Flux site results

Daily fluxes

The two graphs below show three days of 30 minute flux data (dashed line) from February and August in 2009. Over winter when the forest canopy is absent and light levels low (solid line), carbon dioxide is emitted by the soil and vegetation through respiration. This results in a generally small but positive flux, indicating that the forest is acting as a weak source of carbon dioxide. During the summer when the forest canopy is fully developed and ground vegetation abundant, a strong diurnal cycle in forest carbon dioxide flux can be seen. During the day time when light levels are high the carbon dioxide taken up by the forest through photosynthesis is greater than that released by respiration, resulting in a net uptake of carbon dioxide from the atmosphere (shown in this graph as a negative flux). As the day turns to evening and light levels decline, the forest once again reverts back to being a net source of carbon dioxide.

Graph showing 3 days of Net Ecosystem Exchange (NEE) data and corresponding global radiation (light) levels for February 2009. In the absence of a forest canopy winter is dominated by a small, positive flux independent of radiation levels. Graph showing 3 days of Net Ecosystem Exchange (NEE) data and corresponding global radiation (light) levels for August 2009. When radiation levels are high during the day time and the forest photosynthesising NEE becomes negative indicating a net absorption of carbon dioxide by forest from the atmosphere. At night time radiation levels decline and the forest reverts back to being a net source of carbon dioxide (positive flux).

Annual fluxes

The graph below shows the long term half hourly net ecosystem exchange at the Alice Holt site. Understanding the variations in fluxes between years is complex and can partly be accounted for changes in the environmental conditions such a light, temperature and soil moisture. However, changes in forest phenology such as variations in the date of budburst and leaf fall are also likely to have a large impact. Our results have shown that other biological events such as defoliation (caterpillars) and disease outbreak as have occurred in 2009 and 2010 are also having a large effect.

Graph showing half hourly Net Ecosystem Exchange (NEE) data from 1999 to 2011. Points below the zero line indicate that the forest is acting as a net sink for carbon dioxide (summer daytime). Points above the zero line indicate a net loss of carbon dioxide from the forest to the atmosphere (all winter and summer nights). At the annual scale at this site the overall balance is in favour of being a net sink.