Spatial and temporal patterns and dynamics in old-growth northern hardwood and mixed forests of northern Maine

Spatial and temporal patterns and dynamics were studied in the 2000-ha old-growth Big Reed Forest Reserve in northern Maine using stem mapping, dendroecology, GIS, and spatial analyses. Landscape-level analyses indicated that the Reserve was dominated by late-successional community types composed of shade-tolerant species, and mostly mature forests with high canopy closure and trees reaching maximum possible heights. The complex of inter-related topographic and soil characteristics, and predominantly small-scale disturbance regimes afforded a tight link between site factors and vegetation distribution on the landscape.; Stand-level analyses in northern hardwood and mixed forest types of the Reserve indicated smaller maximum-tree sizes, lower basal-area values (26–34 m2/ha) and downed-wood volume (29–64 m3/ha), higher densities (475–649 stems/ha), but similar species longevities compared to other mesic old-growth forests further south and in the Lake States. The stands were dominated by very shade-tolerant species, including Fagus grandifolia, Acer saccharum, Picea rubens and Abies balsamea, with each species found in many crown positions and age and size classes. Most species had reverse-J shaped diameter distributions, but age distributions were indicative of synchronous, episodic recruitment. Most species formed small-scale clusters (≤15 m) perhaps in response to small gap disturbances. Snags, the dominant dead wood type, were randomly distributed.; Disturbance regimes in hardwood and mixed-forest types were delineated using release criteria and estimation methods chosen after a statistical comparison of prevalent methodology. Broad-scale partial disturbances occurred every 60–70 years since the 1780s, in 1791–1810, 1861–1880, 1941–1960; or every 30–40 years with the inclusion of three smaller disturbances in the 1830s, 1910s and 1970s. However, gaps were small (average size being 40–50 m2). The peak disturbance periods with many small single-tree gaps and a few large gaps served as primary venues for recruitment of the dominant shade-tolerant species. The disturbance events affected large areas, but only a small proportion of a given area. Average annual disturbance rates (1.05–1.27%) were higher and canopy turnover times (<100 years) lower than in other northeastern and northcentral mesic forests studied. Dominant known disturbance agents were insect and disease outbreaks, and perhaps the hurricanes of 1788 and 1869.