Predictable Hydrological and Ecological Responses to Holocene North Atlantic Variability

Predictable hydrological and ecological responses to Holocene North Atlantic variability

Bryan N. Shumana,1, Jeremiah Marsicekb, W. Wyatt Oswaldc, and David R. Fosterd

aDepartment of Geology and Geophysics, University of Wyoming, Laramie, WY 82072; bDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706; cInstitute for Liberal Arts and Interdisciplinary Studies, Emerson College, Boston, MA 02116; and dHarvard Forest, Harvard University, Petersham, MA 01366

Edited by David J. Sauchyn, University of Regina, Regina, SK, Canada, and accepted by Editorial Board Member David W. Schindler February 5, 2019 (received for review August 20, 2018) Climate variations in the North Atlantic region can substan- AMOC is strong, heat is transferred northeast into the subpolar tially impact surrounding continents. Notably, the Younger Dryas region, while the NAM cools (1, 6). In winter, the NAO expresses chronozone was named for the ecosystem effects of abrupt similar atmospheric temperature anomalies (Fig. 1D). Multi- changes in the region at circa (ca.) 12.9–11.7 ka (millennia before century variability in the NAM is recorded by alkenones from 1950 AD). Holocene variations since then, however, have been cores such as OCE326-GGC30 off Nova Scotia (7) (Fig. 1A,dark hard to diagnose, and the responsiveness of terrestrial ecosystems blue line) and by correlated changes in pollen from coastal lakes continues to be debated. Here, we show that Holocene climate (8). Consistent with AMOC or NAO dynamics, the centennial variations had spatial patterns consistent with changes in Atlantic variability shows an antiphased relationship with temperatures in overturning and repeatedly steepened the temperature gradient Greenland, which is sensitive to the state of the subpolar North > between Nova Scotia and Greenland since 8 ka. The multicentury Atlantic [as represented by the Greenland Ice Sheet Project 2 changes correlated with hydrologic and vegetation changes in the (GISP2) ice core (9); light blue line in Fig. 1A]. northeast United States, including when an enhanced temperature Three patterns of temperature variation in the Greenland– gradient coincided with subregional droughts indicated by water- NAM gradient exist at 102–104 y (Fig. 1 A and B). First, long- level changes at multiple coastal lakes at 4.9–4.6, 4.2–3.9, 2.8–2.1, term progressive cooling of the NAM, apparent in the original SCIENCES and 1.3–1.2 ka. We assessed the variability and its effects by rep- alkenone time series from OCE326-GGC30 (7), would have ENVIRONMENTAL licating signals across sites, using converging evidence from mul- > tiple methods, and applying forward models of the systems decreased the temperature gradient by 5 °C over the Holocene involved. We evaluated forest responses in the northeast United (Fig. 1B, orange line). However, the cooling does not appear in States and found that they tracked the regional climate shifts in- adjacent continental records, which indicate lower (rather than cluding the smallest magnitude (∼5% or 50 mm) changes in effective precipitation. Although a long-term increase in effective precipita- Significance > > tion of 45% ( 400 mm) could have prevented ecological commu- ECOLOGY nities from equilibrating to the continuously changing conditions, The importance of climate change arises from effects on nat- our comparisons confirm stable vegetation–climate relationships ural resources like water and ecosystems. Diagnosing the pre- and support the use of fossil pollen records for quantitative dictability of these effects in the past can help to anticipate paleoclimate reconstruction. Overall, the network of records indi- future changes, while also clarifying the paleoclimate record. cates that centennial climate variability has repeatedly affected We examined >8,000 y of climate variations and their effects in the North Atlantic region with predictable consequences. the North Atlantic region, where ocean and atmosphere processes have often produced global consequences and altered drought | vegetation | North Atlantic | Holocene | climate climates from one century to the next. We found that repeated temperature variations across the region followed patterns his paper examines the expression of Holocene climate var- consistent with climate dynamics that affected water and for- Tiability in the North Atlantic region and then tests hypotheses ests in North America. Forest plant communities changed rap- about the attendant ecological responses on adjacent continents. idly and predictably, which underscores both the risk of future Both aspects of the paper relate to the overarching hypothesis that change and the potential to reconstruct paleoclimates using multicentury variability in Atlantic Meridional Overturning Cir- fossil evidence. culation (AMOC) (1, 2) or in the atmospheric dynamics underlying Author contributions: B.N.S., W.W.O., and D.R.F. designed research; B.N.S. and J.M. per- the North Atlantic Oscillation (NAO) (3) had substantial influ- formed research; B.N.S. and J.M. analyzed data; B.N.S. wrote the paper; and J.M., W.W.O., ences on synoptic climates and thus the ecosystems of the North and D.R.F. contributed to drafting the paper. Atlantic region during the Holocene. If so, quantitative paleo- The authors declare no conflict of interest. environmental records across the region should contain replicable This article is a PNAS Direct Submission. D.J.S. is a guest editor invited by the and synoptically articulated signals, although the importance of Editorial Board. these climate signals to ecological dynamics has been uncertain. Published under the PNAS license. Marine records of Holocene AMOC vary (2, 4, 5), but the Data deposition: All data analyzed for this study are publicly available through the Na- climate dynamics affecting the hydrology and ecology of adjacent tional Oceanic and Atmospheric Administration Paleoclimate database, including the temperature reconstructions from GISP2 (https://www.ncdc.noaa.gov/paleo-search/study/ landmasses must have also included large-scale atmospheric 2475)andGGC30(https://www.ncdc.noaa.gov/paleo-search/study/6409). The lake-level changes even if multicentury climate variability in the region reconstructions are archived at https://www.ncdc.noaa.gov/paleo-search/study/16094 originated in the ocean. Therefore, we first describe changes in the (for Davis Pond), https://www.ncdc.noaa.gov/paleo-search/study/16095 (for Deep Pond), and https://www.ncdc.noaa.gov/paleo-search/study/23074 (for New Long Pond). The marine and atmospheric temperature gradient across the north- pollen-inferred precipitation reconstructions are archived at https://www.ncdc.noaa. west Atlantic, which may relate to the spatial temperature “fin- gov/paleo-search/study/23072. gerprint” of AMOC variability (6) (Fig. 1 C and D). AMOC 1To whom correspondence should be addressed. Email: [email protected]. variability produces contrasting temperature changes in the North This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Atlantic Ocean along the North American margin (NAM) and in 1073/pnas.1814307116/-/DCSupplemental. the larger subpolar region surrounding much of Greenland. When

