Andrew Christopher Oishi

The Forest Soil Moisture Monitoring Network (FSMMN) provides a coordinated, multi-depth dataset of soil moisture and temperature measurements from three eastern U.S. Forest Service experimental forests, including Hubbard Brook (New Hampshire), Fernow (West Virginia), and Coweeta (North Carolina). This latitudinal gradient spans distinct soil types, vegetation communities, and precipitation regimes across the Appalachian Mountains, capturing the variability in soil water dynamics. The network currently includes 44 monitoring sites and 262 soil moisture sensors distributed across the three forests, with hourly records beginning in 2022 at Coweeta and Fernow, and in 2023 at Hubbard Brook. At each forest, paired in situ volumetric water content (VWC) and soil matric potential (SMP) sensors were installed at three depth intervals (10–20 cm, 50 cm, and 60–100 cm) within multiple soil profiles across several catchments, enabling the characterization of soil moisture dynamics and hydraulic function at multiple scales. The data undergo automated and manual quality control procedures described in this paper and are updated annually in public data repositories. The dataset enhances the spatiotemporal coverage of soil moisture and temperature observations in forested headwater catchments where long-term, spatially distributed soil moisture records have historically been scarce. By capturing both vertical and lateral soil water variability across contrasting forest ecosystems, the FSMMN provides a foundation for cross-site studies linking soil hydraulic properties and catchment water balance at scales relevant to ecological and hydrological modeling.

Over 95% of original longleaf pine (Pinus palustris) (LLP) forests have been converted to other land uses, including loblolly pine (Pinus taeda L) (LOP), croplands, urban uses during the past two centuries in the southeastern United States (U.S.) for socioeconomic developments. Restoring the LLP forests represents a contemporary forest management objective to improve wildlife habitat, water yield, and overall ecosystem services and resilience to a changing climate. Given the importance of understanding ecohydrological processes for guiding restoration efforts, this study compared evapotranspiration (ET) measurements at eight eddy covariance flux sites dominated by LLP or LOP forests in the southeastern U.S. In addition, we developed a “paired stands” approach to compare remote sensing based ET estimates and associated site biophysical properties for approximately 1,600 LLP‐LOP pairs. We found significant differences in ET, ET/Precipitation ratio, and water yield/precipitation ratio between the two types of pine forests, and these differences are explained by surface properties and management histories. Compared to LOP, the LLP forests generally had lower ET due to their significantly (p < 0.05) lower leaf area index but higher land surface temperature and albedo. Regionally, forest ET differences increased with the increase in atmospheric dryness index (reference ET/precipitation ratio). Therefore, we conclude that large‐scale restoration of LLP forests has the potential to reduce ET and augment water yield in the long run, especially in relatively drier watersheds. Maintaining low stand tree density and understory leaf area characteristic of natural LLP ecosystems through active forest management is critical for enhancing forest water supply. Our study provides the scientific basis for large scale restoration of a diminishing ecosystem for benefiting water resources in the southeastern U.S.

Terrestrial evapotranspiration is the second‐largest component of the land water cycle, linking the water, energy, and carbon cycles and influencing the productivity and health of ecosystems. The dynamics of ET across a spectrum of spatiotemporal scales and their controls remain an active focus of research across different science disciplines. Here, we provide an overview of the current state of ET science across in situ measurements, partitioning of ET, and remote sensing, and discuss how different approaches complement one another based on their advantages and shortcomings. We aim to facilitate collaboration among a cross‐disciplinary group of ET scientists to overcome the challenges identified in this paper and ultimately advance our integrated understanding of ET.

