A re-evaluation of biomass dynamics and riparian functions in late-successional forests of the northeastern U.S.

Background/Question/Methods

Riparian forests regulate linkages between terrestrial and aquatic ecosystems, yet relationships among riparian forest development, stand structure, and stream habitats are poorly understood in many temperate deciduous forest systems.  Our research in the Adirondack Mountains of New York has 1) described structural attributes and gap dynamics associated with old-growth riparian forests; and 2) assessed linkages between these characteristics and in-stream habitat structure, light availability, and periphyton production.  The study has also evaluated competing hypotheses regarding late-successional biomass dynamics in northern hardwood-conifer forests.  To explore these questions we sampled forest structure and in-stream attributes along 29 first and second order stream reaches.  Average age of the largest, dominant trees ranged from approximately 81 to 410 years.

Results/Conclusions

Our results indicate that old-growth riparian forest structure is more complex than that found in mature forests and exhibits significantly greater accumulations of aboveground tree biomass, both living and dead.  Aboveground biomass was positively (p < 0.001) and logarithmically correlated with dominant tree age; this held for live trees (r² = 0.52), standing dead trees (r² = 0.36), total trees (r² = 0.63), and downed woody debris (r² = 0.24).   Biomass approached maximum values in stands with dominant tree ages of approximately 350 to 400 years.  In-stream large woody debris volumes were significantly greater at old-growth sites (200 m³ ha^-1) compared to mature sites (34 m³ ha^-1) and were strongly related to the basal area of adjacent forests.  AIC models incorporating large log density, debris dam density, boulder density, and bankfull width had the most support for predicting pool density.  Canopy gaps were abundant, positively correlated with concave landforms, and well-distributed along stream reaches.  Chlorophyll a concentrations, indicative of periphyton abundance, were highly correlated with the spatial distribution of light availability, suggesting that structurally complex riparian forests create a mosaic of heterotrophic (shaded) and autotrophic (lighted) microhabitats.  This finding challenges our understanding of in-stream energy dynamics in low order, late-successional riparian systems. 

That aboveground biomass and carbon storage have the potential to accumulate very late into succession in northern hardwood-conifer forests suggests a need to reexamine theoretical biomass development models for these systems. Old-growth riparian forests provide in-stream habitat features that have not been widely recognized in eastern North America.  Riparian management practices – including buffer delineation and restorative silvicultural approaches – that emphasize development of late-successional characteristics may be useful where the associated in-stream effects are desired.