We reconstructed historical fire regimes of montane forest-grassland ecotones in the ~40,000 ha Valles Caldera National Preserve, New Mexico. We used a targeted approach to sample ancient fire-scarred trees along the ecotone, and compared variations in historical fire occurrence within and among valles in the grassland-forest. The resulting tree-ring record extends from 1240-2008 C.E., comprised of 2,443 fire scars from 330 trees representing 238 fire years during the period of analysis, 1601-1902 C.E. Our results confirm pre-1900 historical occurrence of high-frequency, low-severity surface fires over multiple centuries in the ecotone. Mean fire intervals for all fires were 5.5-22.5 years (~6-123 ha) at individual sites, 2.7-10 years (~67-4955 ha) in individual valles, and 1.6 years (~10 386 ha) across the landscape. Synchronous fires burned extensively and occurred at ~10 year intervals during years with significantly low PDSI. Results will be useful in planning forest/grassland restoration actions and reinstituting fire regimes.

The ongoing drought in the US Southwest (SW) has led to particularly large and severe wildfires, tree die-off events, insect outbreaks, and increased forest stress levels. These disturbances underscore the vulnerabilities of SW dry conifer forests to climate change and past land-uses. Climate projections show a clear upward trend in regional temperatures, which will lead to accelerated heat-related stressors and disturbances in the coming decades. Already, more than 20% of the dry conifer forests of the SW have been severely impacted. This number is likely to grow, but we lack a clear picture of where, when, and to what degree other forest areas will be affected. Here, I apply dendroecological methods to evaluate patterns and processes that might determine greater or lesser vulnerability in dry conifer forests. Much of this work stems from critical concerns voiced by the Navajo Forestry Department (NFD). Long-term and representative data are necessary for the NFD, as they are responsible for closely managing over 250,000 hectares of forests and woodlands for the traditional products and ecosystem services that their forests provide for the Navajo people. The first study takes a multi-century perspective on changing fire regimes across Navajo forests, and places current forest structure and recent severe events in a long-term context. We found that surface fires were frequent across the landscape from at least the late-1500s until 1880. Navajo settlement of the area began to affect the fire regimes with added small fires in some areas beginning in 1700. By 1832, the rise of traditional pastoralist practices and transhumant migrations reduced fire activity in areas of greater use. Conditions changed following the establishment of the Navajo reservation in 1868, as livestock herds grew rapidly and initiated a near-synchronous and widespread collapse of fire regimes across the study area by 1880. The legacies of this change in land use are greater forest densities and higher fuel loads in some areas, raising the vulnerability of the forest to more severe fires. The second study assesses one of the most dramatic long-term consequences of recent high-severity fires in the SW, the rapid post-fire transition of dry conifer forest to oak-dominated shrubfields. To assess probable successional trajectories and interactions with climate change of recently converted forests, we reconstructed the age structures and fire regimes of some of the largest and oldest shrubfields in the Jemez Mountains of northern New Mexico. We found that shrubfields are a resilient configuration to drought and fire, historically burning at the same rates and under similar climate as dry conifer forests. Dense shrubfields pose a significant challenge to conifer recruitment from competition effects, with our sites persisting for over 100 years in the absence of burning and through periods of favorable climate to conifers. Given future warming favoring oak over pine, and projected trends in fire activity and high-severity fire behavior, we expect much more forest area to convert to shrubfield, especially in the absence of restoration efforts to reduce crown fire potential.Finally, we evaluate landscape-scale variability in tree growth response to regional climate across the Navajo forest. Projections of climate-induced forest decline often omit upper-elevation and mesic sites because they are not represented in regional tree-ring chronology networks. We found that these stands had much lower response to 20th century droughts than mid to lower elevation stands, and that targeted tree-ring sites are consistently more correlated with regional climate. However, as temperature-driven atmospheric moisture demand has remained above average since ~1997 in the study area, the upper-elevation trees are now nearly as responsive to regional climate as lower elevation xeric sites, probably due to increased moisture limitations. Recent warming has thus synchronized tree growth to an unprecedented extent across this large landscape and regionally.