www.pnas.org/cgi/doi/10.1073/pnas.1814307116 PNAS Latest Articles | 1of6 Downloaded by guest on September 25, 2021 Fig. 1. Holocene temperature histories (A) from Greenland, GISP ice core (9), and the NAM of the North Atlantic, core GGC30 (7), include negatively correlated multicentury departures from the shared long-term trends (black dashed line in A). The NAM temperatures in A have been linearly detrended to account for a >5 °C cooling trend (8, 10) to visually highlight the short-term variability, which is consistent with the temperature contrast produced today (C and D) by AMOC changes in the Gulf Stream and North Atlantic Current regions (1, 6). The difference in the nondetrended Holocene temperatures from the two regions (orange line in B) correlates with the effective precipitation (Eff. Precip.) changes indicated by lake-level changes in Massachusetts, United States (14) (blue line in B). The scatter plot in B shows the correlation (Pearson’s product moment correlation: r = 0.90; P < 0.0001; n = 220) between the two series over the last 8,000 y after the regional influence of the Laurentide ice sheet rapidly diminished (8). Maps show winter (December to February) sea-surface temperature (SST) (C) and air temperature (D) differences between AD 2008–2017 and AD 1958–1967: a decade of weak AMOC (6) and positive NAO (33) minus a decade of strong AMOC (6) and negative NAO (33). Shades of blue indicate lower than normal temperatures during the weak AMOC period, and red indicates higher than normal temperatures during the weak AMOC period based on the Hadley Centre Sea Ice and Sea Surface Temperature data set (34) in C and the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis Project (35) in D. The blue line in B is mean of the reconstructions from Deep and New Long ponds (8, 14); shading indicates the 95% uncertainty in the mean. Insets in C and D show the location of the lake-level records and the inset map in Fig. 2. A generalized additive model represents the mean multimillennial trends as the dashed line in A.Calyr,calendaryear.

