Conclusions and Future Directions

Although most studies of the impact of elevated [CO2] on trees have been made on young seedlings or small trees in various enclosures (see Table 11.3), our interest is with 'real' forests. In real forests, trees experience severe competition; therefore, competition should be included in future experimental designs. The question also arises whether or not the knowledge gained at the individual tree level can be applied to the stand level, and, if so, under which conditions? This issue is especially important with regard to leaf area. Most studies reviewed in this chapter have found that leaf area is enhanced under elevated CO2 conditions, provided nutrient supply is not a strong limiting factor. It is questionable whether or not this stimulation of leaf area would still occur under closed canopy conditions. If there is a stimulating effect of elevated [CO2] in a closed canopy, then the increase in leaf area index might, or might not, result in a significant increase in canopy photosynthesis. Therefore, careful attention should be paid to the proper up-scaling from the leaf to the tree, and to the ecosystem levels.

'Real' forests include not only natural or native forest ecosystems, but also planted and managed forest plantations. As these managed forest plantations (as well as agro-forestry systems) are crops of major global significance, the effects of global climatic changes on their yield or productivity are as important as, or even more important than, the effects on native or natural forests. Drought and nutrient limitation are not likely to be a major concern in plantations where intensive management practices such as irrigation and fertilization are already used to optimize yield in fast-growing stands destined for harvest on 10-12-year rotations. Likewise, seed germination and early plant competition, as key elements of forest regeneration, are less likely to be a concern of forest geneticists who now have at their disposal advanced pedigrees of loblolly pine, poplar, eucalyptus, etc. for the mechanized establishment of forest plantations. Challenging questions for the future remain. How will fast-growing trees in managed plantations respond to global change? What are the unique aspects of short-rotation forests that might cause them to respond differently and perhaps more unpredictably than natural forest ecosystems? The answers to these questions might have important implications for the kind of management options that need to be taken by forest managers either to overcome or to take advantage of changes in future climate.

This chapter has demonstrated that a great deal can be learned from detailed studies of the existing literature, since there are large amounts of available data. However, long-term field-based studies are necessary and should be continued into the future to understand the feedback processes and to predict future forest production from CO2 enrichment. Several free-air CO2 enrichment (FACE) studies that have been started over the last 4-5 years have the main advantages of focusing on entire ecosystem responses to elevated atmospheric [CO2] and of being long-term experiments. Interesting and relevant results will come out of these FACE studies in the near future, but some patience is needed before long-term complex ecosystem responses (both above and below ground) can be properly evaluated.

In order to predict and evaluate the responses of forests to global climatic changes, a better understanding of the C and nutrient cycles in managed or natural forest ecosystems is badly needed. It should also be evident from this chapter that below-ground processes and components play a crucial role in the response of trees and forest ecosystems to global climatic changes, particularly their pronounced responses and their feedback control systems. Finally, whether or not tree or forest productivity will increase in the future also depends on societal attitudes toward forests. Good management and decreased deforestation will, without any doubt, result in improved worldwide forest productivity. Geo-political and geo-economic changes will play as big a role as global climatic changes.

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