Fire, logging, livestock grazing, and insect outbreaks are disturbances that have significantly influenced both the historic and present fire regimes. The composition and structure of vegetation communities within Great Smoky Mountains National Park (GSMNP) have likely changed in response to these disturbances. Two study sites (CRX, the near site, and CRT, the far site) were chosen along the Cooper Road Trail based on topographic separation, presence of mixed oak-pine communities, presence of fire-scarred yellow pine trees, and GSMNP land acquisition records. To quantify and evaluate fire regimes, individual fire histories were developed for each site from fire-scarred yellow pine trees, and two 1000 m2 (0.1 ha) study plots were established for vegetation surveys. Fire history analysis yielded mean fire intervals of 6.2 years at the near site, 3.4 years at the far site, and 3.2 years when combined. Spatial analysis showed significant differences in fire activity between study sites. Temporal analysis showed significant differences in mean fire intervals between the pre-settlement (1720–1818) and post-settlement periods (1819–1934). Superposed epoch analysis showed the over-riding influence of climate at these sites. At the near site, trees displayed greater species diversity, larger diameter, and older age. Eastern white pine, pitch pine, red maple, and black gum were the dominant species. At the far site, tree species diversity was lower and trees were generally younger. Mixed oak-pine communities are succeeding to a canopy dominated by shade-tolerant, fire-sensitive species such as eastern white pine and red maple. Without fire disturbance, yellow pine communities will cease to regenerate, as will oak species that prefer a fire-maintained habitat.

Beginning in the late 1930s, fire exclusion has drastically altered the vegetation dynamics of the southern Appalachian Mountains. Extremely low fire frequency has allowed for more shade-tolerant species to invade once predominantly open forests and has made it almost impossible for fire-dependent species to establish on a site. One such species is the endangered Peters Mountain mallow (Iliamna corei Sherff.) located on Peters Mountain in The Nature Conservancy s Narrows Preserve in Giles County, Virginia. This paper focuses on the fire history and stand dynamics of Peters Mountain and how fire exclusion has altered the forest composition. The historic fire frequency and successional changes discovered here may provide an insight into management strategies for the mallow. Seventy-nine fire scarred cross-sections were taken and aged to determine fire history dates and frequencies. Three 50x20 meter plots were set up on opposing aspects: northwest and southeast. The aspects were chosen at the direction of The Nature Conservancy personnel. All trees within were identified, cored and aged to determine species composition and the establishment dates of all trees. Fire history analysis revealed a mean fire interval of 2.48 years, a Weibull median fire interval of 2.18 years and a 25 percent scarred class mean fire interval of 12.5 years. Stand dynamic results show that Quercus montana has established on Peters Mountain prior to fire exclusion and remains the dominate species on the landscape. An increased number of fire intolerant species (including Acer rubrum, Sassfras albidum, Nyssa sylvatica) have been establishing on Peters Mountain during the decades of decreased fire frequency, suggesting a shift in forest composition. Frequent fires are suggested for mallow management and oak forest maintenance.

The role of fire in determining the structure and composition of many forested ecosystems is well documented (e.g. North American boreal forests; piñon-juniper woodlands of the western US). Fire is also believed to be important in temperate forests of eastern North America, but the processes acting here are less clear, particularly in xerophytic forests dominated by yellow pine (Pinus, subgenus Diploxylon Koehne) and oak (Quercus L.). In this study, I use dendroecological techniques to investigate fire history and vegetation dynamics of mixed pine-oak forests in the central Appalachian Mountains of Virginia. The study addresses three objectives: (1) develop a lengthy fire chronology to document fire history beginning in the late presettlement era and extending throughout the period of European settlement, industrialization and modern fire exclusion; (2) explore fire-climate relationships; and (3) investigate vegetation dynamics in relation to fire occurrence. The study was conducted on three study sites within the George Washington National Forest. I used fire-scarred cross-sections from yellow pine trees to document fire history. Fire-climate relationships were investigated for each study site individually and all sites combined using superposed epoch analysis (SEA). Fire-history information was coupled with dendroecological data on age structure to explore stand development in relation to fire occurrence. Results of fire history analysis reveal a long history of frequent fire with little temporal variation despite changes in land use history. Mean fire intervals (MFI) ranged from 3.7–17.4 years. The most important change in the fire regime was the initiation of fire suppression in the early twentieth century. Results of SEA show that periodic droughts may be important drivers of fire activity. Drought the year of fire was important at two of the three study sites and when all sites were combined. Results of age structure indicate that vegetation development was clearly influenced by fire. Frequent burning maintained populations of yellow pine throughout the period of study until fire suppression allowed fire-sensitive hardwood trees and shrubs to establish. It is clear from this study that continued fire suppression will likely result in fire-tolerant pines and oaks being replaced by more mesophytic trees and shrubs.

