposes of all the LAI-dependent ecophysiological and biophysical processes of canopies.

Ground‐based measurements of leaf area index: a review of methods, instruments and current controversies

During low water levels, habitats in river-floodplain systems are isolated from each other and from the main river. Oppositely, floods tend to connect water bodies with distinct hydrological characteristics and, as a result, ecological processes and biological communities tend to be more similar among the distinct habitats that comprise a river-floodplain system. Based on a literature review and also using unpublished data obtained in tropical floodplains, the aim of this paper is to highlight the effects of floods as a process that reduce spatial variability. The usual negative relationship between the coefficient of variation of any ecological indicator (e.g., chlorophyll-a or total phosphorus) and water level is the main result demonstrating a reduction in spatial variability due to floods. Considering physical, chemical or biotic data gathered in distinct habitats within the floodplain, this pattern was found in temperate and tropical regions, subjected to distinct levels of anthropogenic impacts, and at different spatial extents. The main mechanism that accounts for this pattern may be stated as follow. During low water level, the biological communities of each habitat in the floodplain (e.g. lagoons, backwater, streams) follow distinct temporal trajectories due to the effects of local driving forces (e.g. an efficient predator trapped in a lagoon but not in another). Management plans and biodiversity conservation in river floodplain systems will benefit by considering the effects of flood homogenization and increased connectivity peculiar to these unique ecosystems.

Floods increase similarity among aquatic habitats in river-floodplain systems

Variation in forest canopy structure influences both understory light availability and its spatial distribution. Because light is a major environmental factor limiting growth and survival of many forest species, its distribution may affect stand-level regeneration patterns. We examined spatial patterning in light availability and seedling regeneration in old-growth, second-growth, and selectively logged stands of tropical moist forest in northeastern Costa Rica. Our objectives were to determine how the frequency distribution and spatial pattern of understory light “microsites” differ among tropical wet forests; whether patterns of seedling regeneration are linked to spatial patterning of light availability; and whether these relationships differ among old-growth, second-growth, and selectively logged forest stands. We used both sensor-based and hemispherical photograph-based methods to measure light availability along three 130–160 m long transects in each of eight stands (three old-growth, three second-g...

Spatial heterogeneity of light and woody seedling regeneration in tropical wet forests

http://faculty.geog.utoronto.ca/Chen/Chen%27s%20homepage/PDFfiles/p91_lebl5_AFM.pdf

Methodology comparison for canopy structure parameters extraction from digital hemispherical photography in boreal forests

The study shows that leaf area index (LAI), leaf chlorophyll content (LCC) and canopy chlorophyll content (CCC) can be mapped in a heterogeneous Mediterranean grassland from canopy spectral reflectance measurements. Canopy spectral measurements were made in the field using a GER 3700 spectroradiometer, along with concomitant in situ measurements of LAI and LCC. We tested the utility of univariate techniques involving narrow band vegetation indices and the red edge inflection point, as well as multivariate calibration techniques, including stepwise multiple linear regression and partial least squares regression. Among the various investigated models, CCC was estimated with the highest accuracy ( R cv 2 = 0.74 , nRMSE cv = 0.35 ). All methods failed to estimate LCC ( R cv 2 ≤ 0.40 ), while LAI was estimated with intermediate accuracy ( R cv 2 values ranged from 0.49 to 0.69). Compared with narrow band indices and red edge inflection point, stepwise multiple linear regression generally improved the estimation of LAI. The estimations were further improved when partial least squares regression was used. When a subset of wavelengths was analyzed, it was found that partial least squares regression had reduced the error in the retrieved parameters. The results of the study highlight the significance of multivariate techniques, such as partial least squares regression, rather than univariate methods such as vegetation indices in estimating heterogeneous grass canopy characteristics.

LAI and chlorophyll estimation for a heterogeneous grassland using hyperspectral measurements

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, rangi...

http://max2.ese.u-psud.fr/publications/2003walte_integr.pdf

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

Regulation works in the Upper Rhine carried out since the 19th century have drastically modified the floodplain forest ecosystem in the Alsace Plain. These works have disconnected the largest part of the riparian forest from the river and interrupted the flooding. This study aims at assessing the effects of the flood interruption on woody regeneration in three hardwood stands: one still liable to flooding, the second one unflooded for 30 years and the third one unflooded for 130 years. Fieldwork was carried out to determine the species richness of seed rain, seed bank and extant vegetation of the regeneration understorey. The potential species richness was calculated using the first-order jackknife estimator. In addition, floristic composition and diversity indices of extant vegetation were calculated. Results show that species richness of the seed bank is lower than seed rain and extant vegetation. The species richness of the extant vegetation increases with the duration of interruption of the floods in the Rhine forest as a result of introduction of flood-intolerant species in the unflooded forest. Although species richness of the extant vegetation was the lowest in the flooded site Shannon-Weaver diversity and equitability were found to be the highest, because the disturbance caused by floods allows a more equal coexistence of several species. Conversely, in the unflooded sites, a small number of species represent most of the woody individuals of the regeneration phase. Implications of flood restoration on species richness and diversity of the regeneration compartment are discussed.

Effects of flood interruption on species richness, diversity and floristic composition of woody regeneration in the upper rhine alluvial hardwood forest

Regeneration strategies in a temperate hardwood floodplain forest of the Upper Rhine: sexual versus vegetative reproduction of woody species

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.

Indirect Measurement of Forest Canopy Structure from In Situ Optical Sensors

Leaf area index (LAI) measured with fish-eye sensors is inevitably affected by slope. In order to quantify this effect, LAIs measured in contrasted forests were corrected for slope. Digitised hemispherical photographs were analysed to compute LAI according to azimuth and as a function of slope. The topographic mask (part of hemisphere obstructed by slope) and the path length of a ray of light at a given zenith angle (longer upslope and shorter downslope) were taken into account. Results indicate a strong slope effect, both in temperate conifer forest plantation and in tropical rain forest. On regular slope, this effect is easily corrected, although the topographic mask affects data. On irregular topography, corrected and uncorrected LAIs strongly differ, suggesting a need for reference measurements; corrected figures appear nevertheless realistic and normality is reached only for corrected values. LAIs should systematically be corrected for slope above 15-20 degrees, more so if topography is irregular or the forest cover heterogeneous.

Jean-Michel N. Walter

Papers

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

Effects of flood interruption on species richness, diversity and floristic composition of woody regeneration in the upper rhine alluvial hardwood forest

Regeneration strategies in a temperate hardwood floodplain forest of the Upper Rhine: sexual versus vegetative reproduction of woody species

Indirect Measurement of Forest Canopy Structure from In Situ Optical Sensors

The computation of forest leaf area index on slope using fish-eye sensors.