M. K. Cleaveland

North Carolina Climate Changes Reconstructed from Tree Rings: A.D. 372 to 1985

Millennium-old bald cypres trees (Taxodium distichum [(L.) Rich.] have ben used to develop a 1614-year reconstruction of the June Palmer drought severity index for North Carolina. This proxy paleoclimatic record indicates that the growing season climate of North Carolina has undergone many changes between significantly different regimes of drought and wetness that persist for approximately 30 years. Alternating wet and dry regimes were particularly well developed during the Medieval Warm Epoch (A.D. 1000 to 1300). The record June droughts in 1985 and 1986 and the preceding three decades of much wetter than average conditions both appear to have been rare climatic events, equaled only five times each since A.D. 372.

Management implications of annual growth rings in Pterocarpus angolensis from Zimbabwe

Dendrochronological analyses indicate that Pterocarpus angolensis produces anatomically distinctive annual growth rings in the indigenous forests of western Zimbabwe. Annual growth rings in P. angolensis can be identified on highly polished crosssectional surfaces on the basis of the semi-ring porous tree rings; the reasonably continuous band of initial parenchyma; and by smaller changes in vessel diameter, wood density, and color from the early-wood to late-wood portion of the growth rings. Together, four lines of evidence indicate that these growth bands are annual: P. angolensis is strongly deciduous and the phenology of this species is tightly synchronized with the seasonality of rainfall in western Zimbabwe; the growth rings are semi-ring porous; ring-width time series are highly correlated between trees at a single forest location, and between forests separated by >100 km; and also because the mean ring-width chronologies derived from these trees are significantly correlated with regional rainfall totals during the wet season from 1901‐1990. P. angolensis is one of the most valuable timber species in south tropical Africa, and the discovery of exactly annual growth rings will allow detailed analyses of the growth and yield under different ecological and management settings. It may also be possible to develop centuries-long tree-ring chronologies from P. angolensis, which would be useful for the reconstruction of past climate and streamflow. # 1999 Elsevier Science B.V. All rights reserved.

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^ ^

A 450-year drought reconstruction for Arkansas, United States

A 450-year reconstruction of the June Palmer Drought Severity Index1 (PDSI) has been derived for Arkansas using annual tree-ring chronologies from selected old-growth baldcypress trees (Taxodium distichum (L.) Rich.) found in floodplain swamps. This is the first palaeoclimate reconstruction achieved with baldcypress, and is some 150 years longer than any previous dendroclimatic estimate for eastern North America. The reconstruction is characterized by substantial fluctuation in the intensity and duration of past wet and dry spells, and is well verified against independent climate data. While there has been some very limited previous use of wet site species for dendrochronology2,3, this is the first verified climate reconstruction based exclusively on a swamp-grown species. Several 500–800-year-long baldcypress chronologies are under development, and some may be extended into mid- to late-Holocene times with tree-ring records from preserved subfossil cypress logs4.

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Niño/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Niña and warm Atlantic SSTs prevailed, and the 1980–1990s when El Niño and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Niño and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.