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.