Groundwater subsidizes tree growth and transpiration in sandy humid forests

Abstract

As drought variability increases in forests around the globe, it is critical to evaluate and understand ecosystem attributes that ameliorate drought impacts. Trees in arid and semi‐arid ecosystems can sustain tree growth and transpiration during drought by accessing shallow groundwater, yet the extent to which groundwater influences forest growth and transpiration in humid environments has largely been unexplored. We quantified groundwater's influence on tree growth and transpiration in northern humid forests with sandy soils. We hypothesized that even in wet regions, soil droughts occur relatively frequently in forests with sandy soils and result in water stress and reduced tree growth. Further, we hypothesized these reductions in productivity are ameliorated if the forest can access shallow groundwater during dry conditions. We evaluated tree growth responses using tree cores in Pinus resinosa trees and estimated forest groundwater use from diel water table fluctuations across sites covering a 1‐ to 9‐m depth‐to‐groundwater (DTG) gradient. In areas of shallow groundwater (DTG < 2.5 m), we observed twice as much tree growth and high, frequent groundwater use (up to 81% of non‐rainy summer days). Groundwater's influence on tree growth and transpiration declined as groundwater deepened along the DTG gradient in the range 1–5 m below land surface. These findings suggest that water provided by a shallow water table subsidizes evapotranspiration in humid forests and results in enhanced tree growth. Our research provides a basis for understanding the role of groundwater in conferring drought resistance in humid forests to help guide sustainable water and forest management decisions.

Evaluating the spatial and temporal variability of groundwater uptake by riparian vegetation in a humid southeastern US catchment

“…Even humid regions are subject to droughts (e.g., Schubert et al, 2021; Seager et al, 2009), and, while they often are of shorter duration compared with more arid regions like the Western United States, they can induce vegetation stress and mortality (Berdanier & Clark, 2018, 2016). Ciruzzi and Loheide (2021) demonstrated that when trees in a humid region could access shallow groundwater, greater growth occurred, thus suggesting that a better understanding of groundwater–tree interactions in humid regions could improve riparian zone management.…”

The ecosystem wilting point defines drought response and recovery of a Quercus‐Carya forest

“…It is clear that greater emphasis on how plants interact with the belowground environment (Warren et al, 2015), and considering the entire critical zone from aquifers to treetop is needed (Dawson et al, 2020) to develop accurate predictions of ecosystem responses to climate change. Tree roots may delve deep enough below the surface to reach aquifers (Jackson et al, 2000; Miller et al, 2010), which can backstop water availability during exceptionally dry conditions to sustain the ecosystem (Baldocchi et al, 2021; Ciruzzi & Loheide II, 2021; Domec et al, 2010; Montaldo & Oren, 2022).…”

Drought responses in floodplain forests obtained by remotely sensed and tree‐ring data

“…Ground-based ecological data to compare to drought indicators can be difficult to obtain, but tree-ring records have proven valuable in forests across a wide range of climatic conditions (Bhuyan et al, 2017), even in humid environments (Ciruzzi & Loheide, 2021).…”

“…Ground‐based ecological data to compare to drought indicators can be difficult to obtain, but tree‐ring records have proven valuable in forests across a wide range of climatic conditions (Bhuyan et al, 2017), even in humid environments (Ciruzzi & Loheide, 2021). Variations in annual tree‐ring widths indicate growth response to drought at the individual‐tree or stand scale (Camarero et al, 2015), while remote‐sensing imagery provides information on photosynthetic activities from stand to forest scales.…”