About

Todd Scanlon is an Associate Chair and Professor in the Department of Environmental Sciences at the University of Virginia. His primary research interests include catchment hydrology, focusing on hydrological and geochemical transport processes, as well as land-atmosphere interactions involving the exchange of water, energy, and gaseous compounds such as carbon dioxide, nitrous oxide, and methane between the terrestrial surface and the atmosphere. He seeks to develop an integrated understanding of how the hydrological cycle, vegetation processes, and atmospheric dynamics are interconnected, and how these relationships influence nutrient cycling and ecosystem function. His research employs a combination of field studies, remote sensing, and numerical modeling methods to address these issues across various spatial and temporal scales. Current field research locations include Shenandoah National Park, Ireland, and southern Africa.

Research topics

• Environmental science
• Atmospheric sciences
• Geography
• Environmental chemistry
• Ecology

Selected publications

• Divergent water use efficiency trends among eastern North American temperate tree species

  Oecologia · 2025-07-29

  articleOpen accessSenior author

  Abstract Both theory and observations suggest that tree intrinsic water use efficiency (iWUE)—the ratio of photosynthetic carbon assimilation to stomatal conductance to water—increases with atmospheric CO 2 . However, the strength of this relationship varies across sites and species, prompting questions about additional physiological constraints and environmental controls on iWUE. In this study, we analyzed tree core carbon isotope ratios to examine trends in, and drivers of, iWUE in 12 tree species common to the temperate forests of eastern North America, where forests have experienced changes in CO 2 , climate, and atmospheric pollution in recent decades. Across all site-species combinations, we found that tree iWUE increased 22.3% between 1950 and 2011, coinciding with a 25.2% increase in atmospheric CO 2 . iWUE trajectories varied markedly among tree functional groups and within species across sites. Needleleaf evergreen iWUE increased until circa 2002 before declining in recent years, while iWUE of broadleaf deciduous species continued to increase. The analysis of environmental controls on iWUE trends revealed smaller increases in iWUE in trees subjected to higher atmospheric pollution loads. Our results suggest that tree functional characteristics and atmospheric pollution history influence tree response to atmospheric CO 2 , with implications for forest carbon and water balance in temperate regions.

  PublisherOA PDFDOI

• Constraining light dependency in modeled emissions through comparison to observed biogenic volatile organic compound (BVOC) concentrations in a southeastern US forest

  Atmospheric chemistry and physics · 2024-11-12 · 1 citations

  articleOpen access

  Abstract. Climate change will bring about changes in meteorological and ecological factors that are currently used in global-scale models to calculate biogenic emissions. By comparing long-term datasets of biogenic compounds to modeled emissions, this work seeks to improve understanding of these models and their driving factors. We compare speciated biogenic volatile organic compound (BVOC) measurements at the Virginia Forest Research Laboratory located in Fluvanna County, VA, USA, for the year 2020 with emissions estimated by the Model of Emissions of Gases and Aerosols from Nature version 3.2 (MEGANv3.2). The emissions were subjected to oxidation in a 0-D box model (F0AM v4.3) to generate time series of modeled concentrations. We find that default light-dependent fractions (LDFs) in the emissions model do not accurately represent observed temporal variability in regional observations. Some monoterpenes with a default light dependence are better represented using light-independent emissions throughout the year (LDFα-pinene=0, as opposed to 0.6), while others are best represented using a seasonally or temporally dependent light dependence. For example, limonene has the highest correlation between modeled and measured concentrations using an LDF =0 for January through April and roughly 0.74–0.97 in the summer months, in contrast to the default value of 0.4. The monoterpenes β-thujene, sabinene, and γ-terpinene similarly have an LDF that varies throughout the year, with light-dependent behavior in summer, while camphene and α-fenchene follow light-independent behavior throughout the year. Simulations of most compounds are consistently underpredicted in the winter months compared to observed concentrations. In contrast, day-to-day variability in the concentrations during summer months are relatively well captured using the coupled emissions–chemistry model constrained by regional concentrations of NOX and O3.

