5.1. The Tech River landscape: dominance of biotic forcing in an extremely irregular torrent context
A spatially explicit retrospective GIS analysis of the landscape dynamics of the Tech River, from a situation of complete abiotic reinitialization at the scale of the fluvial corridor – following the catastrophic flood of October 1940 – made it possible to specify the intensity and spatial modalities of responses and effects of vegetation on fluvial morphodynamics in a hydrologically irregular torrential Mediterranean context with moderate anthropogenic pressure (Corenblit et al., 2010).
The hydrological configuration (no major floods for ten years) that followed immediately after the devastating flood of 1940 allowed to clearly distinguish two modalities of plant succession specifically in relation to the effect of close location to the water resource on the growth rate and density of plant cover. Distance and relative altitude from main and secondary channels acted in this Mediterranean context as key parameters controlling vegetation growth rates. Indeed, (i) close to channels and the water resource, in low-lying areas, plant growth/succession is strongly stimulated. Dense tree formations develop in less than ten years; the stimulation and success of the recruitment of pioneer herbaceous and woody species that were measured at the local scale on alluvial bars in the highly exposed zones (Corenblit et al., 2009a) corroborate and clarify the processes underlying these dynamics. (ii) Far from wet channels, plant growth/succession is slower and follows a more progressive successional trajectory through a shrubby and sparse tree stage that lasts about thirty years. A dendrochronological analysis of adult P. nigra individuals growing close to and far from the active channel showed a contrast in the growth rate of the rings (on average 1.3 cm of annual growth at the edge of the channel against 0.4 cm far from it). During this period, avulsions favoured by the dense tree band at the edge of the channel led to the rejuvenation of succession over a significant area of the corridor (about 40%). When the floodplain was covered with a dense forest after thirty years, the system switched to a new domain of stability because the rejuvenation process was strongly constrained at the scale of the entire corridor, even for morphogenic floods with a return period of between twenty and thirty years. The active tract (active channel and bare alluvial bars) then contracted and the channel incised, potentially irreversibly in the absence of an exceptional flood with a frequency >100 years.
It appears that the relationship between the speed, quality, and location of vegetation closure and the frequency/amplitude of floods determines the conditions of geomorphological equilibrium at the scale of the corridor. The combined effect of the two modalities of plant succession favours the progression of the riparian forest within the entire fluvial corridor (from the outside to the inside and vice versa) and the contraction and confinement of the active tract. The spatial configuration which results from biotic-abiotic feedbacks after thirty years on the Tech represents a biogeomorphological attractor state. The shift of the geomorphological equilibrium system towards stabilization is explained by the fact that, although plant growth is slower on the floodplain, a dense tree formation eventually develops at the medium term (30 years on the Tech), thus reducing the probability of destruction by floods. The areas near the channel are frequently flooded, but the growth of vegetation well adapted to hydraulic constraints is stimulated there. The very rapid and efficient development of pioneer vegetation in the backwater along the active channel maintains the continuity of the coupling with sediment accretion processes and thus minimizes the width of the active band while the floodplain becomes progressively more densely wooded. This type of phenomenon is involved when a river evolves from an unstable braiding style to a more simplified and entrenched stable style.