3. Tree growth

Historically, carbon limitation, through a shortage of photoassimilates has been argued to limit the growth of trees at the upper altitudinal treeline. We used CO₂ enrichment in combination with a defoliation treatment to test the effect of altered source-sink relationships on tree growth and leaf level responses of Larix decidua and Pinus uncinata. While Larix showed a stimulation in shoot growth and ring width over all four growing seasons when exposed to elevated CO₂ supporting carbon limitation, Pinus did not. However, when trees were defoliated in spring 2002, this response was reversed in the two species. We conclude that the expected changes in growth of these treeline conifers with improving carbon availability as atmospheric CO₂ continues to increase will thus depend on both the interplay between biotic and abiotic processes, and the species or tree functional type involved.