Fire, logging, livestock grazing, and insect outbreaks are disturbances that have significantly influenced both the historic and present fire regimes. The composition and structure of vegetation communities within Great Smoky Mountains National Park (GSMNP) have likely changed in response to these disturbances. Two study sites (CRX, the near site, and CRT, the far site) were chosen along the Cooper Road Trail based on topographic separation, presence of mixed oak-pine communities, presence of fire-scarred yellow pine trees, and GSMNP land acquisition records. To quantify and evaluate fire regimes, individual fire histories were developed for each site from fire-scarred yellow pine trees, and two 1000 m2 (0.1 ha) study plots were established for vegetation surveys. Fire history analysis yielded mean fire intervals of 6.2 years at the near site, 3.4 years at the far site, and 3.2 years when combined. Spatial analysis showed significant differences in fire activity between study sites. Temporal analysis showed significant differences in mean fire intervals between the pre-settlement (1720–1818) and post-settlement periods (1819–1934). Superposed epoch analysis showed the over-riding influence of climate at these sites. At the near site, trees displayed greater species diversity, larger diameter, and older age. Eastern white pine, pitch pine, red maple, and black gum were the dominant species. At the far site, tree species diversity was lower and trees were generally younger. Mixed oak-pine communities are succeeding to a canopy dominated by shade-tolerant, fire-sensitive species such as eastern white pine and red maple. Without fire disturbance, yellow pine communities will cease to regenerate, as will oak species that prefer a fire-maintained habitat.

Using dendroecological and archaeological methods and data we investigated the temporal dynamics of forest regeneration and fire history following depopulation of four large Pueblo IV period (1300-1600) villages on the Jemez Plateau, New Mexico. With tree rings we reconstructed the timing of reforestation on village footprints after depopulation–a novel approach to terminus ante quem dating of site occupation. Our tree-ring based forest age structure and fire history chronologies enabled us to reduce by 51 to 70 years the range of previous estimates of village depopulation dates derived primarily from terminal ceramic assemblages. One of the four village sites we investigated was depopulated in 1696, two were depopulated between 1625 and 1700 CE, while the fourth village was depopulated earlier (pre 1500), but the area was likely in continued use for agriculture or other seasonal purposes until the mid-1600s. Our results indicate that the Jemez were highly influential ecological agents. Forest structure and fire regime dynamics changed greatly after the departure of most people from these landscapes after circa 1650 CE. The terminus ante quem methods that we demonstrate in the Jemez Mountains have strong potential to constrain and refine low temporal resolution chronologies of human occupation at archaeological sites within other forested ecosystems of the Southwest and elsewhere.

In the past few decades, wildfires have increased in size and severity in the Southwest and across the western US. These recent trends in fire behavior are a drastic change in arid, ponderosa pine and mixed conifer forests of the Southwest compared with tree-ring records of fire history for the past ~ 400 years. This study presents a late Holocene record (~ 3,000 years) of fire history and related changes in fire regimes with climate variability over annual to multi-decadal time scales. Tree-ring and alluvial-sediment sampling sites were paired in four small, tributary basins located in the western San Juan Mountains of Colorado. In our study sites, tree-ring records show that fire return intervals were longer and fire behavior was more severe on the north-facing slopes with relatively dense mixed conifer stands. Increased fire barriers and steep topography decreased the fire frequency and extent relative to gentle terrain elsewhere in the range and leading to a lack of synchrony among fire years in different parts of the study area. The alluvial-sediment record showed four peaks in high-severity fire activity over the past 3,000 years ranging between 200 - 400 years in length. The timing of peaks coincided with decadal-length drought episodes and were often preceded by multiple decades of above average winter precipitation. The sampling of alluvial-sediment and tree-ring data allowed for site-level comparisons between recent alluvial deposits and specific fire years interpreted from the tree-ring records. We found good correspondence between the type of fire-related sediment deposit (i.e. geomorphic response) in the alluvial record and the extent of mixed and high-severity fire estimated from the tree-ring record, and the correspondence was well-supported by the debris flow probability model results. The two paleofire data tend to represent particular components of the historical fire regime, with alluvial-sediments biased towards infrequent, high-severity events during recent millennia, and the tree-ring record biased toward lower severity fires during recent centuries. The combined analyses of different paleofire proxy types in the same study sites, therefore, can enhance and expand our understanding of fire and climate history beyond what is possible with either proxy alone.

