Whereas competition between species has traditionally been considered the driving process for selection and diversity, it is now postulated that in a world experiencing climate change and frequent human impacts in ecosystems, disturbance may become the driving process. Disturbance of a habitat may be abiotic (flooding, storms, fire) or biotic (grazing, insect outbreak). Plant resistance and resilience to disturbance can be reflected in a certain number of traits, including resistance to nutrient and water stress, response to temperature and light. In the case of a mechanical, abiotic disturbance e.g. storms, landslides and avalanches, plant biomechanical properties may determine whether or not a species will be adapted to the disturbance, depending on its frequency and intensity.

Some pioneer clonal species e.g. bamboo, have evolved poor root anchorage even though they are endemic to mountain slopes where frequent substrate disturbance occurs. By not fixing the soil, shallow landslides were found to be common, but pioneer plants were able to resprout from root fragments buried in the soil entrained downslope. Frequently occurring landslides therefore allowed the opening up of the landscape to pioneer heliophile species, allowing such species to remain dominant. Bearing in mind this strategy we are currently examining mechanical properties of three coexisting species e.g. Alnus viridis, Fagus sylvatica and Betula alba growing along disturbance (small-scale avalanche) gradients in the French Alps. Of the proposed species, only A. viridis grows preferentially in the centre of avalanche tracks and scree slopes. Aerial architecture changes from a bushy shrub to a medium sized tree, depending on its position along the disturbance gradient. Where stems had been bent permanently by the weight of the snow/avalanche, roots were observed growing from stems where they touched the soil. The Modulus of Elasticity (E) was found to be lower at the base of the stem in shrubby trees, compared to further up the stem, enabling stems to bend under the weight of the snow, and thus resist uprooting. Even if the stem is damaged but the root system remains intact, stem resprouting will enable an individual to stay alive. Local foresters have suggested that as snows melt in the springtime, stresses which have built up in the buried branches of A. viridis are released, causing the branches to return to the vertical with a whip-like movement, which in turn actually triggers avalanches. If this is true, then like bamboo, A. viridis can remain dominant in frequently formed gaps in the landscape, and it is to its advantage to display particular biomechanical traits allowing it to occupy this 'disturbance niche.' A biomechanical and architectural analysis of A. viridis with F. sylvatica and B. alba will allow us to determine why the latter two species do not grow in frequently perturbed avalanche tracks.