The ecology of Atlantic white cedar wetlands - USGS National ...

Figure 7. Atlantic white cedar logs in exposed freshwater peat underlying a salt marsh on Buzzard's Bay,Massachusetts. Note that many trunks and roots remain as they grew in the forest floor. Photo by I. Laderman.Atlantic white cedar appears to have movedsouthward to refugia on the Gulf Coastal Plain duringfull glaciation (Belling 1977; Delcourt and Delcourt1977). It probably began its northward migrationfrom the Gulf refugia during the late glacial period,between 17,000 and 10,000 years B.P (Belling 1977and unpubl.). Some evidence for this view is thatcedar (Cupressaceae) pollen grains are found inNorth Carolina sediments that predate the mostrecent glaciation (25,000 yrs B.P.), but are absentduring the glacial epoch (21,000 to 10,000 yrs B.P).Cupressaceae pollen reappears there at 10,000 yrsB.P (the beginning of the present interglacial period),and is continuously recorded in the peats until thepresent time (Whitehead 1981 ).Dated macrofossils of Atlantic white cedarfrom as early as 9500 yrs B.l? (Watts 1979) and 7700yrs B.P. (Psuty et al. I 983) were recorded from unglaciatedsites (Table 1; Figure 8). Mostpalynologists do not distinguish between the pollengrains of Thuja, Juniperus, and Chamaecyparis,which are all in the family Cupressaceae and are verysimilar in pollen morphology (see Figures 2 and 3).Belling (1977 and unpubl.) uses macrofossilevidence in conjunction with pollen data to separatethe three genera and outlines a probable sequenceof cedar migration in the glaciated region. Arrival ofthe species at specific sites during postglacial timewas determined by radiocarbon dating results forpeats containing both macrofossil and pollen evidence.Belling (unpubl.) postulates that northwardmovement of Atlantic white cedar was influencedmore by the distance from the nearest refugium (i.e.,the seed source) and the availability of suitablegrowth sites than solely by warmer climate. Themost suitable sites are those with a favorable waterregimen (discussed in Section 3.2 [silvical habits]and Section 4.1 [hydrology]) and a consolidatedpeat substrate.Basin depths range from 3 to 9 m in glacialsites; the build-up of peat is evidence of the rise inwater tables throughout the region. Belling (1977and unpubl.) noted that Atlantic white cedar was virtuallycontinuous in all sampled glaciated sites fromthe time of its establishment to the present.Peat contains an excellent record of eventsand biological succession. Sediment cores fromcedar bogs in the glaciated region reveal a welldefinedvertical stratigraphy (Figure 9). At most sites,the overlying organic layer consists, in descendingSequence, of woody cedar peat, woody-fibrous or fibrousshrub peat, sedge peat, mossy peat (rarely),and finally gyttja formed from benthic and planktoniclake flora and fauna. The inorganic basal sedimentsare composed of sand and/or clay. Water layers mayinterrupt the sediments.