Determining photosynthetic responses of forest species to elevated [CO2]: Alternatives to FACE

Free air CO2 enrichment (FACE) experiments are considered the most reliable approach for quantifying our expectations of forest ecosystem responses to changing atmospheric CO2 concentrations [CO2]. Because very few Australian tree species have been studied in this way, or are likely to be studied in the near future because of the high installation and maintenance costs of FACE, there are no clear answers to questions such as: (1) which species will be the winners in Australia's natural forests and what are the implications for biodiversity and carbon (C) sequestration; and (2) which will be the most appropriate species or genotypes to ensure the sustainability of Australia's plantation forests.We examined possible experimental approaches that may provide insights into, and more rapid assessment of, responses to elevated [CO2]. Our main conclusions were: (1) better understanding the extent to which species are C-limited could indicate when elevated [CO2] might be expected to increase photosynthesis and biomass production. Plant tissue carbohydrate concentrations can be used to assess any C limitation. Consistently high levels of carbohydrates indicate that plants are not C limited, but rather that growth is determined by other limiting resources or by rates of cell development and expansion; (2) historical examination of forest responses to increasing atmospheric [CO2] using stable isotopes in wood cores can provide clues as to which species may respond favourably to increasing [CO2], although it may remain difficult to distinguish between the environmental conditions under which favourable responses occurred. Undertaking stable isotope studies close to anthropogenic CO2 sources has the potential to provide insights into how species may respond to the higher [CO2] that is predicted during this century; (3) by focusing on genetic and metabolomic regulation of source and sink activity, selection for greater biomass production under elevated [CO2] is possible.

Research highlights▶ Free air CO2 enrichment (FACE) experiments are considered the most reliable approach for quantifying our expectations of forest ecosystem responses to changing atmospheric CO2 concentrations [CO2]. Because very few Australian tree species have been studied in this way, or are likely to be studied in the near future because of the high installation and maintenance costs of FACE, there are no clear answers to questions such as: (1) which species will be the winners in Australia's natural forests and what are the implications for biodiversity and carbon (C) sequestration; and (2) which will be the most appropriate species or genotypes to ensure the sustainability of Australia's plantation forests. ▶ We examined possible experimental approaches that may provide insights into, and more rapid assessment of, responses to elevated [CO2]. Our main conclusions were: (1) better understanding the extent to which species are C-limited could indicate when elevated [CO2] might be expected to increase photosynthesis and biomass production. Plant tissue carbohydrate concentrations can be used to assess any C limitation. Consistently high levels of carbohydrates indicate that plants are not C limited, but rather that growth is determined by other limiting resources or by rates of cell development and expansion; (2) historical examination of forest responses to increasing atmospheric [CO2] using stable isotopes in wood cores can provide clues as to which species may respond favourably to increasing [CO2], although it may remain difficult to distinguish between the environmental conditions under which favourable responses occurred. Undertaking stable isotope studies close to anthropogenic CO2 sources has the potential to provide insights into how species may respond to the higher [CO2] that is predicted during this century; (3) by focusing on genetic and metabolomic regulation of source and sink activity, selection for greater biomass production under elevated [CO2] is possible. ▶ While the paper describes methods that are in general already developed, the novel aspect of the work is in highlighting how these methods might be used to address questions around forest responses to elevated [CO2], including detailed discussion of the advantages and disadvantages of various approaches. It discusses how these approaches might be used to assist forest managers in understanding tree and ecosystem responses to increasing [CO2], and to help with specie/genotype selection for future climates.