Moisture and temperature changes associated with the mid-Holocene Tsuga decline in the northeastern United States
Introduction
Davis (1981) described an abrupt hemlock (Tsuga spp.) decline across eastern North America at ca 5.5 ka (thousands of calibrated radiocarbon years before 1950 AD), noting sharp decreases in hemlock pollen abundances from Michigan to Maine. The decline was initially attributed to a pathogen or pest outbreak because of 1) the range-wide abrupt nature of the decline, 2) the apparent change in only one taxon, 3) the similarity of the event's palynological signature to recent forest disease outbreaks, and 4) a lack of evidence for concurrent climate change or other drivers (Allison et al., 1986). The change, therefore, appeared to represent an abrupt ecological event independent of progressive Holocene climate changes. Since then, additional work has linked the decline to insect outbreaks (Anderson et al., 1986, Bhiry and Filion, 1996), dated the mid-point of the regional decline to 5.5 ± 0.38 ka (Bennett and Fuller, 2002), and documented the decline as one of the most abrupt vegetation changes of the past 8 ka in North America (Shuman et al., 2005, Shuman et al., 2009).
New evidence, however, challenges the hypothesis that the period of low hemlock abundance resulted from biotic interactions independent of climate. Evidence for regional to hemispheric climate change at ca 5.5 ka now includes reduced lake levels and other evidence of drought from Ontario to New Hampshire (Yu et al., 1997, Haas and McAndrews, 1999, Lavoie and Richard, 2000, Newby et al., 2000, Newby et al., 2009, Newby et al., 2011, Shuman et al., 2001, Shuman et al., 2004, Shuman et al., 2005, Shuman et al., 2009, Muller et al., 2003, Booth et al., 2012), changes in atmospheric circulation (Yu et al., 1997, Kirby et al., 2002), and shifts in sea-surface temperatures (SSTs) (deMenocal et al., 2000, Sachs, 2007). Pollen records from areas near the range limits of hemlock also indicate climate change (Calcote, 2003, Foster et al., 2006), and importantly, show that hemlock did not change independently of other taxa.
The hemlock decline was concurrent with increases in Fagus (beech) and Pinus (pine) and decreases in Quercus (oak) populations in coastal Massachusetts (Tzedakis, 1992, Foster et al., 2006), as well as potentially synchronous (within dating uncertainties) beech declines (Williams, 1974, Shane and Anderson, 1993), Ulmus (elm) declines (Williams et al., 2004, Nelson et al., 2006, Grimm et al., 2009), and prairie expansion (Nelson et al., 2006) in the Midwest. However, the relative timing of the various climate and ecological changes has only been examined in detail at a few locations where simple cause-and-effect relationships were not apparent (Booth et al., 2012). The detailed site-specific analyses raise questions about the relative rates and timing of vegetation and moisture changes at local spatial scales and fine time scales (Booth et al., 2012), but the range-wide maintenance of low hemlock abundance and other contemporaneous features of the regional vegetation (e.g., high coastal beech abundance) for millennia after the decline could be consistent with the persistence of a new climatic regime, which also contributed to low lake levels and other changes (Shuman et al., 2004, Foster et al., 2006).
The role of temperature during these mid-Holocene changes has not been examined and may be essential to understanding the associated coastal oak-beech dynamics, in particular. In fact, pollen data from New England show different vegetation changes at inland and coastal sites (Foster et al., 2006), and suggest a persistent change in climate gradients across southern New England at the time of the hemlock decline. Decreases in oak and increases in beech populations might indicate that an increase in fog or other forms of effective moisture buffered the coast from drought experienced inland (e.g., Yu et al., 1997, Newby et al., 2011, Booth et al., 2012). Alternatively, the temperature gradient between coastal and inland sites across southern New England may have changed; a decrease in temperatures at the coast and an increase in temperatures at inland sites (ca 5.5 ka BP) could be consistent with the observed changes. Similar shifts in moisture and temperature gradients have been observed historically in association with atmospheric and ocean circulation changes in the AD 1960s when both temperatures and precipitation in New England were below normal during the most severe historic drought (Namias, 1966).
Testing these hypotheses requires investigations of both inland and coastal effective moisture and temperature histories. Effective moisture, discussed here as the balance of precipitation (P) and evapotranspiration (E), can be reconstructed by evaluating lake sediments to locate the position of lake shorelines through time using ground-penetrating radar (GPR) and various sedimentary analyses (Pribyl and Shuman, Digerfeldt, 1986, Dearing, 1997, Shuman, 2003, Shuman et al., 2005). For this study, we provide the record of lake-level changes at Deep Pond, a kettle lake located on Cape Cod, MA (Table 1, Fig. 1A) where the mid-Holocene oak decline and beech rise were first documented (Foster et al., 2006), and compare the lake-level record with effective moisture (P–E) data from inland sites to address the hypothesis that varying moisture levels during the mid-Holocene contributed to or sustained the hemlock decline.