Restoring and preserving the world's forests are promising natural pathways to mitigate some aspects of climate change. In addition to regulating atmospheric carbon dioxide concentrations, forests modify surface and near‐surface air temperatures through biophysical processes. In the eastern United States (EUS), widespread reforestation during the 20th century coincided with an anomalous lack of warming, raising questions about reforestation's contribution to local cooling and climate mitigation. Using new cross‐scale approaches and multiple independent sources of data, we uncovered links between reforestation and the response of both surface and air temperature in the EUS. Ground‐ and satellite‐based observations showed that EUS forests cool the land surface by 1–2°C annually compared to nearby grasslands and croplands, with the strongest cooling effect during midday in the growing season, when cooling is 2–5°C. Young forests (20–40 years) have the strongest cooling effect on surface temperature. Surface cooling extends to the near‐surface air, with forests reducing midday air temperature by up to 1°C compared to nearby non‐forests. Analyses of historical land cover and air temperature trends showed that the cooling benefits of reforestation extend across the landscape. Locations surrounded by reforestation were up to 1°C cooler than neighboring locations that did not undergo land cover change, and areas dominated by regrowing forests were associated with cooling temperature trends in much of the EUS. Our work indicates reforestation contributed to the historically slow pace of warming in the EUS, underscoring reforestation's potential as a local climate adaptation strategy in temperate regions.

In this study, we investigate how seasonal streamflow and soil moisture patterns have responded to variability in vegetation phenology in humid, temperate forested watersheds without significant seasonal snowmelt over the last four decades. We characterize spring streamflow peaks using 50th percentiles of cumulative daily precipitation, streamflow, and soil moisture measurements, and investigate interactions with remotely sensed, greenup anomalies. After removing a dominant precipitation control, 1‐day earlier greenup is usually associated with about 1‐day early spring flow peak at four low‐elevation deciduous catchments using both sequential and multiple linear regressions. This indicates that the strong dependency of seasonal flow regimes on precipitation is mediated by vegetation seasonality, especially by greenup variability. In contrast, we find less significant correlations of the greenup anomalies on flow percentiles from two paired evergreen and two high‐elevation deciduous catchments. At a plot scale, similar correlations were found only at an upslope topographic position, where precipitation also showed tighter coupling with moisture seasonal patterns than downslope. Our study suggests that rainfall‐runoff and rainfall‐soil moisture relations have been closely mediated by vegetation seasonality in deciduous forests, especially by greenup anomalies, but patterned along topoclimate and hillslope gradients. This study emphasizes that it is important to understand phenological responses to ongoing climate change (in both long‐term and interannual variability) for prediction of seasonal flow regimes especially in deciduous forested catchments.

Ongoing warming due to climate change has generally led to lengthened growing seasons and subsequent changes in evapotranspiration (ET) and streamflow seasonality. This has been well studied in seasonally dry, snowmelt dominated watersheds, but not in humid, temperate forested watersheds without significant seasonal snowmelt. In this study, we investigate how seasonal streamflow patterns have responded to variability in vegetation phenology in the southern Appalachians over the last four decades. We characterize seasonal shifts in low-frequency streamflow peaks using 50th percentiles of cumulative daily precipitation, streamflow, and soil moisture measurements, and investigate interactions with remotely sensed, long-term greenup anomalies in the deciduous-dominated forested watersheds. After removing a dominant precipitation control, one-day earlier greenup is associated with about one-day early spring flow peak at the low-elevation deciduous catchment. This indicates that the strong dependency of seasonal flow regimes on precipitation is mediated by warming-induced extended growing season, especially by early greenup. In contrast, we find less significant correlations of the greenup anomalies on flow percentiles of an adjacent evergreen and a high-elevation deciduous catchment. At a plot scale, similar correlations of cumulative soil moisture days were found only at an upslope topographic position, where precipitation also showed tighter coupling with soil moisture patterns than downslope. This indicates that early greenup in deciduous forests leading to early ET increase, in turn results in early soil moisture dry-down patterned by hillslope positions, and earlier seasonal streamflow peaks and subsequent declines. Our study suggests that spring flow peaks are likely to shift earlier by warming-induced early greenup even in snow-free regions, which has great implications for future freshwater availability in the southeastern US.

