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.
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.