Abstract
Floodplain forests along the Upper Mississippi River System (UMRS) are highly influenced by the human-altered river flow regime. More intense and frequent floods are inducing population decline in less flood-tolerant tree species. Northern pecan (Carya illinoinensis (Wangenh. K.Koch)) is showing increasing defoliation and mortality with a lack of regeneration. We investigated the biometric and site factors related to the decline in C. illinoinensis by recording species composition, health status, diameter at breast height, and tree location for a total of 15 plots along the UMRS floodplain. Additionally, we collected increment cores to study the disturbance regime, and the effects of climate and river flow on radial growth. We applied a principal component analysis and a mixed effect regression model to determine the factors related to C. illinoinensis health at site and individual tree level, respectively. Two major disturbances occurred during the 20th century (in 1965 and 1993), both related to extraordinary discharges in the river that led to significant tree mortality in the overstory. C. illinoinensis exhibited growth release after the disturbances, however, the incidence of recruitment for this species was very scarce during the 20th century. Our analysis revealed that healthy C. illinoinensis are predominately located in the southern portion of our study area, which is characterized by less frequent extraordinary floods and less stand competition. We thus suggest C. illinoinensis-oriented management efforts, especially in the northern extent of the species range, to prevent C. illinoinensis extirpation and subsequent loss of tree diversity in the UMRS.