Vertical and altitudinal distribution patterns of hydrophilic saxicolous lichens across French streams

Abstract

We collected 252 samples in 53 French streams at 3 different heights (low-flow channel, upper limit of streambed, and intermediate zone) across a 190–2200 m altitudinal range, from which we identified and determined the abundance of freshwater lichens to test hypotheses of assemblage zonation. A total of 149 lichenic taxa, including 42 hydrophilic species together with 6 environmental parameters (relative height to stream water, altitude, general and specific orientation, slope, and substratum) were recorded. Hydrophilic species richness was relatively homogenous across height categories and altitudinal classes. Using Canonical Correspondence Analyses, we showed that lichen species, particularly hydrophilic ones, were strongly discriminated along gradients of both exposure to stream water and altitude. Consequently, we proposed a new denomination of freshwater lichens based on their affinity with exposure to stream water: (i) hyperhydrophilic (submersion >9 mo/yr; 14 sp.), (ii) mesohydrophilic (15 sp.), and (iii) subhydrophilic (submersion <3 mo/yr; 15 sp.). We also introduced a 2D typology of freshwater lichens relying on both crossed environmental parameters and showing continuous shifts in species assemblage along gradients.