Rationale: New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential. Methods:We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory.Results: In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Intermethod deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities. Conclusions:We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential. | INTRODUCTIONPlant transpiration fuels the hydrological cycle by returning 35% to 90% of water from land surfaces to the atmosphere. 1,2 Therefore, transpiration greatly influences our climate and impacts water availability in consequence of land use and climate change. 3,4 However, why is the quantification of this essential water flux so uncertain? This comes down to knowledge gaps in the mechanistic functioning of root water uptake (RWU) as well as plant rooting depth that persist despite its crucial role in predicting the future of one of our most important resources. 5 A major constraint is the practical difficulty in observing below-ground processes. Moreover, the magnitude and location of RWU are the results of multiple influencing factors, such as extent of the root system and its hydraulic properties,

Spatially and seasonally distributed information on transpiration (T) fluxes is limited in the tropics. Here, we applied a coupled isotope mass balance model to separate water fluxes of T and evapotranspiration (ET) from precipitation (P). The mean annual T was estimated at a resolution of 100 m for Costa Rica (51,100 km2) and a partitioning of monthly T and evaporation (E) for the 2370‐km2 San Carlos catchment. The dominant flux in the forest ecosystems was T with a mean annual T of 1086 mm that ranged from 700 mm in Tropical Montane Very Humid Forest to 1400 mm in Subtropical and Tropical Low Montane Rainforests. We estimated an average 85% of ET was T, which is concurrent with expectations for forested tropical regions, but varied according to model parameterization and data sources. A model comparison exercise showed a range of mean annual T estimates from 988 to 1465 mm and a range of T/P from 0.35 to 0.5 with temperature and relative humidity exhibiting the highest impact on the model results. Across Costa Rica, we estimated an average loss of precipitation by T of 38% (1085 mm), whereas interception (I) constitutes 10% (230 mm) and direct evaporation (E) only 7% (192 mm). Similarly, the results at the catchment scale indicated that monthly T contributes 76% (85‐mm monthly average) to total ET and E corresponds to 24% (24‐mm monthly average). The T rates exhibited an opposite seasonality to rainfall with highest T over the drier months from December to April with a peak in March (101–144 mm) and the minimum T in September (53–71 mm). Around 17% (79–130 mm) of precipitation over the catchment area is lost to T, both E (10–35 mm) and I (15–38 mm) correspond to 5%. Despite the inherent uncertainties of the data assumptions and simplifications, including data interpolation errors, the coupled isotope mass balance model showed in comparison to other global products reasonable water partitioning for different ecosystems in Costa Rica and the San Carlos catchment area. These results can help to evaluate the impact of land cover conversion on the hydrological cycle in Costa Rica, and the simple isotope‐based model could be transferred to different biomes of the tropics.

<p>Analyzing water stable isotopes in soils and plants is a key method to identify the water sources for transpiration. However, the spatial representation of such studies is often limited and typically data from one or only a few soil water isotope profiles are used for analyzing plant water sources for much larger areas.   </p> <p>Contrary, it is well known from soil sciences that soil physical and hydraulic properties are highly heterogeneous, even over small areas. Only few studies have investigated the spatial variability of soil water isotopes, despite its potential importance for water uptake depth analysis. Goldsmith et al., (2018) showed that vegetation can have a substantial influence on the spatial pattern of soil water isotopes in a tropical cloud forest. We extend the hypothesis that vegetation does not only have an influence on soil water isotopes in wet environments, but also under dry conditions: The isotopic enrichment of soil water isotopes under steady-state dry conditions is controlled by vegetation (canopy parameters). </p> <p>In order to test this hypothesis, we undertook a spatial sampling of ten soil water isotope depth profiles (at 6 depths up to 2m depth) and ~60 evergreen and deciduous trees at the peak of dry season in February 2019 in a tropical dry forest in the northwest of Costa Rica. We then correlated the spatial patterns of water content and isotopes of the soil with 12 vegetation indices and surface (leaf/soil) temperature derived from UAV (Unmanned aerial vehicles; drones) overflights (Jan-Apr 2019) in order to investigate if spatial patterns of soil water isotopes can be predicted using additional information. Finally, we interpolated (external drift kriging) the soil water isotope values using the highest correlated vegetation indices in order to provide a spatially distributed map of soil water isotope depth profiles.</p> <p>Our findings indicate that i.) soil water isotopes are (highly) spatially heterogeneous, even under steady-state conditions (no rain); ii.) this heterogeneity is particularly pronounced for the near-surface soil (first 50 cm) and diminishes with soil depth; iii.) there is a significant correlation between soil water isotopes and multiple vegetation indices. Surprisingly, the highest correlations (0.82 for water content, 0.75 for

El análisis y gestión de ecosistemas urbanos es crucial para construir resiliencia en ciudades y avanzar hacia su sostenibilidad. Uno de los componentes fundamentales de estos ecosistemas son los bosques urbanos. Para determinar los aportes y efectos de los árboles en ciudades se analizó una muestra del casco urbano de la ciudad de Turrialba en la zona central de Costa Rica, a través del modelo iTree Eco, que permite valorar los principales servicios ecosistémicos que proveen los árboles en las ciudades. Aunque en este estudio no fue posible cuantificar todos los servicios ecosistémicos que incluye el modelo, se logró analizar los elementos y contexto estructural de la zona de estudio, y el valor de los servicios ecosistémicos de secuestro y almacenamiento de carbono, así como el valor estructural de los árboles. Anualmente, en la zona estudiada, con 18 especies de árboles identificadas, se generan más de 60 000 USD solamente derivados de estos tres servicios ecosistémicos. De las especies identificadas la Zygia longifolia, se destacó por mayor abundancia y área foliar, Erythrina poeppigiana en altura y mayor almacenamiento de carbono, Mangifera indica en mayor secuestro de carbono y producción de oxígeno y Veitchia sp con el mayor valor estructural. Asimismo, se evidenció que, mantener las franjas ribereñas del río Turrialba y sus tributarios, en su paso por el casco urbano de esta ciudad, aporta la mayor cantidad de servicios ecosistémicos derivados de los árboles que todavía alberga y donde existe aún un importante potencial de incluir nuevas plantaciones.