Populus nigra L. establishment and fluvial landform construction: biogeomorphic dynamics within a channelized river

Abstract

Populations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants. We propose a conceptual model of biogeomorphic unit construction for channelized, lateral stable rivers. We consider the resultant biogeomorphic units as functional from an ecological point of view because P. nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity as a result of successful colonization, especially downstream of mature poplar stands.

Groundwater controls on biogeomorphic succession and river channel morphodynamics

“…Following Corenblit et al's definition, a fluvial deposit begins in a geomorphic phase largely devoid of vegetation; passes through a pioneer phase where high flow disturbance and water stress challenge the persistence of young vegetation (e.g., willow propagules establishment); enters a biogeomorphic phase, where the shrubby vegetation becomes capable of influencing fluvial form and ambient environmental conditions, actively interacting with riverine processes; and ends with an ecological phase. In the latter, the influence of fluvial processes on the landform is much reduced and the vegetation structure becomes dominated by woody willows and mature successional species [e.g., Corenblit et al ., , , , ]. A reduction in disturbance may lead to the dominance of higher biogeomorphic successional stages, which stabilize river banks at the reach scale, reducing the rate of lateral erosion and so the active width.…”

Poplar plantations along regulated rivers may resemble riparian forests after abandonment: a comparison of passive restoration approaches

“…The river has a pluvio‐nival regime with a mean annual discharge of approximately 200 m 3 /s. The mean channel gradient in the study reach is 1 ‰ and the bankfull active channel width is approximately 125 m. Due to bank management, sinuosity of the entire reach is low, with a mean value of less than 1.3 (Corenblit et al 2015 a ). The climate in the Toulouse area is Atlantic‐Mediterranean, with a total annual precipitation of 638 mm and a mean annual temperature of 13 °C.…”

A bivariate model for estimating reciprocal causality within biogeomorphic ecosystems: The cross‐lagged panel model

“…(b) Changes of the number of stems along the transverse gradient from the main water channel (line of plots ‘1’) to the top of the wooded point bar (line of plots ‘6’) on the Garonne River. (c) Changes in surface elevation along the transverse gradient on the six lines of plots (modified from Corenblit et al ., ). [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…We provide an example of application of a crossed‐lagged panel correlation to study biogeomorphic feedbacks in the Garonne River, France. We measured geomorphic and biological variables on two point bars from 2010 to 2012 (Corenblit et al ., ). The objective was to define the relationship between Populus nigra L. establishment and fluvial landform construction.…”