Patterns of woody vegetation along the active zone of a near-natural Alpine river

In many European rivers regulation measures have led to the decline of braided channel systems and associated pioneer ecosystems. Knowledge of their natural functioning is necessary to develop appropriate management and restoration strategies. We examine how woody vegetation within the active zone of the near-natural Tagliamento, NE-Italy, is impacted by environmental variables and patch age. Within 65 plots in 13 river segments the basal area of all woody species was determined, and minimal patch age (8.4 +/- 1.6 years) obtained by counting annual rings of the largest trees. Canonical correspondence analyses suggested that the woody vegetation was mainly structured by the longitudinal gradient incorporating air temperature and altitude. Small-scale variables, such as patch age, distance from the nearest channel and elevation above water level, also had significant but smaller effects. All four variables together explained 38% of the variation in species composition. This suggests that additional processes might be important, e.g. specifics of plant dispersal or spatial separation of suitable regeneration sites. The low age of woody vegetation within the active zone of the Tagliamento suggests that hydroengineering measures for restoration of regulated rivers should allow frequent and severe disturbance.