A forest that exhales
The Montseny Massif, a protected area and UNESCO Biosphere Reserve since 1978, rises dramatically between Barcelona and Girona. Its slopes are covered in a diverse mosaic of forests, with European beech (Fagus sylvatica) dominating the cooler, more humid elevations above 1,000 meters. Beneath the canopy of these ancient trees, the forest floor is constantly "breathing." This process, known as soil respiration, releases carbon dioxide (CO2) into the atmosphere. It results from two main activities: the metabolic activity of plant roots and the decomposition of organic matter by a vast community of soil microbes.
In Montseny's Mediterranean climate, with its characteristically hot and dry summers, the forest floor's breath can be shallow. The lack of water forces soil bacteria and fungi into a dormant state. But when the first autumn rains arrive, the forest doesn't just absorb the water—it responds with a massive exhalation of CO2.
The Birch effect in action
This sudden burst of CO2 following the rewetting of dry soil is a documented phenomenon named the "Birch effect," after the scientist H.F. Birch who first characterized it in the 1950s. In the Montseny beech forests, this effect is particularly pronounced. Researchers using automated instruments called flux chambers have measured the startling aftermath of a significant rainfall event. These devices, which look like small, open-bottomed containers placed on the soil, record the rate of gas exchange between the soil and the atmosphere.
The data shows that after a prolonged dry spell, the arrival of rain can cause soil respiration to spike by as much as 300%. This intense pulse happens because the sudden availability of water awakens the dormant microbial communities. These microbes rapidly begin to decompose the organic matter that has accumulated during the dry period, releasing a large volume of CO2 in a short time. This process is so intense that it temporarily shifts the forest's carbon balance, turning the soil from a net carbon sink into a significant carbon source. This pulse is primarily driven by heterotrophic respiration—the activity of microbes breaking down dead organic material. Studies show that the intensity of the pulse is often negatively correlated with the pre-rain moisture level; the drier the soil, the bigger the subsequent CO2 burst.