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M ost plants require a similar balance of resources-ener-gy, water, and mineral nutrients-to maintain optimal growth. Natural environments, however, differ by at least two orders of magnitude in the availability of these resources. Light intensity varies 100-fold from the canopy to the floor of a rainforest (Bj6rkman 1981); annual precipitation ranges 500-fold (10-5000 mm/yr) from deserts to tropical rainforests; and the amount of nitro-gen available to plants varies from

0.09 g/m2* yr in polar desert (Dowding et al. 1981) to 22.8 g/m2* yr in a rich tropical rainforest (Vitousek