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Land cover governs the biogeophysical and biogeochemical feedbacks between the land surface and atmosphere. Holocene vegetation–atmosphere interactions are of particular interest, both to understand the climate effects of intensifying human land use and as a possible explanation for the Holocene temperature conundrum, a widely studied mismatch between simulated and reconstructed temperatures. Progress has been limited by a lack of data-constrained, quantified, and consistently produced reconstructions of Holocene land cover change. As a contribution to the Past Global Changes (PAGES) LandCover6k Working Group, we present a new suite of land cover reconstructions with uncertainty for North America, based on a network of 1445 sedimentary pollen records and the REVEALS pollen–vegetation model (PVM) coupled with a Bayesian spatial model. These spatially comprehensive land cover maps are then used to determine the pattern and magnitude of North American land cover changes at continental to regional scales. Early Holocene afforestation in North America was driven by rising temperatures and deglaciation, and this afforestation likely amplified early Holocene warming via the albedo effect. A continental-scale mid-Holocene peak in summergreen trees and shrubs (8.5 to 4 ka) is hypothesized to represent a positive and understudied feedback loop among insolation, temperature, and phenology. A last-millennium decrease in summergreen trees and shrubs with corresponding increases in open land was likely driven by a spatially varying combination of intensifying land use and neoglacial cooling. Land cover trends vary within and across regions, due to individualistic taxon-level responses to environmental change. Major species-level events, such as the mid-Holocene decline in Tsuga canadensis (eastern hemlock), may have altered regional climates. The substantial land cover changes reconstructed here support the importance of biogeophysical and biogeochemical vegetation feedbacks to Holocene climate–carbon dynamics. However, recent model experiments that invoke vegetation feedbacks to explain the Holocene temperature conundrum may have overestimated land cover forcing by replacing Northern Hemisphere grasslands >30° N with forests, an ecosystem state that is not supported by these land cover reconstructions. These Holocene reconstructions for North America, along with similar LandCover6k products now available for other continents, serve the Earth system modeling community by providing better-constrained land cover scenarios and benchmarks for model evaluation, ultimately making it possible to better understand the regional- to global-scale processes driving Holocene land cover, carbon cycle, and climate dynamics.

Limited numbers of high-resolution records predate the Last Glacial Maximum (LGM) making it difficult to quantify the impacts of environmental changes prior to peak glaciation. We examined sediments from Last Canyon Cave in the Pryor Mountains of Montana and Wyoming to construct a >45 ka environmental record from pollen and stable isotope analysis. Artemisia pollen was hyper-abundant at the beginning of the record. Carbon isotope values of bulk organic matter (>40 ka) showed little variation (-25.3 ± 0.4‰) and were consistent with a arid C3 environment, similar to today. After 40 cal ka BP, Artemisia pollen decreased as herbaceous taxa increased toward the LGM. A significant decrease in δ13C values from 40–30 cal ka BP (~1.0‰) established a new baseline (-26.6 ± 0.2‰), suggesting cooler, seasonally wetter conditions prior to the LGM. These conditions persisted until variation in δ13C values increased significantly with post-glacial warming, marked by two spikes in values at 14.4 (-25.2‰) and 13.5 cal ka BP (-25.4‰) before δ13C values dropped to their lowest values (-26.9 ± 0.2‰) at the onset of the Younger Dryas (12.8 ka). These results provide insights into late Pleistocene conditions and ecological change in arid intermontane basins of the Rocky Mountains.

Questions: Functional interactions between fire and fire-dependent plant communities have been considered to select for increased community flammability. We address this concept by resolving: (a) can fossil pollen records be used to examine past variations in functional attributes; (b) can community-level, functional responses to fire be obtained by coupling fire history with pollen derived plant traits; and (c) has directional selection promoted attributes that increase community flammability? Location: Breitenbush Lake, Oregon, USA. Methods: We developed a framework based on ecological understanding of functional traits and pollen records to analyse variations in functional attribute expressions through time. Fire-related functional attributes that indicate sensitivity to changes in fire activity were identified and associated with taxa from a pollen record. Results: Nine of the 14 functional attributes were significantly related to fire frequency (number of fires 1,000 per year). When combined with fire history data, variation in the abundances of functional attributes suggest selection of plant community expressions that indicate community-level responses to fire related to changes in structural development and changes in fire adaptation. Fire frequency variations may drive directional selection for fire-adapted attributes, and against fire-sensitive attributes. Our results indicate increased Holocene fire activity may have been linked to vegetation functional interactions with fire. Conclusions: Our method of combining paleoecological data with functional traits allowed reconstruction of community-level changes in the expression of functional characteristics, providing evidence on structural development and changes in fire adaptation. Our results (a) highlight the capacity of paleoecological records to track plant community trait composition; (b) provide novel information on fire–vegetation relationships, independent of and complementary to conventional methods of disturbance-based paleoecological interpretations; (c) suggest the Holocene fire trend may be linked to the type of biomass being burned through directional selection; and (d) suggest an application that may be applied to quantify community responses to various disturbances across broad temporal and spatial scales.