We use regional sea-surface temperature (SST) reconstructions (Sachs, 2007) to help evaluate possible coastal temperature trends, but calibrate the pollen data themselves to quantify what temperature changes (if any) would be consistent with the abrupt vegetation changes. By doing so, we build on previous efforts to use pollen data to reconstruct regional climate gradients (Webb et al., 1993), but simply use the modern analog technique (MAT) (Overpeck et al., 1985, Guiot et al., 1993) to evaluate how the temperature and precipitation preferences of the regional vegetation changed. Because inferring a climatic cause of the vegetation changes from the pollen data would be circular, we use the approach to ask what magnitudes and directions of climate change could be consistent with the pollen record. Comparisons with the lake-level and SST records then enable us to evaluate whether the vegetation changes are consistent with the independently constrained climatic history.
We will evaluate three aspects of environmental change at the time of the hemlock decline: 1) past hydroclimatic trends at Deep Pond for comparison with inland water-level declines; 2) past changes in the temperature preferences of the coastal and inland vegetation; and 3) past agreement among vegetation climate preferences and independent climate indicators. We use our sediment-based lake-level reconstruction approach to pursue the hydroclimate changes, and then apply the MAT to pollen data to reconstruct the gradients and consistency of the vegetation climate preferences. We first apply the MAT to well-dated, detailed pollen data from a pair of inland and coastal sites in Massachusetts, Little Pond-Royalston (LPR) and Deep Pond, respectively (Table 1, Fig. 1A), and then use a wide array of available data (e.g., SST, lake level, and temperature preference data) from southern New England to examine the spatial patterns and potential drivers of mid-Holocene climate changes during the hemlock decline.
Section snippets
Primary study site
We reconstruct the lake-level history of Deep Pond (41.581°N, −71.640°W, 23 m above sea level), a 0.89 ha, ∼3.5-m-deep, closed-basin kettle lake at the base of Cape Cod near Falmouth, MA on the Buzzards Bay Moraine (Fig. 1). The pond lies within glacial till and outwash on a perched aquifer underlain by fine-grained sediments. The fine sediments isolate the pond from interactions with deep groundwater and sea-level fluctuations (Oldale, 1992), and allow lake-level fluctuations to result
Lake-level changes at Deep Pond
Stratigraphic changes at Deep Pond show evidence of a long-term rise in water levels based on shoreward expansion of deep-water sediments (Figs. 6 and 7). The rise culminated in high water after ca 3 ka consistent with other regional reconstructions (e.g., Shuman et al., 2001). Additionally, 17 stratigraphic units, including 8 high LOI units (even numbered events) and 9 low LOI units (odd numbered events), interrupt the long-term shoreward expansion of sediments (Figs. 6 and 7). The low LOI
Reconstruction similarities and their implications
Many of the lake-level changes interpreted from stratigraphic units at Deep Pond correspond with similar evidence of lake-level changes at Davis and Crooked Ponds, MA (Newby et al., 2009, Newby et al., 2011, Shuman et al., 2001, Shuman et al., 2009) as well as other sites in the region (see discussion of the comparison of New Long Pond, MA and White Pond, NJ with Davis Pond in Newby et al., 2011). All of these sites experienced brief episodes of low water recorded by sand layers in intermediate
Conclusions
The hemlock decline represents the most abrupt vegetation change of the Holocene in eastern North America, and although extensively studied, it remains poorly understood. Evidence of a synchronous oak decline, and beech and pine increases at coastal Massachusetts, raise questions about potential climatic drivers of these extensive vegetation changes. Even if climate did not cause the declines, our results provide support for the interpretation that drought helped to maintain the subsequent
Acknowledgments
This work was supported by National Science Foundation grants to Shuman (DEB-0816731) and Foster and Oswald (DEB-0815036). We also thank S.T. Jackson, P. Heller, T.A. Minckley, B.R. Marsicek, J.L. Gill, and three anonymous reviewers for helpful comments that greatly improved the manuscript; G.E. Carter, J. Calder, J. Donnelly, P. Pribyl, T. Miller, and D. Colvard for field and lab assistance; D.J. Marsicek and L.A. Marsicek for moral support.
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