This paper shares an early-career perspective on potential themes for the upcoming International Association of Hydrological Sciences (IAHS) scientific decade (SD). This opinion paper synthesizes six discussion sessions in western Europe identifying three themes that all offer a different perspective on the hydrological threats the world faces and could serve to direct the broader hydrological community: “Tipping points and thresholds in hydrology”, “Intensification of the water cycle”, and “Water services under pressure”. Additionally, four trends were distinguished concerning the way in which hydrological research is conducted: big data, bridging science and practice, open science, and inter- and multidisciplinarity. These themes and trends will provide valuable input for future discussions on the theme for the next IAHS SD. We encourage other Early-Career Scientists to voice their opinion by organizing their own discussion sessions and commenting on this paper to make this initiative grow from a regional initiative to a global movement.

Purpose of Review: Agroforestry and urban forestry have evolved mainly as separate disciplines, although they share a long history of tree cultivation in man-made environments. Here, we review their common threads, exploring how trees in both systems interact with and shape their environments. We examine common themes and methodologies – ranging from tree growth dynamics to environmental stressors, ecosystem services provision, and questions of governance – and identify opportunities for synergies between these fields. Recent Findings: We emphasize the potential of agroforestry and urban forestry for enhancing multifunctional landscapes. Geographical divides in research are evident, with agroforestry predominantly studied in the Global South and urban forestry receiving more attention in the Global North. However, significant research gaps provide avenues for collaboration, for instance, addressing challenges in capturing the monetary and socio-cultural value of ecosystem services and environmental justice considerations. Summary: In light of the growing need for integrated approaches in addressing contemporary challenges, from climate change mitigation and adaptation to community well-being, our review explores what these research fields can learn from each other and provides recommendations for fostering greater interdisciplinary dialogue and new avenues for collaborations in a meaningful and synergistic manner, aiming to advance policy, research, and practice in agroforestry and urban forestry.

The rapid expansion of impermeable surfaces in cities has a major impact on urban hydrology by reducing rainwater infiltration and increasing runoff rates and peaks. The use of urban trees as stormwater management tools is gaining recognition for their potential to mitigate flood risk and provide additional ecosystem services. We conducted an in‐situ field experiment to measure throughfall on Norway maple (Acer platanoides) and small‐leaved lime (Tilia cordata) to assess the interception capacity of solitary urban trees under different degrees of surface sealing in the city of Freiburg, Germany. We examined the relationships between rainfall characteristics, tree morphological traits, and the interception behaviour, analysing eight trees per species over 76 recorded rainfall events from April to September 2021. Average interception values were higher for small‐leaved lime trees (70.3% ± 6.6%) than for Norway maple (54.8% ± 10.3%) surpassing those in typical forested environments. The average interception loss of all recorded events was 2.6 ± 0.6 mm for Norway maple and 3.7 ± 0.3 mm for small‐leaved lime. For both tree species, significant linear correlations were found between the relative interception and factors such as rainfall depths, the leaf area index (LAI), and the plant area index (PAI) (adj. R² >0.45). Unlike Norway maple, small‐leaved lime demonstrated significant relationships between several tree morphological parameters and interception (adj. R² >0.43). The LAI of both species significantly decreased with the increasing degree of surface sealing, which in turn affected interception rates. Our results enhance the understanding of the interception process by solitary trees in urban contexts and enables the parameterization of interception rates using measurable properties. To develop a comprehensive database for modelling parameters and aid urban planners in stormwater management, further field experiments involving various tree species are essential.

Urban trees provide essential ecosystem services, notably air cooling through transpiration, which helps mitigate the urban heat island effect and enhances cities' climate resilience. However, the complex spatial variability within urban areas and extreme weather events like droughts can disrupt trees' ecohydrological dynamics. In a study conducted in Freiburg, Germany, we investigated transpiration processes in Norway maple (Acer platanoides) and small‐leaved lime (Tilia cordata) across diverse urban locations, including parks, parking lots, grass verges and tree pits. We assessed the effects of four distinct drought periods on transpiration and compared differences between tree species and growing sites. Small‐leaved lime exhibited a 5% greater reduction in transpiration during drought periods compared to Norway maple, which experienced a 34% decline in transpiration during peak sap flow compared to nondrought periods. Tree pits with 90% surface sealing induced the most significant drought‐induced transpiration reduction for small‐leaved lime (58%), with both species displaying the lowest transpiration to potential evapotranspiration ratio in these locations. Significant differences were observed in the diurnal sap velocity patterns for both species. We highlighted the site‐specific impact of surface sealing on transpiration during droughts, as well as the significant relationship between soil water deficit and relative transpiration rates. This study provides crucial insights into common urban tree species' responses to drought‐induced transpiration across varied urban settings, emphasising the role of surface sealing. Continuous monitoring of diverse urban tree species is essential for building extensive databases and enhancing our understanding of tree water relations in diverse urban landscapes.

The leaf area index (LAI) is a key parameter in the context of monitoring the development of tree crowns and plants in general. As parameters such as carbon assimilation, environmental stress on carbon, and the water fluxes within tree canopies are correlated to the leaves surface, this parameter is essential for understanding and modeling ecological processes. However, its continuous monitoring using manual state-of-the-art measurement instruments is still challenging. To address this challenge, we present an innovative sensor concept to obtain the LAI based on the cheap and easy to integrate multi-channel spectral sensor AS7341. Additionally, we present a method for processing and filtering the gathered data, which enables very high accuracy measurements with an nRMSE of only 0.098, compared to the manually-operated state-of-the-art instrument LAI-2200C (LiCor). The sensor that is embedded on a sensor node has been tested in long-term experiments, proving its suitability for continuous deployment over an entire season. It permits the estimation of both the plant area index (PAI) and leaf area index (LAI) and provides the first wireless system that obtains the LAI solely powered by solar cells. Its energy autonomy and wireless connectivity make it suitable for a massive deployment over large areas and at different levels of the tree crown. It may be upgraded to allow the parallel measurement of photosynthetic active radiation (PAR) and light quality, relevant parameters for monitoring processes within tree canopies.