Low latitudes support a higher plant species diversity in riparian forests, surpassing other regions (Wantzen, 2008; Slik et al., 2015). In tropical zones, it is common to find an average of around 100 species of trees in 1 ha (e.g., França et al., 2009; Wittmann et al., 2010; Gonçalves et al., 2014), whereas in other parts of the world, like temperate regions, riparian forests typically host between 5 and 10 tree species (Swan and Palmer, 2004). The species composition of riparian plants and their phenotypic responses are strongly related to the heterogeneity environmental, such as natural variations of edaphic characteristics of soil pH, soil fertility and water availability, temperature, rainfall and ecological interactions in the community defining the high tropical biodiversity (Haridasan, 2008; Bambi et al., 2016; Miatto et al., 2016; Sena et al., 2022) (Figure 1).

Plant litter entering streams in high latitudes commonly reflects the diversity and composition of the adjacent riparian community of tree species (Swan and Palmer, 2004). Nonetheless, this is often not true in tropical streams because of their higher riparian plant diversity (Gonçalves et al., 2014; Tonin et al., 2017), the phenological variation of leaf fall of species (Sakai, 2001) and the pronounced seasonality (Tonin et al., 2017). Due to this complex dynamic, tree species from the riparian forest contribute with different amounts and composition of plant litter to the aquatic ecosystem throughout the year in streams in the tropics (Gonçalves and Callisto, 2013; Gonçalves et al., 2014). This results in a plant litter pool instream with a continuous variety of litter types with diverse chemical characteristics (Covich, 1988; Tonin et al., 2021). Also, the composition of the plant litter pool in these streams varies widely throughout the year (Gonçalves and Callisto, 2013; Gonçalves et al., 2014). Therefore, the litter mixtures' decomposition may play a key role, i.e., the way to know how the energy flow and nutrient reloading for the food web of the tropical aquatic ecosystem.

Climate emerges as a principal determinant guiding the distribution and composition of plant species within terrestrial ecosystems (Reich and Olesksyn, 2004; Wright et al., 2004) (Figure 1). This climatic influence gives rise to a latitudinal gradient in leaf litter properties worldwide, resulting in tropical riparian forest litter exhibiting heightened recalcitrance, as demonstrated by Boyero et al. (2017). Consequently, leaf litter from tropical riparian forests may have higher toughness, heightened chemical defenses (such as elevated phenolic content), and a more pronounced phosphorus limitation, characterized by higher nitrogen-to-phosphorus (N:P) ratios, in comparison to litter from riparian forests in other regions (Boyero et al., 2017).

The climate also makes riparian forests in tropical biomes exhibit natural seasonal changes in the amount of litter input streams (Gonçalves and Callisto, 2013; Gonçalves et al., 2014; Tonin et al., 2017). For example, the Amazon and the Cerrado have a marked seasonality in the dynamics of the litter input, relating to precipitation as a limiting factor (Tonin et al., 2017). These seasonal variations in the litter input also suggest a change in the chemistry of the litter budget that has been input in the streams (Gonçalves and Callisto, 2013; Tonin et al., 2017; Tonin et al., 2021), as observed for the Cerrado. The concentration of structural compounds (carbon, fibers, lignin and cellulose) and nutrients (N and P) is highest in wet and wet-dry transition seasons and decreased toward the dry season or until the beginning of the dry-wet transition in this biome (Tonin et al., 2021).

In addition to climate variations, soil characteristics strongly influence the chemical characteristics of green leaves (Reich and Olesksyn, 2004) (Figure 1), and these characteristics are often retained as leaves transition into their senescent phase (Killingbeck, 1996). However, leaf senescence allows efficient remobilization of nutrients from leaves to other parts of the plant, serving as a survival mechanism during adverse environmental conditions such as drought or water stress (Taiz et al., 2017). For example, litter from the riparian forests of Cerrado biome are more refractory (higher C:N and C:P mass ratios and tougher tissues) than those from the Amazon and Atlantic Forest biomes (Sena et al., 2022) due to the harsh climatic and edaphic factors for plants found in the Cerrado, such as low pH, low fertility and reduced water availability in some periods of the year (Kraus et al., 2004; Haridasan, 2008; Miatto et al., 2016). Also, much of the tropical riparian forest litter biomass comes from a few dominant species (about 20 species dominate litter phenological patterns and are responsible for more than 70% of annual litter biomass; França et al., 2009; Gonçalves et al., 2014; Bambi et al., 2016; Rezende et al., 2017; Tonin et al., 2021). This means that shifts in the identity of dominant plant species in these riparian forests significantly influence the litter chemistry within the streams (Tonin et al., 2021).

