Fróðskaparrit - 01.01.2007, Side 121

GRÓÐURIN Á FLAGTEKJUM í FØROYUM OG Á GRASLENDINUM UTTANUM
- EIN KANNING ÚR SANDOYNNI
119
Detrended correspondence analysis
(DCA; Hill and Gauch, 1980) is used to
detennine the possible ecological factors
underlying the distribution of the plant
taxa present on the turf roofs. The roof data
are also combined with that of the ground
vegetation in order to determine the rela-
tionships between the types of plant com-
munity represented by these data sets. Cor-
relations were then performed between the
DCA sample scores and the variables as-
pect, grazing and slope in order to deter-
mine their influence upon the vegetation.
Results
Diversity
Species richness comparison shows a dif-
ference between the plant diversity of the
turf-roof and ground vegetation (Fig. 4).
The average number of species per quadrat
is 5.6 (n=76; SD 2.4) for the roofs and 17.6
(n=39; SD 7.17) for the ground vegetation.
A t-test confirms that this difference is very
significant (at the 0.001 level; t=9.2;
df=42.).
An additional z-test (that allows for dif-
ferences in the size of the two sub-sets) of
the grazed and non-grazed roof vegetation
was undertaken to determine whether the
number of quadrats (11 for grazed and 65
for non-grazed) might be responsible for a
difference in diversity. The results of this
test confirmed that no significant differ-
ence could be found between these two
sets of roof samples (at the 0.05 level; z=-
1.9; df=ll).
Ordination
The sample and taxa ordinations produced
by DCA of the roof-turf and the infield and
outfield ground vegetation are presented in
Figures 5 and 6. In Figure 6, polygons are
drawn around samples from the same habi-
tat, i.e. the infield (A), outfield (C) and
roof (B) turfs. For the sake of clarity, only
taxa whose abundance is greater than 10%
in at least one quadrat are labelled.
The DCA analysis of the turf-roof vege-
tation (Fig. 5) reveals a gradient of inferred
condition (which may be a function inter
alia of factors such as maintenance, aspect
Figure 4. Comparison of
spccies richness between
infield/outfield vegetation
and turf roof vegetation on
the island of Sandoy. Eiror
bars indicate two standard
deviations.