Where acidic humus embraces calcareous rock: What makes subalpine^spruce forests champions in plant diversity?

Despite their superficial apperance as coniferous “monocultures” subalpine
Picea abies-forests of the Calcareous Alps harbour remarkable fine-scale
plant species richness. In fact, a meta-analysis has shown that their
vascular understorey figures among the richest in Europe. Thus, subalpine
spruce forests on calcareous (Adenostylo glabrae-Piceetum) and mixed
bedrocks (Homogyno-Piceetum) harboured 45-50 or 40-50 vascular plant,
and 15-23 or 20-25 soil dwelling bryophyte species per 12 x 12 m plot. The
large calciphytic species pool, relatively open tree canopies and microsite
diversity of forest floor thickness, morphology and chemistry have been
proposed as explanations.
Based on regional studies in the Bavarian Alps I discuss forest pasture
(Ewald, 2000), forest continuity and high stand age, macronutrient (NPK)
limitation (Reger et al., 2014), succession (Ewald, 1999), deadwood
legacies, forest floor heterogeneity and bimodal frequency distributions of
acidophytes and calciphytes as concurrent drivers of plant species
richness. While these forest types enjoy legal protection as annex I habitat
type 9410 in the European Natura 2000 system, climate change is likely to
promote the meltdown of forest floors and the advance of deciduous,
shade-tolerant European beech to higher elevations. While highly
specialised habitats for threatened bird species like capercaillie (Tetrao
urogallus) and three-toed woodpecker (Picoides tridactylus) decline, lack of
soil development and alpine pastures limit the potential for upward
migration of subalpine spruce ecosystems.
References:
Ewald J (2000) Long-term impact of forest pasture on the understorey of mountain
forests in the Tegernsee Alps (Bavaria). Z. Ökologie und Naturschutz, 9, 161–171.
Ewald J (1999) Relationships between floristic and microsite variability in coniferous
forests of the Bavarian Alps. Phytocoenologia, 29, 327–344.
Reger B, Mellert K-H, Ewald J (2014) Indikatorarten für nährstoffarme Standorte in
den Bergwäldern der Bayerischen Alpen. Tuexenia, 34, 39–51.