Although litter characteristics are primarily driven by environmental variations (Boyero et al., 2017), species phylogeny also influences the chemical characteristics of the litter budget (LeRoy et al., 2020; Sena et al., 2022) (Figure 1). Litter from closer phylogenetic species has more similar chemical characteristics than litter from distant species due to the conservation of characteristics during their evolution species lineages (LeRoy et al., 2020; Sena et al., 2022). For example, in tropical riparian forest closely related species have similar P contents in the Atlantic Forest, which suggests that phylogeny is essential for determining P content within this tropical biome (Sena et al., 2022). Furthermore, plant species in riparian forests across different tropical biomes display phenotypic plasticity, leading to distinct variations in litter chemical characteristics within the same species (Sena et al., 2022) (Figure 1). This demonstrates that in tropical riparian forests functional responses of individual traits are induced by environmental conditions (Boyero et al., 2017; Sena et al., 2022). Thus, litter budget chemical and physical variations are influenced by climate variations, edaphic factors, composition of riparian plant species, species phylogeny and phenotypic plasticity in tropical riparian ecosystems.

The established correlation between plant species diversity and various ecosystem processes is well-documented, as demonstrated by studies by Hooper et al. (2005) and Hector and Bagchi (2007). This diversity significantly influences the chemical characteristics of litter, including nutrient content, phenolic compounds, and structural elements, subsequently impacting decomposer and detritivore activity and leaf decomposition in streams (Figure 1). Evidence from previous studies shows that litter species mixtures can accelerate decomposition by up to 30% compared to single species in these ecosystems (e.g., Kominoski et al., 2007; Lecerf et al., 2007; Handa et al., 2014; Oliva et al., 2019), especially at low latitudes where more generalist detritivores appear to be relatively more efficient when more varied resources are available, through a balanced diet effect (Boyero et al., 2016; Boyero et al., 2021). However, negative or no effects of litter species diversity in mixtures on decomposition in streams were also recorded (Moretti et al., 2007; Ferreira et al., 2012; Frainer et al., 2015; Mori et al., 2020; Boyero et al., 2021). In this context, it is important to note that an increase in plant species diversity does not uniformly translate to a rise in the chemical diversity of the litter stock. Some species may have functional redundancy or unique characteristics compared to others in the community (Violle et al., 2017; Dee et al., 2019). When functional redundancy exists, species extinction may not substantially affect the ecosystem, as the remaining species can compensate for their loss (Violle et al., 2017).

Conversely, the extinction of species with distinct sets of characteristics can significantly impact ecosystem functioning, as other species in the community cannot readily replace their unique roles (Lyons et al., 2005; Dee et al., 2019). For example, the absence of plant species with specific chemical attributes in the litter stock in tropical streams may decrease fungal biomass and cause difficulty in leaf decomposition (Rabelo et al., 2022; Rabelo et al., 2023). Thus, while plant species diversity remains a key for aquatic ecosystem functioning, as observed in tropical streams (e.g., Handa et al., 2014), the chemical compounds of the litter stock hold more significant way over the decomposition process than plant diversity alone (Handa et al., 2014; Gonçalves et al., 2017; Rabelo et al., 2022; Rabelo et al., 2023). This underscores the importance of considering plant diversity and chemical characteristics in studies of ecological systems.

The dominant plant species in the litter (higher relative contribution) are widely used by decomposition studies evaluating diversity effects, since they contribute most to ecosystem processes according to the mass ratio hypothesis (Grime, 1998). These species are the main species determining litterfall chemistry patterns (or average quality; Tonin et al., 2021). Therefore, they may direct decomposition in litter mixtures through resource concentration effects, that is, litter mixtures with a higher concentration of nutrients and lower concentration of secondary compounds, and/or litter toughness (García-Palacios et al., 2017; Boyero et al., 2021). In general, the dominant species with a higher nutrient concentration accelerate decomposition in litter mixtures, whereas those with a higher toughness or secondary compound concentration delay this process (Graça et al., 2015), as found by several studies in tropical and worldwide streams (Gonçalves et al., 2007; Gonçalves et al., 2012; Handa et al., 2014; Gonçalves et al., 2017; Zhang et al., 2019).

