From the CO2Science.org archive: Many of the cases we report on here involve multiple experiments in a laboratory setting of seedlings or juvenile plants. This week we look at Larix decidua L. but there is only one experiment to report on, and it was performed not on seedlings but on 27 year old trees in the Swiss Alps at the tree line. The researchers set up a system to fumigate the trees with CO2 and ran it for four years. The result of an extra 300 ppm CO2 was a 142% increase in growth compared to the untreated trees. So if you are hiking in the Alps and you see a stand of larches much taller than the others nearby, you’ll know what happened.
And that's how the hockey stick graph came about. Michael Mann attributed accellerated growth in bristlecone pine trees high in the Sierra Nevada to increased temperature, whereas the growth spurt was most likely caused by increased atmospheric CO2. Mann-made warming!