Modern pollen–vegetation studies are crucial for the calibration and interpretation of fossil pollen assemblages. In eastern India, knowledge regarding the relationship between pollen deposition and the modern-day vegetation is required to make use of fossil pollen data, which is still scant. To fill this gap, the present study deals with the pollen analysis of 20 surface sediment samples from the Sajnekhali Island Wildlife Sanctuary, Sundarbans, India. The palynological results show that 20 genera in 18 families are identified, of which 16 major pollen taxa are recorded under mangrove and its associate groups. To understand potential drivers changing pollen abundance in the surface samples, a detrended correspondence analysis (DCA) is conducted to typify the environmental gradients among samples. The DCA of the modern pollen assemblages reveals the taxa defining the environmental gradient from inland to the margin of the island. The primary gradient appears to be the salinity tolerance of mangroves from near the water margin to the less salt-tolerant taxa. Axis 1 describes 62.5% of the variance in the dataset. The secondary axis appears to describe the gradient in subtaxa away from the mangrove. Axis 2 describes 4.5% of the variance in the dataset. Rhizophora mucronata, Avicennia marina, Sonneratia apetala, Excoecaria agallocha, Heritiera fomes, Bruguiera gymnorrhiza, Xylocarpus granatum and Nypa fruticans are recorded as the dominant pollen taxa that follow a salinity gradient. All modern pollen rain appears compatible with the actual floristics of the area. Thus, the present study indicates that the mangrove environment is well represented in the pollen record and displays close linkages between pollen composition and native parent plants in the area. This pollen dataset fills an information gap and is useful in understanding and interpreting the palaeoecology and palaeoenvironment of tropical marshes of similar environments.

Aim: Our understanding of how climate and fire have impacted quaking aspen (Populus tremuloides Michx.) communities prior to the 20th century is fairly limited. This study analysed the period between 4500 and 2000 cal. yr bp to assess the pre-historic role of climate and fire on an aspen community during an aspen-dominated period. Location: Long Lake, south-eastern Wyoming, central Rocky Mountains, USA. Methods: Sedimentary pollen and charcoal were analysed to reconstruct the vegetation and fire history for a subalpine catchment currently dominated by lodgepole pine. Modern pollen-climate relationships were applied to the fossil pollen spectra to interpret past climate variability. Nonparametric ANOVA and Tukey HSD tests were used to determine whether the reconstructed climate and fire parameters were different throughout the study period. Results: The modern pollen-climate data suggest a c. 150-year long drought centred on 4200 cal. yr bp, which caused the aspen ecotone to shift upslope. Between 3950 and 3450 cal. yr bp, an anomalous period of abundant quaking aspen pollen (Populus) occurred at the study site. Optimal climatic conditions coupled with frequent fires facilitated local quaking aspen dominance for roughly 500 years. After 3450 cal. yr bp, Populus pollen declined coincident with a return to less frequent fires and conifer dominance. Reconstructed climate variables from 550 cal. yr bp to present suggest conditions were not favourable for quaking aspen establishment at Long Lake. The Tukey HSD test confirms that the period of abundant Populus pollen was significantly different than any other period during this study. Main conclusions: Quaking aspen shifted upslope in response to warmer temperatures, and persisted for roughly 500 years as a result of optimal climatic conditions and frequent fire events.

Rapid hydroclimatic shifts repeatedly generated centuries to millennia of extensive aridity across the headwaters of three of North America's largest river systems during the Holocene. Evidence of past lake‐level changes at the headwaters of the Snake‐Columbia, Missouri‐Mississippi, and Green‐Colorado Rivers in the Rocky Mountains shows that aridity as extensive and likely as severe as the CE 1930s Dust Bowl developed within centuries or less at ca. 9 ka (thousand years before CE 1950), and persisted across large areas of the watersheds until ca. 3 ka. Regional water levels also shifted abruptly at >11.3 and 1.8‐1.2 ka. The record of low water levels during the mid‐Holocene on the Continental Divide links similar evidence from the Great Basin and the Midwestern U.S., and shows that extensive aridity was the Holocene norm even though few GCMs have simulated such a pattern.

