Terrestrial plants vary widely in their yearly yields per hectare. Yields are depen­dent on insolation, temperature, the presence of nutrients and water and the nature of plants (Coombs et al. 1987). In natural ecosystems on average, the efficiency of photosynthesis in converting solar energy into plant material is usually in the or­der of 0.1-0.3% (Mezhunts and Givens 2004; Rosing et al. 2006). In the case of cultivated plants, higher conversion efficiencies are achievable. The highest yields are usually achieved in experiments under ‘excellent’ conditions that are highly con­ducive to plant growth. In large-scale commercial cultivation, yields are much lower. In the following, we will use data from large-scale cultivation, as this should be the basis for substantial feedstock production. As there is a tendency of gradual yield increases over time, such data may be biased in favour of crops that have a long tradition of large-scale cultivation. After a similar history of cultivation, the yields of relatively new crops that may serve as biofuel feedstocks such as Miscanthus and switchgrass may well be substantially higher than those that will be presented here.

In practice, the C4 plant sugar cane is relatively efficient in converting solar en­ergy into biomass (Sinclair and Muchow 1999). In subtropical areas, sugar cane may annually yield about 80 Mg per hectare of harvestable biomass (dry weight) when the conditions are excellent (Bastiaanssen and Ali 2003; Braunack et al. 2006). Av­erage sugar cane yields during the mid 1990s in Brazil were about 36.8 tons of biomass (dry weight) ha-1year-1 (Kheshgi et al. 2000). Under excellent condi­tions, another C4 plant, Miscanthus, may yield annually up to about 30-60 Mg of dry weight harvestable biomass per hectare (Long et al. 2006; Heaton et al. 2008a), but more commonly, yields are in the range of 10-13 Mg aboveground dry weight biomass ha^year-1 (Lemus and Lal 2005; Christian et al. 2008).

Oil palms in Southeast Asia yield about 20 Mgyear-1 ha~1 as fresh fruit bunches (dry weight) (Reijnders and Huijbregts 2008a). For sugar beets, good yearly dry weight yields of biomass from large-scale commercial cultivation are also in the or­der of 20 Mgha-1 (Sahin et al. 2004; Tzilivakis et al. 2005). For eucalyptus, yearly biomass yields per hectare tend to be in the order of 10-20 tons (Sims et al; 1999; van den Broek et al. 2001). Yearly dry biomass yields of large-scale cultivation un­der good conditions for switchgrass are in the order of 10-15 Mgha-1, for willow 9Mgha-1, and for poplar 11Mgha-1 (Lemus and Lal 2005; Heaton et al. 2008a). Total yearly (dry weight) aboveground biomass accumulation per hectare in the USA is in the order of 17-18 Mg for corn (Heaton et al. 2008a), and under good conditions, 10-11 Mg for wheat (world average is 5.5 Mg; Wright et al. 2001), in the order of 9 Mg for peas and 4-5 Mg for canola (Lemus and Lal 2005; Malhi et al.

2006) . High yields of photosynthesis in practice usually depend on substantial in­puts of synthetic nutrients derived from non-renewable natural resources (Samson et al. 2005). Sustainable yields that can be achieved when only recycling nutrients that are present in biomass tend to be much lower as will be discussed in Chap. 3 (also Pimentel et al. 2002; Reijnders 2006). Table 2.1 shows the overall energy con­version efficiency (taking account of inputs of fossil fuels) for a variety of crops with relatively good yields.

The overall solar energy conversion efficiencies in Table 2.1 are below 1% and range roughly between 0.15% (for rapeseed/canola) and 0.9% (for sugar cane). For comparison, a percentage is added for sustainably grown wood in Western Russia (Nabuurs and Lioubimov 2000). In this case, the conversion efficiency is about 0.05%.

There have been efforts to improve the solar-energy-to-biomass conversion by transgenic approaches. These have focused on increasing the net carboxylation ef­ficiency of 1,5-biphosphate carboxylase and the introduction of enzymes charac­teristic for C4 plants in C3 plants (Heaton et al. 2008; Raines 2006). So far, such efforts have not led to a substantial improvement in the conversion of solar energy to biomass (Raines 2006).