What drives riparian vegetation encroachment in braided river channels at patch to reach scales? Insights from annual airborne surveys (Drôme River, SE France, 2005–2011)

Abstract

Understanding riparian vegetation encroachment and its drivers is crucial for predicting alluvial river channel morphodynamics in different hydro‐climatic contexts. We investigated fine‐scale vegetation encroachment patterns and underlying physical controls along the Drôme River, France, following a 50‐year flood in 2003. Our study addressed the following questions: (a) What is the relative timing and the respective importance of vegetation patch recruitment versus patch expansion? (b) How well do regional/climatic factors explain the spatio‐temporal patterns of recruitment and expansion versus local and reach‐scale variation in geomorphic position and habitat? Along a 3‐km reach, we analysed a chronosequence of high‐resolution aerial images acquired yearly between 2005 and 2011. From each image series, we digitized patches of newly recruited vegetation and their lateral extent in following years to determine their expansion. This was complemented by elevation and plant structure data from airborne LiDAR data and field observations. The initial post‐flood vegetated area doubled within 6 years, with a pronounced recruitment peak in 2006–2007. Patch expansion showed a positive linear relationship to growing season temperature. Vegetation encroachment was greatest in a slightly aggrading sub‐reach compared to an incising sub‐reach. Initially, it occurred principally along low flow channel margins and on bar surfaces, later in intermittent channels. Our results confirmed that extensive recruitment can succeed floods with recurrence intervals smaller than 1‐in‐5 to 10 years, when the flood history resets the geomorphic framework. Recruitment extent depends in part on local hydrological connectivity, whereas higher temperatures can boost plant growth where there is a perennial water supply.

Understanding processes of island development on an island braided river over timescales from days to decades

“…Furthermore, although the 2014 deposited trees could not be located on the 2013 LiDAR, it is very likely that they were deposited at an elevation of approximately 0.1 to 0.3 m, comparable to that of the 2004 pioneer islands almost 10 years after deposition. A similar wide range of deposited tree elevation was reported also by Räpple et al (2017) (when transformed from elevation above low flow to elevation above the mean). This reinforces the crucial role played by flood history.…”

“…A similar wide range of deposited tree elevation was reported also by Räpple et al . () (when transformed from elevation above low flow to elevation above the mean). This reinforces the crucial role played by flood history.…”

“…The timescale is probably long enough to filter out the impact of annual changes in temperature and precipitation, masking the controls highlighted by Räpple et al . (). More importantly, once the trees have survived initial deposition and early establishment, which is affected by access to groundwater and thus river stage (e.g.…”

“…The annual growth rate of trees on the 2000 and 2004 pioneer islands is quite constant when measured on a time scale of three to five years, with no apparent marked changes between particular years ( Figure 6A). The timescale is probably long enough to filter out the impact of annual changes in temperature and precipitation, masking the controls highlighted by Räpple et al (2017). More importantly, once the trees have survived initial deposition and early establishment, which is affected by access to groundwater and thus river stage (e.g.…”

Short‐term geomorphological and riparian vegetation responses to a 40‐year flood on a braided, dryland river

“…We see this as a consequence of some sites being rejuvenated by the flood, eroded, and simplified and others maintaining nearly intact vegetation and ultimately underlies the key role of floods in maintaining a shifting mosaic of habitats in riparian systems (Bormann & Likens, 2012;Décamps, 1996;Latterell, Bechtold, O'Keefe, Van Pelt, & Naiman, 2006). Vegetation will develop in the absence of a subsequent large flood (Friedman, Osterkamp, & Lewis, 1996;Loucková, 2012;Räpple et al, 2017) and presumably decrease vegetation heterogeneity (and therefore beta diversity) over time. Vegetation will develop in the absence of a subsequent large flood (Friedman, Osterkamp, & Lewis, 1996;Loucková, 2012;Räpple et al, 2017) and presumably decrease vegetation heterogeneity (and therefore beta diversity) over time.…”