higher) than present temperatures during the Younger Dryas shoreline sediments in small lakes, which shifted through time and and early Holocene (10). Removing the linear cooling trend, produced direct stratigraphic evidence of past water levels (12). Dry however, reveals the second set of long-term (>103 y) changes in- periods lowered water levels and expanded shoreline sands out cluding warming before ca. 8 ka and subsequent cooling (Fig. 1A, toward deep areas. Wet periods raised lake levels and expanded black dashed line). These trends affected the NAM (Fig. 1A,dark deep-water muds over shoreline sands. The ages and elevations of blue line), Greenland (Fig. 1A, light blue line), and eastern North the resulting layers of sand and mud in multiple cores from different America (8) similarly and track the expected regional effects of water depths thus enabled previous studies to quantitatively re- North American deglaciation and orbital change (8, 11). The third construct lake-level histories (13–15). The lake-level changes were pattern often appears as antiphased departures from the long trends correlated among lakes in the coastal NE US (Fig. 2) but were (Fig. 1A; Pearson product moment correlation of the departures sometimes negatively correlated with hydrologic changes in adja- from the regional mean trend: r = −0.29; 95% CI: −0.41 to −0.17; cent regions (14, 15); such patterns are consistent with the spatial n = 220), which routinely changed the steepness of the Greenland– heterogeneity of drought and spatially antiphased climate changes NAM gradient on multicentury scales (Fig. 1B,orangeline). associated with AMOC or NAO dynamics (Fig. 1 C and D). Here, Consistent with related atmospheric dynamics, the multi- for comparison with the Greenland–NAM gradient (Fig. 1B), we century changes in the Greenland–NAM temperature difference use an average of the well-correlated reconstructions from two correlated with hydrologic changes in the northeast United closed-basin lakes (Deep and New Long ponds; Fig. 2) in the States (NE US) (Fig. 1B, blue line; Pearson product moment coastal NE US. Each reconstruction was based on three to five correlation: r = 0.90; 95% CI: 0.87–0.93; n = 220). The hydro- sediment cores from different water depths, >50 calibrated 14C logic changes were first reconstructed using the elevations of ages, and supporting geophysical surveys (14).

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Fig. 2. Fossil pollen (A) and sedimentary evidence of lake-level change (B) reveal regional changes in effective precipitation. Overlapping green areas in A indicate individual pollen-inferred precipitation reconstructions from five lakes [Little Pond, Royalston (LPR); Blood Pond (BP); Rogers Lake (RP); Winneconnett Pond (WP); and Deep Pond; large green circles in the inset map]; overlapping blue areas in B represent the individual lake-level reconstructions, converted to effective precipitation, from Davis, New Long (NLP), and Deep Ponds (blue circles in the inset map). Gray shading (overlapping green and blue areas in A and B, respectively) shows the 95% range of multisite means produced from 1,000 iterations of leave-one-out resampling of the records with replacement. Principal component scores from the pollen (green) and lake-level (blue) datasets are plotted by time in C. For comparison, pollen percentages of oak (Quercus spp.) and hemlock (Tsuga canadensis) (plotted ×10) in B indicate abrupt ecological changes at Deep Pond (25). Vertical gray lines indicate 8.2 ka and the beginning and end of synchronously deposited low-water sand layers at the lake-level sites at 4.9–4.6, 4.2–3.9, 2.8–2.1, and 1.3–1.2ka(14).Calyr,calendaryear.