Shifts in phenological timing have important implications for ecosystem processes, with spring leaf emergence as a dominant control of carbon, water, and energy cycling. Phenological events are predominantly determined by weather and climate, therefore dynamic in time and sensitive to climate feedbacks. Improving our understanding of how ecosystems respond to changes in phenological timing will enhance our ability to assess summer soil water availability, since the timing of spring leaf emergence may lead to soil moisture deficits later in the growing season. We leveraged data from five AmeriFlux towers in central and eastern United States to investigate the extent spring leaf emergence (i.e., start of spring, SoS) influences rates at which forest canopies develop and how this impacts summer soil moisture (θ_JJA) variability. Our results indicate that ecosystem processes, specifically gross primary production (GPP) and evapotranspiration (ET), exhibit compensatory responses to varying leaf emergence; with delayed spring‐onset, the canopy developed more quickly, resulting in rapid GPP and ET increases, consistent across sites. Nonetheless, early SoS is a relatively good indicator for potential summer soil water deficits, particularly when it occurs together with meteorological conditions (i.e., lower‐than‐average precipitation, hot summer temperatures) that contribute to soil water deficits. When these meteorological conditions coincide with early SoS, θ_JJA deficits are exacerbated. To the extent that these extreme conditions occur more frequently under future climate scenarios, the dynamics of spring phenology and hydroclimate may play an increasingly important role in portending the likelihood of summer water deficits, which are projected to become more severe.

Restoring and preventing losses of the world’s forests are promising natural pathways to mitigate climate change. In addition to regulating atmospheric carbon dioxide concentrations, forests modify surface and near-surface air temperatures through biophysical processes. In the eastern United States (EUS), widespread reforestation during the 20th century coincided with an anomalous lack of warming, raising the question of whether reforestation contributed to biophysical cooling and slowed local climate change. Using new cross-scale approaches and multiple independent sources of data, our analysis uncovered links between reforestation and the response of both surface and air temperature in the EUS. Ground- and satellite-based observations showed that EUS forests cool the land surface by 1-2 °C annually, with the strongest cooling effect during midday in the growing season, when cooling is 2 to 5 °C. Young forests aged 25-50 years have the strongest cooling effect on surface temperature, which extends to the near-surface air, with forests reducing midday air temperature by up to 1 °C. Our analyses of historical land cover and air temperature trends showed that the cooling benefits of reforestation extend across the landscape. Locations predominantly surrounded by reforestation were up to 1 °C cooler than neighboring locations that did not undergo land cover change, and areas dominated by regrowing forests were associated with cooling temperature trends in much of the EUS. Our work indicates that reforestation contributed to the historically slow pace of warming in the EUS, highlighting the potential for reforestation to provide local climate adaptation benefits in temperate regions worldwide.

Assessing nutrient critical load exceedances requires complete and accurate atmospheric deposition budgets for reactive nitrogen (N_r). The exceedance is the total amount of N_r deposited to the ecosystem in excess of the critical load, which is the amount of N_r input below which harmful effects do not occur. Total deposition includes all forms of N_r (i.e., organic and inorganic) deposited to the ecosystem by wet and dry pathways. Here we present results from the Southern Appalachian Nitrogen Deposition Study (SANDS), in which a combination of measurements and field-scale modeling was used to develop a complete annual N_r deposition budget for a deciduous forest at the Coweeta Hydrologic Laboratory. Wet deposition of ammonium, nitrate, nitrite, and bulk organic N were measured directly. The dry deposited N_r fraction was estimated using a bidirectional resistance-based model driven with speciated measurements of N_r air concentrations (e.g., ammonia, ammonium aerosol, nitric acid, nitrate aerosol, bulk organic N in aerosol, total alkyl nitrates, and total peroxy nitrates), micrometeorology, canopy structure, and biogeochemistry. Total annual deposition was ∼ 6.7 kg N ha⁻¹ yr⁻¹, which is on the upper end of N_r critical load estimates recently developed for similar ecosystems in the nearby Great Smoky Mountains National Park. Of the total (wet + dry) budget, 51.1 % was contributed by reduced forms of N_r (NH_x = ammonia + ammonium), with oxidized and organic forms contributing ∼ 41.3 % and 7.6 %, respectively. Our results indicate that reductions in NH_x deposition would be needed to achieve the lowest estimates (∼ 3.0 kg N ha⁻¹ yr⁻¹) of N_r critical loads in southern Appalachian forests.