Kirschbaum, M.U.F. (1999). Modelling forest growth and carbon storage with increasing CO2 and temperature. Tellus 51B: 871-888.

The response of plant growth to increasing climate change remains one of the unresolved issues in understanding the future of the terrestrial biosphere. It was investigated here by using the comprehensive forest growth model CenW 1.0.5 which integrates routines for the fluxes of carbon and water, interception of radiation and the cycling of nutrients. It was run with water and/ or nutrient limitations on a background of naturally observed climate at Canberra, Australia. It was parameterised for Pinus radiata, the commercially most important plantation species in Australia.

The simulations showed that under water-limited conditions, forest growth was highly sensitive to doubling CO2, with growth increases of over 50% on average and even greater increases in dry years. In contrast, when water supply was adequate, but nutrients were limiting, growth increases were smaller, with an initial increase of about 15% during the first year after CO2 was doubled. This growth increase diminished further over subsequent years so that after 20 years, there was virtually no remaining effect. This diminishing response was due to developing nutrient limitations caused by extra carbon input which immobilised nutrients in the soil. When both water and nutrients were adequate, growth was increased by about 15-20% with no decrease over time. Increasing ambient temperature had a positive effect on growth under nutrient limited conditions by stimulating nitrogen mineralisation rates, but had very little effect when nutrients were non-limiting.

Responses were qualitatively similar when conditions were changed gradually. In response to increasing CO2 by 2 m mol mol-1 yr-1 over 50 years, growth was increased by only 1% under nutrient-limited condition but by 16% under water-limited conditions. When temperature and CO2 were both changed to emulate conditions between 1950 and 2030, growth was enhanced between 5-15% over the 80-year period due to the effect of CO2 on photosynthesis and water economy especially under water-limited conditions, and due the effect of increasing temperature in mineralising greater amounts of nutrients.

These results show that there is not one universally applicable biological growth response to increasing temperature and CO2, but that they interact in complex ways with a number of other growth limiting factors. Any response factor of plants to CO2 can only be quantified if the important interacting factors can be independently characterised for different situations.

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