It is possible to separate pollen grains within the genera Acer and Juglans reliably, but palynological identification of species is more problematic in Fraxinus. We compared pollen grains of the eight native Acer species found in our study regions (Table 2). A. spicatum is the most readily identified as it is the only tricolporate Acer grain in the region. Tricolpate types are divided into striate (A. pensylvanicum, A. rubrum), reticulate (A. nigrum, A. saccharum) and rugulate (A. negundo, A. saccharinum), and then to individual species on the basis of the patterns made by the exine structure and sculpturing elements (Helmich 1963; Richard 1970; Adams & Morton 1972–1979; McAndrews et al. 1973). The pollen of A. barbatum Michx. is not found in the pollen databases nor in most pollen treatments, but it is similar to A. saccharum and future taxonomic analyses may place them in the same species (Burns & Honkala 1990). Acer barbatum pollen are included in A. saccharum-type although Helmich (1963) indicates that its reticulum is coarser than that of pollen grains of A. saccharum or A. nigrum.
Table 2. List of species of Acer, Fraxinus and Juglans in eastern North America (Gleason & Cronquist 1963). Pollen characteristics are taken from Cushing (1963), Richard (1970) and McAndrews et al. (1973). Note: Acer pseudoplatanus L. and A. platanoides L. are omitted because they are introduced. A. barbatum Michx., F. caroliniana Mill. and F. profunda (Bush) Bush (= F. tomentosa Michx. f. in Gleason & Cronquist 1963) are omitted because they are not found in pollen keys nor in the NAPD, but note that A. barbatum has A. saccharum-type pollen and F. caroliniana and F. tomentosa have tetracolpate pollen grains
|Species||Distinguishing pollen characters|
|Acer negundo L.||Tricolpate, more finely rugulate, no colpus membrane|
|A. nigrum Michx. f.||Tricolpate, reticulate, colpi do not meet at poles|
|A. pensylvanicum L.||Tricolpate, striate, striae in ‘digital’ pattern|
|A. rubrum L.||Tricolpate, striate, striae parallel|
|A. saccharinum L.||Tricolpate, coarsely rugulate, with colpus membrane|
|A. saccharum Marsh.||Tricolpate, reticulate, colpi meet at poles|
|A. spicatum Lam.||Tricolporate|
|Fraxinus americana L.||Tetracolpate, finer reticulum, thick exine|
|F. nigra Marsh.||Tricolpate or rarely tetracolpate, colpi do not meet equator at right angles, coarser reticulum than F. americana-type|
|F. pennsylvanica Marsh.||Tetracolpate, coarser reticulum, larger and thinner exine|
|F. quadrangulata Michx.||Tricolpate, as in F. nigra|
|Juglans cinerea L.||Periporate, 7–11 pores|
|J. nigra L.||Periporate, 12–18 pores|
Eastern North American Fraxinus pollen is divided into two types, referred to throughout our study as tricolpate (F. nigra, F. quadrangulata) and tetracolpate (F. americana, F. pennsylvanica) (Cushing 1963; Adams & Morton 1972–1979; McAndrews et al. 1973). Richard (1970) further divided F. americana from F. pennsylvanica on the basis of a tighter reticulum and slightly thicker exine in F. americana (Table 2). He notes, however, that secure distinctions between these two types depend on the simultaneous viewing of pollen grains of each type, and we confirmed this by studying reference material. Therefore, we limit our discussion to tri- and tetracolpate Fraxinus pollen types. The two south-eastern Fraxinus species, F. caroliniana P. Mill. and F. profunda (Bush) Bush (F. tomentosa in Gleason & Cronquist 1963) also have tetracolpate pollen grains but these are not found in the NAPD nor are they described in pollen keys. In rare cases, F. nigra-type pollen can be found in a tetracolpate form but it may be distinguished from the F. americana-type grains on the basis of a coarser reticulum and colpi that do not cross the equator at right angles (Cushing 1963).
There are two species of Juglans in eastern North America: J. cinerea and J. nigra (Gleason & Cronquist 1963). The distinction between pollen grains of these two species is determined by the number of pores; J. nigra has 12–18 pores, whereas J. cinerea has 7–11 (McAndrews et al. 1973). Cushing (1963) indicates that 10–11 pores should be used to divide the two Juglans species. Pore size may also be used as a distinguishing character (Adams & Morton 1972–1979).
