Stemflow, despite being a small proportion of the gross rainfall, is an important and understudied flux of water in forested areas. Recent studies have highlighted its complexity and relative importance for the understanding of soil and groundwater recharge. Stemflow dynamics offer an insight into the rain water that is stored and released from the stems of trees to the soil. Different attempts have been made to understand the variability of stemflow under different types of vegetation, but rather few have focused on the combined influence of both biotic and abiotic factors that affect the inter and intra-storm stemflow variability, and none known in Mediterranean climates. This study presents stemflow data collected at high temporal resolution for two species with contrasting canopy and bark structures: Quercus pubescens Willd.(downy oak) and Pinus sylvestris L. (Scots pine) in the Vallcebre research catchments (NE of Spain, 42º 12'N, 1º 49'E). The main objective was to understand how the interaction of biotic and abiotic factors affected stemflow dynamics. Mean stemflow production was low for both species (~1% of incident rainfall) and increased with rainfall amount. However, the magnitude of the response depended on the combination of multiple biotic and abiotic factors. Both species produced similar stemflow volumes, but funneling ratios of some trees diverged significantly. The combined analysis of biotic and abiotic factors showed that, for events of the same rainfall amount, funneling ratios and stemflow dynamics in each species were highly controlled by the interaction of rainfall intensity and tree diameter (DBH).

11Drought limits tree water use and growth of Mediterranean trees. However, growth and 12 water use strategies are rarely addressed simultaneously across species and drought 13 conditions. Here, we investigate the link between stem diameter variations and sap flow which allow them to cope with seasonal and interannual drought. 34A c c e p t e d m a n u s c r i p t 3

Abstract. Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The “sapfluxnetr” R package – designed to access, visualize, and process SAPFLUXNET data – is available from CRAN.

Most hydrological studies based on stable water isotopes ( 18 O and D) use the isotopic composition of rainfall as input signal. Although stable water isotopes are conservative tracers, previous studies have shown that canopies modify the isotopic composition of rainfall. At present, there is a scientific agreement about the factors involved in isotopic modification, but the effect of each factor and the magnitude of the isotopic shift are still not clear. In this study, we analyse at an interevent and intraevent basis the spatio-temporal differences between the isotopic composition of rainfall, throughfall, and stemflow for two different species (Pinus sylvestris L. and Quercus pubescens Willd). The aim of the study is to analyse the isotopic modification that takes place in throughfall and stemflow and how meteorological variables and structural forest characteristics influence the observed changes. Rainfall and throughfall were sampled by a combination of bulk and sequential collectors, whereas stemflow was collected only by bulk collectors. Results showed that the isotopic modification occurred in both directions, although stemflow was consistently more enriched than throughfall. Despite the contrasting canopy structures, no significant differences between species were found. Moreover, the intraevent analysis suggested that all fractionation factors could occur during one event, but evaporation or isotopic exchange would have a higher impact at the beginning of rainfall, whereas canopy selection processes would be more important at the end of rainfall. Our results emphasize the importance of considering the isotopic composition of throughfall and stemflow in isotope-related studies in forested catchments.

This paper presents the main results obtained from the study of hydrological processes in the Vallcebre Research Catchments since 1988. Distributed hydrometric measurements, environmental tracers and hydrological modelling were used to understand Mediterranean catchment behaviour and to provide new data to help assess the global change effects on these catchments' water resources. Thirty years of hydrological processes observation in the Vallcebre Research Catchments have increased understanding not only of their hydrological response, but also of the main hydrological and erosion processes characteristic of Mediterranean mountain catchments. This paper briefly summarises the main results obtained since 1988 on ecohydrological processes, hydrological response, runoff generation processes, erosion and sediment transport. Some of the main findings from this research are (i) the importance of temporal variability in precipitation to determine the hydrological processes; (ii) the paramount role played by forest cover in reducing soil water content; (iii) the marked influence of antecedent wetness conditions on runoff generation that determine different runoff responses; (v) the dominant contribution of pre-existing water during floods; (vi) the importance of freezing-thawing processes in badland areas on erosion and the role of summer convective storms in controlling sediment transport.