  PublisherOA PDFDOI

• Supplementary material to "Constraining Light Dependency in Modeled Emissions Through Comparison to Observed BVOC Concentrations in a Southeastern US Forest"

  2024-06-18

  preprintOpen access
  PublisherOA PDFDOI

• Summer aridity decouples growth from carbon assimilation in temperate oaks

  2024-03-08

  preprintOpen accessCorresponding

  Forest biomass resulting from tree radial growth can remain on the landscape over decadal to centennial timescales and plays a critical role in forest carbon cycling. However, visually green vegetation may not be a good proxy for carbon allocation to growth as the phenology and environmental sensitivity of photosynthesis may be different from radial growth. Here we investigate the decoupling between photosynthesis and tree radial growth across intra to interannual timescales for seven North American oak species (Quercus spp.) at four sites (Lamont Sanctuary, NY; Morton Arboretum, IL; Pace Forest, VA; & Tonzi Vaira, CA, USA). Using point dendrometers and wood anatomy, we find that oak trees generally commenced radial growth (cell division and expansion) one month prior to full canopy development and peak carbon assimilation estimated using eddy covariance, satellite and in-situ remote sensing, and leaf-level chlorophyll fluorescence. Further, radial growth was essentially completed by early summer, two to three months prior to the early autumn end of the photosynthetic activity, and before the annual peak in temperature and vapour pressure deficit (VPD) and lowest soil moisture. This suggests that high summer aridity limits carbon allocation to growth more strongly than assimilation. Tree-ring width chronologies for these species across North America further supports that results that earlywood growth depends on prior season climate and assimilated carbon while latewood growth ends by early-summer and responds primarily to current year climate variability. In summary, temporal decoupling between radial growth and photosynthesis and the stronger constraint of summer aridity on growth than photosynthesis appears to be widespread among multiple North American temperate oak species. As summers continue to warm and dry under climate change, this source-sink (or photosynthesis-growth) decoupling needs to be better resolved to constrain forest carbon cycling, as increasing aridity will likely influence the ability of trees to allocate carbon to long-term storage as woody biomass.

  PublisherDOI

• Factors influencing seasonal chemistry patterns in Virginia mountain streams

  Biogeochemistry · 2024-08-06 · 2 citations

  articleOpen access

  The relative influence of seasonal patterns in hydrological flow and seasonal differences in biological and geochemical activity on stream chemistry patterns is difficult to discern because they covary; temperate systems are characterized by lower mean flow in the summer (i.e. corresponding to deeper flow paths, elevated temperature, and biological activity), and higher mean flow in the winter (i.e. corresponding to shallower flow paths, depressed temperature, and biological dormancy). Using 2018 data, when seasonal stream flow conditions reversed, and two prior conventional water years, the relationship between monthly acid-relevant analyte concentrations and streamflow were compared within and between winter and summer to provide insight into controls on characteristic seasonal chemistry patterns at two mid-Appalachian sites with distinct geology (weatherable mafic and weather resistant siliciclastic). Acid neutralizing capacity (ANC) increased (1) with lower flow, in both seasons and (2) in summer, for all flow conditions. The compounding impacts resulted in a doubling of concentration from typical winter with high flow to summer with low flow at both sites. Base cation patterns tracked ANC at the mafic site, resulting in an ~ 60% increase of from winter with high flow to summer with low flow; distinctions between summer and winter contributed more to the seasonal pattern (72%) than changes in flow. Sulfate increased at the mafic site (1) with higher flow, in both seasons and (2) in winter, for all flow conditions, resulting in an ~ 50% increase from summer with low flow to winter with high flow; distinctions between winter and summer conditions and flow contributed similarly (40-60%) to the typical seasonal chemical pattern. The biogeochemical mechanism driving differences in stream chemistry between summer and winter for the same flow conditions is likely increased rates of natural acidification from elevated soil respiration in summer, resulting in greater bedrock weathering and sulfate adsorption. Findings highlight the significance and consistency of growing vs dormant season variations in temperature and biological activity in driving intra-annual patterns of stream solutes. This data set informs parameterization of hydro-biogeochemical models of stream chemistry in a changing climate at a biologically relevant, seasonal, timescale. Supplementary Information: The online version contains supplementary material available at 10.1007/s10533-024-01163-x.

