Beech phenology and productivity at the Vielsalm Terrestrial Observatory

Temperate forests, as other ecosystems and oceans, mitigate the accumulation in the atmosphere of CO2, the main responsible for the current climate change. It is, therefore, a necessity to understand how these forests react and will react in a changing environment. That requires long-term simultaneous monitoring of the forest productivity and phenology as well as of climatic variables.
Using the 20 years dataset from the Vielsalm Terrestrial Observatory (VTO), an eddy covariance site located in a mixed forest in east Belgium and dominated by beeches, the relations between phenological and productivity indicators were analyzed.
Phenological indicators (leaf out beginning, duration and ending as well as leaf fall date) were derived from the relative light transmissivity through the canopy and from the temporal dynamic of the Net Ecosystem Productivity (NEP). No trend was observed for these indicators during the last two decades, but significant relations were found between them and between them and climatic variables.
Productivity indicators were derived from eddy covariance measurements (NEP), tree ring widths (growth index) and masting intensity. Growth index was significantly related to the NEP and a reverse proportionality to the masting intensity was found.
The analysis of the relation between the beech stand NEP and the phenology gives us evidences that annual NEP is significantly related to the length of the carbon uptake period. A model combining the length of the carbon uptake period and the photosynthetic capacity of the ecosystem was able to predict more than two third of the interannual variability at the VTO.
This work underlines the multiplicity of the relation between phenological and productivity indicators.