Floodplain forest structure and the recent decline of Carya illinoinensis (Wangenh.) K. Koch (northern pecan) at its northern latitudinal range margin, Upper Mississippi River System, USA

Abstract

Floodplain hardwood forests that exist on islands and immediately adjacent to channels along the Upper Mississippi River System (UMRS) are ecologically important due to high floral and faunal biodiversity. The recent decline and regeneration failure of hard mast species—e.g. Quercus spp. and Carya spp.—along the UMRS has been well documented, but causes of these trends have not been widely examined and are thus poorly understood. To investigate the mechanisms of this decline, we installed three 0.20-ha plots within the floodplain forests of the UMRS amongst communities known to contain the northernmost documented individuals of Carya illinoinensis (Wangenh.) K. Koch, a regionally imperiled, yet ecologically, economically, and culturally important hard mast species. Tree density (>5 cm dbh) averaged 355 stems ha−1 (n = 239, 12 representative species across 9 genera) and mean basal area was 77 m2 ha−1 across the three sites. Acer saccharinum L. was the most dominant species (49.0 m2 ha−1). The diameter distribution of trees demonstrated a negative exponential pattern, generally indicative of uneven-aged, regenerating forests. Over the past ca. 120 years, decadal-scale pulses of tree establishment across species show linkages with riparian engineering installments and flood and drought events. C. illinoinensis demonstrated a consistent establishment pattern since at least the 1860s through the 1950s, but experienced regeneration failure over the past ca. 50 years. Widespread ecological growth releases in C. illinoinensis are linked with known large-scale flood events along the UMRS (1965, 1993, 2001, 2011 CE). Correlation analysis revealed relationships between C. illinoinensis growth rings and drought, precipitation, maximum temperature, and river flow to be temporally unstable with an inflection point at the beginning of the lock and dam era (ca. 1930s–1940s). The coincidental decoupling of climate and flow variability with tree growth in the post-lock and dam era suggests that peripheral, disjunct populations of C. illinoinensis are sensitive to impacts from anthropogenic land and hydrologic modifications. Forest structure, disturbance history, and climate analyses demonstrate that without management intervention, C. illinoinensis will no longer have a significant presence within floodplain forests of the UMRS in the near future, contributing to loss in diversity. Direct forest restoration techniques, such as planting and creation of canopy gaps via regeneration-focused harvesting, are needed if C. illinoinensis is to return to historical abundance and distribution.