**Section 1**

**Land Cover** 

**1** 

*University of Helsinki* 

*Finland* 

**Narrowband Vegetation Indices for** 

**Estimating Boreal Forest Leaf Area Index** 

Veli-Heikki Vesanto, Titta Majasalmi and Pauline Stenberg

Ellen Eigemeier, Janne Heiskanen, Miina Rautiainen, Matti Mõttus,

The green photosynthesizing leaf area of a canopy is an important characteristic of the status of the vegetation in terms of its health and production potential. At stand level, the amount of leaf area in a canopy is represented by a variable called the leaf area index (LAI), which is one of the key biophysical parameters in the global monitoring and mapping of vegetation by satellite remote sensing (Morisette et al., 2006). In this paper we adopt the, by now widely accepted, definition of LAI as the hemi-surface or half of the total surface area of all leaves or needles in the vegetation canopy divided by the horizontal ground area below the canopy. The definition is in line with the original definition of LAI, formulated for flat and (assumedly) infinitely thin leaves (Watson, 1947), as the one-sided leaf area per unit ground area. For coniferous canopies, the question arose on how to define the "one-sided" area of non-flat needles. While projected needle area formerly often has been used erroneously as a synonym to one-sided flat leaf area, it is now commonly accepted that the hemi-surface needle area represents the logical counterpart to the one-sided area of flat leaves (e.g. Chen

LAI controls many biological and physical processes, driving the exchange of matter and energy flow. Because LAI responds rapidly to different stress factors and changes in climatic conditions, monitoring of LAI yields a dynamic indicator of forest status and health. The link between forest productivity and LAI, in turn, lies in that LAI is the main determinant of the fraction of incoming photosynthetically active radiation absorbed by the canopy (fAPAR). The absorbed photosynthetically active radiation (APAR) quantifies the energy available for net primary production (NPP) and is thus a critical variable in NPP and carbon flux models. NPP is related to APAR by the light-use-efficiency originally introduced by

Traditionally, ground-based measurements of LAI have typically involved destructive sampling and determination of allometric relationships, e.g. between leaf area and the basal area of stem and/or branches carrying the leaves (the pipe model theory) (Shinozaki et al., 1964; Waring et al., 1982). However, such "direct methods" are quite laborious and indirect measurements of LAI using optical instruments are today the preferred choice (Welles &

**1. Introduction 1.1 Leaf area index** 

& Black, 1992; Stenberg, 2006).

Monteith (1977) for agricultural crops.