  PublisherOA PDFDOI

• Constraining Light Dependency in Modeled Emissions Through Comparison to Observed BVOC Concentrations in a Southeastern US Forest

  2024-06-18

  preprintOpen accessCorresponding

  Abstract. Climate change will bring about changes in meteorological and ecological factors that are currently used in global-scale models to calculate biogenic emissions. By comparing long-term datasets of biogenic compounds to modeled emissions, this work seeks to improve understanding of these models and their driving factors. We compare speciated BVOC measurements at the Virginia Forest Research Laboratory located in Fluvanna County, VA, USA for the 2020 year with emissions estimated by MEGANv3.2. The emissions were subjected to oxidation in a 0-D box-model (F0AM v4.3) to generate timeseries of modeled concentrations. We find that default light-dependent fractions (LDFs) in the emissions model do not accurately represent observed temporal variability of regional observations. Some monoterpenes with a default light dependence are better represented using light-independent emissions throughout the year (LDFα-pinene=0, as opposed to 0.6), while others are best represented using a seasonally or temporally dependent light dependence. For example, limonene has the highest correlation between modeled and measured concentrations using LDF=0 for January through April and roughly 0.74–0.97 in the summer months, in contrast to the default value of 0.4. The monoterpenes β-thujene, sabinene, and γ-terpinene similarly have an LDF that varies throughout the year, with light-dependent behavior in summer, while camphene and α-fenchene follow light-independent behavior throughout the year. Simulations of most compounds are consistently underpredicted in the winter months compared to observed concentrations. In contrast, day-to-day variability in the concentrations during summer months are relatively well captured using the coupled emissions-chemistry model constrained by regional concentrations of NOx and O3.

  PublisherDOI

• Growing season dynamics of net photosynthesis and leaf respiration for a mixed hardwood forest as inferred from flux-variance similarity

  2024-03-09

  preprintOpen access1st authorCorresponding

  Ecosystem-scale estimates of net photosynthesis may be derived from eddy covariance measurements of net ecosystem exchange through the application of flux-variance similarity theory. Net photosynthesis, which is defined as carboxylation minus photorespiration and leaf respiration, differs from gross primary production by the leaf respiration term, which has been implicated as a potential source of error for traditional flux partitioning approaches. Here, we focus on seasonal dynamics of net photosynthesis and leaf respiration by deriving relevant variables (e.g. magnitude of dark respiration, light-saturated rate of net photosynthesis, sensitivity of leaf respiration to light intensity) through rectangular hyperbolic fits of net photosynthesis to photosynthetically active radiation (PAR) throughout the growing season. We find that the magnitude of dark leaf respiration decreases throughout the growing season, while the sensitivity of leaf respiration to light intensity and light-saturated net photosynthesis remain relatively stable. The level of PAR required for carboxylation minus photorespiration to exceed leaf respiration increases over the course of the growing season. We examine how environmental variables, specifically air temperature and volumetric soil moisture, influence these aspects of net photosynthesis. Estimates of leaf-level water use efficiency, a key parameter in the flux-variance similarity theory approach, are evaluated through comparisons with co-located measurements of solar induced fluorescence and sap flux.

  PublisherDOI

• Impact of atmospheric dryness on solar-induced chlorophyll fluorescence: Tower-based observations at a temperate forest

  Remote Sensing of Environment · 2024-04-01 · 13 citations

  articleOpen access
  PublisherOA PDFDOI

• Decoupling of Ecological and Hydrological Drought Conditions in the Limpopo River Basin Inferred from Groundwater Storage and NDVI Anomalies

