Forests are much more than simply a sink for and store of carbon. They are important climate regulators.
Forest is not a static state. It is a process in constant revival. A process which sustains the conditions favorable for its own perpetuation, including its own local environment and climate. This is a complex task best illustrated by the example of the biotic pump. In order to sustain the levels of condensation that keep air pressure low on land -so that moist winds blow inland from the ocean- intense evaporation from the forest canopy must occur. However, evaporation reduces the amount of moisture in the soil and further vital moisture is also lost from the soil through gravitational runoff into rivers. When there is no moisture left in the soil, evaporation ceases, as does the transport of atmospheric moisture.
This highlights the importance of maintaining a complex balance of moisture-related forest processes. Forest evaporation must be such that it never fully depletes the soil moisture but, at the same time, is intense enough to ensure that the amount of moisture brought from the ocean by the winds compensates for moisture losses in the soil. Natural forests skillfully manage this delicate balance. Neglecting the water balance in greening efforts might lead to the loss of soil moisture and growing aridity instead of an enhanced water cycle.
Native species that form natural forest communities have evolved a complex set of biophysical and morphological traits that make the biotic pump possible. These traits took millions of years to evolve. For example, the root system of forest trees facilitates both the storage and the extraction of moisture from the soil. Cloud seeds produced by trees control the intensity of water vapor condensation over the forest. The height of the trees determines the vertical temperature gradient under the canopy, keeping soil evaporation under biotic control. The tallest trees are essential for creating surface friction that prevents damaging winds from developing. In this way, natural forests not only create an ocean-to-land flow of moist air, but also stabilize winds at an optimum level and prevent extreme fluctuations like hurricanes, tornadoes, severe droughts or floods. Species other than plants (bacteria, fungi, animals) are also essential for the stability of the forest ecosystem itself.
In recognizing the complexity of pristine forests, re-greening aims to restore the land´s capacity wisely and ecologically, so as to stabilize the water cycle as well as other aspects of the climate.