The movements of Earth in relation to the Sun generate a strong annual cycle of radiation and temperature that further affects other environmental factors such. Environmental factors affect physiological processes and therefore, identification and understanding of their nature is needed in order to understand and predict the biological actions of a forest.

The diurnal and annual cycle in solar radiation is reflected into air temperature by the absorption of solar radiation. Soil temperature follows the changes in the air temperature with a time lag that increases with soil depth. Close to the soil surface, the variation in temperature is larger than, for example, half a meter deep in the soil. In summertime the soil surface warms up much more than the deeper soil layers. In summer the temperature in soil surface can reach 15 to 20 °C, while in deeper soil layers the temperature stays around 10°C. In midwinter, the temperature remains above zero in deeper soil layers, approximately at +1–2°C. The snow cover efficiently insulates the soil surface keeping the temperature close to zero.

The current atmospheric CO₂ concentration is around 385 ppm (0.0385 %). The concentration increases annually by approximately 2 ppm.

In the Northern Hemisphere, the CO₂ rises in the winter and declines in the summer, mainly as a response to the seasonal activity of forests, other land vegetation and algae which all absorb atmospheric CO₂ both in oceans and in terrestrial biospheres and release it by heterotrophic respiration. In addition, the house heating in winter contributes to the seasonal cycle of CO₂ concentration.

There are plenty of methods available to measure photosynthesis. The method we use is actually a measurement of the carbon dioxide catch:  We enclose a small part of the tree inside a chamber and follow the change of CO₂ concentration in the air of this chamber. If the plant is taking in CO₂, the concentration will decrease and if it is mostly releasing CO₂, then the concentration will increase. From the difference in concentration in a given time, we can calculate the rate of CO₂ exchange between the plant and the air.

Many years of measurements have provided a lot of information on the behaviour of the forest, how it photosynthesizes, transpires and respires. We know more or less what to expect the tree will do according to the time of the year, the available light, the air and soil temperature and the water availability in the soil. We can express this knowledge as mathematical models that can forecast the overall behaviour of the tree. Mathematical modelling is a typical tool to describe the phenomena in an exact and concise format. However, always bear in mind that the model reflects only such variables in the reality than exist in the model.