Based on the lake-level histories, the NE US was dry when the Because changes in the regional temperature gradient corre- Greenland–NAM temperature gradient was steepest (most late with water-level changes in multiple lakes (Fig. 2) and, thus, negative differences in Fig. 1B, orange line), such as when the appear to represent ocean–atmosphere dynamics that could have “8.2 ka event” reduced AMOC (16). Greenland and the subpolar been ecologically significant, we also examined the responses of region cooled (16), while at least portions of the NAM warmed forest plant communities in the NE US. Three ecological hy- modestly (7) (Fig. 1A). Conversely, repeated warm episodes in potheses embody the range of possible ecological responses: (i) Greenland at 4.2, 3.3, 2.0, and 0.9 ka coincided with modest the composition of plant communities could have remained NAM cooling (Fig. 1A) and increased moisture in the NE US correlated with climate through time because vegetation re- (Fig. 1B). The 8.2 ka event also permanently reduced the cli- sponse times, λ, were short (∼100 y) and lags, Λ, were small (18); matic influence of the Laurentide ice sheet and coincided with a (ii) vegetation composition could have slowly equilibrated and state shift in the NE US (8), central North Atlantic (2), and other developed a predictable correlation with climate only after long regions (8, 17). The shift appears to have established the cor- periods of climatic stability (λ > 1,000 y and Λ decreased over relation between the Greenland–NAM gradient and NE US time) (19); and (iii) plant assemblages could have remained in hydroclimate variability (Fig. 1B). disequilibrium with climate (Λ was persistently large) (20, 21).