Eastern North American Acer species differ in terms of functional type, stature, shade tolerance, regeneration requirements and other ecophysiological characters. A. saccharum is the most shade-tolerant (Burns & Honkala 1990), capable of long suppression in the understorey, and has high water and nutrient requirements (Sipe & Bazzaz 1994). A. rubrum is characterized by architectural and physiological plasticity, making it well suited to gap or understorey environments, and a good competitor on exposed microsites (Sipe & Bazzaz 1994). It can be found at drier sites than other Acer species and its distribution is bimodal, with populations found in wet, dry, shaded or open habitats (Curtis 1959; Warren et al. 2004). Acer negundo is similarly widely distributed; A. saccharinum is moderately shade-tolerant and is the Acer species associated with the wettest conditions (Curtis 1959). A. spicatum has a more boreal and high-elevation distribution (Curtis 1959) and is of subcanopy stature, whereas A. pensylvanicum is a shade-tolerant shrub or subcanopy tree (Burns & Honkala 1990). Acer nigrum is more common in the western part of the study region where a warmer and drier climate prevails (Burns & Honkala 1990).
Acer pollen is rarely found outside of the range of the tree, except in the cases of A. saccharum and Acer undifferentiated (Fig. 2). Incidences of A. saccharum pollen outside the range of the tree may represent long-distance transport but could also reflect the presence of scattered populations of A. saccharum across the American south-east or stands of A. barbatum, which has A. saccharum-type pollen. In all cases, within the range of the trees, there are regions where the corresponding pollen is lacking, indicating that palynological distinctions have not always been made for those types. In the central to south-eastern United States, there are few incidences of undifferentiated Acer pollen (Fig. 2), and higher counts of specific Acer species, reflecting the higher taxonomic resolution of studies from that region.
Figure 2. Modern distribution of pollen of Acer, Fraxinus and Juglans species in lake surface sediments from eastern North America. Percentages are based on a sum of arboreal and non-arboreal pollen. ‘Undifferentiated’ includes pollen in the category ‘undiff.’ as well as the category genus name only. Fraxinus nigra type includes F. nigra and F. quadrangulata, and F. americana type includes F. americana and F. pennsylvanica. Range of the taxa included is shown in grey (Thompson et al. 1998). Although pollen from all of North America are plotted, only the range of eastern species is indicated on the undifferentiated category. Values equal to zero are not plotted.
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The paucity of species-level distinctions of Acer pollen types across most of eastern North America makes it difficult to see the range of environmental conditions associated with particular species, based on pollen data (Fig. 3). Nevertheless, two of the less common Acer species show different tolerances. A. pensylvanicum pollen is found in moist, cool habitats, and A. negundo pollen has important representation in warmer areas. Because pollen counts are lacking from large parts of the ranges of the species, these response surfaces must be interpreted with caution.
Figure 3. Response surfaces depicting the relative abundance of pollen from modern samples as a function of July temperature (°C) and annual precipitation (mm).
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Pollen morphotypes in Fraxinus do not correspond to the habitat associations within the genus: F. nigra and F. pennsylvanica are species of wet ground or swamp environments whereas F. quadrangulata and F. americana are associated with mesic or moist habitats (Gleason & Cronquist 1963). F. nigra in particular is associated with a cooler, humid climate (Burns & Honkala 1990).
Tetracolpate Fraxinus pollen grains (F. americana, F. pennsylvanica) are found throughout the range of the two trees producing them, although the western range limit of the pollen is slightly to the east of the tree limit (Fig. 2). Tricolpate Fraxinus pollen (F. nigra, F. quadrangulata) is recorded at sites across all of eastern North America, including the south-eastern United States, where Thompson et al. (1998) report that the trees absent. Gleason & Cronquist (1963), however, indicate that F. quadrangulata can be found from Oklahoma to Alabama, explaining the presence of tricolpate Fraxinus pollen in that region. Response surfaces (Fig. 3) do not show a clear difference between the two Fraxinus pollen types, which is expected given the broad climatic ranges associated with each one, and the lack of correspondence between pollen morphotypes and habitat associations.
responses to holocene environmental changes
Within single sites, species of Acer, Fraxinus and Juglans responded differently to environmental changes through the Holocene. For the site with the highest temporal resolution (Table 1) within each of our three focal regions, PCAs (Fig. 4) show that congeneric species do not necessarily follow the same patterns through time, thus confounding the interpretation that we might make if genera only are considered.
Figure 4. Principal components analysis of fossil pollen data from three sites with high-resolution palaeoecological records. Ordination of fossil pollen taxa with species-level pollen counts for Acer, Fraxinus and Juglans (a, c, e) or lumped into generic categories (b, d, f) for Crawford lake, Ontario (a, b), Tourbière Lanoraie St Jean, Québec (c, d) and South Rhody Peatland, Michigan (e, f). References for the data sources are given in Table 1.
