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Response of Alligator Juniper (Juniperus deppeana Pinaceae) to Historic Environmental Variability in South Central New Mexico
Richard A. Earl and Dallas L. Bash
The Southwestern Naturalist
Vol. 41, No. 3 (Sep., 1996), pp. 227-238
Published by: Southwestern Association of Naturalists
Stable URL: http://www.jstor.org/stable/30055118
Page Count: 12
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Fringing many of the mountain ranges in south central New Mexico is a zone of large > 5 m high) dead alligator junipers (Juniperus deppeana) that are as much as 100 m lower in elevation than similar-sized live trees. Employing tree-ring to diameter correlated ages, the point quarter method, and elevation values, we analyzed the age, elevation, and socioecology of J, deppeana along a transect from 1520 m to 1690 m elevation on the east slope of the Organ Mountains, New Mexico. For trees established before 1890, there has been an 80 m upward displacement of their lower treeline whereas younger trees have not experienced a similar die off. From the late 1800s to 1943 the region experienced an eleven-fold increase in livestock, episodes of fuelwood cutting, and intense, droughts of less than seven years. Precipitation records document that the 1943-1971 period (with a 39 percent decrease in precipitation) was the longest drought period to affect the region since continuous precipitation records were initiated in the region; the 1947-1956 decade was 46 percent drier than the 1890 to 1991 mean. Tree-ring records indicate that 1950-1959 was the driest decade since 680 AD. During the 1943-1971 drought, the occasional wetter years produced major episodes of J. deppeana establishment; 36 of the 72 sampled trees were established in six isolated events during the 1943-1971 drought. Trees were established at lower elevations during the 1943-1971 drought than were established during the post-1971 "mesic" period. Favorable conditions for establishment and survival of younger trees were created by the reduction in grasses by drought and grazing as well as the near elimination of fires. Apparently, severe climatic fluctuations of less than 30 years duration combined with grazing do not cause lasting changes in the lower treeline of J. deppeana.
The Southwestern Naturalist © 1996 Southwestern Association of Naturalists