Henri D. Grissino-Mayer

Tree‐ring data document 16th century megadrought over North America

The two most severe, sustained droughts in the continental United States during the 20th century occurred in the 1930s and 1950s. The 1950s drought was most extreme over the southwest and southern Great Plains, where ecological consequences are still evident on the landscape [Swetnam and Betancourt], 1998].The Dust Bowl,vividly recounted in John Steinbecks The Grapes of Wrath, was the nations most severe, sustained,and widespread drought of the past 300 years, according to tree-ring reconstructions of the Palmer drought severity index (PDSI) across the continental United States [Cook et al., 1999] (http://www.ngdc.noaa.gov/paleo/pdsiyear.html. Droughts during the 1750s, 1820s, and 1850s–1860s estimated from tree rings were similar to the 1950s drought in terms of magnitude, persistence, and spatial coverage, but these earlier episodes do not appear to have surpassed the severity or extent of the Dust Bowl drought. However, longer tree-ring reconstructions of PDSI for the United States and precipitation for northwestern Mexico and western Canada indicate that the “megadrought” of the 16th century far exceeded any drought of the 20th century (Figure 1) [also see Wood-house and Overpeck, 1998], and is considered to be the most severe prolonged drought over much of North America for at least the last 500 years [Meko et al., 1995].

Century scale climate forcing of fire regimes in the American Southwest

Interannual time-scale associations between fire occurrence anddrought indices, the Southern Oscillation, and other synopticpatterns demonstrate that large-scale, long term atmospheric featuresare precursors to regional fire activity. However, our knowledge offire-climate relations over longer (century) timescales is fragmentary because of the rarity of comparable climate and fire time-series with sufficient resolution, length and regional extent. In this study, we develop reconstructions of wildfire occurrence from tree-ring data collected from northwestern New Mexico to compare with a millennium-length dendroclimatic reconstruction of precipitation. Reconstructions of both wildfires and climate show simultaneous changes since AD 1700 that indicate climate forcing of wildfire regimes on interannual to century timescales. Following a centuries-long dry period with high fire frequency (c. AD 1400-1790), annual precipitation increased, fire frequency decreased, and the season of fire shifted from predominantly midsummer to late spring. We hypothesize that these shifts in fire regimes reflect long-term changes in rainfall patternsassociated with changes in synoptic-scale atmospheric circulation patterns and the Southern Oscillation. Our evidence supports century-scale climate forcing of fire regimes in the American Southwest, providing a useful analogue of future wildfire regimes expected uinder changing global climate conditions.

Tree-ring isotope records of tropical cyclone activity

The destruction wrought by North Atlantic hurricanes in 2004 and 2005 dramatically emphasizes the need for better understanding of tropical cyclone activity apart from the records provided by meteorological data and historical documentation. We present a 220-year record of oxygen isotope values of α-cellulose in longleaf pine tree rings that preserves anomalously low isotope values in the latewood portion of the ring in years corresponding with known 19th and 20th century landfalling/near-coastal tropical storms and hurricanes. Our results suggest the potential for a tree-ring oxygen isotope proxy record of tropical cyclone occurrence extending back many centuries based on remnant pine wood from protected areas in the southeastern U.S.

CLIMATIC AND HUMAN INFLUENCES ON FIRE REGIMES OF THE SOUTHERN SAN JUAN MOUNTAINS, COLORADO, USA

Fire severity, frequency, and extent are expected to change dramatically in coming decades in response to changing climatic conditions, superimposed on the adverse cumulative effects of various human-related disturbances on ecosystems during the past 100 years or more. To better gauge these expected changes, knowledge of climatic and human influences on past fire regimes is essential. We characterized the temporal and spatial properties of fire regimes in ponderosa pine forests of the southern San Juan Mountains of southwestern Colorado by collecting 175 fire-scarred tree samples from nine sites across a wide range of topographic settings. All tree rings and fire scars were dated using standard dendrochronological techniques. Fire-free intervals were statistically modeled using the Weibull distribution to provide quantitative measures that characterized the historical range of variation in pre-EuroAmerican fire regimes. Fires during our reference period were more frequent in the low elevation ponderosa pine forests (6-10 yr) than in the high elevation, mixed conifer forests (18-28 yr). Fires at lower elevations were predominantly low-severity, isolated fires. Fires during some years (e.g., 1748) were spatially extensive throughout the entire mountain range. Intervals that delimited significantly long fire-free periods ranged from 10-19 yr (low elevation) to 27- 50 yr (high elevation). Fire histories were similar between the eastern and western portions of the mountain range, although we found significant evidence of topographic isolation on fire regimes at one site. Pre-1880 fires primarily occurred in the dormant season, and we found no temporal changes in past fire seasonality. We found no compelling evidence that Native Americans influenced fire regimes in our study sites. We found a hiatus in fire occurrence between 1750 and 1770 that we believe was likely related to weakened El Nino-Southern Oscillation activity, an extended series of cool-phase Pacific Decadal Oscillation events, and weakened monsoonal moisture, all possibly en- trained in an invasive air mass typical of locations that are more northerly. In addition, pre-1880 fires occurred during years of severe drought, conditioned by above average moisture conditions in preceding years. The 20th century is characterized by a near complete absence of fires (fire-free interval of .100 yr), suggesting future wildfires may be more widespread and ecologically severe compared to pre-1880 fires.