Shuman et al. PNAS Latest Articles | 3of6 Downloaded by guest on September 25, 2021 Evaluating these hypotheses for the Holocene can help to affirm NAM temperature differences) than expected from the Holocene- the ecological significance of the climate variability, test the length trend (Fig. 1B). Underlying species’ responses include ability of fossil pollen records to detect such changes, and an- minima in hemlock abundance at Deep Pond (13), one of the ticipate the rates and predictability of future ecological changes. coastal ponds, at 4.9–4.6, 4.2–3.9, and 2.8–2.1 ka (Fig. 2B). Correlations among oceanic, hydrologic, and ecological variability A principal components analysis (PCA) confirms a high de- would both validate patterns of paleoclimate change and affirm the gree of shared variance (84.1%) among the eight effective pre- first hypothesis and its expectation of a close coupling of forest cipitation reconstructions (Fig. 2 A and B). Based on the PCA, composition and climate. Under a stable Holocene climate, the the three lake-level records shared 93.7% of variance (blue line, second hypothesis would predict a long-term convergence of Fig. 2C) and the five pollen-inferred reconstructions shared hydroclimate and ecological trends. However, the hypothesis is 80.6% (green line, Fig. 2C). The two sets of principal component testable in the NE US because large, long-term hydroclimate trends 1 scores also correlate closely (Pearson’s product moment cor- prevented the environment from stabilizing (8, 15) (Fig. 1B)and relation: r = 0.95; 95% CI: 0.94–0.96; n = 220). The largest should result in persistently poor correlations between climate and differences exist during multicentury events (Fig. 2C) but may vegetation composition. The third hypothesis would produce a only exist because of factors like the different geographic dis- similar outcome. It may apply to the NE US because several key tributions of the pollen and lake-level reconstructions. The plant taxa increased there later than others, hypothetically in- datasets emphasize different inland versus coastal geographic dicating dispersal lags (22), which potentially combined with fre- areas where comparisons of the individual lake-level records also quent disturbances, including anthropogenic fire and tropical reveal antiphased droughts during the mid-Holocene (14, 15). storms, to limit vegetation correlations with climate (23). Some tree The reconstruction differences rarely extend to geographically taxa also experienced abrupt declines similar to those attributed to focused comparisons at individual sites (Fig. 3). biotic (e.g., disease) (24, 25) rather than climatic causes. If so, we The lake-level and pollen-inferred changes from the same site expect persistently large differences in hydroclimate and ecological (Deep Pond) or from the averages of all coastal sites show few trends, possibly lacking systematic patterns. Because continental significant differences (Fig. 3). Scatter plots show a correlation temperatures have been difficult to reconstruct separately from consistent with a 1:1 relationship (Fig. 3, Middle), and the dif- fossil pollen records (8), we focus on hydroclimatic responses to ferences between reconstructions (Λ) indicate that lags rarely Atlantic variability to test the ecological hypotheses. exceeded the uncertainties around zero (Fig. 3, Right). The To do so, we use an index of past forest composition calibrated reconstructed effective precipitation changes are not as abrupt as to annual precipitation. When evaluated against the independent some species’ responses (24, 25) (Fig. 2B), but the abrupt lake-level data, such an index can show the degree of coupling changes do not change the community relationship to climate, as between plant communities and the hydroclimate (21). We indicated by persistent near-zero values of Λ (Fig. 3). The rela- generated the index by matching fossil and modern pollen as- tionships are consistent with plant communities assembled from semblages with equivalent compositions and then assigning each individual taxa with unique, nonlinear, but overlapping niches fossil sample the mean annual precipitation associated with the and responding to multiple climate variables. closest matching modern assemblages (analogs) (10, 13). Be- Forward modeling further confirms that vegetation predict- cause assemblages of mesic taxa indicate humid climates and ably tracked regional climate responses to Atlantic variability. xeric assemblages dry conditions, the method has been used Specifically, using the pollen-inferred precipitation changes from broadly to reconstruct paleoclimates (10), but, as noted recently Deep Pond to drive models of lake depth and associated sedi- (21), agreement with independent climate observations depends ment changes predicts both the water level history (SI Appendix, upon the predictability of plant community composition and the Fig. S1) and the major sedimentary patterns in cores from the stability of climate–vegetation relationships (Materials and Meth- same site (Fig. 4). The results correctly capture a shoreward ods). If factors such as species dispersal lags (22) or widespread increase in mineral content among cores and sequences of sand anthropogenic burning (23) altered the climate–vegetation rela- layers representing paleoshoreline deposits in cores 16 and 23 m tionships, pollen-inferred records would fail to predict the lake from shore of the ∼110-m-diameter lake (Fig. 4). Scatter plots changes. We, therefore, use the differences between lake-level and indicate a near 1:1 relationship (slope, β, of 1.05 ± 0.08) between pollen-inferred records as a measure of any ecological offsets, Λ. the predicted and observed sediment composition. To do so, we compiled the five best-resolved annual pre- That fossil pollen, analyzed in a deep-water core with its own cipitation reconstructions inferred from pollen from Mas- age uncertainties (13), can predict the first-order stratigraphic sachusetts and Connecticut, United States (13) (each layer of features in shallow-water cores sensitive to lake-level change (14) green shading represents an individual reconstruction in Fig. 2A), and highlights the strength of the relationships. A hierarchy of un- compared them with the set of quantified lake-level reconstructions certainties in the pollen counts, the pollen-inferred reconstruction, from the same area. Because these records extend inland, we include the ages of the pollen samples and core sediments, and the chain a third lake-level site, Davis Pond, from western Massachusetts (14) of simple hydrologic and sedimentary models could have readily (Fig. 2). Estimates of past lake levels enable independent calculations obscured any weak climate–vegetation–hydrology relationships. of effective precipitation (12, 13) (SI Appendix). The results confirm that climates were continuously changing All of the hydroclimate records indicate a long-term increase in and that ecological changes did not take place amid a stable effective precipitation of >400 mm (Fig. 2), which equals the Holocene environment. Despite the lack of climatic stability, how- modern difference in precipitation between the eastern Great ever, stable climate–vegetation relationships persisted throughout Plains and the NE US. Pollen-inferred changes represent in- the Holocene (Fig. 3). The findings conflict with hypotheses that creasingly mesic forest composition, which after ca. 10–8ka,became modern climate–vegetation relationships exist only because Holo- dominated by mixed and temperate deciduous forest taxa such as cene climates were stable (19) or that the vegetation changes rep- hemlock (Tsuga canadensis)andoak(Quercus spp.) (26) (Fig. 2B). resented disequilibrium dynamics (20, 23). Instead, the apparent The reconstructions also contain multicentury variations of dynamic equilibrium between climate and vegetation (18) agrees ∼50 mm superimposed upon the long trend. Dry intervals inferred with other comparisons that found small biotic lags (27) and indi- from pollen conform with the calibrated radiocarbon age distri- cates rapid responsiveness to the broad range of late-Quaternary butions of sandy paleoshorelines in the lakes, which indicate climate forcing. Correlations exist in response to both multimillen- synchronous low water levels at 4.9–4.6, 4.2–3.9, 2.9–2.1, and 1.3– nial climate trends (Figs. 2 and 3) and rapid events such as during 1.2 ka (14) (gray vertical lines, Fig. 2) when the Greenland–NAM the Younger Dryas (27). Factors such as soils may alter the rela- temperature gradient was steeper (more negative Greenland– tionships at fine spatial scales and changing atmospheric carbon

