- Rowland, Erika L.
University of Maine Graduate School
Numerous factors have the potential to influence the effects of disturbance events on forest stands. A subset of these factors includes stand composition and structure and topographic setting. This study compared the natural disturbance patterns in conifer-northern hardwood stands (n=8) in slope and lowland topographic settings by examining current compositional and structural characteristics, disturbance histories reconstructed using dendroecological methods, and late-Holocene vegetation dynamics from closed canopy pollen sites.
Red spruce (Picea rubens Sarg.) was the dominant species in all stands, mixing primarily with northern white-cedar (Thuja occidentalis L.) in the lowland conifer- dominated sites and sugar maple (acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh.), and balsam fir (Abies balsamea (L.)) in the mixedwood and upland sites. The modern structural characteristics of the sites were remarkably similar, regardless of landscape setting or composition. No differences in the disturbance histories of the stands were clearly associated with composition and landscape setting. Low-intensity disturbances (<10%/decade) creating small canopy openings have prevailed in the sites since at least the 1850s. While significant variability in disturbance rates occurred during the decades from 1850 to 1980, average decadal rates (percent canopy area removed) were low, ranging from 6.1% to 9.7 %. Most stands had at least 2-3 decades during which -20%-30% of the canopy area was opened.
Even during these pulses of moderate intensity disturbance, most o1'the canopy openings were <50-100 m2, favoring the regeneration and canopy accession of shade-tolerant tree species. The palynological studies suggest that the disturbance patterns of the last 150-200 years extend through the past 1000 years. Consistent with the disturbance histories reconstructed from tree rings, no evidence of stand replacing fire or other catastrophic disturbance was observed in the sedimentary and pollen records. Spruce (and, perhaps, northern-white cedar) appears to have dominated in the conifer stands since the regional expansion of spruce species in late Holocene. Inferred from the compositional stability evident in the pollen record and the associated evidence, the disturbance dynamics reconstructed for the historical period. small canopy gaps combined with periodic moderate intensity events, have maintained stands dominated by shade-tolerant, late-successional tree species throughout the last millennium.