Background/Question/Methods    The mesas of the Pajarito Plateau in the Jemez Mountains of northern New Mexico exhibit transitions along an elevational gradient between ponderosa pine (Pinus ponderosa) forests, mixed woodlands dominated by piñon (Pinus edulis) and one-seed juniper (Juniperus monosperma), and juniper savannas.  Using multiple lines of evidence, a history of ecological regime shifts since ca. 1500 A.D. is reconstructed for a dynamic transition zone on one such mesa (Frijolito Mesa).  Evidence includes intensive archeological surveys, dendrochronological reconstructions of the demographic and spatial patterns of establishment and mortality for these three main tree species, dendrochronological reconstructions of fire regimes and climate patterns, broad-scale mapping of vegetation changes from a sequence of historic aerial photographs since 1935, long-term monitoring of vegetation from permanent transects since 1991, detailed soil maps and interpretations, intensive ecohydrological studies since 1993 on portions of this mesa, and research on the ecosystem effects of an experimental tree-thinning experiment conducted in 1997. 
Results/Conclusions    Frijolito Mesa was fully occupied by large numbers of ancestral Puebloan farmers from the A.D. 1200’s until the late 1500’s, when they left these mesas for settlements in the adjoining Rio Grande Valley.  Archaeological evidence and tree ages indicate that the mesa was likely quite deforested when abandoned, followed by episodic tree establishment dominated by ponderosa pine during the Little Ice Age.  By the late 1700’s Frijolito Mesa included ponderosa pine in open stands maintained by frequent surface fires burning through herbaceous ground cover adequate to maintain ancient (>100,000 year old) soils, interspersed with young piñon-juniper savannas and woodlands on rockier fire-safe sites.  Intensive livestock grazing from the late 1800’s thru 1932 reduced the herbaceous ground cover, interrupting the surface fire regime, triggering massive establishment of fire-sensitive piñon and juniper throughout much of the 1900’s.  Severe drought in the 1950’s killed all the ponderosa across an irregular ecotone shift zone up to 2 km wide, with no subsequent regeneration, leaving piñon-juniper woodland with accelerated, unsustainable erosion in desertified areas between tree clumps.  Warm drought in the early 2000’s caused mass mortality of essentially all overstory piñon, leaving juniper as the only remaining tree dominant across huge areas.  Ecohydrological processes are shifting again with declining runoff/erosion trends since 2003 as dead piñon skeletons fall and with increased abundances of shrubs and herbaceous surface cover, decreasing the connectivity of bare soil patches.  The history of Frijolito Mesa illustrates multiple ecosystem regime shifts since 1500, particularly since 1850.