Cool- and Warm-Season Precipitation Reconstructions over Western New Mexico

Precipitation over the southwestern United States exhibits distinctive seasonality, and contrasting ocean‐ atmospheric dynamics are involved in the interannual variability of cool- and warm-season totals. Tree-ring chronologies based on annual-ring widths of conifers in the southwestern United States are well correlated with accumulated precipitation and have previously been used to reconstruct cool-season and annual precipitation totals. However, annual-ring-width chronologies cannot typically be used to derive a specific record of summer monsoon-season precipitation. Some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring, and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool- and warm-season precipitation and their associated large-scale ocean‐atmospheric dynamics. Two 2139-yr-long reconstructions of cool- (November‐May) and early-warm season (July) precipitation have been developed from ancient conifers and relict wood at El Malpais National Monument, New Mexico. Both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large-scale SST and 500-mb height fields using instrumental precipitation data from New Mexico. Above^ ^

THE CONTEMPORARY FIRE REGIME OF THE CENTRAL APPALACHIAN MOUNTAINS AND ITS RELATION TO CLIMATE

This paper uses records of wildland fire to investigate the contemporary fire regime on federal lands in the central Appalachian Mountains of Virginia and West Virginia. During the study period (1970-2003), 1557 anthropogenic fires and 344 natural fires occurred on these lands. Most were small, low-intensity burns. However, fires of moderate to high intensity also occurred, and because of their larger sizes they were responsible for most of the area burned. Fire size also differed between natural and anthropogenic fires (median size 1.2 ha vs. 0.4 ha). A few of the anthropogenic fires were quite large, however (up to 6484 ha), whereas the largest natural fire measured only 1188 ha. Anthropogenic fires burned more area than natural fires and consequently they had a shorter fire cycle (1196 years for anthropogenic fires, 6138 years for natural fires). These fire cycles appear to be much longer than in the past, prior to fire suppression. Nonetheless, despite suppression efforts, a substantial amount of fire activity occurred during the study period when conditions were sufficiently dry. The dry conditions of spring and fall were especially favorable for burning. Moreover, on an interannual level, drought had a strong influence on the amount of fire activity.

Record of the North American southwest monsoon from Gulf of Mexico sediment cores

Summer monsoonal rains (the southwest monsoon) are an important source of moisture for parts of the southwestern United States and northern Mexico. Improved documentation of the variability in the southwest monsoon is needed because changes in the amount and seasonal distribution of precipitation in this semiarid region of North America influence overall water supply and fire severity. Comparison of abundance variations in the planktic foraminifer Globigerinoides sacculifer in marine cores from the western and northern Gulf of Mexico with terrestrial proxy records of precipitation (tree-ring width and packrat-midden occurrences) from the southwestern United States indicate that G. sacculifer abundance is a proxy for the southwest monsoon on millennial and submillennial time scales. The marine record confirms the presence of a severe multicentury drought centered ca. 1600 calendar (cal.) yr B.P. as well as several multidecadal droughts that have been identified in a long tree-ring record spanning the past 2000 cal. yr from west-central New Mexico. The marine record further suggests that monsoon circulation, and thus summer rainfall, was enhanced in the middle Holocene (ca. 6500–4500 14 C yr B.P.; ca. 6980–4710 cal. yr B.P.). The marine proxy provides the potential for constructing a highly resolved, well-dated, and continuous history of the southwest monsoon for the entire Holocene.

