Abstract
AbstractRiparian vegetation survival and establishment in gravel‐bed rivers depends on the balance between vegetation growth and flood disturbance. We present four examples of vegetation and landform development in gaps (linear open spaces) between established islands and/or floodplain within a reach of the middle Tagliamento River, Italy. Gaps offer shelter to vegetation, supporting higher colonization success and different vegetation‐landform evolution pathways. Time sequences of aerial images track vegetation development over 30 years in the four gaps. In combination with the flood disturbance time series, we interpret vegetation dynamics and identify the fate of sexual and asexual reproduction strategies by observing vegetation expansion from lines of young plants and shrubs and from uprooted deposited trees and pioneer islands, respectively. Analysis of image sequences reveals common features across the four gaps that are generalized to extend a conceptual model of island development. Growing conditions, disturbance energy and time (window of opportunity) between major floods are the main controls on vegetation colonization. These vary among rivers, among reaches along the same river and locally, as in the investigated gaps, allowing different tree species with different life history traits (e.g., Populus nigra, Alnus incana) to engineer local river landforms in different and complementary ways. Although the conceptual model is inspired by observations on the Tagliamento River, consideration of species life history traits and the joint influences of growing conditions, disturbance energy and windows of opportunity provide a framework that may be applied to other temperate rivers where trees drive landform development.
Highlights
The importance of riparian trees for the development and stability of river islands and floodplain margins has been a subject of considerable research attention, especially over the last two decades
The conceptual model incorporates three pathways along which Salicaceae species may colonize the surfaces of river bars and initiate island development: (1) germination and growth of widely dispersed seeds when they are deposited at suitable germination sites; (2) germination and growth of seeds that accumulate in sheltered locations such as in the lee of wood piles and (3) sprouting of shoots and roots from deposited wood pieces or entire uprooted trees
Our observations of physical ecosystem engineering by P. nigra and A. incana within the study reach of the Tagliamento, emphasizes that on this high energy river, patches of vegetation that survive within the braid plain are mainly initiated by vegetative reproduction from large propagules
Summary
(grey alder), a member of the Betulaceae family that produces seeds with a longer period of viability than the riparian Salicaceae, for river bed landform development in the middle reaches of the Tagliamento They investigated broad spatio-temporal and topographic changes across the river bed associated with woody vegetation in general (dominated by P. nigra) and with A. incana in particular. A. incana appears to be associated with landform building in specific locations where it may complement physical engineering by the dominant species P. nigra, and its importance may vary through time because of its greater susceptibility to removal during large floods Following from this previous research, we investigate the potential complementary role of A. incana (Betulaceae) to that of the dominant species, P. nigra (Salicaceae), in physically engineering the development of islands and floodplains along the middle reaches of the Tagliamento. We generalize the outcomes of the above investigations to extend the conceptual “island development model” described by Gurnell et al, 2001 and Gurnell & Petts, 2006
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