Richard P. Guyette

Dynamics of an anthropogenic fire regime

Human interaction with fire and vegetation occurs at many levels of human population density and cultural development, from subsistence cultures to highly technological societies. The dynamics of these interactions with respect to wildland fire are often difficult to understand and identify at short temporal scales. Dendrochronological fire histories from the Missouri Ozarks, coupled with human population data, offer a quantitative means of examining historic (1680–1990) changes in the anthropogenic fire regime. A temporal analysis of fire scar dates over the last 3 centuries indicates that the percent of sites burned and fire intervals of anthropogenic fires are conditioned by the following four limiting factors: (a) anthropogenic ignition, (b) surface fuel production, (c) fuel fragmentation, and (d) cultural behavior. During an ignition-dependent stage (fewer than 0.64 humans/km2), the percent of sites burned is logarithmically related to human population (r2 = 0.67). During a fuel-limited stage, where population density exceeds a threshold of 0.64 humans/km2, the percent of sites burned is independent of population increases and is limited by fuel production. During a fuel-fragmentation stage, regional trade allows population densities to increase above 3.4 humans/km2, and the percent of sites burned becomes inversely related to population (r2 = 0.18) as decreases in fuel continuity limit the propagation of surface fires. During a culture-dependent stage, increases in the value of timber over forage greatly reduce the mean fire interval and the percent of sites burned. Examples of the dynamics of these four stages are presented from the Current River watershed of the Missouri Ozarks.

Fire Frequency on an Oak-Hickory Ridgetop in the Missouri Ozarks

-Wedges taken from 24 post oaks (Quercus stellata Wang.) growing on a ridge in an oak-hickory stand were used to reconstruct the fire history in the Houston Ranger District in Missouris Mark Twain National Forest. A chronology was constructed dating from 1734 to 1991. Fire frequency was greatest between 1740 and 1850 with a mean return interval of 2.8 yr. After 1850, the fire return interval increased to 24 yr. This change in fire return regimes is coincident with settlement of the area by Anglo-Americans.

Fire history of oak–pine forests in the Lower Boston Mountains, Arkansas, USA

Perspective on present day issues associated with wildland fire can be gained by studying the long-term interactions among humans, landscape, and fire. Fire frequency and extent over the last 320 years document these interactions north of the Arkansas River on the southern edge of the Lower Boston Mountains. Dendrochronological methods were used to construct three fire chronologies from 309 dated fire scars that were identified on 45 shortleaf pine (Pinus echinata) remnants. Fire frequency increased with human population density from a depopulated period (the late 1600s and early 1700s) to a peak in fire frequency circa 1880. Fire frequency then decreased as human population continued to increase. Fire frequency and human population density were positively correlated during an early period (1680–1880) with low levels of population, but negatively correlated during a later period (1881–1910) with high levels of population. We hypothesized that this difference is due to limits on fire propagation and ignition caused by land use and culture, as well as human population density. Relatively high human population densities (>5 humans/km 2 ) were associated with a peak in the maximum number of fires per decade in this highly dissected, ‘bluff and bench’ landscape compared to less dissected regions of the Ozarks. # 2002 Elsevier Science B.V. All rights reserved.

HISTORICAL CO2 GROWTH ENHANCEMENT DECLINES WITH AGE IN QUERCUS AND PINUS

Despite experimental evidence showing that elevated C02 levels increase growth in most plants, the isolation of a signal consistent with anthropogenically caused increases in atmospheric C02 from the dendrochronological record has shown mixed results. Our extensive sets of tree ring data from the Ozark Mountains in Missouri showed that since 1850, Quercus velutina Lam., Quercus cocc?nea Muench., and Pinus echinata Mill, trees increased in stem growth coincidently with increases in atmospheric C02. Those long-term increases in radial growth appear unrelated to historical disturbance levels for the region, to long-term changes in relevant climatic variables, or to productivity of sites sampled for the purpose of creating a time sequence of tree ring growth. It is still unclear what the potential role of nitrogen deposition might have been for tree growth. We cross-dated a large number of increment cores and aligned the ring width data by pith date for accurate age constant assessments of growth over the past 150 years. Thus, we circumvented changes in growth trend associated with differences in physiological functioning during development, as well as the need for statistical detrending that removes an unknown degree of long-term environmental signal, the so called segment length curse that applies to standard dendrochronological investigations. When the positive relationship between C02 and ring width was examined at different ages, an ontogenetic decline in the rate of growth stimulation was found. Specifically, both the pooled Quercus spp. and P. echinata were characterized by a negative exponential pattern of response over a developmental sequence through age 50. Further knowledge of an intrinsic decline in C02 sensitivity with tree age or size such as this may be important for increased accuracy in estimating terrestrial carbon stocks across successional landscapes.

Fire and human history of a barren-forest mosaic in southern Indiana

Abstract The purpose of this paper is to provide quantitative fire history information from a historically unique region, the oak barrens of the Interior Low Plateau Ecoregion. We sampled 27 post oak (Quercus stellata Wangenh.) trees from the Boone Creek watershed in southern Indiana. The period of tree-ring record ranged in calendar years from 1654 to 1999 and fire scar dates (n = 84) ranged from 1656 to 1992. The mean fire interval for the period 1656 to 1992 was 8.4 y and individual fire intervals ranged from 1 to 129 y. The average percentage of trees scarred at the site was 19% or about 1 in 5 trees sampled. No significant relationship was identified between fire years and drought conditions however, variability in the fire record coincided with Native American migrations and Euro-American settlement periods. Temporal variability in the fire record illustrates not only the dynamic nature of anthropogenic fire regimes but also the importance of humans in culturing presettlement barrens communities.