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1814307116 Shuman et al. Downloaded by guest on September 25, 2021 Fig. 3. Comparisons of effective precipitation reconstructions show minimal lags in the vegetation response (red). Comparisons at a single site with both data types (Deep Pond) (Top) and using multisite averages from the coastal area (Bottom) are based on mean lake-level changes (blue shading) and fossil pollen analogs (red lines). Scatter plots (Middle) and residual plots of Λ (Right) show that the lake level-derived estimates of effective precipitation accurately predict the vegetation community moisture response. Uncertainties are indicated by gray lines in the scatter plots (Middle) and combined to create signif- SCIENCES

icance intervals (black lines) in the residual plots (Right). Cal yr, calendar year. ENVIRONMENTAL

dioxide concentrations, or shifts in megafaunal herbivory may have Overall, the persistence of predictable relationships between altered the relationships before the Holocene, but within the bounds temperature gradients, paleohydrology, and vegetation changes defined by such influences, the relationships here indicate that indicates a common underlying cause, likely AMOC or NAO temperate forest communities can respond rapidly and predictably variability, on multicentury time scales and similar response times ECOLOGY to climate change. The predictability of the pollen–precipitation of the different systems relative to our ability to detect them (i.e., relationship supports the use of fossil pollen to reconstruct Holo- λ < 100 y). Long response times would have obscured the cen- cene climates (10). tennial variability (18). Instead, the replicated signals provide a

Fig. 4. Observed and predicted stratigraphies of sediment cores (A–C) from Deep Pond, Massachusetts (13). The cores are labeled by distance from shore (m), constrain past shoreline elevations (SI Appendix, Fig. S1), and are predicted using the pollen-inferred precipitation history (Fig. 3, Top) to drive a combination of simple hydrologic and sediment models (SI Appendix). The observed (tan) and predicted patterns of loss-on-ignition (LOI) at 550 °C (black lines) are plotted with depth for each core (A–C) as the inverse (100%-LOI), percent mineral content. The inset scatter plot (D) shows the relationship between observed and predicted LOI. Thin black lines represent uncertainties equivalent to the root-mean-square error of the pollen-inferred effective precipitation. Horizontal black lines mark 350 y before 1950 AD, the approximate timing of European settlement (ESH), which favored dry, tolerant weedy plant species (26).

