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

AbstractUnderstanding 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.