Testing for assembly rules along disturbance gradients in a riparian broadleaved forest.
Riparian habitats often represent highly disturbed sites with relaxed competitive plant integrations and increased resource supply, which frequently lead to promoted colonization of alien and invasive plant species. Unfortunately, there is a knowledge gap about the assembling mechanisms that govern vegetation structure and composition along disturbance gradients. This study tried to answer the question if riparian vegetation assembling is governed by abiotic or biotic forces, as well as whether the strength and nature of assembling mechanisms change along local disturbance gradients. We used species-based approach in which species data of sampled vegetation were compared with a null model assuming random organization. The resulting variance ratios were regressed on the local flooding and logging disturbance gradients. We found that the studied riparian vegetation is dominated by abiotic assembling rules. Their nature did not change along the two disturbance gradients, but their strength did. It seems that the overwhelming factor that rules the riparian vegetation structure and composition in the area is the periodical flooding from the nearby river. It is most prominent in most frequently flooded habitats, then it slowly decays towards the more distant uplands. Its influence is so powerful that it can overshadow other potential assembling factors, and can promote the establishment of native and exotic (including invasive) species. Perhaps, the biotic filter never plays a significant role in the assembling of this highly disturbed and dynamic vegetation. In order to stop further colonization and to preserve these valuable sites from degradation, additional human-induced disturbances should be completely restricted, and natural ones be kept in its historical range. These findings should not be overlooked in the prospective management activities anymore, but should be incorporated in future action plans. Only then, we can hope that these ecosystems could be conserved at their current state for the upcoming generations.