GEOMORPHIC AND VEGETATION DYNAMICS ALONG A GREAT LAKES RIDGE PLAIN: ASSESSING THE UTILITY OF COASTAL JUNIPERS AS DENDROCHRONOLOGICAL PROXIES OF LAKE-LEVEL CHANGE

An eco-geomorphologic study is underway along a high-sand supply portion of the Zion Beach-ridge Plain, a migrating strand system along the wave-dominated SW Lake Michigan coast. We are evaluating linkages between lake-level, coastal geomorphic, and vegetative dynamics (growth of common juniper, an important dune succession species). We herewith present preliminary insights into potentially lake level-driven patterns across different strand elevations within a backbarrier swale complex that is becoming increasingly separated from Lake Michigan by accretion of new coastal landforms. The primary goal is to assess the role of lake-level variations (on the order of 2 m over decadal timespans) on ecological development in the cross-shore direction. The research approach is two-fold. First, a GIS-based analysis integrates existing geospatial information from 5 years of active topographic assessment of this portion of the strand (by the Illinois State Geological Survey), with historical aerial photographs and Federal LiDAR-based digital elevation models. Second, quantitation of clonal juniper patches by GIS-based information extraction (mean elevations and diameter assessments), in-field morphometrics (stem thicknesses and patch sizes), and stem diameter growth dynamics revealed by increment cores. Trends thus far in juniper patch areas and individual stem diameters mostly reflect a post-2012 lake-level rise of up to 2 m over a previous 15-year period of lower than average levels. Areas of juniper patches increased by factors from ~60% to more than 20 times. Increment cores of the small and convoluted stems are proving challenging to assess, but more than half of them reveal distinct corresponding growth rate increases. All vegetation is in full sun, so the more parsimonious hypothesis is water availability changes rather than any shade dynamic. The creation of a new overwash berm between 2018 and 2020, coincident with the lake-level peak, may be providing an opportunity for a future reiteration of this fascinating geomorphic, hydrologic and ecological linkage.