Sally P. Horn

Predictability of biomass burning in response to climate changes

Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Holocene fires in Costa Rica

Charcoal fragments in soils and sediments document Holocene fires in the rain forests of the La Selva Biological Station in the northern Caribbean lowlands of Costa Rica, and in the paramo surrounding Cerro Chirrip6 in the Cordillera de Talamanca. Radiocarbon determinations on soil charcoal from La Selva and charcoal-rich lake sediment from Chirrip6 cluster at 2430 yr B.P. and at 1110-1180 yr B.P.; dates in each cluster are coeval, suggesting that the rain forest and paramo fires occurred at similar times. Fires at La Selva were likely set by human activity but may have spread into intact rain forest during exceptionally dry periods; fires at Chirrip6 were set by people or lightning during what may have been lower lake stands. The drought periods suggested by our charcoal samples may have been associated with short-term atmospheric anomalies such as El Niiio, or with longer-term shifts in climate.

Pre-Columbian land-use history in Costa Rica: a 3000-year record of forest clearance, agriculture and fires from Laguna Zoncho

We analysed pollen and charcoal in a lake-sediment core from Laguna Zoncho, a small mid-elevation lake in southernmost Costa Rica. The record provides evidence of 3000 years of human occupation, forest clearance, agriculture and fires in the area. Laguna Zoncho is located 2 km from the Las Cruces Biological Station, and results are relevant to understanding the likely extent of prehistoric disturbance within the premon tane rain forests of this biological reserve. Pollen grains ofZea mays in the basal sediments of the Zoncho core constitute the oldest botanical evidence of maize cultivation in southern Costa Rica. The presence of maize pollen in almost every sample analysed indicates nearly continuous occupation of the lake basin for 3000 years. Charcoal abundances and percentages of pollen grains and spores of forest and disturbance taxa fluctuate strongly downcore, reflecting variable intensities of past human impact. Forest clearance and burning by indigenous people were most marked between 3240 and 460 cal. yr BP. Sediments deposited subsequently indicate forest regeneration and few, if any, fires. However, indigenous cultivation of maize continued on a small scale until European settlement in the twentieth century.

Modern pollen spectra from Costa Rica

Abstract We analyzed pollen spectra preserved in surface lake and mire sediments and soils to provide information on modern pollen rain across a range of vegetation types in Costa Rica. We collected samples at 29 sites located within ten principal vegetation types: mangrove, tropical dry forest, derived savanna, tropical moist forest, tropical wet forest, lowland freshwater swamp, premontane rain forest, montane rain forest, montane bog, and paramo. Samples from sites with similar vegetation generally had similar pollen assemblages and clustered together in detrended correspondence analysis. Results indicate that major vegetation types in Costa Rica have distinct pollen spectra.

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.

Prehistoric agriculture and forest clearance in the Sierra de los Tuxtlas, Veracruz, Mexico

Abstract Analysis of the pollen and charcoal content of a 9.4 m sediment core from Lago Catemaco in the Sierra de los Tuxtlas of southern Veracruz, Mexico, has provided a record of prehistoric agriculture and forest clearance in the Olmec region. Problems with radiocarbon dating have precluded the establishment of a firm chronology, but the volcanic ash stratigraphy suggests that the core represents approximately the last three thousand years. Maize pollen was encountered in all but two of the samples analyzed, and is especially common in the middle section of the core. This section is also characterized by low arboreal pollen counts and high percentages of herbaceous taxa, particularly Ambrosia. We interpret this interval to represent a period of intensified agricultural activity during the Late Classic and Post‐classic. In the upper section of the core maize pollen counts are generally low, even in the near‐surface samples. The most striking feature of this section is a sharp increase in arboreal pollen...

Fire history from soil charcoal in a mixed hardwood forest on the Cumberland Plateau, Tennessee, USA1

Abstract In this study, we documented the presence of macroscopic (> 2 mm) charcoal, quantified charcoal mass, and radiocarbon-dated charcoal macrofossils in 10 soil cores to develop a coarse-resolution fire history for a mixed hardwood forest on the Cumberland Plateau in Tennessee. Macroscopic charcoal occurred in all 10 soil cores. Total dry mass of macroscopic charcoal varied by core and by depth layer. Charcoal fragments were most abundant in two non-adjacent cores (separated by ca. 80 m), a finding that may be evidence of a patchy fire regime in the study area. AMS radiocarbon dating of the five deepest charcoal samples indicated that the earliest recorded fire in the study site occurred around 6735 cal yr BP (calibrated years before 1950). Charcoal in surface soils was not dated but one deep sample indicated a fire during the historic period at approximately 174 cal yr BP. No overlap occurred within the 2-sigma calibrated age ranges of the dated charcoal samples, indicating a minimum of five separate fire events have occurred on the site during the last 6700 plus years. This was the first study to use soil charcoal to document past fire events in hardwood forests of the Cumberland Plateau and the first to examine the prehistoric fire regime of Quercus stands in the region at a local-scale. Our results provide a basis for reconstructing long-term fire histories at the stand-scale in Quercus-dominated forests of eastern North America.

