Environmental predictors of annual incremental Populus deltoides growth, and riparian forest structure of the South Platte River in northeastern Colorado

Abstract

Riparian forests are biologically diverse systems that provide essential ecological services, such as flood attenuation, bank stabilization, habitat, nutrient cycling, temperature regulation, etc., for the landscapes they occupy. The present-day South Platte River riparian forest is dominated by native phreatophytes (Populus and Salix species) which require hydrologic disturbance to reproduce. However, with changing water-use patterns and hydrology in the South Platte basin of Colorado, the future riparian forest status is unknown. This study describes the contemporary forest composition and age structure. Data was collected along transects from seven randomly selected sites within three randomly positioned 30-km river sections between Kersey and Julesburg, Colorado on the South Platte River. A ring width chronology was developed using cores from 237 Populus deltoides (plains cottonwood) trees and was used in linear mixed modeling to describe relationships between climate, hydrology, and site attributes that affect annual biomass production (Basal Area Increment). Populus deltoides dominate the riparian forest overstory, while later successional species (Ulmus pumila and Fraxinus pennsylvanica) are present at low densities. Though the timing of recruitment has varied among sites, overall recruitment of P. deltoides is abundant, reflecting the ongoing flow-related channel change occurring in this system. Summer mean flows, as well as November and March mean flows, and climate factors (Palmer Drought Severity Index) during the growing season were responsible for variations in P. deltoides annual tree growth, as were tree age, site, and attributes of the individual trees. Our findings contrast with previously hypothesized successional trajectories for this system, which predicted the replacement of the Populus-Salix overstory by later successional species (Ulmus pumila and Fraxinus pennsylvanica). The linear mixed model results highlight the importance of not only summer season flows and climate, but also the potential effects of off-season flow variables in a changing system.