Environment-driven changes in diversity of riparian plant communities along a mountain river

Abstract

The study of changes in species richness and composition along rivers has focused on large spatial scales. It has been ignored that in different sections of the river (high mountain area, middle zone, and mouth of the river) the specific environmental conditions can generate different longitudinal patterns of the species richness and composition. In this study, we determine whether species richness and composition of the riparian plant communities change along a mountain river and whether these changes are related to environmental variables. We expect an increase in species richness and turnover along the river, that the upstream communities would be a subset of the downstream communities, and that such would be related to edaphic and hydrologic conditions. To test this, we sampled three strata of the riparian vegetation (upper: individuals with <1 cm of ND, middle: individuals with >1 cm of ND, low: individuals with >1 m tall) in a set of 15 sites that we place along a mountain river. Additionally, we recorded topographic, hydrological, morphological, and soil variables. We performed correlation analyzes to determine whether changes in species richness and turnover were related to increased distance to the origin of the river. Also, we obtained the nestedness and evaluated the importance of environmental variables with GLM, LASSO regression, and CCA. With the increase in distance, the species richness decreases in the upper stratum, but not in the middle and the low stratum (although the highest values were observed near the origin of the river), the turnover increase in all strata and the upstream communities were not a subset of the downstream communities. The changes in species richness and composition were related to topographic (altitude), hydrological (flow), and edaphic (conductivity and pH) variables. Our results indicate that at small spatial scales the patterns of richness and composition differ from what has been found at larger spatial scales and that these patterns are associated with environmental changes in the strong altitude gradients of mountain rivers.

Keywords: River; River Collector Hypothesis; altitudinal gradient; diversity of species; longitudinal dimension; mountain river.

FIGURE 1

Location of the sampling sites along a river in the upper part of the Papaloapan River basin, Oaxaca, Mexico. (a) 15 sites were established along one river, and the Strahler stream order is shown in parentheses for site 1 and 15. (b) In each site, riparian vegetation was inventoried in two sets of plots of different sizes. The upper stratum in plots of 100 m², the middle stratum in plots of 50 m², and the low stratum in plots of 1 m². (c) The position of the sites in an altimetric profile is shown. *Sites in which all the environmental variables were recorded

FIGURE 2

Relationship between species richness of riparian plant communities and geographical distance from the origin of the river. The species richness (q0) in the upper stratum decreases with distance to the origin of the river, for the low stratum the richness in site 1 was higher than the rest of the sites. (a) Upper stratum. (b) Middle stratum. (c) Low stratum. (*) Significant correlations

FIGURE 3

Relationship between community dissimilarity, nestedness, and distance to the origin of the river. Dissimilarity increases and nestedness decreases with the distance to the origin of the river in upper stratum (a) and low stratum (c) but not for the middle stratum (b). The filled circles, squares, and triangles are the dissimilarity values while the unfilled ones are the nestedness values. (*) Significant correlations

FIGURE 4

Presence of species and RIV in the sampling sites from the origin of the river to downstream. No nestedness of the species composition of the sites near the river starting point is observed in the composition of the downstream sites. (a) Upper stratum, (b) middle stratum, (c) low stratum. The species are arranged in the order of frequency, the most frequent species on the left side and the less frequent species on the right side. The RIV is presented for species with RIV > 10 (black bar, the size of the bar represents its VIR value), species with RIV < 10 as shown with diagonal lines. The names of the species are shown in Appendix S4

FIGURE 5

Ordination of the study sites based on the Canonical Correspondence Analysis, using the composition of species of sampling sites and environmental variables. The differences in the species composition of the sites near to the origin of the river and the downstream sites were related in the upper and in the middle stratum with flow, altitude, and conductivity; in the low stratum, it was related with flow and pH. (a) Upper–middle stratum. (b) Middle stratum. (c) Low stratum. Alt, altitude; Con, conductivity; Flo, flow; Mc, moisture content. The most frequent species in the study area are displayed (species names are in Appendix S4)