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.