Relationship between species distribution and environmental
variables
The distribution of the three species studied showed somewhat similar
associations to environmental variables, which is expected due to the
close phylogenetic relationship among the species (Losos, 2008). In
general, the distribution of the three Myrcia species was
negatively related to elevation and terrain slope, and positively
related to canopy opening. Flooding is expected to be more frequent,
last for a longer time, and/or reach higher heights at lower parts of
the study plot due to periodic water table flood (Diniz, 2009). As shown
in Fig. A1, the lower part of the study plot does present higher soil
moisture and flooding height. Therefore, we believe elevation is a good
surrogate for flooding at the study site. Additionally, we detected
positive relationships between the distribution of small and large trees
and soil moisture for M. multiflora , which reinforces our
hypothesis that the three congeneric species are restricted to patches
subject to flooding within the plot. We probably did not find more
relationships between species distribution and flooding height and/or
soil moisture because both variables were measured only once and thus we
probably could not capture flooding seasonality properly. The
distribution of the three species studied was also restricted to
better-lit patches, where individuals are likely to show higher survival
probability, growth rate, and fecundity (Dahlgren and Ehrlén, 2009).
Interestingly, we were able to detect the influence of temporal changes
in the light environment on the distribution of different size classes
of the three Myrcia species. Although smaller trees were
correlated with recent and/or past light environment, large trees were
correlated only with the past light environment, which probably reflects
more closely the conditions of the time plants were smaller.
Myrcia brasiliensis , which is well distributed in the study plot,
showed weak associations to the environmental variables. Conversely, the
other two species, whose distributions follow a flood-dry gradient,
showed strong associations to elevation. For the three species,
environmental requirements shifted through plant development, as also
observed for other species (e.g. Comita et al. , 2007). ForM. brasiliensis , the light environment was important during early
development (Poorter, 2007). However, as trees reach large sizes, they
no longer depend on canopy opening, as the species occupies the first
canopy stratum and also occurs as an emergent tree (Valladares et
al. , 2016). As individuals grow in better-lit environments, some become
restricted to patches subject to flooding. On the other hand, M.
racemosa showed an opposite pattern, whereby smaller trees get
restricted to patches subject to flooding and later on development the
past light environment becomes detectable on the distribution of large
trees. The shift in environmental requirements likely explains the
change in distribution from small and medium trees (flood-dry gradient)
to large trees (from northwest to southeast of the plot) for M.
racemosa . Myrcia multiflora showed an intermediate pattern, with
species distribution restricted to patches subject to flooding from
early to late development, and light influencing the distribution of
small and medium trees.
Even though flooding is the most important environmental characteristic
driving species distribution in the plot, the fundamental niche of the
three Myrcia species potentially encompasses a much wider
variation of conditions than the environments of the study site, as the
species occur in different vegetation types and are not restricted to
areas subject to flooding at the geographic scale (Fig. 1). Thus, in therestinga forest studied, the species are capable to use the most
common environmental condition (niche position, Marino et al. ,
2019), and patches subject to flooding may work as areas of refuge from
strong competitors and/or natural enemies (Baraloto et al. ,
2007), which drives the species’ realized niche. We suggest future
studies should evaluate flood tolerance of the different species of the
community and the presence of natural enemies to better understand the
spatial distribution constraint in a gradient of soil moisture in
flooded forests.