To understand the fundamental processes and mechanisms driving stand productiv­ity, we need to introduce the so-called production ecology equation (after Landsberg and Waring 1997), which states that

NPP = iPAR * ac * R,

where

NPP = net primary production

iPAR = intercepted photosynthetically absorbed radiation

ac = canopy quantum efficiency (mol of C sequestered per mol radiation absorbed) R = Respiration (a fairly constant value in young tree crops).

Figure 5.2 shows the theoretical development of leaf area index over time in short rotation eucalypt pulpwood crops with relatively high stand densities (after data from du Toit and Dovey 2005; du Toit et al. 2008; White et al. 2009). The three scenario’s in Fig. 5.2 are: (i) low-level silvicultural inputs, (ii) intensive silvicultural inputs that temporarily improve resource availability, and (iii) Intensive site or silvicultural treatments that ensured a prolonged improvement in resource availability. The latter two responses have also been labelled as Type A (Type II) and Type B (Type I) responses (Snowdon and Waring 1984; Snowdon 2002; Rubilar et al. 2008). The areas under the curves represent the cumulative leaf area that is deployed over the rotation, which is responsible for radiation interception and hence, photosynthesis.