Floodplain forests are species-rich, endangered habitats, which provide numerous ecosystem services such as the retention of flooding water and pollutants. Healthy floodplain forests function as carbon sinks and can therefore contribute to counteracting climate change. Box 1: My study side, Biosphärenreservat Niedersächsische Elbtalaue The biosphere reserve “Niedersächsische Elbtalaue” (Lower Saxonian floodplain of the River Elbe) lies 100 km southeast of Hamburg, Germany, and is part of the 3430 km² UNESCO biosphere reserve “Flusslandschaft Elbe” (Elbe stream landscape). The unique landscape stands out with a high biodiversity of 250 bird and 1300 plant species. The objective of my project was to test the capability of trees of temperate floodplain forests to transfer water between soil layers of different water content through their roots. I aimed at assessing the biological importance of the process (called hydraulic redistribution) for the surrounding vegetation, especially during dry periods. The project was conducted at Hamburg University, Biozentrum Klein Flottbek, and the research area lay within the UNESCO biosphere reserve “Flusslandschaft Elbe” (Elbe stream landscape). The results showed that hydraulic redistribution indeed does occur, but to a minor extent. The reason for this was found to be the surprising occurrence of a competing process in the trees, the transpirational water loss through leaves during the night. Floodplain forests are thus even more endangered by future drought events than hitherto anticipated. Tree species for restoration must therefore be chosen carefully, and the findings of this project will aid conservation efforts for temperate floodplain forests. Trees as Water Dispensers for their Drought-stressed Plant Neighbours Due to climate change, summer droughts are expected to increase in frequency and severity in Central Europe – a prospect that particularly threatens the fragile ecosystems along streams and rivers, the floodplain forests (Figure 1 & 2, Box 1). Figure 2: Softwood forest with willow close to the riverbank (left) and hardwood forest with oak (right) farther inland from the Elbe River. Photo: Dennis Schulze. Box 2: Hydraulic redistribution: a challenge to the soil-plant-atmosphere continuum Water moves from the soil through plant roots and stems to finally be transpired out of the leaves into the atmosphere. This concept has been challenged by the discovery of hydraulic redistribution, when water enters and exits roots passively without being released to the atmosphere through the leaves. The functionality of a floodplain forest is depending on the vitality of its vegetation, especially the juvenile trees, which secure the continuity of these ecosystems. Juvenile trees, however, are shallow-rooted with limited access to deeper water sources, and restoration efforts have proven to be challenging (Küßner 2003). Prolonged drought periods are one likely reason for the high juvenile tree mortality. The aim of my project, funded by the Carlsberg Foundation, was to study plant-water-relations of juvenile trees and to investigate, if a closer proximity to an adult tree can provide juveniles with more water. When the soil surface is dry, only the groundwater is accessible. Deep roots from adult trees are capable of bridging this moisture gradient and passively transport the groundwater to the dry soil surface. Thereby, the water becomes accessible also to the shallow-rooted vegetation. This process is called “hydraulic redistribution” (Burgess et al. 1998, Box 2, Figure 3) and although it is very likely to be of importance in temperate floodplain forests, the occurrence of hydraulic redistribution has so far mainly been documented in arid ecosystems. Hydraulic redistribution is occurring over a moisture gradient resulting in the transport of water through roots from deeper wet soil layers to drier surface areas. Shallow-rooted vegetation may access the redistributed water and benefit from it especially during drought periods. Never Far from the Tree – Some Juvenile Trees Benefit from Growing Close to their Adults Figure 4: Experiment to study hydraulic redistribution in ash and elm tree seedlings. Roots can access water only from the lower compartment; the upper pot contains dry soil. Here, photosynthesis is measured. Photo: Franziska Eller. The carbon isotopic signature is a useful indirect measure of drought-stress. During my project, which was funded by the Carlsberg Foundation, I analysed the carbon isotopic signature of tree seedlings growing in different distances from an adult tree to estimate their drought stress. I found that the drought response of 25% of all investigated seedling transects was affected by their distance to an adult tree. Since hydraulic redistribution is more likely to appear with closer proximity to a deep-rooting plant, it is also likely that at least a quarter of all juvenile trees analysed benefited from growing closer to an adult tree. To show the occurrence of hydraulic redistribution in nature, I measured the sapflow in three adult oak trees in a floodplain forest at the Elbe River in Lower Saxonia (Germany) throughout an entire growing season. Although there was little precipitation during summer 2015 in the study area, the average temperature was rather low, and hence soil moisture