Dynamic processes require appropriate methods to capture them: Why in-situ water stable isotope monitoring needs to become a standard method in forest hydrological research

As a consequence of global change, forests worldwide are undergoing a restructuring process. The expectations for forests of the future are ambitious: Providing resilient forest ecosystems that capture large amounts of carbon but also provide stable groundwater recharge rates. To balance the interests of both forestry and water management authorities, scientists and practitioners need to be able to investigate and predict which forest types and combinations of tree species are most likely to fulfill these needs.

Methods based on the analysis water stable isotopes have been used extensively for studying water uptake depths of vegetation, groundwater recharge, transit times, and water sources in general. Despite being arguably the superior tool when not only amounts but also knowledge of the sources of an (eco-)hydrological flux are needed, the highly dynamic nature of water transport processes within the soil-plant-atmosphere continuum (SPAC) could hardly be captured in the past. With the advent of  laser spectroscopy in the last decade, we are now able to measure water stable isotopes continuously and in all compartments of the SPAC.

In this keynote, we present and review the most recent advances (2016-now) of combined soil and plant in-situ water isotope measurements carried out in different ecosystems worldwide. We then critically discuss the gain in process-understanding of in-situ monitoring approaches and demonstrate how in-situ methods could be integrated with traditional and novel methods to advance forest hydrology.

In-situ and semi-in-situ (i.e., sampling of water vapor) water stable isotope methods have been greatly improved within the last five years. Initial disadvantages (e.g., comparability to traditional methods, complicated & laborious setup & maintenance, expensive) have been carefully addressed, and improvements have been implemented. Recent research has proven that i.) highly dynamic and heterogenous processes (e.g., stem flow, groundwater recharge through preferential pathways, change of uptake depths in response to rainfall/drought, disentangling water use of different tree species in mixed forests) can be captured exceptionally well using in-situ isotope methods; ii.) water-vapor equilibration methods represent the isotope composition of mobile water better compared to destructive methods, and iii.) using continuous water isotope data reduces parameter uncertainties in SPAC modeling.

In summary, we state that the benefits of using in-situ or semi in-situ techniques outweigh the disadvantages by far and strongly encourage the water stable isotope community to integrate them regularly into studies of dynamic soil-plant-atmosphere feedbacks.