W. Wyatt Oswald

A statistical approach to evaluating distance metrics and analog assignments for pollen records

Abstract The modern analog technique typically uses a distance metric to determine the dissimilarity between fossil and modern biological assemblages. Despite this quantitative approach, interpretation of distance metrics is usually qualitative and rules for selection of analogs tend to be ad hoc. We present a statistical tool, the receiver operating characteristic (ROC) curve, which provides a framework for identifying analogs from distance metrics. If modern assemblages are placed into groups (e.g., biomes), this method can (1) evaluate the ability of different distance metrics to distinguish among groups, (2) objectively identify thresholds of the distance metric for determining analogs, and (3) compute a likelihood ratio and a Bayesian probability that a modern group is an analog for an unknown (fossil) assemblage. Applied to a set of 1689 modern pollen assemblages from eastern North America classified into eight biomes, ROC analysis confirmed that the squared-chord distance (SCD) outperforms most other distance metrics. The optimal threshold increased when more dissimilar biomes were compared. The probability of an analog vs no-analog result (a likelihood ratio) increased sharply when SCD decreased below the optimal threshold, indicating a nonlinear relationship between SCD and the probability of analog. Probabilities of analog computed for a postglacial pollen record at Tannersville Bog (Pennsylvania, USA) identified transitions between biomes and periods of no analog.

Changes in nitrogen cycling during the past century in a northern hardwood forest.

Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.

Correspondence of pollen assemblages with forest zones across steep environmental gradients, Olympic Peninsula, Washington, USA

The use of pollen records to document vegetation responses to climatic change in mountains relies on the ability of pollen assemblages to differentiate among elevationally stratified vegetation zones. Comparisons among modem pollen assemblages within mountainous areas provide a basis for assessing this potential. We examined relationships between pollen assemblages, forest vegetation and climate at 65 small lake sites distributed across five forest zones on the Olympic Peninsula, Washington. Sample sites spanned nearly the full range of climatic gradients on the Peninsula, c. 900-6000 mm annual precipitation and c. 17-8C July mean temperature (from 9 to 1981 m a.s.l.). The pollen percentages of most arboreal taxa showed a strong relationship with elevation, especially for taxa with poor dispersal potential (Abies and Picea) or high elevation ranges (Tsuga mertensiana and Alnus sinuata). Tsuga heterophylla was the only pollen taxon abundant across all forest zones. Both detrended correspondence analysis (DCA) and linear discriminant analysis (DA) arranged pollen assemblages along temperature and precipitation gradients approximating the actual distribution of vegetation along these gradients. DA classified 90% of the sites into their correct forest zones, but was very sensitive to sample size, suggesting that caution should be exercised when using DA for classifying fossil pollen assemblages. Pairwise comparisons using the squared-chord-distance metric (SCD) showed that the SCD threshold that best distinguishes neighbouring forest zones is affected by the number of samples in each forest zone and by the patchiness of species distributions within each zone, suggesting that SCD is also sensitive to sample size and that thresholds used with the modem analogue technique must be calibrated for each vegetation zone. This study indicates that pollen assemblages on the Olympic Peninsula record local forest zones despite steep environmental gradients and the close proximity of neighbouring forest zones.

Abrupt cooling repeatedly punctuated early-Holocene climate in eastern North America

Climate proxy records and general circulation models suggest that Atlantic Meridional Overturning Circulation (AMOC) plays a key role for global climate changes. Paleoceanographic data document multiple episodes of prominent AMOC weakening during the early Holocene. However, proxy records at adjacent continents have not been demonstrated to fully capture the climate responses to multiple AMOC variation due to temporal resolution and/or the proxy sensitivity. Here we present decadal- to multidecadal-resolution hydrogen isotopic records of aquatic biomarkers from Blood Pond, Massachusetts during the early Holocene. Our data reveal a full series of prominent and abrupt cooling events centered on 10.6, 10.2, 9.5, 9.2, 8.8 and 8.4 ka. These abrupt climatic reversals coincide with key intervals of weakened AMOC, suggesting an apparent relationship between AMOC oscillations and the abrupt continental climate changes in northeastern North America. The noticeable connection implies that the AMOC variation did play an important role in the abrupt climate changes during the early Holocene. Our data also suggest that northeastern North America may experience significant climatic variations should the predicted major disturbance of AMOC occur in the coming century as a result of anthropogenic greenhouse gas emissions.

