Multi-stemming and mechanical traits ensure persistence of subalpine woody plants exposed to a disturbance gradient

Question: Many woody plants persist in a temporal ‘persistence’ niche, through resprouting after disturbance events. Will multi-stemming (as a consequence of resprouting or life form) and biomechanical traits enable certain species to survive disturbance and occupy a persistence niche? Location A 1200-m long avalanche corridor in the French Alps, with disturbance events ca. every 2 yr. Methods: We measured tree/shrub size and multi-stemming in transects along and around the avalanche corridor. The mechanical traits, wood density, modulus of elasticity (Ed), bending strength (σ) and stiffness (EI), along stems of several subalpine tree/shrub species were measured to test for differences in flexibility and strength. Results: The multi-stemmed pioneer shrubs Alnus viridis, Salix appendiculata and to a lesser extent, Corylus avellana, were more abundant where disturbance was most severe. Multi-stemming broadleaf species, in particular Fagus sylvatica, had more tapered, shorter and more numerous stems in severely disturbed zones. Single-stemmed conifers were usually found furthest from the avalanche track. A. viridis, codominant at the centre of the track, had the lowest Ed, σ and EI. In A. viridis, Alnus incana, F. sylvatica and Betula pendula, Ed was lower at the stem base and increased with height up the stem. Alnus viridis and C. avellana stems were most flexible (low EI), whereas single-stem tree forms, Picea abies and B. pendula, were stiffest. Corylus avellana had the highest σ and lowest wood mechanical construction cost (ρ) per unit function (Ed), whereas P. abies had highest construction cost. Conclusions: High flexibility of A. viridis, and to a lesser extent C. avellana and the multi-stemmed form of F. sylvatica, allows individuals to bend during snow loading, before an avalanche; therefore, stem damage and uprooting during an avalanche are minimized. Increased stem basal flexibility in these species will also facilitate bending during snow loading. More rigid distal sections will confer extra mechanical advantage to stems that need to grow quickly upwards, dominate and maintain their position in the canopy during the growing season