Climate response among growth increments of fish and trees

Significant correlations were found among the annual growth increments of stream fish, trees, and climate variables in the Ozark region of the United States. The variation in annual growth increments of rock bass (Ambloplites rupestris) from the Jacks Fork River was significantly correlated over 22 years with the ring width of four tree species: white oak (Quercus alba), post oak (Quercus stellata), shortleaf pine (Pinus echinata) and eastern red cedar (Juniperus virginiana). Rock bass growth and tree growth were both significantly correlated with July rainfall and stream discharge. Variations in annual growth of smallmouth bass (Micropterus dolomieu) from four streams were significantly correlated over 29 years (1939–1968) with mean May maximum air temperature but not with tree growth. The magnitude and significance of correlations among growth increments from fish and trees imply that conditions such as topography, stream gradient, organism age, and the distribution of a population relative to its geographic range can influence the climatic response of an organism. The timing and intensity of climatic variables may produce different responses among closely related species.

Barium and manganese trends in tree-rings as monitors of sulfur deposition

Barium and manganese concentrations in radial growth increments of heartwood from Juniperus virginiana L. are used to examine changes in soil reaction and sulfur deposition. Barium and Mn trends from trees growing on 2 sites with acid, low-Ca soils, signaled sulfur-induced changes in soil acidity after 1870, while Ba and Mn trends before 1870 did not. Factors controlling Ba availability are hypothesized to have changed from soil acidity before 1870 to precipitation of BaSO4 by increased soil sulfate after 1870.

Fire Regime of an Ozark Wilderness Area, Arkansas

Abstract Many of the present day issues associated with fire management in wilderness areas are addressed by studying past interactions among fire, humans, vegetation and climate. In this paper we describe three centuries of the fire regime in the Lower Buffalo River Wilderness Area, Arkansas, USA. We reconstructed fire events from 159 tree-ring dated fire scars on 26 shortleaf pine (Pinus echinata Mill.) remnants and live trees. During the late-17th Century and early 18th Century the mean fire return interval (MFI) was 7.7 y. Fire frequency increased abruptly circa 1820 with fires burning every 2 y on average until 1920. The number of fires decreased during the 1900s as cultural values changed to favor fire suppression over multiple-use burning. Analyses of the influence of human ignitions and drought on the fire regime resulted in two important findings: (1) that fire frequency was positively correlated to human population density up to 1920 and (2) the influence of drought seemed to be masked by frequent anthropogenic fires and fire suppression. Fire events were associated with droughts only prior to Euro-American settlement. Studies of climate-fire relationships should consider the potential for anthropogenic influence and future studies should attempt to quantify the historic role of humans in the fire regime.

Spring temperature responses of oaks are synchronous with North Atlantic conditions during the last deglaciation

Paleoclimate proxies based on the measurement of xylem cell anatomy have rarely been developed across the temperature range of a species or applied to wood predating the most recent millennium. Here we describe wood anatomy-based proxies for spring temperatures in central North America from modern bur oaks (Quercus macrocarpa Michx.). The strong coherence of temperature signals across the species range supports the use of these proxies across thousands of years of climatic change. We also used 79 subfossil oak log cross sections from northern Missouri, 14C-dated to 9.9–13.63 ka (ka is 1000 cal yr BP), to assess the frequency of oak deposition into alluvial sediments and a subset of these oaks for a wood anatomy-based reconstruction of spring paleotemperatures. Temperatures during the Younger Dryas cold period (YD) were up to 3.5°C lower than modern temperatures for that region, equivalent to or lower than those experienced at the northern edge of the modern species range. Compared to extant oaks growing a...

Drivers of radial growth and carbon isotope discrimination of bur oak (Quercus macrocarpa Michx.) across continental gradients in precipitation, vapour pressure deficit and irradiance

Tree-ring characteristics are commonly used to reconstruct climate variables, but divergence from the assumption of a single biophysical control may reduce the accuracy of these reconstructions. Here, we present data from bur oaks (Quercus macrocarpa Michx.) sampled within and beyond the current species bioclimatic envelope to identify the primary environmental controls on ring-width indices (RWIs) and carbon stable isotope discrimination (Δ(13) C) in tree-ring cellulose. Variation in Δ(13) C and RWI was more strongly related to leaf-to-air vapour pressure deficit (VPD) at the centre and western edge of the range compared with the northern and wettest regions. Among regions, Δ(13) C of tree-ring cellulose was closely predicted by VPD and light responses of canopy-level Δ(13) C estimated using a model driven by eddy flux and meteorological measurements (R(2)  = 0.96, P = 0.003). RWI and Δ(13) C were positively correlated in the drier regions, while they were negatively correlated in the wettest region. The strength and direction of the correlations scaled with regional VPD or the ratio of precipitation to evapotranspiration. Therefore, the correlation strength between RWI and Δ(13) C may be used to infer past wetness or aridity from paleo wood by determining the degree to which carbon gain and growth have been more limited by moisture or light.