## INTRODUCTION

The aggregation of narrow needles in clumped shoots is a common architectural strategy in several *genera* of conifers such as *Abies*, *Picea*, *Pinus*, *Pseudotsuga* and *Larix*. Needle clumping in shoots has a profound impact on ecosystem processes, as it affects the radiative regime, the photosynthesis and the aerodynamics of coniferous canopies (Norman & Jarvis 1974; Leverenz & Jarvis 1980; Carter & Smith 1985; Stenberg 1996a).

The relationships between shoot morphology and microclimate have been investigated extensively by several authors, both from an experimental (Smith & Carter 1988; Leverenz & Hinckley 1990) and theoretical point of view (Oker-Blom & Kellomäki 1983; Cescatti 1998). The plasticity of conifers in developing variable architectures according to the light microclimate suggests that shoot structure is relevant to the overall acclimation process of these canopies (Niinemets & Kull 1995a).

Several empirical coefficients have been proposed to account for the effect of needle clumping in shoots on the interception efficiency of the leaf area and on the canopy gap fraction (Oker-Blom & Smolander 1988; Chen 1996). These clumping indices, based on the ratio of the shoot silhouette area over different view angles to the needle area, can be readily integrated into light interception models based on the turbid medium analogy (Stenberg 1996a). The ratio of the spherical mean shoot silhouette to total needle area ( ) has also been proposed as a correction factor in the indirect estimation of leaf area index (LAI) based on the inversion of gap fraction data (Stenberg 1996b). Furthermore, the angular description of the shoot to needle area ratio, together with a detailed reconstruction of the light microclimate based on hemispherical photographs, have been used to estimate the light intercepted by single shoots over the whole growing season (Stenberg *et al*. 1998).

Nilson & Ross (1997) presented a first approach to the application of the turbid medium analogy at the shoot scale, assuming a random needle distribution in cylindrically idealized shoots. On the basis of these assumptions, the mean number of contacts was estimated as the ratio of the observed needle silhouette area to the projected area of the cylindrical shoot.

In order to overcome the limitations of silhouette analysis, several attempts have been made to describe the shoot radiative field with geometrical models based on a deterministic description of needle shape and position (Oker-Blom 1985; Wang & Jarvis 1993). In contrast, detailed experimental descriptions of the light microclimate have been obtained with the application of innovative multipoint light sensors to Scots pine shoots (Palmroth *et al*. 1999).

The scientific relevance of describing the shoot radiative field is mainly linked to the typical non-linearity of the physiological responses to light intensity. In fact, a detailed reconstruction of the shoot radiative regime is required to estimate the needle photosynthetic capacity from shoot level measurements in direct light, or to upscale needle responses to shoot and canopy levels (Smolander *et al*. 1987; Wang & Jarvis 1993).

In this study, a new methodology to investigate the architectural features and radiative regime of conifer shoots is presented. In comparison with the current available methodologies, this indirect approach has the following advantages:

- 1it is based on a consistent procedure to retrieve architectural parameters from standard measurements such as shoot dimensions and silhouette areas;
- 2it allows the analytical description of the light distribution on the leaf area, and therefore is appropriate for upscaling photosynthetic responses from needle to shoot level;
- 3it can be embedded in canopy radiative transfer models based on the turbid medium analogy in order to predict light distribution at the canopy scale.