Shuman et al. PNAS Latest Articles | 5of6 Downloaded by guest on September 25, 2021 benchmark for diagnosing the millennial-to-centennial variability, number of different possible dynamics (20, 21) when λ is substantially larger which links ocean, atmospheric, and continental (hydrologic) than the rate of climate change (18). Alternatively, Λ(t) may be near zero if changes and could involve feedbacks through factors like conti- systems can respond rapidly relative to the rate of climate change (18). Thus, – nental runoff. These patterns need to be examined further. Based climate changes as long or longer than the processes of succession (200 600 y) may produce small values of Λ(t) as forest mortality, turnover, and on the past dynamics, future changes in the North Atlantic will regeneration under the new conditions matches the rates of climate varia- likely also produce important, but potentially predictable, conse- tion or when exponential processes, such as population changes of trees or quences for water and forests on adjacent continents. their pathogens, outpace the underlying climate change. Stratigraphic records of past lake-level changes (12, 28) can be used to detect Materials and Methods Fo(t) separately from the fossil pollen records [the ecological indicator of Cr(t)] be- Paleoclimate Reconstructions. Our analyses of the Greenland–NAM temper- cause hydrologic and sedimentological equilibration processes have small annual- ature gradient and the hydrologic variations in the NE US rely on existing scale values of λ (29). Even in large lake systems globally, λ is often <5y(30). datasets as described in the main text. Details about the datasets and By applying a simple water-balance model (WBM) (SI Appendix) to estimate methods employed for lake-level reconstruction and inferring precipitation past changes in effective precipitation from reconstructed water-level changes at from fossil pollen are provided in SI Appendix. the three lakes in Massachusetts, United States (Fig. 2A), we compare the pollen and lake-level records directly in terms of mm of effective precipitation across a Measuring Ecological Lags. To test our ecological hypotheses, we use the pollen- range of elevations and proximities to the coast in the same region (Fig. 2B). As a inferred precipitation reconstructions to provide an index of the community further test, we reverse the modeling scheme and use the WBM to predict the composition of past vegetation as it relates to climate (13, 21). The approach past lake-level changes from the pollen-inferred changes in precipitation for Λ builds upon the concept that lags, , in the climate response of a given system comparison with the lake-level reconstructions (SI Appendix,Fig.S1). Further, we (e.g., ecological communities; lake hydrology) can equal the difference be- compare the sedimentary data that form the basis of the lake-level reconstruc- tween an observed climate (forcing), Fo, for a location at a given time, t, and tions with the output of a simple sedimentary model (an empirical logistic re- the climate (response), Cr, inferred from the state of the system (e.g., the mix lationship between sediment sand content and water depth; SI Appendix)that of taxa and their individual distributions with respect to climate) (21): predicts the sequence of sediments found in individual sediment cores (Fig. 4) based on the simulated lake-depth changes (SI Appendix,Fig.S1). Combining the ΛðtÞ = C ðtÞ–F ðtÞ. [1] r o WBM and the simple sedimentary model follows the framework of simulating “ ” In the case of logarithmic equilibrium processes that require some response the responses of both a sensor (the lake water level), which directly interacts “ ” time, λ, the lags develop because the rate of response from a given system with the hydroclimate, and an archive (the distribution of organic and in- organic lake sediments), which records the sensor response to produce the ob- state [Cr(t), e.g., the ecological community or a lake’s water level at time t] is slow relative to the rate of climate change over time steps of Δt (18): servations made in the lake sediment cores (31, 32).

ΛðtÞ = ðCrðt − ΔtÞ – FoðtÞÞ * expð−Δt=λÞ. [2] ACKNOWLEDGMENTS. We thank T. Webb and P. Bartlein for comments. We thank the 300 Committee for access to Deep Pond. Funding was provided by Hypotheses resulting from combining Eqs. 1 and 2 expect that Λ(t) can National Science Foundation Grants DEB-1146297 (to B.N.S.) and DEB- persistently remain greater than zero for some systems as the result of a 1146207 (to W.W.O. and D.R.F.).

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