Denser and more diverse: Vegetation change in unmanaged mountain forests of the Calcareous Alps since the mid-1980s

Unmanaged natural forest reserves are important reference ecosystems for
detecting environmental change. We resurveyed permanent vegetation plots
in the strict forest reserves Schrofen and Wettersteinwald (Bavarian Alps) to
detect changes in the vegetation structure and composition of these
mountain forests since the mid-1980s.
The gradient covered by the investigations ranges from mixed mountain
forests of the montane zone to natural spruce forests of the lower
subalpine zone and larch-pine forests of the upper subalpine zone.
Particularly noticeable changes were an increase in the canopy closure of
the stands and the establishment of a shrub layer. In addition, an increase in
frequency was observed for many species of the herb layer (including
woody regeneration), resulting in an increase in alpha diversity. The species
with a positive reaction in the study period (“winners”) were predominantly
competitive and vigorous forest species with comparatively higher nutrient
and lower light requirements. In many cases, the increase in the frequency
of species was accompanied by an expansion of their altitudinal ranges. As
a result, the relevés of the subalpine coniferous forests and the mixed
mountain forests today show an overall higher floristic similarity than in the
1980s, which can be interpreted as a homogenization of the species
composition of the understorey vegetation.
In a Central European context, the combination of canopy closure and
increased understory diversity is unusual. In our opinion, the main driving
forces behind these changes are increasing temperatures due to climate
change and changes in land use practices (cessation of timber harvesting,
reduced browsing by grazing livestock and wild ungulates) and continued
high atmospheric nitrogen inputs.