Human impact on the vegetation of limestone cliffs in the northern Swiss Jura mountains

Cliffs provide unique habitats for many specialised organisms, including
chamaephytes and slowly growing trees. Drought, high temperature
amplitude, scarcity of nutrients and high insolation are general characteristics
of exposed limestone cliff faces. The vegetation of limestone cliffs in the
Swiss Jura mountains consists of plants of arctic-alpine, continental and
Mediterranean origin. Several populations exhibit relicts from post- or
interglacial warm or cold climatic periods. Grazing goats and timber
harvesting influenced the forests surrounding the limestone cliffs in northern
Switzerland for many centuries. During the twentieth century, however, these
traditional forms of forest use were abandoned.
In recent years, rock climbing enjoys increasing popularity in mountain
areas at low elevation, where this sport can be performed during the whole
year. The limestone cliffs of the Jura mountains provide unique opportunities
for sport climbers. As a consequence, more than 2000 climbing routes with
fixed protection bolts have been installed on 48 rock cliffs in the region of
Basel, Switzerland.
Overgrowing forest, due to the abandonment of forestry and damages
due to recreational activities including rock climbing reduce the quality and
size of the cliff habitats. In my dissertation, I examined quantitative, spatial
and temporal patterns of human impacts on the cliff flora and on the genetic
population structure of two plant species on isolated cliffs in the Jura
mountains of northern Switzerland.
The assessment of plant cover and species density at various distances
from frequently used climbing routes in the region showed that plant cover
was significantly reduced at the base of climbing routes. Furthermore, species
density (number of species per m2) at the cliff base as well as plant cover and
species density on the cliff face tended to increase with distance from the
route.
The comparison of the vegetation at the cliff base and on the cliff face of
five frequently climbed cliffs with that of seven unclimbed cliffs indicated that
rock climbing significantly altered the plant composition. Specialised rock
species occurred less frequently on climbed cliffs than on unclimbed cliffs.
At the Gerstelflue, a popular recreational climbing site with rock climbing
activities since more than 40 years, plant cover and species density (number
of species per m2) were reduced in climbed areas. Rock climbing also
reduced the density (number of individuals per m2) of forbs and shrubs,
whereas the density of ferns tended to increase in climbed areas. In addition,
rock climbing caused a significant shift in plant species composition and
altered the proportions of different plant life forms.
Species diversity and cover of lichens, and possible associations
between lichens and lichen-feeding land snails were assessed in climbed and
unclimbed areas of 10 isolated cliffs. Total lichen species density was not
correlated with the complexity of the rock surface, climbing frequency and age
of the climbing route. The species density of epilithic lichens was lower along
climbing routes than in unclimbed areas, whereas no difference in species
density of endolithic lichens was found between climbed and unclimbed
areas. Furthermore, climbed and unclimbed areas did not differ in total lichen
cover. The dissimilarity of the lichen communities between climbed and
unclimbed areas increased with increasing climbing intensity on the focal
route in climbed areas, but not with the age of the climbing route. Within cliffs,
plots along climbing routes harboured fewer snail species and individuals than
plots in unclimbed areas.
The effects of forestry practices on the species richness and abundance
of vascular plants on the face, at the base and on the talus have been
investigated by comparing two different forestry practices (clear-cutting and
shelter tree cutting) with forest reserve (i.e. no management in the past 80
years)) on three cliffs. Plant species density and vegetation cover was higher
in the shelter-cut areas than in the forest reserves on the talus as well as at
the cliff base. Clear-cut areas showed a higher vegetation cover than forest
reserves on the talus. Shelter-cut areas harboured a larger proportion of
plants with high light demands and plant indicator species showed a higher
mean light score than in clear-cut areas and forest reserves.
The analysis of time-series of air photographs taken between 1951 and
2000 at six cliffs revealed an increase in tree cover from 60% to 85% between
1951 and 1964 after which the increase levelled off. The increase in tree
cover showed a distinct spatial pattern. It was significant in the talus and on
the cliff face, but not on the plateau (at the top of the cliffs).
Possible effects of isolation and the presumed colonisation history of
cliffs as well as of anthropogenic activities on the genetic population structure
of two plant species with different life-histories were assessed using RAPDpolymorphisms.
Fourteen populations of Draba aizoides L. and 12 populations
of Melica ciliata L. living on isolated limestone cliffs were examined. Analysis
of molecular variance revealed a high among-population variation of each
27% in the gene pools of both species. A clear isolation-by-distance pattern
and a separation of populations from the Jura mountains and the Alps were
found in D. aizoides. This provides evidence for glacial relict endemism in this
species, resulting from nunatak survival in the Jura mountains. In M. ciliata,
UPGMA-analysis showed clusters of plant populations growing on cliffs with
castles with shared historical incidents, indicating zoochorical dispersal
related to human settlements.
The various studies emphasise the uniqueness and vulnerability of the
limestone cliff ecosystem of northern Switzerland. Protection measures in
several fields of activity are needed to preserve the unique relict vascular
plant, lichen and animal communities. Adequate management actions should
be developed and implemented. Actions should particularly be directed to
cliffs with numerous arctic-alpine plant species to protect them from mechanic
disturbances by sport climbing and hiking. The prohibition of sport climbing on
cliffs with a high number of specialised plant or animal species and the
establishment of climbing-free protection zones in popular areas are the most
effective and adequate measures in this context. However, any management
plan should include a comprehensive information campaign to show the
potential impact of intensive sport climbing on the specialised flora and fauna
and to increase the compliance of these measures by the climbers. Forestry
practices that keep the supply of light on a high level at the lower parts of the
cliffs are required to preserve the relict plant species. Selective thinning on the
talus results in relatively large plots with good light conditions and therefore
promotes the rare, relict plant species with high light demands. Self
evidentely, forestry actions and climbing prohibitions should be executed in
coordination. Another measure to manage the lower parts of cliffs could be to
use them as temporal pastures for goats. Finally, the preservation of
mediaeval sites also connotes the conservation of plant species introduced
into the area during the time of human activities.