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At Crawford Lake (McAndrews & Boyko-Diakonow 1989), A. saccharum and A. rubrum show almost opposite trends on PCA axis 1, as do J. cinerea and J. nigra, indicating that the abundances of pollen of individual species within a genus have varied in a dissimilar fashion over the past 1700 years (Fig. 4a). By contrast, the two Fraxinus types show similar loadings on axis 1. When genera only are considered, Acer is closely correlated with Fagus, as is A. saccharum when species distinction is available (Fig. 4a). Juglans is apparently primarily composed of J. cinerea and is well correlated with Ulmus (Fig. 4b). When pollen counts are lumped into generic groupings for Acer and Juglans, associations of A. rubrum and J. nigra with Betula and Populus in the upper right quadrat of the ordination diagram can no longer be detected (Fig. 4a,b). The ordination shows the changes in species composition that occurred in the forest surrounding Crawford Lake beginning about 500 years ago, coincident with the onset of the Little Ice Age as well as increased human activity in the surrounding area (Clark & Royall 1995). These disturbances resulted in canopy openings that decreased the competitive advantage of slow-growing, shade-tolerant A. saccharum and lowered its pollen percentages. Acer rubrum is able rapidly to exploit canopy openings and other disturbances, and this is reflected in the rapid increases in its pollen percentages at this time. Pollen of J. nigra increased at this time of disturbance relative to that of J. cinerea, which can be explained on the basis of the association between J. nigra and more open forest canopies (Curtis 1959; Szeicz & MacDonald 1991).
Five Acer species were recorded in the 5500-year record at Tourbière Lanoraie St Jean (Comtois 1982). In the ordination showing all species, the first axis separates A. saccharinum and A. pensylvanicum, with positive loadings on axis 1, from A. rubrum, A. spicatum and A. saccharum, with negative loadings (Fig. 4c). The three Fraxinus species recorded at this site are again poorly differentiated on axes 1 and 2 (Fig. 4c). When counts for all Acer species are grouped, Acer remains correlated with Quercus and Ulmus (Fig. 4d), as were A. saccharum and A. spicatum in the species-level ordination (Fig. 4c), but evidence for the correlation of A. pensylvanicum and A. saccharinum with Pinus strobus at the positive end of axis 1 is lost. At South Rhody Peatland (Booth et al. 2004), A. saccharum and A. rubrum, and F. americana and F. nigra, are differentiated on axis 1 only weakly (Fig. 4e). When the Acer and Fraxinus species are grouped into generic categories (Fig. 4f), the position of Acer suggests an averaging effect: neither Acer species reflects closely the position of the Acer genus, which is intermediate in location on the ordination plot with respect to the two species. Similarly, the genus Fraxinus is correlated with Cupressaceae (Fig. 4f), indicating PCA axis 1 values between those of F. americana and F. nigra in the species-level ordination (Fig. 4e).
synchrony of changes
Within any region, pollen of Acer, Fraxinus and Juglans species show synchronous changes, when accounting for uncertainties in chronology and resolution. We compared the species pollen curves in the three regions (Figs 5–7) to determine if the broad-scale, regional variations in the abundance of these pollen types are synchronous, which would suggest an overall climatic influence on these taxa. However, given the chronological constraints of the datasets available, it is unlikely that exact temporal correlations will exist in pollen curves for the species of interest. Thus, we can only ask whether the curves suggest that similar changes occurred at the regional scale.
Figure 5. Normalized per cent pollen diagrams for Acer saccharum (a, c, e) and A. rubrum (b, d, f) in three regions. Note the shorter time scale for Crawford Lake. For curves with continuous or near-continuous pollen representation, a lowess smoothing function is superimposed. The onset of the Tsuga pollen decline at each site is indicated by horizontal black lines in (a). Sites are ordered geographically, from east to west. References for the data sources are given in Table 1.
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The onset of the Tsuga pollen decline is plotted for sites across Ontario (Fig. 5a). This decline presumably occurred synchronously in response to the rapid spread of a pathogen (Davis 1981a; Fuller 1998), yet at the 14 sites considered, the chronologies supplied in the NAPD suggest that its date falls within a 1100-year window. It is unclear if these differences are due to a time-transgressive decline or due to insufficient dating control, but the lack of synchrony does suggest that many profiles in the NAPD are inadequately dated, and/or have poor temporal resolution between samples.
