Christopher H. Baisan

Tree-Ring Reconstructions of Fire and Climate History in the Sierra Nevada and Southwestern United States

Most of the fire history research conducted in the past century has focused on case studies and local-scale assessments of pattern and process, with an emphasis on describing typical fire frequencies in forest stands and watersheds. Dominant research themes have included the characterization and analyses of fire frequencies across ranges of topographic settings and habitats. In general, these “histories” have been more about describing time-averaged processes, than elucidating the events, narratives, and contingencies of “history.” Now that many crossdated fire chronologies have been developed from tree-ring analyses of firescarred trees, it is possible to assemble regional to global-scale networks of fire occurrence time series. These networks and time series can be used in quantitative, historical analyses that identify and separate broad-scale climate-driven patterns of fire occurrence from local, nonclimatic features of individual sites. The seasonal to annual resolution of tree rings facilitates historical fire climatology because the high temporal resolution of these data allows us to connect multiple events in space and time. The importance of climatic influence is reflected in the degree of synchrony in specific fire events and decadal to centennial trends among widely distributed sites (Swetnam and Betancourt 1990, 1998; Swetnam 1993; Veblen et al. 1999; Veblen, Kitzberger, and Donnegan 2000; GrissinoMayer and Swetnam 1997, 2000; Heyerdahl, Brubaker, and Agee 2001, in press; Kitzberger and Veblen 1998; Kitzberger, Veblen, and Villalba 1997; Kitzberger, Swetnam, and Veblen 2001; Brown, Kaufmann, and Shepperd 1999; Brown et al. 2001; Allen 2002).

Fire history along environmental gradients in the Sacramento Mountains, New Mexico: Influences of local patterns and regional processes

Abstract Patterning in fire regimes occurs at multiple spatiotemporal scales owing to differences in scaling of local and regional influences. Local fire occurrence and behavior may be controlled largely by site factors, while regional climate and changes in human land use can synchronize fire timing across large areas. We examined historical patterns in fires during the past five centuries across gradients in forest types and physiography and in relation to regional climate variability and land use change in the Sacramento Mountains in southern New Mexico. Forest stand-level chronologies of fires were reconstructed for 19 pinyon-juniper, ponderosa pine, and mixed-conifer stands using fire-scar records in crossdated tree-ring series. The fire history documents both local and regional factors effected fire occurrences in stands. Lower-elevation stands recorded more frequent fire than higher-elevation stands, although there were not significant differences between means of fire frequencies from clusters of ponderosa pine and mixed-conifer stands. Mean fire intervals ranged from approximately 3 to 11 years in ponderosa pine sites to 4 to 14 years in mixed-conifer sites. Sites on the steeper west side of the range, where fire spread more readily between forest types, recorded significantly more frequent fire than sites on the more physiographically heterogeneous east side. Fires were also synchronized by regional factors. Fire occurrences and fire-free years are related to variability in both annual Palmer Drought Severity Indices and El Niño-Southern Oscillation events. Fire regimes in the stands were also profoundly effected by changes in human land use patterns, with fire cessation in all sites following intensive Euro-American settlement beginning in the 1880s.

Spatial and temporal corroboration of a fire-scar-based fire history in a frequently burned ponderosa pine forest.

Fire scars are used widely to reconstruct historical fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (n = 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (> 100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x + 0.0087, r2 = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by < 3 yr from documentary records (29.6 yr); mean fire return intervals (MFI) for large-fire years (i.e., > or = 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of tree-ring growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed fire-scar samples.

Paired charcoal and tree-ring records of high-frequency Holocene fire from two New Mexico bog sites

Two primary methods for reconstructing paleofire occurrence include dendrochronological dating of fire scars and stand ages from live or dead trees (extending back centuries into the past) and sedimentary records of charcoal particles from lakes and bogs, providing perspectives on fire history that can extend back for many thousands of years. Studies using both proxies have become more common in regions where lakes are present and fire frequencies are low, but are rare where high-frequency surface fires dominate and sedimentary deposits are primarily bogs and wetlands. Here we investigate sedimentary and fire-scar records of fire in two small watersheds in northern New Mexico, in settings recently characterised by relatively high-frequency fire where bogs and wetlands (Chihuahuenos Bog and Alamo Bog) are more common than lakes. Our research demonstrates that: (1) essential features of the sedimentary charcoal record can be reproduced between multiple cores within a bog deposit; (2) evidence from both fire-scarred trees and charcoal deposits documents an anomalous lack of fire since ~1900, compared with the remainder of the Holocene; (3) sedimentary charcoal records probably underestimate the recurrence of fire events at these high-frequency fire sites; and (4) the sedimentary records from these bogs are complicated by factors such as burning and oxidation of these organic deposits, diversity of vegetation patterns within watersheds, and potential bioturbation by ungulates. We consider a suite of particular challenges in developing and interpreting fire histories from bog and wetland settings in the Southwest. The identification of these issues and constraints with interpretation of sedimentary charcoal fire records does not diminish their essential utility in assessing millennial-scale patterns of fire activity in this dry part of North America.

