A Comparison of the Composition and Diversity of Tree Populations along a Hydrological Gradient in Floodplains (Southern Québec, Canada)

Jean-Sébastien Berthelot, Diane Saint-Laurent, Vernhar Gervais-Beaulac, Aurélien Présent
2015 Forests  
With the current climate changes, it is essential to understand the mechanisms that govern floods and flow regimes and their effects on the dynamics of riparian forests. The aim is to assess the effects of new hydrological conditions (increase in flood frequency) on forest stands subject to frequent floods. The sampling sites (total of 94 quadrats) are located in riverine woodlands, and the choice of location corresponds to the boundaries of the flood-risk zones established by official
more » ... t maps. Our study shows that there are significant differences in the composition and diversity of forest communities following differences in the flood recurrence zones. In the active floodplains (i.e., recurrence interval of 0-20 years), the tree population stands are clearly distinguished from other intermediate flood zones (interval of 20-100 years). Differences are also noted in the structure of the communities, in particular in the frequent flood zones, which are characterized by a low renewal rate, low density and less-diversified forest stands. The frequent floods risk forest stand rejuvenation and creating decline as a result of increased tree mortality and the low renewal OPEN ACCESS Forests 2015, 6 930 rate. With the expected increases in the number of flood events in the coming decades, there may be greater tree mortality and a gradual disappearance of the forest communities. A number of studies have dealt with variations in the water regime and their effects on the composition and vitality of riparian ecosystems [1] [2] [3] . With global warming, it is known that these impacts can vary at the local and regional levels, as well as based on the type of catchment and flow regime [3, 4] . The hydroclimatic variations (e.g., river flow, temperature and rainfall) related to global warming could thus induce major changes in flow regimes and riparian ecosystems [4] [5] [6] . Also, river regulation has a direct impact on river flow, which in turn can have an adverse effect on the vitality of riparian ecosystems [6, 7] . Among the many factors that regulate the structure of wetland communities and plants, environmental heterogeneity easily stands out as the key factor. Heterogeneity in wetlands is created and maintained mainly through the interaction between hydrology and geomorphology [8] [9] [10] [11] . Various authors [9, [12] [13] [14] [15] have shown that patterns of species richness and floodplain productivity are significantly influenced by physical factors (e.g., drainage, floods, fires) and by the processes operating on the lateral, longitudinal and vertical dimensions of rivers. Topographic elevation appears to be the key variable that accounts for the vegetation composition of several riverside environments [16] [17] [18] . Land elevation affects flood rate and duration, for instance. Sites with an average elevation that are flooded on an intermediate basis are usually those with the richest plant species. Some studies on species richness, conducted along the rivers' longitudinal axis (upstream-downstream), have shown that the greatest richness is most often found on the sites of floodplains with a greater variability of substrate [14, 19, 20] . Riparian ecosystems are known to be critical environments for maintaining biodiversity. Floods are also generally considered active factors in the enrichment of floodplain ecosystems (e.g., fine sediments, transport of seeds, organic particles, nutrients), especially along rivers crossing through wetlands, marshes or forests [21] [22] [23] . The concentration of these various elements (e.g., humus, nitrogen, phosphorus) in the riparian environment is used as an indicator of soil fertility and the vitality of plant life in fluvial environments [23] [24] [25] . However, certain studies report that changes in the water regime, in particular an increase in heavy floods, have a direct impact on the vitality of riparian populations [26] [27] [28] . Heavy or frequent floods can result in the premature demise of trees found in low river terraces [26] . Various studies stress the role of disturbances in the development and maintenance of a rich vegetation mosaic [6, 13, 14] . In riparian environments, one of the greatest disturbances results from flood dynamics [4, 9, 14] . Different intensities of flood events over time and space can create a mosaic involving successive stages [28, 29] since the various plant species have different levels of tolerance to disturbances and variations in the hydrological regime (floods vs. drought), as well as different competitive abilities. Factors such as frequency, season, duration, flood extent, flood period and seasonality contribute to the variability observed in wetland riparian communities [30] [31] [32] [33] [34] .
doi:10.3390/f6040929 fatcat:wvwlp5giv5fpnhwpubtnzm46km