Stand dynamics and structure of old-growth Fraxinus nigra systems: Critical baselines for a foundation species in peril

Background/Question/Methods

Black ash (Fraxinus nigra Marsh.) is a foundation species dominating millions of hectares of lowland forest ecosystems in north temperate and sub-boreal regions of North America.  Despite being a dominant feature on the landscape in many regions, little is known about the natural stand dynamics or structures characterizing these systems.  Such information is becoming increasingly important given the threats posed to these ecosystems by the introduced emerald ash borer (EAB).  The goal of this study was to characterize the developmental dynamics and structure of six old-growth black ash forests in northern Minnesota prior to their infestation by EAB.  Each area contained one, large (0.7 ha) stem-mapped plot within which increment cores were extracted from all trees > 10 cm DBH resulting in a total of 1,890 core samples.  In addition, the size and spatial distribution of live and dead trees and volume and characteristics of coarse woody debris (CWD) were measured within each plot.  Increment cores were cross-dated, measured, and analyzed for growth releases indicating past disturbances.

Results/Conclusions

The old-growth systems examined had either double-cohort or multi-cohort age structures with distinct establishment peaks indicative of past gap-scale disturbances.  There was synchrony in establishment peaks across several of sites, particularly during the 1860s and 1930s.  These peaks corresponded with drier growing season conditions, which may have provided greater establishment opportunities in these lowland ecosystems.  Canopy trees ranged in age from 150-270 years and live tree diameter distributions were either reverse-J or rotated sigmoid in nature. Coarse woody debris volumes ranged considerably across sites (58-202 m³ ha^-1) reflecting past variation in canopy disturbance rates.  In particular, sites with higher accumulations of coarse woody debris had experienced greater levels of canopy disturbance (10-18% of canopy area) during the 1970s and 1980s reflecting localized wind disturbance and the loss of canopy American elm (Ulmus americana L.) to Dutch elm disease. A large proportion of the regeneration in these systems was American elm and black ash suggesting little resilience to the impending loss of overstory black ash to EAB, particularly given the already pervasive effects of Dutch elm disease in these forests.  These findings represent an important baseline for the development of conservation and management strategies for these highly threatened ecosystems.