Temporary increases in NPP are likely to occur in increased CO 2 . The initial slope of the growth curve is more steep but we don't know what will happen out into the future. Short term studies with short-lived plants gives us little insight into what will happen down the road. In particular, questions in temporal scale could hold surprises. One important difference between experiments and reality is that in reality, CO 2 is increasing gradually in the environment compared to experiments that go from ambient levels to doubled levels immediately.
Growth is a multivariant response and therefore it is difficult to sort out.
We could also use paleodata on trees, to see how changes in CO 2 that have already occurred have affected growth. Zak disputes the claim that there could be a negative feedback on vegetation growth in the short term due to nutrient availability. The majority of evidence suggests a positive feedback on photosynthesis and a negative feedback on ambient CO 2 . We do not have enough information to discern long-term impacts.