**3. Nutrient cycling in forests**

following section, we show some features of the main forest formation in the Atlantic Forest according to Veloso [12] and the Brazilian Institute of Geography and Statistics [13, 14].

116 Tropical Forests - The Challenges of Maintaining Ecosystem Services while Managing the Landscape

The Ombrophilous Forest is classified as Dense, Open and Mixed formation. Dense Ombro‐ philous forest is characterized by the presence of medium and large trees, in addition to lianas and epiphytes in abundance, due to the constant moisture from the ocean. The coastline extends from the Northeast to the extreme South of Brazil. Its occurrence is connected to hot and humid tropical climate without dry season, with rainfall well distributed throughout the year (eventually there may occur in some regions dry periods until 60 days) and average temperature is 25°C. In Open Ombrophilous Forest, we find arboreal vegetation more sparse and with lower shrubby density. It occupies areas with climatic gradients ranging between two and four dry months. Average temperatures range between 24°C and 25°C. Finally, Mixed Ombrophilous Forest is strongly characterized by the predominance in the upper stratum of *Araucaria angustifolia* and genera of the family Lauraceae (e.g., *Ocotea* and *Nectandra*). It consists of 2776 forest species, and 946 are endemic [10]. The physiognomy occurs in areas of wet climate and without water deficit. The average annual temperature is around 18°C. The Dense and Open Ombrophilous Forests had most forest species (9661) as well as most endemic species

Seasonal Forest is classified as Deciduous, Semideciduous and Evergreen. For the first, De‐ ciduous Seasonal Forest, it is characterized by a large number of deciduous trees, account‐ ing for more than 50% of individuals of the forest component. It consists of 165 endemic forest species of the total of 1113 found in the forest typology [10]. In the tropical region, its occurrence is conditioned to a long dry period (more than seven months). In the subtropical region, however, this forest formation occurs in areas with long cold periods, for more than five months with average temperatures below 15°C. On the other hand, Semideciduous Sea‐ sonal Forest is composed of deciduous trees, which represent 20–50% of individuals of the forest component. It has the second largest number of forest species (3841) of the Atlantic Forest of which 1081 are endemic [10]. Their occurrence in the tropical region is defined by two well‐defined pluviometric periods, one dry and one rainy with average annual temper‐ ature around 21°C. However, in the subtropical region, this formation occurs in a short dry period followed by a sharp drop in temperature, with averages below 15°C in the cold peri‐ od. The last type is the Evergreen Seasonal Forest, which is composed of deciduous trees, which account for less than 20% of individuals of the forest component. This forest occurs under tropical climate with a rainy and dry season, with about four to six months of dry weather. Still, the arboreal component does not seem to undergo water stress, which causes

Currently, approximately 7% of the biome natural areas are well preserved in fragments larger than 100 ha [15]. The biome consists of about 20,000 plant species of which 8000 (i.e., 40%) are endemic [16]. The analysis of species distribution in the different forest formations [10] showed that more than half of the wealth (60%) and most endemics (80%) are found in the Atlantic Forest. Due to their high levels of richness and endemism, the Atlantic Forest is among the top

(5164) [10].

low leaf shedding.

five hotspots in the world [16].

Biomass production in a forest ecosystem is conditioned to several factors, namely light, water, CO2 concentration, chlorophyll content, temperature, nutrients, genetic adaptation and competition, among others [18, 19]. Among these factors, nutrients stand out as an essential element for the primary productivity of the forest ecosystem [20]. Nutrient cycling in forests is defined as the transfer of elements between the different components of the ecosystem. This transfer is controlled by climate, site, abiotic factors (topography, source material) and biotic agents [21]. Therefore, nutrient cycling in tropical forests is distinct from that in temperate zones. For example, the amount of nutrients on the forest floor and the length of deposition are shorter in tropical forests than in boreal forests, due to slow decomposition in regions of cold climate and high altitudes [21].

**Figure 2.** Scheme of nutrient cycling dynamics in a forest. Adapted from Refs. [24, 25].

Nutrient cycling in forests can be generalized into three models: geochemical, biogeochemical and biochemical cycling [22]. Geochemical cycling is characterized by the input and output of nutrients in the ecosystem. Atmospheric deposition (wet and dry), fertilization, biological fixation and rocks weathering are responsible for most nutrients input [23]. While, leaching, volatilization and harvest biomass are responsible for most nutrients output [24]. The biogeo‐ chemical cycle is characterized by the transfer of nutrients between the plant and the soil. In this cycle, plants absorb nutrients form soil reserves and then return them to the soil via litterfall (litter decay), roots decay or plant death [24]. Biochemical cycling is the translocation of nutrients inside the plant (internal cycle). Once soil nutrients are absorbed, some of these elements are in constant mobilization within the plant, mostly from older to younger tissues.

The dynamic process of nutrient cycling in native or exotic forest ecosystems is shown in **Figure 2**.
