Open top chamber climate impact studies

 

The atmospheric conditions and pollution environment that plants will be exposed to in the future is likely to be different from that of today. For most climatic variables, impact studies can utilise natural variation in climate to investigate how climate change is likely to affect the growth, function and health of trees. The gaseous environment to which trees are subjected is far more difficult to manipulate, in most cases requiring purpose-built controlled environment facilities. This is particularly the case for carbon dioxide, the principal driver of climate change, because concentrations in the atmosphere vary little (~10 ppm) compared to the magnitude of change predicted, rising from the current value of ~370 ppm to between 500 and 800 ppm; we cannot even infer from the past, as concentrations have not reached the levels predicted for the last 100 million years. 

The Headley OTC facility (pictured below) enables young trees, planted in the ground, to be exposed to enhanced concentrations of carbon dioxide and ozone. In the past, the facility has also been used to identify how ambient air pollution affects tree growth by comparing growth of trees in normal, ambient, air, with that in clean, charcoal-filtered air. Work carried out at Headley made a significant contribution to the EU fourth framework ECOCRAFT project, while the data were also integral to the setting of new concentration and flux-based critical loads for ozone.

Why carbon dioxide and ozone?

Carbon dioxide

The atmospheric carbon dioxide concentration has increased from pre-industrial revolution levels of ~270ppm (parts per million) to a current value of ~370ppm, mainly as a result of fossil fuel burning. This increase in concentration is expected to continue to rise to between 525ppm and 950ppm by the end of this century. The indirect effects of increased atmospheric carbon dioxide concentration resulting in the enhanced greenhouse effect are largely well understood. However, rising atmospheric carbon dioxide concentrations are likely to have a direct effect on tree growth since present day concentrations do not saturate photosynthesis, the mechanism by which plants obtain carbon from the atmosphere.

Ozone

Many people are well aware of the decline of ozone high up in the atmosphere, where it acts as a natural shield filtering out harmful ultra-violet rays from reaching the earth’s surface. Some industrial pollutants such as chloroflurocarbons (CFCs), chlorine and bromine are damaging this layer. The result is a thinning of the protective layer, particularly over Antatartica - the so-called ozone hole. Ozone is also present at ground level usually in low concentrations. However, through a series of complex chemical reactions, pollution in the atmosphere can raise these background concentrations to such a level that it can have important consequences for green plants. Ozone diffuses into the leaf through stomata, the small openings in the leaf surfaces through which gas exchange occurs. Once inside the leaf it is transformed to a variety of reactive cell damaging compounds. Ozone can cause damage to the internal structures of the leaf as well as disrupting photosynthesis and the ability of plants to regulate water loss.

Open Top Chamber (OTC) experiments

To date, four individual experiments have been carried out at the Headley OTC facility.

Headley I

nvestigated the effects of ambient air pollution on the growth of beech, Norway spruce and Scots pine. This experiment was part of a larger programme, with sister sites in the English Midlands (Chatsworth: beech, Scots pine, Norway spruce, Sitka spruce) and the Scottish uplands (Glendevon: Sitka spruce, Scots pine, Norway spruce). The experiment lasted for seven years (1987-1993).

Using growth as the effect indicator, the only statistically significant negative impact was observed on beech at Headley. In southern England, the most important pollutant was (and is) ozone, and thus the conclusion was drawn that ambient ozone pollution was reducing tree growth. Indeed, needle retention of Scots pine at Headley was also reduced as a result of ambient air pollution, and this is a common impact of enhanced ozone levels in experimental studies. Growth in this species was not reduced, statistically. For some species at some sites, ambient air pollution actually led to an apparent increase in growth, which may have been the result of the ‘clean air’ treatment having a lower input of atmospheric nitrogen, which can act as a fertiliser.

Headley II

This experiment investigated the interactive effects of elevated carbon dioxide levels, enhanced ozone pollution and two water supply treatments in a fully factorial, split block design. Three species were planted, oak (Quercus rober), ash (Fraxinus excelsior) and Scots pine (Pinus sylvestris). The treatments were ambient or 700 ppm carbon dioxide, ambient or 100 ppb ozone for four hours a day and droughted (minimum 2 bar SWP) or well watered (minimum 0.1 bar SWP). No additional nutrients were supplied. The experiment lasted for three years, between 1994 and 1996. The major conclusions were that:

• elevated CO2 enhanced growth in oak and Scots pine, but not ash;
• nitrogen deficiencies were apparent in ash and, to a lesser extent, oak in the elevated CO2 and well watered treatments;
• in oak, maximum rates of photosynthesis were not reduced in the elevated CO2 treatments;
• ozone reduced chlorophyll content in all three species, and reduced maximum rates of photosynthesis in oak and ash;
• ozone significantly reduced needle retention in Scots pine
• elevated CO2 reduced stomatal conductance and transpiration in oak and ash;
• both ozone and drought treatments ameliorated the effects of ozone pollution;
• elevated CO2 had no effect on the timing of spring flushing
• root respiration was reduced at elevated CO2 levels for all three species - the magnitude of the reduction appeared to be linked to growth increment, indicating nutient limitation.

  Headley III

  Three species of oak were planted (pedunculate, sessile, red: Q. robur, Q. petraea, Q. rubra), again under with a full factorial design for the two main treatments, carbon dioxide (700 ppm) and ozone (80 ppb for four hours). All trees were well watered while optimal nutritional requirements were met through the application of fertilisers. The experiment only lasted for two years (1997-1998) as a result of the rapid growth of all three species under optimal conditions. 

  The principal objective of the experiment was to compare the performance of the two native species of oak under the experimental treatments with that of red oak, a species of north American origin, and one that is predicted to perform well as the climate warms. In addition, the experiment was used to investigate the effects of elevated carbon dioxide levels on the parameterisation of process models of tree growth. This included full parameterisation of the Jarvis stomatal conductance model (see Medlyn et al., 2000) and the Farquhar photosynthesis model (see Medlyn et al., 1999). The major conclusions were:

  • under conditions of optimal nutition and water supply, no down-regulation of photosynthesis was observed, contrasting with the results from Headley II, where nutrient supply was limited;
  • for Q. petraea a growth enhancement of 225% in response to elevated CO2 was observed - one of the largest recorded (see photographs below);
  • leaf area increased in response to elevated carbon dioxide levels (see photographs below);
  • elevated carbon dioxide levels delayed late summer reduction in photosynthetic capacity;
  • elevated CO2 had no effect on respiration.
     

  	Three species of oak grown under ambient carbon dioxide (360 ppm CO2) levels
  	Three species of oak grown under elevated carbon dioxide (700 ppm CO2) levels

  Headley IV

  The six species given below were planted in a factorially designed experiment, with carbon dioxide (600 ppm) and ozone (80 ppb for four hours) the main treatments. The six species were chosen for their varying expected response to climate change. Corsican pine and to a lesser extent, oak and Douglas fir are expected to perform relatively well, while sycamore and beech are both predicted to suffer. Southern beech was chosen as a canidate species for climate change adaptation. Water supply was limited to that predicted for the southern UK under the UKCIP98 climate change scenarios, and additional fertilisers were applied to maintain nutrient levels. The experiment was planted in spring 2000 and harvested during the winter of 2003-4. The data are currently being analysed.

  • Corsican pine (Pinus nigra)
  • Douglas fir (Pseudotsuga menziesii)
  • Pedunculate oak (Quercus robur)
  • Beech (Fagus sylvatica)
  • Sycamore (Acer pseudoplatanus)
  • Southern beech (Nothofagus obliqua)