Understanding historic fire regimes to develop benchmarks for emulating historic natural disturbance processes in the interest of conserving biodiversity has been actively pursued for approximately 30 years. Mixed-severity fire regimes are increasingly becoming a recognized component of historic fire regimes. Mixed-severity fire regimes are inherently difficult to classify and characterize given the complexity of the process and the multiple scales at which this complexity is expressed. I utilized a systematic study design to gather fire scar and stand dynamic information in order to describe and classify the historic fire regime. I established the presence of mixed disturbance regime dominated by a mixed-severity fire regime. The historic fire regime was mixed-severity over time dominated by individual high severity fire events occurring at a frequency of 60-130 years with some areas that experienced lower severity fire events occurring at a frequency of 20 - 40 years. Twenty-one per cent of the current landscape was dominated by high-severity fire, 42% by mixed-severity and 37% had an unknown fire history. I developed a fire regime classification scheme that provides a useful tool for considering fire severity in mixed-severity system with forest species that generate strong establishment cohorts. I was able to combine time-since-fire methods with a systematic study design and this combination provided an excellent tool to explore mixed-severity fire characteristics in a complicated mixed-disturbance forest. I found limited relationships between topographic controls and fire severity. I found a number of significant relationships that fit the broadly held perceptions of how fire severity would affect species relative densities and stand structure attributes. The existing stand origin map and the Vegetation Resource Inventory stand age were largely accurate for high-severity 20th century fires but had decreasing accuracy in older forests and for mixed and unknown fire severity. The accuracy of the Vegetation Resource Inventory leading species accuracy was quantified at only 60%. My results have implications for fire and forest management in south-eastern British Columbia and in other forest systems that had historic mixed-severity fire regimes with tree species that have strong establishment cohorts. / Graduate / 0478 / 0329 / rick.kubian@pc.gc.ca

Quantifying charcoal particles preserved in sedimentary environments is an established method for estimating levels of fire activity in the past, both on human and geological timescales. It has been proposed that the morphology of these particles is also a valuable source of information, for example allowing inferences about the nature of the vegetation burned. This thesis aims to broaden the theoretical basis for these methods, and to integrate morphometric study of sedimentary charcoal with its quantification. Three key questions are addressed: firstly, whether the elongation of mesocharcoal particles is a useful indicator of fuel type; secondly, whether different sedimentary archives tend to preserve different charcoal morphologies; and finally, the critical question of how morphology affects charcoal quantification. The results corroborate the idea that grasses and trees produce mesocharcoal with distinctly different aspect ratios. However, the application of this as an indicator of vegetation change is complicated by the inclusion of species which are neither grasses nor trees, and by considerations of the effects of transportation. Charcoal morphotypes in diverse sedimentary environments are shown to be influenced by vegetation types, transportation history, and nature of the fire that produced them. Previous research has treated charcoal quantification and charcoal morphology as separate issues. Here it is shown that understanding morphology is essential for the accurate quantification of charcoal, since it affects the relationship between volumes and the two-dimensional areas from which measurements are taken. Understanding this relationship could allow such measurements to be used not just as relative measures of past fire activity, but to enable the accurate quantification of the charcoal sequestered in soils and sediments. This has important implications for our ability to understand the effects of fire on carbon cycling, and the role that fire plays in the Earth system.