Modeling Riparian Forest Dynamics and Structure on Alluvial Rivers; John Stella

 

 

 

¹ State University of New York College of Environmental Science and Forestry (SUNY-ESF), Syracuse, NY, USA; stella@esf.edu

² Marine Science Institute, University of California Santa Barbara, Santa Barbara, CA, USA; li.kui@ucsb.edu

³The Nature Conservancy, Chico, CA, USA; ggolet@tnc.org

⁴ ESSA Technologies Ltd, Squamish, British Columbia, Canada; fpoulsen@essa.com

 

 

Riparian forest development on alluvial rivers is tightly linked to the morphodynamics of channels and floodplains. Pioneer trees establish on fresh alluvium deposited by floods, and forest structure and composition co-evolve with changes in floodplain morphology and soils. Restoring natural riparian communities on rivers altered by flow regulation and channel modifications requires an understanding of how forests develop in response to physical drivers in order to set realistic ecological targets and track progress toward restoration goals. In this context, we developed a simple dynamic model of riparian forest structure using tree inventory data from a 100-year chronosequence of floodplain sites on the middle Sacramento River (CA, USA). The resulting predictions of tree density and diameter distributions are applicable to forecasting how key ecosystem properties such as biomass, community composition, and inputs of large woody debris change over time along active alluvial rivers.

 

Using an extensive riparian forest inventory of 431 plots distributed throughout a 160-km river reach, we fitted Weibull distributions of stem diameter and density, modeling its shape and scale parameters as time-varying functions of floodplain age. Pioneer and later successional species were modeled as separate groups, as they have distinct trends in their densities and size distributions with forest age. Early-successional willow and cottonwood species had high initial densities that decreased sharply within 20–30 years, whereas later-successional trees including box elder, walnut and oak increased slowly in density and size over 60 years from the time of floodplain creation. As a result, the largest diameter classes were dominated for >50 years by early successional species, which comprised the greatest source of biomass, instream large wood and foundational habitat structure during that timeframe. Coupled with physical models of channel and floodplain dynamics, this relatively simple phenomenological approach to modeling riparian forest dynamics allows floodplain managers to predict general characteristics of riparian forest structure and to compare quantities of biomass, carbon storage or large wood recruited to the river ecosystem under various existing and future restoration scenarios.