Wildlife conservation in urban greenways of the mid-southeastern United States

Goals related to wildlife conservation are stated or implied in many urban greenway plans, but the actual wildlife conservation value of urban greenways is unclear. We surveyed 72 km. of greenway consisting of 38 different greenway segments in six cities in the mid-southeastern United States to determine the presence or absence of red fox (Vulpes vulpes), grey fox (Urocyon cinereoargenteus), and white-tailed deer (Odocoileus virginianus), and to assess characteristics of the greenways and adjacent lands that might promote or limit the presence of these species in each greenway. We focused on fox and deer because these mammals, while habitat generalists, require diverse habitats within close proximity, and are more sensitive to human disturbance and have larger territories than the majority of urban wildlife in this region. Thus, they can serve as indicator species for these habitat values in urban greenways in the southeastern United States. Field observations and scent station inventories revealed that only 18 of the 38 greenway segments had either fox or deer, 12 had only fox, 6 had fox and deer, and none had deer alone. Greenway segments with more forest cover, wider corridors, greater amounts of adjacent natural or seminatural habitat, and forest connectivity between greenways and nonadjacent natural areas were more likely to have fox or deer present. Characteristics associated with fox and deer presence were not independent of each other. Because greenways with such characteristics tend to be surrounded by less urban areas, adjacent land use is a good predictor of fox and deer presence in the surveyed greenways. Whether greenways that are beneficial to fox and deer can be developed in urban settings is strongly site specific, because the greenway variables that appear most essential to these species are often dictated by the preexisting urban form and the vegetation in the vicinity of the greenway. Because the wildlife conservation value of urban greenways depends greatly on factors external to the greenway, greenways that have fox and deer present today may be particularly vulnerable to future changes in surrounding land use.

A 10 000 year record of Páramo fires in Costa Rica

The pairamo ecosystem comprises various types of treeless vegetation that occur above montane forest and below permanent snowline on high mountains of the humid equatorial tropics of Latin America (Baslev & Luteyn 1991, Luteyn 1999). The most extensive pairamos are found above 3000 m in the northern Andes of Venezuela, Colombia and Ecuador, but isolated areas exist in northern Peru and in the Cordillera de Talamanca in southern Costa Rica and adjacent westernmost Panama. Whether pairamo is a human-made or natural ecosystem continues to be debated. The possible role of human-set fires in expanding (or even initially creating) some pairamos has attracted considerable attention (Baslev & Luteyn 1991). Andean paramo vegetation is generally believed to reflect centuries or millennia of burning associated with human occupation, agriculture and livestock grazing that may have begun as early as 5000-6000 y ago in some areas (Hansen & Rodbell 1995), though not until the Spanish Conquest in others (Luteyn 1999, Verweij 1995). The Chirripo pairamo of Costa Rica is the largest of the several islands of paramo vegetation in the Cordillera de Talamanca. Protected as the centrepiece of Chirripo National Park, this pairamo is remote from human settlement and has no history of prehistoric or historic occupation or livestock grazing. Nevertheless, it has long been subjected to periodic fires. Recent fires have been set accidentally by hikers, and by aircraft accidents (Horn 1989). Previous broad-interval sampling of charcoal fragments in a sediment core from a glacial

PreColumbian agriculture and forest disturbance in Costa Rica: palaeoecological evidence from two lowland rainforest lakes

Lake-sediment cores from Laguna Bonilla and Laguna Bonillita provide some of the first evidence of prehistoric human impacts on lowland rainforests in Costa Rica. The longer Bonillita sediment record docu ments permanent settlement of the lake shores by 2560 BP, about 600 years earlier than previously inferred from the archaeological record. Zea pollen and charcoal fragments in cores from both lakes indicate a subsist ence strategy that included maize cultivation and some use of fire. A dramatic decline in Myrsine pollen percentages about 1300 BP suggests local eradication of this woody plant, likely as a result of land clearance and/or the use of Myrsine for construction.