Supporting conservation planning in a national biodiversity hotspot - Projecting species composition across a groundwater level gradient using a demographic forest model

The Leipzig floodplain forest is a biodiversity hotspot of national significance. However, it is an urban forest heavily impacted by human activities, including the alteration of the hydrological regime preventing floods and leading to a lower groundwater level. In parts of the Leipzig Floodplain Forest, the restoration of a near-natural hydrological regime with regular floods and a raise of the groundwater level is considered. However, it is unclear whether raising the groundwater level in particular would ensure the long-term conservation of typical floodplain tree species such as European ash (Fraxinus excelsior) and pedunculate oak (Quercus robur), which are considered key species for biodiversity conservation. To investigate this question, we quantified the relationships between groundwater table distance and the growth, mortality, and recruitment rates for eight common tree species in the Leipzig Floodplain Forest using forest inventory data from 60 plots and a spatial groundwater model. Based on these relationships, we simulated the long-term dynamics of species composition with and without a raise of the groundwater table using the Perfect Plasticity Approximation (PPA) forest model. Under current groundwater conditions, the model projected a substantial decline of the typical floodplain species ash and oak over 100 years. Field maple (A. campestre) and hornbeam (Carpinus betulus) benefited from this decline, as did the less flood-tolerant Norway maple (A. platanoides) on dry sites. When a raise of the groundwater level was simulated, ash and oak continued to decline. However, A. platanoides, which is not a typical floodplain forest species, was projected to be inhibited by a raise of the groundwater level. These results suggest that a raise of the groundwater table alone does not lead to the conservation of ash and oak, and hence the rich biodiversity associated with them. The study illustrates how ecological modeling can support the evaluation of biodiversity conservation strategies and provide the scientific basis for the successful transformation of this unique ecosystem towards a self-sustained biodiversity-rich urban forest.