Tree-ring reconstructions of climate and fire history at El Malpais National Monument, New Mexico

摘要

The purpose of this research was to: (1) reconstruct climate for the malpais region from long-lived trees and remnant wood; (2) reconstruct the fire history of forests in the malpais; and (3) investigate short-term and long-term relationships between wildfire and climate. To reconstruct climate, I calibrated a 2,129 year long tree-ring chronology (136 BC - AD 1992) with annual rainfall (previous July to current July). Since AD 100, seven major long-term trends in rainfall occurred. Above normal rainfall occurred during AD 81-257, 521-660, 1024-1398 and 179 1- 1992, while below normal rainfall occurred during AD 258-520, 661-1023 and 1399- 1790. The prolonged drought from AD 258-520 was unsurpassed in its intensity, while rainfall during the most recent 200 years has exceeded any since AD 660. The reconstruction of long-term climate trends confirmed the general sequence of environmental change over the last 2,000 years for the southern Colorado Plateau. To reconstruct past fire occurrences, 217 fire-scarred trees were collected from nine sites representing the major habitat types of the malpais and dendrochronologically dated. Fire frequency was highest at sites on cinder cones and on the highly-weathered basalt flows (ca. once every five years), and lowest on the isolated kipukas and on the Hoya de Cibola Lava Flow (once every 10-12 years). Fire frequency decreased along a north to south gradient, reflecting changing vegetation properties. Combined information revealed fire occurred once every two years, while more widespread fires occurred once every 2.5 years. Fires were largely asynchronous between sites, suggesting the malpais landscape effectively hinders fire spread. Past fire history at El Malpais was characterized by four temporally distinct periods: (1) FH-1 (prior to 1782): high fire frequency, patchy fires, throughout the growing season; (2) FH-2 (1795 - 1880): longer fire intervals, widespread fires, mostly early season fires; (3) FH-3 (1893 - 1939): even longer intervals, decreased widespread fires; (4) FH-4 (1940 - 1992): longest fire-free periods during the last 600 years. The increase in rainfall and the simultaneous change in fire regimes ca. 1790 was likely related to an increase in summer monsoonal rainfall due to changes in hemispheric circulation patterns. The decrease in fire spread ca. 1880 was most likely due to intense sheep grazing, while the change ca. 1940 reflects greater efficiency in fire suppression techniques. The presettlement fire regime emphasizes that the current absence of fire in the monument exceeds the historical range of variability established for the presettlement period. Unless effects of past humanrelated disturbances are mitigated, fire regimes of El Malpais will continue to favor high-intensity, catastrophic fires.