Understanding and quantifying the carbon and greenhouse gas (GHG) balance of the UK’s woodlands and forests is a key part of our programme on managing forest carbon and greenhouse balances. A major part of the experimental work is focussed on quantifying the carbon dioxide and other greenhouse gas uptake and losses from a planted oak woodland within the Alice Holt Research Forest. Measurements of carbon dioxide, water vapour and energy exchange (or fluxes) commenced in March 1998 at the Straits Inclosure, in addition to numerous other assessments at the site, linked to the Intensive Forest Monitoring Network and the Environmental Change Network. The Straits Inclosure flux site is currently one of only 3 long-term carbon dioxide flux monitoring sites in woodland in the UK.
- To quantify the carbon balance of the woodland ecosystem and its main components (soil, trees, understorey vegetation)
- To understand how this may be affected by climate variation and management regimes
- To provide detailed measurements for the development and evaluation of models of forest growth and productivity, that can be used to assess the likely impacts of climate change and management
The carbon dioxide flux measurement site is located within the Straits Inclosure, Hampshire, UK. This is a commercially managed, lowland oak forest where the main tree species is Quercus robur L., but other species, including European ash (Fraxinus excelsior L.), Q. petraea (Mattuschka) Liebl. and Q. cerris L., are present. The understorey is dominated by hazel (Corylus avellana L.) and hawthorn (Crataegus monogyna Jacq.). The eastern half of the site was thinned in 2007 and the western half in 2015.
The measurement system
Quantifying the carbon balance of the forests requires measurements of the exchange of carbon dioxide between the air passing over the forest and the forest vegetation, using micrometeorological methods. Techniques and instruments available since the late 1980s allow us to measure the rapid turbulence in the air (or ‘eddies’), and the fluctuations in the carbon dioxide concentration to see how they ‘co-vary’ (vary together). We can then calculate the net exchange upwards (when the forest is termed a ‘source’ of carbon dioxide, for example at night) and downwards (when the forest is a sink, for example on a sunny summer day).
The instruments are mounted above the trees on a tall tower located within the 90 ha plantation. These include a high-speed anemometer (using the speed of sound to measure the turbulence) and an infra-red gas analyser (measuring fluctuations in atmospheric carbon dioxide concentration). From very rapid (20 times a second) measurements half-hour average fluxes of carbon dioxide are continuosly computed in all weather conditions. An automatic weather station (AWS) mounted at the same height provides data on changes in environmental conditions such as air temperature, light levels and relative humidity etc. In addition digital cameras are also used to automatically record change in canopy leafiness (‘phenocams’).
The graph below shows the annual partitioned carbon dioxide flux components. From the onset of monitoring in 1998, the mean annual NEP (Net Ecosystem Productivity) has been 4.8 tC/ha/yr, indicating that overall this woodland is acting as a strong sink for carbon.
A short video summarising this work can be found here.