2. PRODUCTIVITY - LEAF AREA CONCEPTS

It is now generally accepted that much of the variation in biomass and wood production can be accounted for by variation in light interception. (Landsberg and Gower 1997, Cannell 1989, Linder 1987). Light interception is principally a function of the amount of leaf area and the duration of leaf area display. Differences in individual tree crown architecture and stand canopy structure can also affect light interception. Empirical data from field studies with Scots pine, Norway spruce (Bergh et al 1998, Linder 1987), radiata pine (Linder et al. 1987, Benson et al. 1992, Snowdon and Benson 1992), southern pines (Colbert et al. 1990, Vose and Allen 1988, Albaugh et al. 1998), and Eucalyptus globulus (Pereira et al. 1994) have shown that leaf area and consequently wood production are below optimum levels in many areas of the world. Low nutrient availability is a principal factor causing suboptimal levels of leaf area in many areas (Albaugh et al 1998, 2004, Colbert et al. 1990, Linder 1987, Vose and Allen, 1988). Low soil water availability, high vapor pressure deficits, and high temperatures have also been shown to adversely affect leaf area production and/or retention (Benson et al. 1992, Hennessey et al. 1992, Pereira et al. 1994, Teskey et al. 1987).

The variation in production per unit of leaf area (generally defined as the slope of the wood production – leaf area relationship and referred to as growth or leaf area efficiency) can also contribute to the variation in production. Growth efficiency can vary do to differences in photosynthetic efficiency, respiration, and partitioning to various biomass components. Improved nutrient and water availability has been shown to increase photosynthetic efficiency (Linder 1987, Murthy et al 1996), and above ground productivity proportionally more than below ground productivity in stand level studies (Albaugh et al. 1998, 2004, Gower et al. 1992, Haynes and Gower 1995). On an individual site basis, changes in growth efficiency generally do not contribute to as much to changes in productivity as changes in leaf area. However, when species (e.g. loblolly pine in the Southeast or radiata pine in Chile) are planted across large areas with substantial regional differences in precipitation and temperature, region variation in growth efficiency may be substantial (Sampson and Allen 1999).

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