Geoscience Reference
In-Depth Information
Figure 12.3
Indirect Ordination Biplot (DCA) of species composition of stream segments coded by their TWINSPAN
group. Each group is represented by a polygon that delimits its area of distribution in the diagram, with subsequent
(and non-influential) projection of environmental co-variables (represented by arrows). Length of gradient- axis 1:
7.183; axis 2: 6.171; eigenvalues - axis 1: 0.804; axis 2: 0.536; species; accumulated variance - axis 1: 5.2; axis 2: 8.6;
total inertia: 15.5. For abbreviations representing parameters, see Table 12.1).
predominant. High bryophyte cover is therefore
indicative of turbulent, clean water and stable
substrata usually associated with cool, steep
mountain streams with natural tree shade along
the margins. This confirms the preference of
bryophytes as a general group for clean, shallow,
undisturbed fluvial micro-habitats (Glime and Vitt,
1984).
Previous studies have shown that bryophyte
species richness tends to be higher at or just above
the air-water interface compared with deeper or
more emergent habitats (Muotka and Virtanen,
1995) or seasonally intermittent watercourses
(Fritz
et al.,
2009). This has been explained by
four main factors: (i) the presence of facultative
and semi-aquatic species; (ii) the existence of
few obligate aquatic bryophyte species capable of
withstanding deep water (Vitt and Glime, 1984);
(iii) the need to obtain carbon from CO
2
in the
air or dissolved in turbulent waters (Bain and
Proctor, 1980)
;
and (iv) competitive exclusion by
vascular macrophytes growing in deeper reaches
characterized by deposits of fine sediment (Madsen
et al.,
2001). In our study area, shallow conditions
and large seasonal variations are part of the
natural flow regime, while the predominance of
bedrock favours the presence of several rheophilic
and semi-aquatic mosses (e.g.
Platyhypnidium
spp.,
Racomitrium
spp.,
Marsupella
spp.).
A greater variety of flow and fluvial
geomorphological features tends to increase
species richness and diversity. This is probably
because the availability of micro-habitats is directly
related to the heterogeneity of river-bed substrata
and also new opportunities for colonist species
following erosion in the late succession phases of
