Possible Increases in Flood Frequency Due to the Loss of Eastern Hemlock in the Northeastern United States: Observational Insights and Predicted Impacts

Abstract

Trees exert a fundamental control on the hydrologic cycle, yet previous research is unclear about the nuanced relationship between forest cover and riverine flood frequency. In the Northeastern United States, warming air temperatures have resulted in a decline of Eastern Hemlock (EH), and subsequent increases in observed catchment water yield. We evaluated the possibility of EH loss leading to a changed flooding regime. We first investigated plant hydraulic regulation by root water uptake in EH and American Beech (AB; a candidate successional species) through stable isotope analysis of stream, soil water, and plant xylem water. EH xylem water showed evidence of deeper soil water uptake than AB during both wet and dry seasons, suggesting species succession may be an important mechanism for altering catchment “plant accessible water.” Next, we estimated catchment flood frequency with mechanistic hydrologic simulations for present conditions, and two hypothetical cases where all EH is succeeded by AB. The largest change to catchment extreme discharge after AB succession coincided with fall season tropical moisture export‐derived precipitation. We observed reduced sensitivity under future climatic forcing with an ensemble simulation of five localized constructed analogs downscaled general circulation models. Thus, the influence of forest composition on the flood regime may be most related to the temporal alignment of the synoptic‐scale processes that generate Atlantic Basin tropical cyclones and regional plant phenology of the Northeast United States. Our results provide a justification for using physically based hydrologic models incorporating plant hydraulic regulation when evaluating future flooding frequency.

Understanding Catchment‐Scale Forest Root Water Uptake Strategies Across the Continental United States Through Inverse Ecohydrological Modeling

“…RWU depth variations have been explained by elevation (Martin et al, ), shallow soil water limitation (Meinzer et al, ), and competition with nearby species (Andrews et al, ; Volkmann et al, ). Knighton, Conneely, et al () found deep RWU by Tsuga canadensis was not well explained by topographic position, shallow soil water availability, or nearby competitors for subsurface waters. Our RWU depth estimates (Figures h and j) provided corroborating evidence of plant RWU depths to those reviewed by Evaristo and McDonnell () through independent, temporally continuous measurements collected across larger spatial scales.…”

“…Understanding forest‐scale RWU strategies is critical for developing a process understanding of hydrologic responses to forest cover change. While individual‐ and stand‐level estimates of RWU strategies facilitated by stable isotopes in water and sap flux have become ubiquitous (e.g., Brinkmann et al, ; De Deurwaerder et al, ; Evaristo et al, ; Knighton, Conneely, et al, ), uncertainties in field methods and “scaling‐up” these findings to the catchment remain intractable problems (e.g., Oerter & Bowen, ; Penna et al, ). The metaanalysis of Evaristo and McDonnell () reviewed 531 studies of tree RWU derived from isotopes in water, of which 354 (~67%) used less than 10 xylem samples, possibly missing infrequent locations or periods of deep RWU.…”

“…There is global evidence that trees exhibit varied rooting strategies to access stored catchment waters (Barbeta & Peñuelas, 2017;Evaristo & McDonnell, 2017). In particular, signatures of stable isotopes ( 2 H and 18 O) in subsurface waters and tree-stored water have facilitated estimates of RWU depths in mixed-species forests of the tropics (Brum et al, 2019;De Deurwaerder et al, 2018;Evaristo et al, 2016), the temperate Northeast United States (Gaines et al, 2015;Knighton, Conneely, et al, 2019), Europe (Brinkmann et al, 2019;Volkmann et al, 2016), the Chinese Loess Plateau (Wang et al, 2018), and urban forests (Gómez-Navarro et al, 2019) among others. Despite the broad application of stable isotopic data to estimate tree RWU strategies, several aspects of these methodologies may be limiting.…”

Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework

“…Catchment tree cover is composed primarily of second‐generation regrowth (stand age approximately 60 years) deciduous American beech (beech) and coniferous eastern hemlock (hemlock; Figure ), though other deciduous species exist at lower densities (Knighton, Coneelly, & Walter, ). Within the mixed hemlock/beech region, hemlock and beech densities are 0.031 and 0.012 m −2 , respectively.…”

“…Time lags between δ GP and δ XYLEM have been observed across catchments with varied geologies and climates (e.g., Allen, Kirchner, Braun, Siegwolf, & Goldsmith, ; Knighton, Coneelly, & Walter, ; Brinkmann et al, ), which each study attributed to the residence time within catchment soils prior to RWU. These studies incorporated the open assumption that δ XYLEM reflects the composition of δ RWU as a reasonable approximation at the catchment‐scale during the growing season.…”