  Hydrology · 2023-08-12 · 6 citations

  articleOpen access

  Droughts are projected to increase in intensity and frequency with the rise of global mean temperatures. However, not all drought indices equally capture the variety of influences that each hydrologic component has on the duration and magnitude of a period of water deficit. While such indices often agree with one another due to precipitation being the major input, heterogeneous responses caused by groundwater recharge, soil moisture memory, and vegetation dynamics may lead to a decoupling of identifiable drought conditions. As a semi-arid basin, the Limpopo River Basin (LRB) is a severely water-stressed region associated with unique climate patterns that regularly affect hydrological extremes. In this study, we find that vegetation indices show no significant long-term trends (S-statistic 9; p-value 0.779), opposing that of the modeled groundwater anomalies (S-statistic -57; p-value 0.05) in the growing season for a period of 18 years (2004–2022). Although the Mann-Kendall time series statistics for NDVI and drought indices are non-significant when basin-averaged, spatial heterogeneity further reveals that such a decoupling trend between vegetation and groundwater anomalies is indeed significant (p-value < 0.05) in colluvial, low-land aquifers to the southeast, while they remain more coupled in the central-west LRB, where more bedrock aquifers dominate. The conclusions of this study highlight the importance of ecological conditions with respect to water availability and suggest that water management must be informed by local vegetation species, especially in the face of depleting groundwater resources.

  PublisherOA PDFDOI

• Shenandoah Watershed <scp>Study‐Virginia</scp> Trout Stream Sensitivity Study (<scp>SWAS‐VTSSS</scp>): Stream water quality and hydrologic monitoring data for <scp>mid‐Appalachian</scp> headwater streams

  Hydrological Processes · 2021-04-01 · 4 citations

  article

  Abstract The Shenandoah Watershed Study (established in 1979) and the Virginia Trout Stream Sensitivity Study (established in 1987) serve to increase understanding of hydrological and biogeochemical changes in western Virginia mountain streams that occur in response to acidic deposition and other ecosystem stressors. The SWAS‐VTSSS program has evolved over its 40+ year history to consist of a temporally robust and spatially stratified monitoring framework. Currently stream water is sampled for water quality bi‐hourly during high‐flow events at three sites and weekly at four sites within Shenandoah National Park (SHEN), and quarterly at 72 sites and on an approximately decadal frequency at ~450 sites within the wider western Virginia Appalachian region. Stream water is evaluated for pH, acid neutralizing capacity (ANC), base cations (calcium, magnesium, sodium and potassium ion), acid anions (sulphate, nitrate and chloride), silica, ammonium, and conductivity with a subset of samples evaluated for monomeric aluminium and dissolved organic carbon. Hourly stream discharge (four sites) and in‐situ measurements of conductivity, water and air temperature (three sites) are also measured within SHEN. Here we provide an overview and timeline of the SWAS‐VTSSS stream water monitoring program, summarize the field and laboratory methods, describe the water chemistry and hydrologic data sets, and document major watershed disturbances that have occurred during the program history. Website links and instructions are provided to access the stream chemistry and time‐series monitoring data in open‐access federal databases. The purpose of this publication is to promote awareness of these unique, long‐term data sets for wider use in catchment studies. The water chemistry and hydrologic data can be used to investigate a wide range of biogeochemical research questions and provide key inputs for models of these headwater stream ecosystems. SWAS‐VTSSS is an ongoing program and quality assured data sets are uploaded to the databases annually.

  PublisherDOI

Recent grants

• CAREER: Linked Hydrologic and Atmospheric Fluxes from Riparian Biogeochemical Hotspots

  NSF · $450k · 2007–2015

• Collaborative Research: Forest Water Use and the Influence of Acid Deposition

  NSF · $270k · 2016–2021

Frequent coauthors

• Ami L. Riscassi

  University of Virginia

  45 shared

• J. D. Albertson

  Cornell University

  26 shared

• James N. Galloway

  McCormick (United States)

  18 shared

• K. K. Caylor

  University of California, Santa Barbara

  15 shared

• Paolo D’Odorico

  University of California, Berkeley

  12 shared

• Daniel L. Druckenbrod

  Rider University

  11 shared

• William P. Kustas

  Agricultural Research Service

  10 shared

• Michael V. Saha

  University of Virginia

  10 shared

Education

• Ph.D., Environmental Sciences

  University of Virginia

  2002

• M.S., Environmental Sciences

  University of Virginia

  1998

• B.A., Earth Sciences

  Dartmouth College

  1995