Tiit Nilson

A Reflectance Model for the Homogeneous Plant Canopy and Its Inversion

Abstract An analytical reflectance model for a statistically homogeneous plant canopy has been developed. The most specific characteristics of the model are: 1) considering both the single and the multiple scattering of radiation in the canopy and on the soil and 2) accounting for the specular reflection of radiation on leaves and canopy hot spot. For the inversion of the model the technique suggested by Goel and Strebel (1983) has been applied. The reflectance model fits well the results of measurements both of the seasonal course of the nadir reflectance and of the angular distribution of the directional reflectance of the winter wheat and barley canopies.

A directional multispectral forest reflectance model.

Abstract The forest reflectance model by Nilson and Peterson has been extensively modified. Leaf optics model PROSPECT2, atmosphere radiative transfer model 6S, and homogeneous canopy reflectance model MCRM have been incorporated into the model. The new model works in the spectral region 400–2500 nm with the same set of input parameters. Any Sun and view directions are allowed. Sample calculations are compared to published data on the old black spruce stand of the BOREAS Southern Study Area. The new model is able to adequately simulate both the reflectance spectrum and angular distribution. The determination of the whole set of input parameters of the model is a difficult practical problem.

Inversion of gap frequency data in forest stands

The inversion of gap proportion data for canopy structure variables has been widely used in plant canopy analysers. However, the results for most forest canopies tend to be biased, due to the foliage clustering at different structural levels. To invert the gap frequency data in forest stands for the canopy leaf area index, new gap fraction formulas are proposed capable of taking explicitly into account the clustering of foliage into crowns as one of the most important clustering levels. The formulas are based on the given mathematical expressions for the crown shape and on the Poisson or binomial tree distribution patterns.

Age dependence of forest reflectance: Analysis of main driving factors

Abstract As an analogy to forest yield tables, successional trajectories of forest reflectance can be established. Examples of such deterministic reflectance age trajectories for common northern temperate forest types in Estonia have been derived from airborne reflectance measurements and simulated by a forest reflectance model. In the simulations, the yield tables were used as input data. The primary controls on the reflectance age course are: changes in canopy closure, tree-storey leaf-area index, species composition, and background reflectance. In long-term monitoring of forest stands, the effects of sun and view angle and phenology must be considered. Successional reflectance trajectories may further form a basis for a forest inventory and monitoring system. Provided that the problems of normalization of multidate satellite imageries and seasonal signature variation are solved, the reflectance change of a particular forest stand may be compared to the change predicted by established reflectance successional trajectories. In case of normal forest development, the measured reflectance changes must be close to those predicted. Some kinds of the disturbance-type forest changes (e.g., extensive thinning due to damage) are detectable by optical methods.

Recent advances in geometrical optical modelling and its applications

Geometrical optical (GO) modelling applies to canopies with distinct architecture and is particularly appropriate for forest canopies. Although sophisticated 3‐dimensional radiative transfer models are developed, GO models remain attractive tools for remote sensing applications for their ability to capture the angular and spatial variabilities of reflectances using simple geometries defined by canopy architecture. In this paper, we review recent developments in GO modelling methodology and a wide range of applications of GO models for vegetation information retrieval. This review covers GO applications in both optical and thermal spectral regions. Existing problems and limitations of GO models will be discussed and future research directions are thus suggested.

A forest canopy reflectance model and a test case

Abstract A brief description of a new forest reflectance model is given. As model input parameters, several ordinary forest mensuration parameters can be used. Results from model calculations compared with those from reflectance measurements with an airborne radiometer for 12 Scots pine-dominated boreal forests in Estonia have shown a rather good agreement in the red region both in summer and winter conditions. The coincidence is the poorest in the green and near infrared regions in summer. In winter with the snow-covered grounds, in spite of the acceptable r2 values of the respective linear regression, there tends to be systematic overestimation of simulated reflectance values. Further tests of the model are needed to confirm its ability to predict the relations of forest reflectance with most important structural and optical model input parameters.

Successional reflectance trajectories in northern temperate forests

Abstract Reflectance change during a management rotation period in monospecific even-aged birch (Betula pendula), (B. pubescens) and pine (Pinus sylvestris) stands is described. Successional reflectance trajectories were fitted to airborne reflectance factor (RF) data for different ages of forest. Changes in RF during secondary succession were mainly directional as they were related to time after clearcutting. There was a monotonic decrease of ‘Brightness’. No evidence of convergence or divergence of successional reflectance trajectories of forests from fertile and infertile sites was noted if the overstory was monospecific.

Modeling Radiative Transfer through Forest Canopies: Implications for Canopy Photosynthesis and Remote Sensing

In order to theoretically describe the radiative transfer inside a forest canopy, information must be obtained on the following basic geometrical and optical characteristics: the geometrical cross section of foliage elements, three-dimensional distribution of their area volume density and the phase function. In coniferous trees and stands, it is reasonable to consider one-yr-old shoots main foliage elements. Theoretical problems related to the determination of optical parameters in the hierarchical levels of needle, shoot, crown and canopy are discussed and a few examples demonstrating the structural and optical complexities of forest communities are presented. A brief description of the basic components of a new forest ecosystem radiation model and some examples of the results obtained are given.

Determination of needle area indices of coniferous forest canopies in the NOPEX region by ground-based optical measurements and satellite images

Abstract Eight coniferous ‘ground truth’ stands were studied in the NOPEX region. Needle area indices (LAI) of the stands were estimated via regressions on the tree parameters and stand density measured on the respective sample plots. Reflectances of the ground truth stands were simulated by a forest reflectance model and related to the canopy LAI. In all short-wave spectral bands of Landsat TM and SPOT, decreasing non-linear trends of stand reflectance along with an increase in LAI were observed. The trends were confirmed by an analysis of satellite images. With the logarithmic regression of stand LAI on Landsat TM or SPOT DNs, the ground truth information can be extended to the rest of coniferous forests in the NOPEX region with approximate relative error in LAI 30–40%. The best spectral bands appeared to be TM5, SPOT XS2, SPOT XS1, TM7 and TM3. Canopy LAI can be successfully estimated from the coefficients of penetration of direct and diffuse radiation to the forest floor. This has been demonstrated by an analysis of the results of ground-based measurements of canopy transmission of the ground truth stands by means of a four-channel field radiometer.

Radiative Transfer in Nonhomogeneous Plant Canopies

The transfer of solar radiation in plant canopies has been explored both theoretically and experimentally during the last three decades. Typically, radiation transfer in plant canopies is examined in terms of three problems: (1) Theoretical modeling of the phyotosynthetic productivity in plant stands and production ecology; (2) heat and mass transfer in vegetation as a boundary layer to the earth’s atmosphere; (3) remote sensing of agricultural crops, forests and other types of man-created or natural vegetation.