RESPONSE OF ALLIGATOR JUNIPER (JUNIPERUS-DEPPEANA-PINACEAE) TO HISTORIC ENVIRONMENTAL VARIABILITY IN SOUTH CENTRAL NEW-MEXICO

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 quar ter method, and elevation values, we analyzed the age, elevation, and socioecology of J. deppeana along a transect from 1520 m to 1690 m ele vation on the east slope of the Organ Mountains, New Mexico. For trees established before 1890, there has been an 80 m upward displacement o f their lower treeline whereas younger trees have not experienced a si milar die off. From the late 1800s to 1943 the region experienced an e leven-fold increase in livestock, episodes of fuelwood cutting, and in tense, droughts of less than seven years. Precipitation records docume nt that the 1943-1971 period (with a 39 percent decrease in precipitat ion) was the longest drought period to affect the region since continu ous precipitation records were initiated in the region; the 1947-1956 decade was 46 percent drier than the 1890 to 1991 mean. Tree-ring reco rds indicate that 1950-1959 was the driest decade since 680 AD. During the 1943-1971 drought, the occasional wetter years produced major epi sodes of J. deppeana establishment; 36 of the 72 sampled trees were es tablished in six isolated events during the 1943-1971 drought. Trees w ere established at lower elevations during the 1943-1971 drought than were established during the post-1971 ''mesic'' period. Favorable cond itions for establishment and survival of younger trees were created by the reduction in grasses by drought and grazing as well as the near e limination of fires. Apparently, severe climatic fluctuations of less than 30 years duration combined with grazing do not cause lasting chan ges in the lower treeline of J. deppeana.