The link between an increase of CO 2, WUE, and productivity, while undoubtedly important with respect to growth and carbon sequestration models, is not fully conclusive. observed an increase of WUE in temperate and boreal forests. Further uncertainty is linked to the effect of increasing CO 2 on water use efficiency (WUE). Some studies have detected a positive effect at sites with limited water supply, while others state the opposite. Studies on the effect of CO 2 on forest productivity are, however, inconclusive, with productivity increasing in young trees but a levelling off effect is seen in mature trees. This might be due to changes in either the photosynthesis rate or a lowered demand for water, explained by a CO 2 fertilisation effect. In general, we found that a decline in productivity aligned with an increase in WUE i.įoresters have a major interest in the annual growth of trees, which is believed to increase due to globally rising CO 2 concentrations. Slopes vary with the provenance from Poland showing the least decline in productivity. Counterintuitively, the populations at the drier site showed low WUE i values at the beginning of the investigation. The gradient of the correlation between WUE i and productivity varies between provenances and sites. While WUE i increased over this period, the productivity decreased, causing an opposing trend. Using stable carbon isotopes derived from tree rings combined with productivity as the product of the annual growth increment and annual density measurements, we compared the change in WUE i over a 15 year period. For forest trees, this question is less clearly understood. In general, the increase is a trade-off for productivity and is therefore not desired. In agricultural research, the question of drought tolerance by increased WUE i has been well studied. It is generally assumed that it is a response to water availability. Intrinsic water use efficiency (WUE i) in trees is an indication of the ratio of carbon assimilation to the rate of transpiration.
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