Middle-Holocene dynamics of Tsuga canadensis (eastern hemlock) in northern New England, USA

The abrupt, range-wide decline of Tsuga canadensis ~5500 calibrated years before present (cal. yr BP) is one of the most-studied events in North American paleoecology. Little attention, however, has been given to an earlier Tsuga decline, dated to ~6000 cal. yr BP in southern Ontario, Canada. To investigate whether this event occurred elsewhere in eastern North America, we analyzed the middle-Holocene interval of a lake-sediment record from Knob Hill Pond, located in northern Vermont, USA, an area of historically high Tsuga abundance. A dramatic, short-lived drop in Tsuga pollen abundance does occur at ~6000 cal. yr BP in the Knob Hill Pond record, indicating that Tsuga populations declined in various parts of its range. We hypothesize that both middle-Holocene declines of Tsuga were caused by the deleterious effects of pronounced droughts on this moisture-sensitive tree. Close examination of pollen data from a transect of sites across New England reveals that the earlier decline of Tsuga is present in other records, although some aspects of the event appear to have varied geographically. While northern and higher-elevation sites exhibit a nearly full recovery of Tsuga populations between the two declines, records further to the south are characterized by a stair-step pattern of progressive decline. At sites near its southern range limit, relatively warm conditions between ~6000 and 5500 cal. yr BP were apparently not conducive to the reestablishment and survival of Tsuga, and thus it was unable to recover between the drought events.

Detecting open vegetation in a forested landscape : pollen and remote sensing data from New England, USA

The proportional cover of forest and grassland vegetation, known as landscape openness, has been particularly difficult to reconstruct because of differences in pollen productivity and transport between the two vegetation types. To begin to calibrate landscape openness in eastern North America, we collected 21 samples of surface sediments from small ponds (less than 60 ha) in the Upper Connecticut River Valley of New England, USA. Pollen assemblages from these surface samples were compared with vegetation composition around each pond assessed with three different remote sensing methods: land cover digitized from aerial photographs, the National Land Cover Database and a spectral mixture analysis of vegetation structure using LANDSAT data. A principal component analysis indicates that most of the variation in land cover among sites is captured by pollen data and each of the three vegetation methods. Because spectral mixture analysis contains at least as much vegetation information as the other two techniques, while avoiding the problem of classifying vegetation into categories, it is better for comparison with pollen data and detection of landscape openness. Even though there is a relationship between non-arboreal pollen percentages and a metric of landscape openness, it is inadvisable to use pollen percentages alone to infer landscape openness across regions either in modern or ancient times. Quantitative assessments and reconstructions of plant abundance at a higher taxonomic resolution will require detailed vegetation surveys to correlate with pollen data from surface samples and possibly simulations of modern and ancient landscapes. Open areas in a landscape that is over 75% forested can be detected, allowing for future quantitative reconstructions of landscape openness.

Pollen Morphology and Its Relationship to Taxonomy of the Genus Sarracenia (Sarraceniaceae)

Abstract Despite nearly a century of research, the systematic relationships among North American pitcher plants in the genus Sarracenia (Sarraceniaceae) remain unresolved. In this study we analyzed pollen morphology of the 11 currently recognized species of Sarracenia and examined how variations in key pollen characteristics relate to our current understanding of the taxonomy of this genus. We used principal components analysis to explore variations in pollen grain size (equatorial diameter and length) and shape (number of colpi) among Sarracenia species, and used cluster analysis to compare systematic groupings of Sarracenia based on floral, vegetative, and pollen characters. We compared these results with a previously published phylogeny based on molecular data. Groupings based on pollen characteristics alone did not align completely with those based on molecular or all morphological data. In clusters based on pollen alone and those using all morphological characters, S. purpurea and S. rosea formed a single group, and S. flava, S. alata, and S. leucophylla grouped together consistently. The pollen morphology of S. jonesii and S. alabamensis differed substantially from that of S. rubra, supporting the current systematic treatment of the genus that recognizes these three taxa as distinct species.

