Tree-ring analysis of buried plains cottonwoods of the floodplain of the Powder River, Montana
Date
2018
Authors
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Publisher
University of Delaware
Abstract
Creating accurate temporal and spatial chronologies of floodplain deposits contributes to a greater understanding of the fluvial processes, ecological implications, and historical records associated with a riparian system. Dendrochronology is a powerful chronologic tool that, when combined with stratigraphic correlations along a floodplain, can be used to create an extensive profile of a river system. Dendrochronological dating of buried trees precisely determines the germination year and identifies the stratigraphic context of germination for the trees. This recently developed application of dendrochronology provides accurate time-averaged sedimentation rates of overbank deposition along floodplains. Previous studies have demonstrated that tamarisk (Tamarix ramosissima), sandbar willow (Salix exigua), green ash (Fraxinus pennsylvanica), and balsam poplar (Populus balsamifera) develop anatomical changes within the tree rings (including increased vessel size, less distinct annual ring boundaries, and decreased ring widths) on burial. However, there have been no observations recorded for trees larger than 60 cm in diameter, older than a hundred years in age, or for plains cottonwood (Populus deltoides, ssp. monilifera) the most abundant riparian tree in the western United States. Dendrochronology can also be used to determine if specific riparian tree species germinate in even-aged stands, providing insight to the geomorphic and hydrologic context of germination. In September 2016, June 2017, and September 2017, six buried plains cottonwoods were excavated along a single transect of a meander bend of the Powder River, Montana. Nineteen additional trees were cored at 1.3 m above ground level in September 2017. Samples from all six buried plains cottonwoods exhibit anatomical changes associated with burial events that are observed in other species, which can be correlated to flood years along the Powder River. Center years of the excavated trees, estimated from cores taken 1.3 m above ground level, range from 2014 to 1863. Accurate establishment years determined by cross-dating the buried section of the tree can add at least an additional 13 years to the cored age. The germination year, total sediment accumulation, and average sedimentation rate at the six sampled trees is: 2011, 35 cm, 7.0 cm/year; 1985, 29, 0.9 cm/year; 1972, 77 cm, 1.7 cm/year; 1947, 123 cm, 1.8 cm/year; 1927, 82 cm, 0.9 cm/year; and 1860, 112 cm, 0.7 cm/year. Four of the six trees germinated within or on bedload deposits, while the remaining two germinated within suspended load deposits. This indicates that plains cottonwoods germinate on bars and within overbank deposits along the Powder River. The precise germination age and depth provide elevations and minimum age constraints for the bar deposits and maximum ages for the overlying sediment, helping constrain past channel positions and overbank deposition rates. The additional trees cored 1.3 cm above ground level range in center year at sample location from 1871 to 1986 and do not necessarily occur in even-aged stands, again indicating that plains cottonwoods germinate both on bar and overbank deposits in this area.
Description
Keywords
Earth sciences, Buried trees, Dendrochronology, Floodplain, Meandering river, Montana, Populus deltoides, Powder River, Ssp. monilifera