Ecology of the Seed Bank in the Amazon Rainforest

*Natali Gomes Bordon, Niwton Leal Filho and Tony Vizcarra Bentos*

### **Abstract**

The seed bank is directly related to forest resilience because it contributes to the greatest number of regenerants after the occurrence of disturbances. Changes in seed density, floristic composition, and life forms completely alter the successional trajectory of forest environments. These changes are directly related to land use. For example, suppression of the seed bank can occur in pastures, that experience frequent fires with increase of density of seeds and predominance of herbs are typical of highly degraded areas, such as Poaceae, Rubiaceae, Asteraceae, and Cyperaceae. Melastomataceae seedlings are an important component of the seed bank in the Amazon rainforest. On the other hand, Urticaceae has greater representation in forests that exhibit low-impact land use. Any change in seed bank functionality is bound to compromise the diversity, regeneration potential and overall maintenance of tropical forests. Therefore, it is necessary to expand studies that investigate seed banks in the Amazon rainforest. It is as important to prioritize sampling methods and pursue standardization of data presentation, as well as improve the identification of species that occur in the seed bank.

**Keywords:** floristic composition, forest disturbance, anthropic changes, forest regeneration, land use

### **1. Introduction**

The seed bank, or stock of viable seeds in the soil, can be defined as a set of latent, or dormant, seeds capable of originating adult plants [1, 2]. Studying the composition of the seed bank and understanding its role in regeneration are important to the conservation and management of tropical forests, as well as the control and eradication of invasive species in agrosilvopastoral systems [3–8]. The seed bank is influenced by the local plant community, history of land use, and forest matrix in general, it also has spatial and temporal variations [5, 9–12]. Spatial variations occur both horizontally and vertically; however, the greatest amounts of seed are observed in the upper layers [13–16]. Temporal variation occurs as a result of both loss and incorporation of seeds in the soil [13, 17]. The incorporation of seeds is the result of seed rain, which also presents seasonality owing to the different fruiting patterns of the species [9, 18, 19]. The rate of seed loss in the soil depends intrinsic loss of viability resulting from dispersal, environmental conditions, predation, and attack of pathogens [17, 20, 21].

Seeds of pioneer species are found in high density in the soils of tropical forests and constitute the main reserve of propagules for the regeneration of areas subject to disturbances [10, 22–24]. Most pioneer species have quiescent diaspores, owing to canopy light conditions or temperature variations [25–27], and compound the persistent seed bank [10, 22–24]. Species that compound the transitional seed bank have a lower density and are composed of late species of the forest succession [10, 14, 22], commonly forming a seedling bank [28, 29]. Consequently, floristic composition of the tropical rainforest seed bank does not reflect the composition of species in the arboreal, or regenerating, strata of old-growth forests [8, 14, 29]. Thus, seed bank serve to allow the establishment of a set of species that do not occur in vegetation or that present in low density in old-growth forests, but persist in the seed bank [18, 30–32].

After formation of a clearing by natural or anthropic disturbance, the quantity of seeds in the soil decreases as a consequence of recruitment rates or loss of seed viability [14, 29, 33]. After the establishment of pioneer species and subsequent fruiting, seed density in the soil increases in the initial stages [9, 21, 24, 29, 33, 34]. The, with the advance of forest succession, the number of seeds in the seed bank tends to decrease and return to pre-disturbance equilibrium [9, 21, 24, 29, 33, 34]. The seed bank plays a major role in the re-establishment of plant communities subjected to medium and high-intensity disturbances and can have a wide impact on the dynamics of plant communities during the process of ecological succession [35–38]. For example, in forest areas of the Amazon burned and converted to pasture, almost no vestiges of the seed bank, remain [39]. Nonetheless, pioneer species of *Vismia* were reported to dominate regeneration [40, 41]. In contrast, areas with some seed bank left intact were initially reported to already be occupied by pioneer species of *Cecropia,* allowing a larger set of plant species to regenerate under its canopy [36, 39, 42–44]. In terms of forest management, the role of the seed bank in the regeneration of forests increases in importance, when compared to the seed rain, both in clearings and trails generated by skidders [45].

The seed bank is known for its low contribution to the establishment of late species in the forest succession in which these groups derived from dispersion and stock seedlings [1, 14]. However, the seed bank can be considered highly diverse in life forms thus contributing to the restructuring of forest strata [33, 46]. The abundance of herbs and shrubs in the seed bank of forest environments can be a consequence of the surrounding matrix, as well as the history of land use [5, 14, 22]. It is a reflection of vegetation that has already undergone some type of anthropic or natural change [5, 29, 33, 47]. In general, herbaceous and shrub species are more commonly found in altered areas and secondary vegetation [14]. However, disturbances that occur around the forest also contribute to the entry of ruderal, or invasive, species in the seed bank [5]. Notwithstanding this phenomenon, forest areas surrounding pastures or agricultural areas change the density and floristic composition of the seed bank in these areas [5]. This gives rise to the entry of common trees and shrubs into the seed bank of forest areas [5]. The seed bank in tropical forests is, therefore, highly variable. At the same time, studies reporting on this natural component of the Brazilian rainforest are scarce. Therefore, this chapter aims to analyze variations in density, family abundance, and life forms of the seed bank in *terra firme* forest of the Amazon rainforest, as well as assess the impact of the main changes in land use in this region on seed bank characteristics.
