• Fraver, Shawn
  USDA Forest Service
  sfraver@fs.fed.us
  
• Palik, Brian J.
  USDA Forest Service
  bpalik@fs.fed.us
  

Questions: What is the natural range of variability in stand and age structure of old-growth Pinus resinosa-dominated forests? Does the spatial pattern of tree ages provide insights into past disturbances that structured these forests?

Location: Old-growth P. resinosa forests at seven sites in Minnesota, USA.

Methods: We applied detailed dendrochronological methods to living and dead material to reconstruct stand and age structures prior to European settlement of the region. We linked dendrochronological data to mapped stem locations to shed light on spatial patterns of past disturbance.

Results: Data pooled across sites revealed a mean living tree basal area of 34.8 m2·ha-1, snag basal area of  6.9 m2·ha-1 and down woody debris (DWD)  volume of 100 m3·ha-1. Living tree diameter distributions varied between sites; however, at all sites, smaller diameter classes were dominated by non-pines, many pre-dating the onset of fire suppression in the 1920s. There were a variety of P. resinosa age structures, including one-cohort, two-cohort (at times with additional sporadic recruitment) and three-cohort stands. For multi-cohort stands, the spatial pattern of cohorts shows considerable within-stand patchiness.

Conclusions: The wide range of stand and age-cohort structures in these old-growth P. resinosa stands depicts pre-settlement forests more complex than those of the single-cohort, post-stand-replacing-fire model that has guided regional forest management. Within-stand patchiness of cohort age structures implies disturbances operating at scales smaller than typically associated with this regional forest type. Presence of non-pine ‘ingrowth’ on all sites might suggest that P. resinosa stands supporting these species lie within the natural range of variability for this community type, representing situations in which surface fires did not occur for extended periods or were spatially patchy at the stand scale. The diversity of reference conditions documented here suggests targets that might guide ecological restoration prescriptions for these ecosystems, with the goal of reintroducing structural and compositional complexity reflecting natural disturbance and stand development.