Forests with canopy trees in excess of 20-30 m height generate almost insurmountable technical and financial problems for long-term CO₂ enrichment research. The minimum functional size of rain forest at La Selva, Costa Rica, is estimated to be in excess of 1 ha. A system has to be maintained under CO₂ control for several seasons to obtain equilibrium with the new environment. No ecosystem has yet been enriched long enough to reach carbon equilibrium under high CO₂ levels. In fact, we do not know how long would be required for an ecosystem to reach equilibrium with an enriched atmosphere. Tropical forests would require hundreds of years while annual desert grasslands would presumably require only decades.

   Useful physiological information could be obtained by doing single-tree canopies in elevated large open-tops or with elevated FACE rings enriching single trees.

   Astrodome-type facilities might be necessary in large-stature forests. The construction costs of such enclosures would be in the millions of dollars ($20- 60 million?) with annual operating costs of $2-5 million.

   Discussion has been under way for several years, in the US Department of Energy CO₂ program, on the feasibility of using a regeneration approach to the study of tropical rainforest response to global change. Assuming that increasing population and development pressures in tropical areas will generate large tracts of regenerating ecosystems, it would seem to be desirable to study early stages of ecosystem regeneration when plants are becoming established and are small enough to be contained in FACE arrays or ecocosms in the 200-400 m² size range. Prices for these units should range from those given for boreal forest to twice those values.