was in a range which merely facilitated low rates of hydraulic redistribution. The trees will be monitored for more growing seasons by Hamburg University. Elm and ash trees are typical species of mid-European floodplain forests. To test whether these two species are capable of hydraulic redistribution I set up an experiment where I manipulated water access to the tree seedlings’ roots (Fig. 4). Box 3: Night-time transpiration: futile water loss? At daytime, plants take up carbon dioxide from the atmosphere, while simultaneously loosing water vapour through transpiration. Nevertheless has night-time transpiration been discovered to be a rather common phenomenon among plants, and scientists are still wondering which purpose it may serve. Only the lower part of the roots had access to water, while the upper part was separated by a water-impermeable layer. I measured soil moisture in the upper, dry soil to see if it became wetter. I discovered that the trees during some nights, when hydraulic redistribution otherwise would occur, were transpiring instead. Transpiration is the release of water as water vapour through the leaves, following water uptake from the soil by roots and the subsequent transport through the whole plant. Transpiration normally occurs at daytime (Box 3, Figure 5) and the transpirational water stream through the plant suppresses the stream of water through the roots that appears during hydraulic redistribution. Thus, hydraulic redistribution occurs in temperate floodplain trees, but it is competing with night-time transpiration. Figure 5: Since carbon dioxide is useless for the plant during the night, when no light can provide the energy needed for photosynthesis, the simultaneous transpiration at night would result in a presumably futile loss of water through transpiration. Enhanced nutrient uptake may be a possible benefit. Certain Trees Do Not Sleep at Night The phenomenon of night-time transpiration has only been discovered recently, and its purpose remains to be fully explained (Caird et al 2007). A possible benefit of water uptake during the night may be the simultaneous uptake of nutrients from the soil. In an experimental study, funded by the Carlsberg Foundation, I found that ash trees indeed seemed to increase their nutrient uptake thereby, whereas elm trees showed no nutritional effects, but suffered from drought due to night-time water loss (Figure 6). Figure 6: Experiment to study night-time transpiration of ash and elm tree seedlings. Photo: Franziska Eller. Our Floodplain Forests Are more Endangered as Hitherto Assumed My finding of night-time transpiration in key-species of floodplain forests has several serious implications. First, hydraulic redistribution will be suppressed while trees are transpiring during the night and therefore, little water will be provided to the neighbouring, shallow-rooted vegetation. Second, not all tree species seem to be capable of regulating their night-time water loss and will therefore suffer more from drought-events than other species. Third, increased eutrophication can lead to even higher night-time water-loss in some tree species and thereby exacerbate their drought-stress responses. The results of my project are shared with the authorities of the biosphere reserve, who can implement the findings in future restoration efforts. Moreover, my results can be extrapolated to other freshwater ecosystems, especially biospheres of the Northern hemisphere where drought impacts already are getting more severe. Franziska Eller about the Support from the Carlsberg Foundation The Carlsberg Foundation’s Internationalisation Fellowship as well as the Postdoctoral Fellowship, which I received from the Carlsberg Foundation, have contributed not only to the significant scientific results I obtained during my project at Hamburg University, I also gained the necessary and valuable experience in an excellent foreign research environment which has allowed me to receive a postdoctoral grant to return and establish my research career in Denmark. I will nevertheless continue cooperating with my colleagues from Hamburg University and look forward to our future bilateral collaboration. As scientist, I have a social responsibility to use my knowledge to help solving some of the challenges that society faces in the 21st century. Clean water, climate action, life on land and in water, and human well-being are amongst the challenges that are associated with this project: The findings of my project will aid in restoration and conservation efforts of floodplain forests, which are hot-spots of high biodiversity, natural wastewater-treatment plants, buffers of climate change and last but not least, recreative areas for enjoyable outdoor activities. To share my nature-enthusiasm, I frequently visit Danish schools to inform the pupils about the value of healthy floodplain forests. References Burgess SSO, Adams MA, Turner NC, Ong CK (1998) The redistribution of soil water by tree root systems. Oecologia 115: 306-311. Küßner R (2003) Mortality patterns of Quercus, Tilia, and Fraxinus germinants in a floodplain forest on the river Elbe, Germany. Forest Ecology and Management 173: 37-48. Caird MA, Richards JH, Donovan LA (2007) Nighttime stomatal conductance and transpiration in C3 and C4 Plants. Plant Physiology 143: 4-10.