The mid-Holocene decline of Tsuga canadensis (hereafter Tsuga ) populations across eastern North America is widely perceived as a synchronous event, driven by pests/pathogens, rapid climate change, or both. Pattern identification and causal attribution are hampered by low stratigraphic density of pollen-sampling and radiometric dates at most sites, and by absence of highly resolved, paired pollen and paleoclimate records from single sediment cores, where chronological order of climatic and vegetational changes can be assessed. We present an intensely sampled (contiguous 1-cm intervals) record of pollen and water table depth (inferred from testate amoebae) from a single core spanning the Tsuga decline at Irwin Smith Bog in Lower Michigan, with high-precision chronology. We also present an intensively sampled pollen record from Tower Lake in Upper Michigan. Both sites show high-magnitude fluctuations in Tsuga pollen percentages during the pre-decline maximum. The terminal decline is dated at both sites ca. 5000 cal yr BP, some 400 years later than estimates from other sites and data compilations. The terminal Tsuga decline was evidently heterochronous across its range. A transient decline ca. 5350 cal yr BP at both sites may correspond to the terminal decline at other sites in eastern North America. At Irwin Smith Bog, the terminal Tsuga decline preceded an abrupt and persistent decline in water table depths by ∼200 years, suggesting the decline was not directly driven by abrupt climate change. The Tsuga decline may best be viewed as comprising at least three phases: a long-duration pre-decline maximum with high-magnitude and high-frequency fluctuations, followed by a terminal decline at individual sites, followed in turn by two millennia of persistently low Tsuga populations. These phases may not be causally linked, and may represent dynamics taking place at multiple temporal and spatial scales. Further progress toward understanding the phenomenon requires an expanded network of high-resolution pollen and paleoclimate chronologies.

Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxy-climate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.

We used pollen and high-resolution charcoal analysis of lake sediment to reconstruct a 7600 yr vegetation and fire history from Anthony Lake, located in the Blue Mountains of northeastern Oregon. From 7300 to 6300 cal yr BP, the forest was composed primarily of Populus, and fire was common, indicating warm, dry conditions. From 6300 to 3000 cal yr BP, Populus declined as Pinus and Picea increased in abundance and fire became less frequent, suggesting a shift to cooler, wetter conditions. From 3000 cal yr BP to present, modern-day forests composed of Pinus and Abies developed, and from 1650 cal yr BP to present, fires increased. We utilized the modern climate-analogue approach to explain the potential synoptic climatological processes associated with regional fire. The results indicate that years with high fire occurrence experience positive 500 mb height anomalies centered over the Great Basin, with anomalous southerly component of flow delivering dry air into the region and with associated sinking motions to further suppress precipitation. It is possible that such conditions became more common over the last 1650 cal yr BP, supporting an increase in fire despite the shift to more mesic conditions.

Shifts in an ecosystem’s state can alter biogeochemical cycling and the extent of nutrient conservation within a terrestrial landscape on multiple time scales. Transient biogeochemical changes may follow disturbance and succession, although persistent long-term differences may exist under different climates and vegetation types. We evaluate the potential for such biogeochemical changes in the context of long-term ecological history by measuring the nitrogen isotope composition of organic matter in a lake sediment core. We targeted Little Windy Hill Pond (LWH) in the Medicine Bow Mountains, Wyoming because reconstructions of the lake level, fire, and vegetation histories from the lacustrine sediments indicated a century-scale transformation from an arid, shrub-dominated landscape to a sub-alpine, tree-dominated ecosystem with extensive woody cover and large, live biomass pools. We demonstrate that the afforestation at the beginning of the Holocene transformed the Artemisia-dominated ecosystem, which had persisted for millennia during the Pleistocene. The changes affected nitrogen cycling dynamics, especially through intensified nutrient conservation when live biomass pools increased with greater woody cover. The LWH sediments record a baseline δ¹⁵N shift from 2.2–3.0 to 0.3–2.0‰ & as less ¹⁵N-enriched organic matter accumulated in the lake. We also observed a transient pattern of maximum nutrient conservation and minimum δ¹⁵N values as terrestrial biomass increased during the aggradation (~175 years) and transition phases of ecological succession. Our nitrogen isotope results support theoretical expectations of long-term biogeochemical dynamics as nutrient conservation increases during afforestation.