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.