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Straits Enclosure long-term carbon flux site

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 C and greenhouse balances, which drew together various related research projects in 2009. A major part of the experimental work is focussed on quantifying the C and other greenhouse gas uptake and losses from a planted  oak woodland within the Alice Holt Research Forest, the Straits Enclosure. Measurements of carbon, water vapour and energy fluxes commenced in March 1998, in addition to numerous other assessments at the site, linked to the Intensive Forest Monitoring Network and the Environmental Change Network. The site is one of only 3 long-term carbon flux monitoring stations in woodland in the UK.

Diagram showing the main organic C stocks and fluxes, and GHG fluxes, in a forest. C enters the trees through photosynthesis. Leaf litter and root loss transfer C to the soil C stock. Respiration by plants, fungi and soil bacteria release stored carbon, as does disturbance and leaching of organic carbon in the soil. Nitrous oxide is released by soil bacteria; methane is both released from and taken up by soil bacteria.

The objectives

  • 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 site

Aerial photo of Straits Enclosure showing thinned eastern halfThe carbon flux measurement site is located within the Straits Enclosure, Hampshire, UK.

This is a commercially managed, lowland oak forest (Q. robur and Q. petraea) which was planted in ~1935 and is interspersed with approximately 10% ash(Fraxinus excelsior). The local soil derives from Gault clay (Denchworth series surface water gley) which supports a varied understorey of woody shrub and herbs dominated by hazel (Corylus avellana) and hawthorn (Crataegus monogyna).

The mean top height and DBH (diameter at breast height) were 19.3 m and 25.9 cm respectively in 1995. The stocking density was 606 stems per hectare, resulting in a basal area of 22 m2 ha-1, the site can therefore be classified as General Yield Class is 6. The eastern half of the site was thinned in 2007, as can be seen in the aerial photograph above, and the western half is due to be thinned in 2011.

The measurement system

Quantifying the C balance of the forests requires measurement of the exchange (or flux) of carbon dioxide (CO2) between the air passing over the forest and the forest vegetation, using micrometeorological methods. Techniques and instruments available since the late 1980’s  allow us to measure the rapid turbulence in the air (or ‘eddies’), and the fluctuations in the CO2 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 CO2, for example at night) and downwards (when the forest is a sink, for example on a sunny summer day).

26m Alice Holt flux towerOpen path carbon dioxide analyser and 3-D sonic anemometer

The instruments are mounted above the trees on a tall tower (28 m) located in the middle of the 90 ha plantation,  including a high-speed anemometer (using the speed of sound to measure the turbulence). The CO2 is measured with an infra-red gas analyser. From very rapid (20 times a second) measurements half-hour average CO2 fluxes are computed. An automatic weather station (AWS) mounted above the trees provides data on changes in environmental conditions such as  air temperature, light and relative humidity etc. In addition digital cameras are also used to record change in canopy leafiness (‘phenocams’).

The following images looking down on to the top of the canopy were all taken on May 4th in (left to right) 2009, 2010 and 2011 and illustrate the variation in canopy development between these years.

Photo of canopy leafiness (‘phenocams’) taken from top of Alice Holt flux tower on 4th May 2009 Photo of canopy leafiness (‘phenocams’) taken from top of Alice Holt flux tower on 4th May 2010 Photo of canopy leafiness (‘phenocams’) taken from top of Alice Holt flux tower on 4th May 2011

Results

The graph below shows the daily (24 h) net CO2 fluxes for every day of the year averaged over several years. The columns above the zero line shows when the forest is acting as a net sink and is removing CO2 from the atmosphere, columns below the zero line show net daily emissions by the forest to the atmosphere. The solid line shows the average accumulated removals from the atmosphere.

Graph showing the long term average daily NEP (Net Ecosystem Productivity) and the average accumulated CO2 removal from the atmosphere at the Alice Holt flux site. Columns below the zero line (day 0 to 140 & 280 to 365) indicate that the forest is acting as a net source for CO2. For the summer period (day 141 to 279) columns are above the zero line indicating that forest is acting as a net sink. The solid line indicates the average accumulated carbon balance for the site.

From the onset of monitoring in March 1998, the mean annual NEE (Net Ecosystem exchange) has been 18 tCO2/ha/yr, indicating that overall this woodland is acting as a strong sink for CO2.

More information can be found on the results page.

Collaborations

At the Straits flux site, Forest Research works in close collaboration with a number of other Research Institutes and University departments, especially the School of Geosciences, University of Edinburgh (software, modelling and 'phenocams') and the Stockholm Environment Institute, University of York (soil respiration and flux partitioning).   

Contact

Matthew Wilkinson