Arizona pine (Pinus arizonica) stand dynamics: local and regional factors in a fire-prone madrean gallery forest of Southeast Arizona, USA

In southwestern North America, large-scale climate patterns appear to exert control on moisture availability, fire occurrence, and tree demography, raising the compelling possibility of regional synchronization of forest dynamics. Such regional signals may be obscured, however, by local, site-specific factors, such as disturbance history and land use. Contiguous sites with similar physical environments, lower and middle Rhyolite Canyon, Arizona, USA, shared nearly the same fire history from 1660-1801, but then diverged. For the next 50 years, fires continued to occur frequently in lower Rhyolite, but, probably as result of flood-induced debris deposition, largely ceased in middle Rhyolite. We related stand dynamics of Arizona pine (Pinus arizonica) to fire history and drought severity and compared the dynamics in the two sites before and after the divergence in fire frequency. Fires occurred during unusually dry years, and possibly following unusually moist years. Arizona pine exhibited three age structure peaks: two (1810–1830 and 1870–1900) shared by the two sites and one (1610–1640) only in middle Rhyolite. The latter two peaks occurred during periods of unusually low fire frequency, suggesting that fire-induced mortality shapes age structure. Evidence was mixed for the role of favorable moisture availability in age structure. As expected, moisture availability had a prominent positive effect on radial growth, but the effect of fire was largely neutral. The two sites differed only moderately in stand dynamics during the period of divergence, exhibiting subtle age structure contrasts and, in middle Rhyolite only, reduced growth during a 50-year fire hiatus followed by fire-induced release. These results suggest that, despite local differences in disturbance history, forest responses to regional fire and climate processes can persist.

A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests

Fire history researchers employ various forms of search-based sampling to target specimens that contain visible evidence of well preserved fire scars. Targeted sampling is considered to be the most efficient way to increase the completeness and length of the fire-scar record, but the accuracy of this method for estimating landscape-scale fire frequency parameters compared with probabilistic (i.e. systematic and random) sampling is poorly understood. In this study we compared metrics of temporal and spatial fire occurrence reconstructed independently from targeted and probabilistic fire-scar sampling to identify potential differences in parameter estimation in south-western ponderosa pine forests. Data were analysed for three case studies spanning a broad geographic range of ponderosa pine ecosystems across the US Southwest at multiple spatial scales: Centennial Forest in northern Arizona (100ha); Monument Canyon Research NaturalArea(RNA)incentralNewMexico(256ha);andMicaMountaininsouthernArizona(2780ha).Wefoundthatthe percentage of available samples that recorded individual fire years (i.e. fire-scar synchrony) was correlated strongly between targeted and probabilistic datasets at all three study areas (r ¼0.85, 0.96 and 0.91 respectively). These strong positive correlations resulted predictably insimilar estimates of commonlyused statistical measures of fire frequency and cumulative area burned, including Mean Fire Return Interval (MFI) and Natural Fire Rotation (NFR). Consistent with theoretical expectations, targeted fire-scar sampling resulted in greater overall sampling efficiency and lower rates of sample attrition. Our findings demonstrate that targeted sampling in these systems can produce accurate estimates of landscape-scale fire frequency parameters relative to intensive probabilistic sampling. Received 16 February 2013, accepted 3 April 2013, published online 6 September 2013

A comparison and integration of tree-ring and alluvial records of fire history at the Missionary Ridge Fire, Durango, Colorado, USA

We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment fire history records were created separately, and then compared and integrated to create a ~ 3000 year record of past fire activity. The tree-ring record showed that from AD 1679 to 1879, there were frequent surface fires, while patches of high-severity fire occurred during widespread fire years. The alluvial record showed that a low- to moderate-and mixed-severity fire regime has likely been dominant over the past ~ 2600 calibrated calendar years before present, as shown by locally episodic deposition of charcoal-rich, fine-grained sediments. Radiocarbon dating suggested that in two stratigraphic sections, there was rapid deposition of several fine-grained sediment layers. One of these episodes occurred during the Medieval Climatic Anomaly (AD 900—1300). A charcoal-rich debris flow deposit in the oldest exposed part of the stratigraphic record dated to ~ 2600 calibrated calendar years before present. This event was potentially equivalent in magnitude to the debris-flow events following the recent wildfire in the study area, and is evidence of a high-severity fire that burned a large proportion of the study basin. The timing of this event coincides with a period of less frequent, yet more severe wildfires in a nearby lake sediment record, and is associated with the end of a Neoglacial period of cooler and wetter temperatures.

Fire History In European Black Pine (Pinus nigra Arn.) Forests of the Valia Kalda, Pindus Mountains, Greece

Abstract The past fire regime of European black pine (Pinus nigra Arn.) forests in Valia Kalda in Greece was investigated by standard dendrochronology methods. The sampled trees contained a record of fires from the early 14th Century through the late 19th Century with the last fire recorded in 1891. Evidence of non-lethal surface fires over the past seven centuries suggests that in addition to its destructive power, fire also plays a role in ecological functioning of the region. This is the first fire history study in Greece and can provide a basis for development of the first fire history network in the region. It also provides insight and perspective that may be useful for planning and justifying future ecosystem management programs.

Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene

Significance Carbon-14 contents in tree rings tell us information of the past cosmic ray intensities because cosmic rays produce 14C in the atmosphere. We found a signature of a quite large increase of incoming cosmic ray intensity in the mid-Holocene (the 5480 BC event) from the measurement of 14C content in North American tree rings. The cause of this event is supposed to be an extremely weak sun, or a combination of successive strong solar bursts and variation of a solar magnetic activity. In any case, 14C variation of the 5480 BC event is extraordinary in the Holocene, and this event indicates the abnormal solar activity compared with other periods. Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of 14C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual 14C data. There is a little understanding about annual 14C variations in the past, with the exception of a few periods including the AD 774−775 14C excursion where annual measurements have been performed. Here, we report the result of 14C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1- to 2-y resolution, and a finding of an extraordinarily large 14C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event). The 14C increase rate of this event is much larger than that of the normal grand solar minima. We propose the possible causes of this event are an unknown phase of grand solar minimum, or a combination of successive solar proton events and a normal grand solar minimum.