Despite the low relative abundance of rare plant species, it has been shown that these species may also play a potentially high role in the decomposition of litter mixtures due to their unique functional characteristics (Mouillot et al., 2013; Dee et al., 2019). These characteristics, such as distinct chemical or physical properties within the ecosystem (different concentrations of nutrients and degrees of toughness), can have a notable effect. One study has tested the impact of the loss of rare plant species on litter decomposition (Rabelo et al., 2022) and found that the loss of these species may reduce litter decomposition by around 20% in tropical streams in Central Brazil, which may lead to a decrease in nutrient reloading in the aquatic ecosystem. This emphasizes the need to consider the broader ecological implications of both dominant and rare species in the context of decomposition dynamics, especially in tropical regions.

In theory, the loss of rare plant species (conceptualized here as those with the lowest contribution to biomass) can reduce the functional diversity of litter mixtures, that is, the variation in species characteristics relevant to ecosystem functions (Tilman, 2001), and influences the resource concentration. These two aspects–functional diversity and resource concentration–are critical in litter decomposition (García-Palacios et al., 2017). It is expected that the loss of rare plant species will have an impact mainly on the functional diversity of litter mixtures since rare species are more likely to have unique functional characteristics than other more common species (e.g., high concentration of scarce resources for the community; Mouillot et al., 2013; Dee et al., 2019). As rare species have a low representation in terms of abundance or biomass of communities (Rabinowitz, 1981), the effect of rare plant species on resource concentration is expected to be low. Previous studies suggest that litter mixtures' functional diversity can accelerate decomposition and other related processes, such as nutrient cycling (Handa et al., 2014; García-Palacios et al., 2017; Rabelo et al., 2022). This acceleration may be attributed to (i) nutrients being transferred between different litter types, facilitated by fungal mediation or leaching (Handa et al., 2014); (ii) complementarity in resource utilization allowing consumers to use a broader range of resources from various plant litter types (Vos et al., 2013); (iii) furthermore, a diverse physical environment benefits detritivore (Sanpera-Calbet et al., 2009).

The potential loss of rare plant species, particularly prevalent in high-richness tropical ecosystems (Slik et al., 2015), could significantly affect decomposition dynamics (Rabelo et al., 2022) (Figure 1). Moreover, the loss of rare plant species can reduce the fungal biomass production in tropical streams, especially if the natural plant species proportion is preserved (Rabelo et al., 2023). This is because mixtures reflecting an environmental gradient may have a higher occurrence of positive and non-additive effects and thus favour more microbial growth (Swan et al., 2009). For example, fungi can benefit from the higher resource variability via nutrient transfer by fungal hyphae, contributing to their growth and activity (Bardgett and Shine, 1999; Chapman et al., 2013).

Exotic species are one of the leading drivers of global biodiversity decline (IPBES, 2019) and often show an initial dominance in communities (Seabloom et al., 2003). Thus, the invasion by these species is an important factor driving changes in dominance patterns (Woitke and Dietz, 2002). When invasive exotic species have functional characteristics that are different from those of the dominant native species, they often affect the ecosystem processes differently. In contrast, when the invader is functionally similar to the dominant native species, it affects the ecosystem similarly (Finerty et al., 2016). Thus, exotic plant species functionally dissimilar to dominant native plant species can significantly alter litter decomposition via changes in mixed litter composition.

Tropical riparian forests are constantly invaded by exotic plant species (van Kleunen et al., 2015), and most studies evaluating the impacts of these species (e.g., Eucalyptus species) on streams have reported changes in the seasonality, quantity, and quality of litter entering streams (Molinero and Pozo, 2004) (Figure 1). These changes can reduce (Pelizari et al., 2022), increase (Gomes et al., 2018) or do not affect (Gonçalves et al., 2012) litter decomposition, which is related mainly to higher or lower chemical and physical compounds in exotic species (i.e., N and P content and presence of refractory compounds such as lignin and cellulose) compared to native species, and to the type of community involvement in the decomposition process (microorganisms and shredder invertebrates) (Ferreira et al., 2019). In regions where shredder invertebrates are not important decomposers and litter decomposition is microbial-driven primarily, as is often the case in tropical regions [e.g., Gonçalves et al. (2007)], the litter decomposition may be less affected by invasive exotic plant species since their effects are more potent for streams where shredders are well represented (Ferreira et al., 2016; Ferreira et al., 2019), although invasion by these species can have significant impacts on microbial colonization and decomposition (Gomes et al., 2018).