On the relationship between crown cover, foliage projective cover and leaf area index

Abstract

Vegetation crown cover is a popular structural descriptor in both the remote sensing and ecological fields. However, several different definitions of vegetation crown cover are commonly measured and used, and relating one against the other is often difficult. In this study we derive simple expressions relating mature forest crown cover (CC), foliage projective cover (FPC) and leaf area index (LAI) using canopy gap fraction and canopy clumping theory. The relationship between FPC and CC is shown to be related to the ratio of the within canopy to between canopy clumping. A comprehensive field data set collected in forests and woodlands across Queensland is used to demonstrate the validity of the models. The results have widespread application in the modelling and analysis of forest structure. Introduction Cover of natural systems is complex to describe and map. In remote sensing, several measurements of vegetation crown cover are commonly used. These are crown cover (CC), foliage projective cover (FPC) and leaf area index (LAI). CC is defined as the percentage of ground area covered by the vertical projection of crowns, which in this case are assumed opaque. FPC is the percentage of ground area covered by the vertical projection of foliage (Walker and Hopkins 1990). LAI is defined here as half the total developed area of leaves per unit ground horizontal surface area (Chen et al. 1997). CC is a primary stand density variable in many forest inventories since it is readily understood by a variety of practitioners. It is relatively easily estimated in the field (Walker and Hopkins 1990) and from aerial photography (Fensham et al. 2003). FPC is more complex to measure in the field and is typically measured using a transect based method (Johansson 1985). FPC is more closely related to the photosynthetic and evaporative potential of a plant community than CC due to the low density foliage typical of many Australian woody species (Specht 1981). Herein, FPC refers to the overstorey component only, which includes woody life forms above 2 m (Specht 1983). The LAI is the main variable used to model many processes, such as canopy photosynthesis and evapotranspiration. It determines the size of the plant–atmosphere interface and thus plays a key role in the exchange of energy and mass between the canopy and the atmosphere (Chen et al. 1997; Simioni et al. 2003). CC treats the crown as an opaque object, however the FPC of individual crowns for most Australian woody plants is between 40% and 70% depending on crown architecture (Walker and Hopkins 1990). For the same leaf area, variation in foliage clumping and leaf orientation angle determines the FPC of individual crowns (Campbell 1990; Henry et al. 2002; Kucharik et al. 1999). Indeed, high variation in leaf orientation angle and foliage clumping across Australian woody species has been reported in the literature and shown to function to reduce levels of intercepted light at high sun elevations (Falster and Westoby 2003; King 1997). Forests are defined as vegetation greater than 2 m in height with a minimum crown cover of 20 %. A CC of 20% is suggested as being equivalent to a FPC approximating of 10 to 12% (Australia National Forest Inventory et al. 2003.). Despite the use of both FPC and CC as indicators of vegetation condition and structure in Australia there is a paucity of work quantifying the relationship across a range of native plant communities in Australia. This research links established models of canopy light interception, and used field data collected from plant communities across Queensland, Australia to fit the parameters in the models, resulting in an improved understanding of the usage, linkage and applicability of these fundamental cover metrics