1. Introduction
Dispersal is the transport of individuals from their site of origin to an establishment site (Cousens et al., 2008; Pijl, 1969). In plants this transport is generally driven by one or several dispersal vectors, for example, if the transport is by wind it’s called anemochory, by an animal it’s called zoochory and if the transport is by water it’s called hydrochory (Boudell, 2016; Matthysen, 2012; Pijl, 1969). Given that for plants the transport of propagules depends on dispersal vectors, the dynamics of these vectors has important implications for plant communities since dispersal has been observed to be important for the patterns of species richness and composition of communities, the coexistence of species and the maintenance of metacommunities (Boudell, 2016; Levine & Murrell, 2003; Nilsson et al., 2010).
Hydrochory is an important source of propagules for riparian plant communities, since it transports the greatest number of different species’ propagules to the riparian zones (Jansson et al., 2005; Leyer, 2006; Moggridge & Gurnell, 2010; Neff & Baldwin, 2005). For this reason, great efforts have been made to understand this dispersal mechanism in rivers (Hyslop & Trowsdale, 2012; Nilsson et al., 2010). These studies have shown that hydrochory has two important characteristics that determine the spatial distribution of dispersed propagules in the riparian zones along rivers: (i) the movement of propagules is unidirectional and (ii) there is a downstream accumulation of the propagules (Andersson et al., 2000; Nilsson et al., 2010). This unidirectional movement and accumulation of species and propagules transported downstream by the river is related to the increase in species richness and the nesting of plant communities along rivers (Kuglerová et al., 2015). This has served as a basis to explain the diversity patterns of plant communities along rivers, synthesized in the ”River Collector Hypothesis” (hereinafter referred to as RCH). The RCH suggests that the species richness increases along the river to a maximum point in the intermediate zones and then decreases towards the mouth of the rivers, in addition, it supposes the nesting of the upstream communities in the downstream ones (Nilsson et al., 2010).
In addition to these spatial changes, temporal variations in the dispersal of propagules by hydrochory have been observed. The most relevant pattern occurs in the season of greatest streamflow, which is correlated with a greater number of propagules and species dispersed along the rivers (Greet et al., 2011; Gurnell et al., 2008; Pettit & Froend, 2001). This increase in the number of dispersed propagules depends on a greater streamflow, but it is also the result of the synchronization of the reproductive phenology of the riparian species with the maximum flow peaks of the water, producing a greater number of propagules that can be dispersed (Pettit & Froend, 2001).
The plants of the riparian communities, also have other vectors of dispersal to move their propagules to other watersheds or riparian zones through anemochory and zoochory (Fraaije et al., 2017). These types of dispersal are important for the spatial distribution patterns of propagules in riparian zones because for some species they are the first dispersal vector that carries propagules from the parental plant to the riparian zone along the river (Esper-Reyes et al., 2018; Fraaije et al., 2017; Nilsson et al., 2010). These types of dispersal could be particularly important for propagule movement in intermittent rivers in which hydrochory propagule transport only occurs at certain times of the year (Boland, 2017; Drezner et al., 2001; Schwab et al., 2018) or in turbulent rivers where the disturbance of the floods limits the establishment of the propagules deposited by hydrochory (Hasselquist et al., 2015). However, there is little empirical studies on the patterns of richness and composition of dispersal types other than hydrochory along rivers and their temporal variations.
Another of the unknown aspects of dispersal in riparian zones is the study of variations in the richness, composition and number of propagules dispersed in rivers with specific geomorphological characteristics, such as mountain rivers. The geomorphological and hydrological characteristics of these rivers, such as the altitudinal gradients, the steep slopes, the large confining sections and the current velocity, increase their downstream transport capacity and reduce their deposition of matter towards the riparian zones (Jacobsen, 2008; Meyer et al., 2007; Wohl, 2010) and thus, it is possible that the deposition patterns of the propagules transported by hydrochory are different from what has been observed in other sections of the rivers.
In this study, we contrast spatial and temporal changes in the number of propagules, species, and species composition deposited by hydrochory and other types of dispersal with increasing distance to the site of origin of rivers in a mountainous system. For the temporal patterns we expect that: (i) the abundance and species richness of propagules deposited by hydrochory is greater in the rainy season than in the dry season, while no changes are observed for other types of dispersal and (ii) differences in composition between seasons for hydrochory but not for other types of dispersal. Regarding spatial patterns, we expect that: (iii) the number of species and propagules dispersed by hydrochory increases with distance to the site of origin of rivers while remaining constant for other types of dispersal, (iv) nesting pattern for hydrochory but not for other dispersal types and (v) the turnover remains constant for hydrochory-dispersed propagule communities and increases for other dispersal types.