Why mountain summits were favoured as reference units
The term summit in the context of GLORIA includes the summit area between the peak down to the nearest 10-m elevation contour line.
There are several reasons to use summit habitats as reference units in a large-scale comparison of climate change effects.
Summits are well-defined topographic units which can provide comparable
conditions; they comprise habitats in all exposures (north, east, south,
west) within a small area.
On summits selected, shading effects from neighbouring land
features can be avoided and therefore, the climatic conditions are
largely defined by the altitude. Any other topographical feature is
likely to be much affected by diurnal and seasonal variation in
insolation as a result of shading by neighbouring features.
The species composition in summit areas is typical for the
respective elevation because the flora is not enriched by elements from
Topographic diversity at summits can result in a high variety
of niches, causing high species richness. The presence of narrow
transition zones between habitats or vegetation types may enable a rapid
recognition of climate-induced shifts of boundaries.
Conversely, summits may function as traps for upward-migrating
cryophilic species with weak competitive abilities. This is particularly
critical on isolated mountains with a high percentage of endemic
species occurring only at the uppermost elevation levels.
Summit areas are not prone to severe disturbance such as debris falls or
avalanches. This enhances their value for long-term observations.
Summits are prominent landmarks which can be easily relocated.
For these reasons, mountain summits are considered as the most
appropriate sites for comparing ecosystems along climatic gradients. For
the selection of monitoring sites, however, certain criteria have to be
considered to avoid possible disadvantages (for details see the GLORIA