Geomorphological controls of Fraxinus excelsior growth and regeneration in floodplain forests

Geomorphological changes can alter river hydrology and thus influence floodplain forest growth and regeneration. In this paper we quantify the effect of changes in channel elevation at the scale of four decades on hydrological conditions, overbank sediment deposits, water availability, and their impacts on common ash (Fraxinus excelsior) growth and recruitment in floodplain forests of the Ain River, France. Ash is a drought-sensitive species, and its regeneration is influenced by flood disturbance. We compared ash growth between 20 sample plots located in two contrasting geomorphological contexts using dendrochronological measurements: 11 along reaches degrading over the last 80 years and nine along stable or slightly aggrading reaches. In each context, half of the plots were located near the channel, and the remainder were within the floodplain. Ash regeneration was also quantified in each plot. This work showed that sites which undergo channel degradation are associated with less frequent overbank flows. Whereas there is no trend in either the climatic or stream flow timeseries over the last four decades, the growth patterns of ashes of these sites are significantly different as opposed to plots located in stable or aggraded reaches (bell shaped curve vs. constant increase in tree ring width). The variance through time due to climatic or stream flow control is masked by variance due to topographical changes. Currently plots located in degraded reaches show a lower ash growth (mean of 0.20 cm/yr vs. 0.34 cm/yr over the last decade) and less frequent ash seedling establishment. We have identified a hydrologic threshold for ash growth response when stand elevation values reach 1.5-2 m above the mean annual flow stage. Our results confirm that, at a many-decade timescale, changes in channel elevation must be considered in addition to other hydrological controls of hardwood species growth and recruitment.