Straits Flux long-term carbon flux monitoring station

 

Eddy correlation measurements of carbon, water vapour and energy fluxes in the Straits Enclosure, Alice Holt commenced in March 1998. The site is one of only four long-term flux monitoring stations in woodland in the UK; across Europe there are over 100 sites across all land-covers, with the majority associated with the EU FP6 CarboEurope IP, to which the Straits flux station contributes.

The objectives

• Quantify the magnitude of the carbon source/sink of the ecosystem and to improve our understanding how this may be constrained by climate variability and interaction with management regimes
• To provide objective data for the validation of forest models related to growth and the partitioning of primary production
• To recommend management strategies for the conservation of carbon stores in forests
• To analyse the response of water and carbon fluxes to climatic factors in order to aid regional scale modelling designed to predict impacts of global environmental change on forest ecosystem function.

The site - Straits Enclosure

The carbon flux station is located within the Straits enclosure, Hampshire, UK (51^o 7’N; 0^o 51’ W). This is a commercially managed, lowland, mature 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). A comprehensive ecological and biomass survey ^(PDF-986K) of the ground vegetation was carried out in 1999/2000.

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 site is due to be thinned in 2005, whilst the most recent thinning was in 1995.

The measurement system

Eddy correlation technique is a micro-meteorological method for surface flux measurements of carbon, water vapour and energy. The system comprises of a sonic anemometer (R2: Gill Instruments) mounted above the canopy on a meteorological mast. Atmospheric sample gas is channelled from an inlet close to the anemometer to a closed path, infrared gas analyser (Licor: LI-6262) located in a weatherproof container at the base of the mast.

Fluxes are calculated by correlating the three dimensional air movement measurements from the sonic anemometer with the measured changes in carbon dioxide and water vapour concentration. Edisol software is used to process the raw data and calculates a user-friendly form of 30-minute flux averages. The raw data is subsequently re-processed in order to account for high frequency fluctuations in gas concentration that would otherwise lead to an under estimation of carbon and water vapour fluxes.

The instrumentation is located at the centre of the 90 ha plantation to provide a 500 m fetch for the flux measurement system.

Results

From the onset of monitoring in March 1998, the peak C02 flux measured during the summer months has shown a progressive increase, with a long-term NEE (Net Ecosystem Exchange) of 4.92 tC/ha/yr, over this period. This increase is presumed to be as a result of canopy closure and ground vegetation growth following thinning of the woodland block in 1996. 

The low NEE (4.5 tC/ha/yr) recorded in 2002, particularly the low peak uptake of carbon in  summer, is not consistent with this pattern; this may be explained by low leaf area index as measured by litter analysis, resulting from poor canopy development in early summer.  

The inter-annual variation of NEE and its component fluxes are currently being examined, this data will be published in Spring 2006.