Historic grazing in southern New England, USA, recorded by fungal spores in lake sediments

Decadal-scale analyses of fungal spores in a lake-sediment core from Ware Pond, located in the town of Marblehead in northeastern Massachusetts, test the potential of this approach for reconstructing past sheep and cattle grazing in southern New England, USA. The influx of spores of Sordaria and other coprophilous taxa increases at ad 1650, which corresponds with the beginning of European settlement, and subsequent peaks in these taxa at ad 1840 coincide with maximum abundances of weedy and agricultural taxa in the pollen record. Historical data from Marblehead and neighbouring towns indicate that maximum numbers of cattle and sheep occurred at this time. These findings suggest that fungal spores in New England lake sediments can be used to reconstruct changes in grazing pressure over time at the landscape scale.

December Leaf Out of Frangula alnus (Rhamnaceae) In Eastern Massachusetts

Frangula alnus Mill. (glossy buckthorn; hereafter ‘‘buckthorn’’) is an invasive shrub that has become widespread across the northeastern US and southeastern Canada over the last century. Buckthorn is native to Europe, and genetic analyses suggest that populations in the northeastern US likely originated in western Europe (De Kort et al. 2016). Buckthorn rapidly forms dense thickets, reducing light availability and thus inhibiting regeneration of native species (e.g., Fagan and Peart 2004; Frappier et al. 2003a). Given the potential impact of buckthorn on forest composition and stand dynamics, a number of studies have examined its ecology and management (e.g., Burnham and Lee 2010; Catling and Porebski 1994; Converse 1984; Cunard and Lee 2009; Frappier et al. 2003b, 2004; Hamelin et al. 2015, 2017; Koning and Singleton 2013; Lee and Thompson 2012; McDonald et al. 2008). In early December of 2015 I observed buckthorn plants leafing out over a span of several weeks in eastern Massachusetts. A strong El Niño contributed to record-breaking warmth across much of the northeastern US (NOAA 2016), with Boston experiencing many days in December with daily high temperatures 5-108C warmer than normal (Figure 1). That observation prompted this opportunistic study in which I paired continued field observations with a laboratory experiment. The objectives of the study were to investigate why leaf out occurred at this time and to evaluate whether leafing out in December had lasting consequences for buckthorn. The findings provide new insights and raise new questions about (1) the causes and costs of off-season leaf out, (2) the ecology of buckthorn, and (3) the dynamics of nonnative species and climatic variability. The study site is located in the northwestern part of the Middlesex Fells Reservation (hereafter ‘‘the Fells’’) in the town of Winchester, ~10 km northwest of Boston. The buckthorns observed in this study

Evaluating the role of insects in the middle-Holocene Tsuga decline1

Abstract The middle-Holocene decline of Tsuga canadensis (L.) Carrière (eastern hemlock) across eastern North America has been attributed to various causes, including the widespread outbreak of an insect pest, such as Lambdina fiscellaria (hemlock looper). We tested this hypothesis by searching for insect remains in sediment cores from Hemlock Hollow, a small basin in north-central Massachusetts. Previous analyses of this site demonstrated that it has been surrounded by Tsuga forest for the past 10,000 yr. We found the remains of chironomids and beetles in the cores but not in sediments dating to the interval of low Tsuga abundance; remains of Lambdina fiscellaria were not encountered. These results are consistent with the interpretation that the decline of Tsuga at Hemlock Hollow was not caused solely by an insect outbreak. The presence of Lambdina fiscellaria remains in middle-Holocene sediments at other sites in the region may reflect local outbreaks, perhaps facilitated by drought or other changes in climate that stressed Tsuga populations.