TREE RINGS, MOSSES, AND OTHER PLANT MACROFOSSILS FROM A LATE-GLACIAL TERRESTRIAL ASSEMBLAGE, COASTAL MAINE, INDICATE VARIABILITY IN MICROENVIRONMENTS AND ATMOSPHERIC RADIOCARBON CONTENT

Segments of white spruce (Picea glauca) logs, dispersed buds of balsam poplar (Populus balsamifera), and plants of 20 mosses were found with other fossils in thin peat mats below glaciomarine clay and above a subaqueous glacial fan at the new Mercy Hospital complex site on the north bank of the Fore River, Portland, Maine.  The mosses document variation in microsite conditions within a ~100 m² area, including forest floor, calcareous seep, and dry soil assemblages.  Ring patterns in the trees are very similar. The trees died at the same time, and their growth ends with a complete ring below the bark.  This latter condition, plus dormant buds on the associated white spruce twigs and balsam poplar buds in their morphologically dormant phase, indicate that the trees died during the winter.  During the 200-year lifespan of the trees, three substantial changes in ring growth occurred: 1) a change from symmetrical, circular, narrow rings to wider ovoid rings with compression cell growth began at 80 years, indicating that the trees underwent a change from growing on a level surface to growing on various slopes; 2) at 140 years compression growth ended, and the ovoid rings were replaced by tiny symmetrical rings and decreased cell density for about 20 years, indicating unfavorable conditions such as water rise or soil slumping; 3) at 160 years the cell growth regained some lost density and about half the trees grew wider rings, indicating improved site conditions favorable to growth.  This continued for the next 40 years until the trees died during a single catastrophic, probably erosional, event.  Seven decadal segments in the last 110 years of the trees' lifespan were radiocarbon dated.  The ages ranged from 11,907 ± 31 to 11,721 ± 41 ¹⁴C BP, with no significant decrease in radiocarbon age over the 110 calendar years.  This implies that atmospheric radiocarbon content varied greatly during that period.  Radiocarbon wiggle-matching for those ages indicate that the trees grew from 13,828 to 13,628 ± 18 Cal BP, based on the IntCal04 calibration curve and calculated by OxCal, version 3.10.