Johannes Kollmann

Channel migration, furcation, and vegetation succession are characteristic processes of braided gravel rivers in Alpine regions. These are associated with a frequent turnover of large parts of the active floodplain. However, more than one century of river regulation has reduced or destroyed most of these ecosystem dynamics. More recently, there have been attempts to restore at least some sections of degraded rivers, while there is little monitoring of the biogeomorphological dynamics of such rivers. Thus, we did a four‐year analysis of four representative sections of the upper River Isar in Southern Germany differing in their degree of naturalness. Dynamics of channels, gravel bars, and vegetation were recorded by drone images, and braiding and gravel indices were calculated. When comparing the near‐natural, semi‐natural, degraded or restored sections, there was a gradient of decreasing channel migration, gravel bank expansion, and bank erosion due to a reduced turnover frequency. Biogeomorphological variation among years correlated with log peak discharge within the four sections. In addition, the cover and height growth of vegetation increased with river degradation, and channel migration was positively related to the braiding index. The total turnover of the active river corridor and of the vegetation were positively correlated with log peak discharge within the four sections. The floodplain dynamics of the restored section were improved compared to the degraded section but did not reach a near‐natural state. Thus, Alpine river sections with contrasting degrees of naturalness differ in terms of habitat turnover and vegetation succession, and these characteristics can only partially be restored by local measures.

Current challenges of functional responses in plant communities to climate change call for multi‐factorial experiments. Moreover, studies on climate change should focus on below‐ground responses since absorptive roots largely control soil C allocation and resource acquisition. Thus, we aimed to understand biomass allocation and traits of absorptive roots in young mesocosm grasslands subjected to simultaneous manipulation of three components of climate change. We tested grassland biomass and root traits under climate change while manipulating functional composition. Using 64 mesocosms with designed grasslands within four chambers of a controlled‐environment facility (‘ecotron’), we simulated two contrasting IPCC climate change scenarios for elevated [CO 2 ] and temperature (‘eCO 2 ’ and ‘eT’). We applied normal vs. reduced precipitation of early summer in Central Europe. We also tested the effect of functional composition by varying the proportion of grasses and forbs in the communities. Specifically, we quantified above‐ and below‐ground biomass, root diameter (RD), root tissue density (RTD), specific root length (SRL), and root length density (RLD). Functional composition played a significant role in biomass allocation of the grasslands, with grass‐dominated communities producing more below‐ground biomass than forb‐dominated ones, and the opposite pattern registered above‐ground. Below‐ground biomass did not respond to climate change factors, whereas root trait values responded significantly during early establishment of the grasslands. A higher RD indicated a more conservative strategy under reduced precipitation, while eT and eCO 2 led to higher RTD. We detected interactive effects between climate change and functional composition on root traits. Moreover, root biomass primarily occupied the upper soil layer, while a warm and CO 2 ‐rich environment promoted root allocation to the lower soil layer. Grass‐dominated communities quickly colonized all available soil volume, while forb‐dominated ones accumulated more root biomass in the upper soil layer. In the mesocosm grasslands, root trait variation rather than root biomass reflected below‐ground adjustments to climate change. Furthermore, functional composition and the associated trait diversity modulated biomass allocation. Thus, establishing plant communities that are more resilient to climate change must consider the functional and taxonomic composition of the seed mixtures designed to restore urban grasslands. Read the free Plain Language Summary for this article on the Journal blog.