What factors tend to decrease plant responses to CO2 fertilization

Some vegetation types respond more strongly to C02 than others, but all seem to show at least some decline in C02 response over time. Various factors seem to be at work in producing both the variation in response and the decline in response. Probably the most important of these is nutrient supply. The more nutrient-deprived the system is. the less responsive it tends to be to CQ2 fertilization. Also, much of the decline in C02 response over time seen in raised C02 experiments is thought to be the result of nutrient shortages. Without the mineral nutrients it is impossible for the plant to build more tissues no matter how much extra carbon it has. and so the response to C02 diminishes. When a plant undergoes an initial burst of growth under raised C02, it is drawing upon a store of nutrients within its own tissues and in the immediate vicinity of its roots. After a while, however, the available nutrients become used up and the plant has difficulty finding more. It is also thought that greater internal shading within the denser growing canopy decreases the gains from C02 fertilization as time goes on, by putting a limit on photosynthesis.

Apart from these factors there is also a poorly defined and poorly understood "acclimation" of the photosynthetic response to C02 that occurs over time. This is involved in the decline in growth rate following an initial response, even in crop plants that have plenty of mineral nutrients. There seems to be some sort of "switching oil" mechanism within the plant over time that causes it to be less responsive to raised C02. It is unclear what advantages there are to the plant in doing this. It might perhaps involve a dismantling of part of the photosynthetic apparatus to allow diversion of nitrogen away to other places within the plant where it is needed.

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