Lisa M. Kennedy

Climatic control of the biomass-burning decline in the Americas after ad 1500:

The significance and cause of the decline in biomass burning across the Americas after ad 1500 is a topic of considerable debate. We synthesized charcoal records (a proxy for biomass burning) from the Americas and from the remainder of the globe over the past 2000 years, and compared these with paleoclimatic records and population reconstructions. A distinct post-ad 1500 decrease in biomass burning is evident, not only in the Americas, but also globally, and both are similar in duration and timing to ‘Little Ice Age’ climate change. There is temporal and spatial variability in the expression of the biomass-burning decline across the Americas but, at a regional–continental scale, ‘Little Ice Age’ climate change was likely more important than indigenous population collapse in driving this decline.

Climate-radial growth relationships of northern latitudinal range margin longleaf pine (Pinus palustris P. Mill.) in the Atlantic coastal plain of southeastern Virginia.

Abstract Climate and longleaf pine (Pinus palustris P. Mill.) radial growth relationships have been documented within its southern and western distribution. However, knowledge of this relationship is lacking along its northern latitudinal range margin (NLRM). Based on the principles of ecological amplitude, limiting factors, and studies of coniferous species in eastern temperate forests of the U.S., we hypothesized that the radial growth of longleaf pine in mixed pine-hardwood forests is responding to winter temperatures in southeastern Virginia. Two longleaf pine chronologies were developed to determine the relationship between radial growth and monthly temperature, precipitation, and Palmer Drought Severity Index (PDSI) via response function analysis (RFA). Results at the 0.05 level yielded significant response function coefficients with a positive response to current winter temperature and precipitation and a negative response to prior August PDSI. In studies of climate and longleaf pine radial growth in other parts of its range, winter temperature and precipitation have not shared a significant positive association with radial growth. Instead current spring and summer precipitation usually share this positive association. These findings add more evidence to an emerging pattern suggesting that winter temperatures contribute to limiting the radial growth of temperate conifers at northern range margins in the Northern Hemisphere.

Boundary-line growth patterns to determine disturbance history of remnant longleaf pine (Pinus palustris P. Mill.) in mixed forests of southeastern Virginia1

Abstract Removal of canopy dominant trees in temperate closed-canopy forests due to natural or anthropogenic disturbance may allow for the release of co-dominant and sub-canopy trees into the canopy. Historical growth releases of these trees can be reconstructed from the analysis of their annual rings and compared with historical disturbance events to better understand forest dynamics. We applied boundary-line growth patterns, a method for the reconstruction of historical release from disturbance, to annual-ring series of co-dominant longleaf pine in two closed-canopy successional forest sites (Everwoods and Seacock Swamp) the Mid-Atlantic Coastal Plain of southeastern Virginia. The somewhat degraded stands were co-dominated by mixed hardwoods and loblolly pine (Pinus taeda L.) after a long fire-free history. Our study documented recruitment patterns through age-class distribution and reconstructed disturbance events (growth releases) using a modified version of the boundary-line growth method. Ages for all cored individuals at Everwoods (n  =  37) ranged from 32–184 years (x¯  =  53 years), and at Seacock Swamp (n  =  32), from 56–175 years (x¯  =  94 years). Longleaf pine has failed to recruit over the past two decades at Everwoods, and over the past half-century at Seacock Swamp, probably due to increased competition and habitat decline in the absence of fire. Boundary-line growth patterns revealed moderate and major release events for longleaf pines at both sites that we linked to anthropogenic disturbances, such as silvicultural operations (1900s to the 1930s) at both sites and logging by the landowner (early 1950s) at Seacock Swamp. We interpreted extremely low growth rates and dramatic growth-change pulses after disturbance as evidence of heavy suppression atypical for this species, which has been conceptualized as shade-intolerant. Our findings fit with other studies that have suggested that longleaf pine may be less shade-tolerant than formerly thought, at least in some areas or sites. Applications of the boundary-line growth patterns method to old-growth and second-growth longleaf pine forests throughout the southeastern U.S. could help to document possible spatial variability in disturbance histories, responses to releases, and suppression patterns.