Gap-scale disturbance processes in secondary hardwood stands on the Cumberland Plateau, Tennessee, USA

Disturbance regimes in many temperate, old growth forests are characterized by gap-scale events. However, prior to a complex stage of development, canopy gaps may still serve as mechanisms for canopy tree replacement and stand structural changes associated with older forests. We investigated 40 canopy gaps in secondary hardwood stands on the Cumberland Plateau in Tennessee to analyze gap-scale disturbance processes in developing forests. Gap origin, age, land fraction, size, shape, orientation, and gap maker characteristics were documented to investigate gap formation mechanisms and physical gap attributes. We also quantified density and diversity within gaps, gap closure, and gap-phase replacement to examine the influence of localized disturbances on forest development. The majority of canopy gaps were single-treefall events caused by uprooted or snapped stems. The fraction of the forest in canopy gaps was within the range reported from old growth remnants throughout the region. However, gap size was smaller in the developing stands, indicating that secondary forests contain a higher density of smaller gaps. The majority of canopy gaps were projected to close by lateral crown expansion rather than height growth of subcanopy individuals. However, canopy gaps still provided a means for understory trees to recruit to larger size classes. This process may allow overtopped trees to reach intermediate positions, and eventually the canopy, after future disturbance events. Over half of the trees located in true gaps with intermediate crown classifications were Acer saccharum, A. rubrum, or Liriodendrontulipifera. Because the gaps were relatively small and close by lateral branch growth of perimeter trees, the most shade-tolerant A. saccharum has the greatest probability of becoming dominant in the canopy under the current disturbance regime. Half of the gap maker trees removed from the canopy were Quercus; however, Acer species are the most probable replacement trees. These data indicate that canopy gaps are important drivers of forest change prior to a complex stage of development. Even in relatively young forests, gaps provide the mechanisms for stands to develop a complex structure, and may be used to explain patterns of shifting species composition in secondary forests of eastern North America.

Fire history, related to climate and land use in three southern Appalachian landscapes in the eastern United States

Fire-maintained ecosystems and associated species are becoming increasingly rare in the southern Appalachian Mountains because of fire suppression policies implemented in the early 20th century. Restoration of these communities through prescribed fire has been hindered by a lack of information on historical fire regimes. To characterize past fire regimes, we collected and absolutely dated the tree rings on cross sections from 242 fire-scarred trees at three different sites in the southern Appalachian Mountains of Tennessee and North Carolina. Our objectives were to (1) characterize the historical frequency of fire in southern Appalachian mixed pine-oak forests, (2) assess the impact of interannual climatic variability on the historical occurrence of fire, and (3) determine whether changes in human culture and land use altered the frequency of fire. Results demonstrate that fires burned frequently at all three sites for at least two centuries prior to the implementation of fire suppression and prevention in the early to mid 20th century. Composite mean fire return intervals were 2-4 yr, and point mean fire return intervals were 9-13 yr. Area-wide fires that burned across multiple stands occurred at 6-13-yr intervals. The majority of fires were recorded during the dormant season. Fire occurrence exhibited little relationship with reconstructed annual drought conditions. Also, fire activity did not change markedly during the transition from Native American to Euro-American settlement or during the period of industrial logging at the start of the 20th century. Fire activity declined significantly, however, during the fire suppression period, with a nearly complete absence of fire during recent decades. The characterization of past fire regimes should provide managers with specific targets for restoration of fire-associated communities in the southern Appalachian Mountains. The fire chronologies reported here are among the longest tree-ring reconstructions of fire history compiled for the eastern United States and support the hypothesis that frequent burning has played a long and important role in the development of forests in the southern Appalachian Mountains.

Occurrence of Sustained Droughts in the Interior Pacific Northwest (A.D. 1733-1980) Inferred from Tree-Ring Data

The occurrence of moderate and severe sustained droughts in the interior Pacific Northwest (PNW) from 1733 to 1980 was mapped using 18 western juniper (Juniperus occidentalis var. occidentalis Hook.) tree-ring chronologies. The frequency and duration of both moderate and severe sustained droughts are substantially greater in the northwest region of the interior PNW. Thus, this area is identified as a drought core region. These droughts are chiefly attributed to the presence of a Pacific blocking high off the NW coast that is associated with significant reductions in cool season precipitation. Specifically, the impacted northwest region lies within a transition zone between the fluxes of marine airflow during the cool season months and interior air during the warm season months. The waxing and waning of the boundaries of this transition zone particularly affect western juniper trees growing in this region. During years in which a blocking high is present, marine airflow is substantially reduced, exposing the trees in the transition zone to substantially drier springtime conditions that limit soil moisture and reduce radial growth. Although the most severe and persistent droughts were concentrated in the northwest region, four large-scale droughts also impacted the entire interior PNW during the study period. These droughts occurred principally during PDO warm phases, suggesting a regional-scale linkage to this climatic oscillation.