Ecohydrology of a Freshwater Coastal Forest Under Fluctuating Lake Levels

Coastal forests of the North American Great Lakes have long provided ecological, economic, and cultural resources. However, extensive logging and shoreline development have greatly reduced the number of well-preserved coastal forests, leaving ridge-and-swale complexes among the most intact examples remaining in the region. These freshwater coastal landscapes formed from ancient beaches, creating sandy, tree-covered ridges separated by low-lying interdunal swales that commonly support wetlands. Ridge-and-swale complexes serve as the transition between upland ecosystems and the Great Lakes along which interaction with groundwater directly controls vegetation composition and function. They are an excellent natural setting to study forest-groundwater interactions as variation in groundwater depth impacts water available to plants. In these systems, groundwater hydrology reflects the combined influence of upland derived regional groundwater flow, local hydrologic inputs (precipitation) and outputs (evapotranspiration), and variability in Great Lakes Water Levels (GLWL). As environmental conditions and GLWLs continue to shift the trajectory of these tightly coupled groundwater-ecosystems interaction remains uncertain.

The goal of this work is to document modern spatial and temporal variation in forest-groundwater interactions, establish the influence of depth to groundwater on historical tree growth, and map forest susceptibility to groundwater conditions in a ridge-and-swale complex at the Ridges Sanctuary in Bailey’s Harbor, WI (45.075163, -87.108379). To investigate modern forest water use and groundwater dynamics, we instrumented a ridge-and-swale complex situated along Lake Michigan with shallow groundwater wells. Over 18 months, analysis of daily fluctuations in groundwater revealed that evapotranspiration from groundwater differs in timing and magnitude between stands of trees. Throughout the ridge-and-swale complex groundwater levels respond differently throughout the year and especially during extended dry periods. This indicates a limit of tree water use based on depth to groundwater and differing influence of regional groundwater and GLWL sources. To investigate historical tree growth, we evaluated tree-ring metrics (basal area increment (BAI), ring widths, earlywood, and latewood) from Pinus strobes and Pinus resinosa to quantify tree growth variability across the ridge-and-swale complex. Chronologies for 120 trees were established with a majority spanning over 125 years from throughout the ridge-and-swale complex with limited trees documenting growth starting in the late 1700s. Comparison of BAI and relative elevation have revealed both tree species experience differing levels of anoxia and water stress based on position in the landscape and depth to groundwater, indicating that annual variability in groundwater is recorded in tree growth. To map future susceptibility to groundwater conditions, we used our established chronologies to decipher where trees showed resilience to the influence of GLWLs. Preliminary analyses suggest that stands situated in areas supported by regional groundwater flow have historically experienced optimal tree growth throughout the period of record. Conversely, the influence of GLWL supporting or hindering tree growth depends on position relative to the coast and the stage of Lake Michigan at the time. These findings not only shed light upon the previously unknown historical influence of GLWL on coastal ridge-and-swale ecosystems but help shape future management for coastal aquifers and forests.