A sediment-based reconstruction of Caribbean effective precipitation during the ‘Little Ice Age’ from Freshwater Pond, Barbuda:

Contemporary climate dynamics of the circum-Caribbean region are characterised by significant precipitation variability on interannual and interdecadal timescales controlled primarily by El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). However, our understanding of pre-industrial climate variability in the region is hampered by the sparse geographic distribution of palaeoclimate archives. Here, we present a high-resolution reconstruction of effective precipitation for Barbuda since the mid-16th century, based on biostratigraphic and stable isotope analyses of fossil ostracods and gastropods recovered from lake sediment cores from Freshwater Pond, the only freshwater lake on the island. We interpret episodic fluctuations in shell accumulation in the sediment record to represent changes in the balance between precipitation and evaporation during the ‘Little Ice Age’ (LIA; ~1400–1850 CE) and Industrial (1850–present) periods. Comparisons between indices of reconstructed ENSO and AMO variability, the abundance of the freshwater gastropod Pyrgophorus parvulus and the δ18O records from ostracod calcite suggest that the relative influence of ENSO and AMO on long-term rainfall patterns in Barbuda has changed over the last 400 years. Our findings are in agreement with other high-resolution palaeoclimate studies that suggest that long-term changes in effective precipitation during the LIA were much more variable, temporally and spatially, than previously suggested.

Temporal generalization of sub-pixel vegetation mapping with multiple machine learning and atmospheric correction algorithms

Temporal generalization allows a trained classification algorithm to be applied to multiple images across time to derive reliable classification map products. It is a challenging remote-sensing research topic since the results are dependent on the selection of atmospheric correction methods, classification algorithms, validation processes, and their varying combinations. This study examined the temporal generalization of sub-pixel vegetation mapping using multiple Landsat images (1990, 1996, 2004, and 2010). All Landsat images were processed with two atmospheric correction methods: simple dark object subtraction (DOS) and the Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) algorithm. For the sub-pixel vegetation mapping of the 2004 Landsat image, we used high-resolution OrbView-3 images as a training/validation data set and compared three machine learning algorithms (neural networks, random forests, and classification and regression trees) for their classification performance. The trained classifiers were then applied to other Landsat images (1990, 1996, and 2010) to derive sub-pixel vegetation map products. For the 2004 Landsat image classification, cross-validation shows similar classification results for neural networks (root mean square error (RMSE) = 0.099) and random forests (RMSE = 0.100) algorithms, and both are better than classification and regression trees (RMSE = 0.123). Pseudo-invariant pixels between 2004 and 2010 were used as validation points to evaluate the temporal generalizability of classification algorithms. Simple DOS and LEDAPS atmospheric correction resulted in similar accuracy statistics. The neural-network-based classifier performed best in generating reliable sub-pixel vegetation map products across time.

Spatial and Temporal Patterns of Tree Encroachment Into a Southern Appalachian Grass/Heath Bald Using Tree Rings

ABSTRACT: Craggy Gardens, a Southern Appalachian grass/heath bald in western North Carolina, is experiencing Quercus rubra L. encroachment on the western slope. Grass balds provide habitat for a number of rare and endemic species and have been ranked by The Nature Conservancy as G1 (Critically Imperiled Globally). We used belt transects and dendroecological analysis to reconstruct Quercus rubra establishment and recruitment over the past 245 years. Tree-ring records from 128 trees yielded distinct spatial and temporal patterns of encroachment that appear to be explained in large part by historical land use. Grazing by European livestock suppressed tree establishment, and its cessation in the early 1930s facilitated tree establishment and recruitment. Reduced rates of tree encroachment after the 1960s appear to be linked to shading of the understory upon canopy closure after the initial surge of establishment. Tree encroachment declined significantly after 1980, and was followed by the initiation of an ecological restoration project aimed at maintaining the grass bald at Craggy Gardens to preserve rare and endangered species and scenic appeal. Active management has reintroduced a strong human influence on the successional development of the bald.

Landscape Transformation During Ceramic Age and Colonial Occupations of Barbuda, West Indies

ABSTRACT This research documented the history of landscape transformation on the island of Barbuda in the Lesser Antilles, Caribbean through cross-disciplinary research approaches. Excavations confirmed a human presence for the seasonal exploitation of conch meat and other molluscs during the Archaic Age (c.3000–500 BC), but more substantial impacts to terrestrial ecosystems likely began during the Ceramic Age (c.500 BC–AD 1500). Our combined sedimentary and charcoal records revealed that human-induced environmental transformations began with Ceramic Age peoples as they cleared vegetation for settlements and gardens with intentional burning. Sedimentary charcoal indicated a dramatic decline in fire during post-Ceramic Age abandonment, continuing through the Colonial Period, as the dominant human activities shifted to herding, farming, and selective wood harvesting. Historical sources showed that during the Colonial Period (post-1492), the island was intermittently settled until the mid-seventeenth century, while the Codrington family of Antigua held the lease to the island from 1681 to 1870. They used the island for farming and stock-rearing, exporting meat and draught animals along with lime, timber, and subsistence crops. Macrocharcoal recovered from Colonial Period archaeological sites reflect the use of a variety of local species and wood imported to the island or harvested from shipwrecks.