The impacts of increased CO2 concentrations on tree growth and function

 

Climate change: Impacts on UK forests - Chapter 9
By Mark Broadmeadow and Tim Randle

Key findings

• Research into the direct effects of rising carbon dioxide concentrations (CO2) on trees presents problems as a result of the size of facility required, and also the timeframe over which the experiments must be conducted; a number of approaches are, however, available.
• Rates of carbon uptake will increase because the primary enzyme of photosynthesis is not saturated at current ambient CO2; at the same time, higher CO2 will inhibit photorespiration; the temperature optimum of photosynthesis will rise.  
• In impact studies, faster growth rates have been shown to result in nutrient deficiencies or imbalances, often manifested as a ‘downregulation’ of photosynthesis. 
• The effect of CO2 on respiration has been variable, and no consensus has been arrived at.
• Stomatal conductance and thus water use on a leaf area basis are generally reduced at enhanced CO2; an increase in leaf area is often observed in impact studies, and thus, water use on a ground area or individual tree basis may not decrease at elevated CO2.
• If the increase in leaf area observed in impact studies is borne out in mature forest stands, there would be a number of important implications over and above changes in transpiration: increased precipitation interception losses; increased wind resistance and potential snow loading; altered forest floor microclimate as a result of increased light interception; increased litter input to the soil. 
• Over a range of impact studies, mostly on saplings and young trees, a doubling of atmospheric CO2 resulted in an average biomass increment of 50% over the course of the experiments; however, there are concerns that this may not be borne out in mature forest stands. 
• Timber quality has been affected in some studies, although the current view is that this impact is largely a result of faster growth rates.
• Rising CO2 and climate change are predicted to result in General Yield Class (GYC) increases of 2 (6 to 8 for oak, 14 to 16 for Sitka spruce) comparing rotations approaching harvest now with new plantings.
• Current forest yield models are likely to require modification if they are to provide accurate yield forecasts in the future.

Next: Chapter 10 - Impacts of climate change on forest growth

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