Nevertheless, the pollen curves for a single species across a region show some overall similarities, and fine-scale variations are seen in sites with higher temporal resolution. In southern Ontario, A. saccharum increased in abundance around 8000–7000 bp and reached maximum abundance between 5000 and 2000 bp. It declines over the most recent 1500 years, and most steeply since the Little Ice Age (Fig. 5a). Similarly, in southern Québec, A. saccharum curves show two peaks around 5000 and 2500 bp (Fig. 5c). Nearby lakes show similar, finer scale details: in Ontario, Lac Bastien, Found and Second Lakes show maxima in A. saccharum pollen around 1000 years ago, whereas the peak at the more south-eastern Nutt, Graham, High, Tonawa and Cranberry Lakes occurs 4000 years ago (Fig. 5a). At the southernmost sites in Ontario, Van Nostrand Lake, Porqui Pond, Wylde Bog and Wylde Lake, A. saccharum peaks even earlier, around 5000 years ago (Fig. 5a). In the western Great Lakes region (Fig. 5e), A. saccharum peaks at about 7000 bp in the easternmost lakes (Wood, Gass, Radtke Lakes and Lake Mendota), and around 3000 years ago in the northernmost lakes situated on the Upper Peninsula of Michigan (Spirit, Kitchner, Canyon, Lost, Camp 11 and Mud Lakes). The profiles for the easternmost Michigan sites (Ryerse, Wolverine Lakes and South Rhody Peatland) show similar profiles, with rises in A. saccharum pollen particularly since 2000 bp.
The curves for A. rubrum pollen are more difficult to interpret because of the overall scarcity of pollen counts of this type (Fig. 5b,d,f). The Québec sites contain the highest counts of A. rubrum (Fig. 5d) and illustrate the dissimilarity in the pollen curves for these two Acer species. At Lac à Sam and Lac Castor, for example, A. rubrum peaks around 4200 bp, while A. saccharum peaks 300 years later (Fig. 5c). Mont Shefford and Lac Geai, located further south, have a similar peak in A. rubrum 4000 years ago, but a later peak in A. saccharum, occurring around 3000 years ago. At several sites across the three regions, a very recent increase in A. rubrum occurs (e.g. Second, Van Nostrand Lakes, Lac Romer, Mont Shefford, Spirit, Kitchner, Mud, Gass and Radtke Lakes). The short, very high-resolution dataset from Crawford Lake shows a pronounced increase in A. rubrum 500 years ago, and a corresponding decline in A. saccharum (Fig. 5a,b). Finally, at 28 of the 32 sites with data for both A. saccharum and A. rubrum, A. rubrum pollen appears in the records following A. saccharum, indicating dissimilar arrival and migration histories for these taxa in the early Holocene (Fig. 5).
The pollen curves for tri- and tetracolpate Fraxinus (Fig. 6) are more complex than those of Acer spp. In Ontario, three classes of tricolpate Fraxinus curves can be distinguished. At Lac Bastien and Found Lake, tricolpate Fraxinus decline from an early maximum; at Graham, High, Tonawa Lakes and Wylde Bog, the curve rises to a mid Holocene maximum, and at some lakes (e.g. Nutt Lake), few changes are observed (Fig. 6a). The data from Québec show some synchrony in Fraxinus pollen curves. Tricolpate pollen types peak between 4000 and 2000 years ago at Lac Geai, St Calixte and Lac Manitou (Fig. 6c), whereas a synchronous peak occurs earlier in tetracolpate Fraxinus types at Lac à Sam, Lac Castor, St-Gabriel and Lac Romer, around 4500 years ago (Fig. 6d). In the western Great Lakes region, tricolpate Fraxinus pollen has decreased since the early Holocene at many of the sites, whereas tetracolpate grains have oscillated through time (Fig. 6e,f). Tricolpate Fraxinus pollen appears earlier in the records than the tetracolpate Fraxinus types in 24 of 34 profiles shown here (Fig. 6).
The curves for Juglans cinerea are broadly similar among the regions (Fig. 7). In Ontario and Québec, where the pollen is more abundant, maxima are well defined between 6000 and 4500 bp at Cranberry Lake, Barry Lake, Van Nostrand Lake, Wylde Bog and Wylde Lake (Fig. 7a) and earlier at the Québec sites Saint-Gabriel, Lac Romer and Lac des Atocas, where an abrupt rise occurs around 7000 years ago (Fig. 7c). Counts are low for J. nigra across the whole study region (Fig. 7b,d,f), limiting the interpretability of those pollen curves. A pronounced rise in J. nigra can, however, be seen in the very high-resolution record from Crawford Lake, coincident with a decline in J. cinerea 500 years ago (Fig. 7a,b). In 24 of 27 sites shown here, pollen of J. cinerea appears earlier in the records than that of J. nigra, suggesting dissimilar migration chronologies for the two species (Fig. 7).