Across most topographic positions, mortality was greatest at lower elevations. Topographic position indices ranged from small values for exposed ridgetops to large values for sheltered valley bottoms ( 22, 26). ( B) Field observations of the persistence ( ⋅) and mortality (⧫) of ponderosa pines, determined from remains of dead trees as a function of elevation and topographic position. Forest reduction was greatest at lower elevations. In addition, we confirmed the relationship between the local elevation/moisture gradient and water stress for ponderosa pine, as measured by the changes in stem diameter of 30 trees monitored continuously since 1991 (10 trees at each of three sites along this gradient).įigure 3 ( A) Forest reduction between 19, estimated from GIS analyses. We verified our mapping results with field observations of the persistence and mortality of ponderosa pines, which included documenting the presence of live ponderosa trees and the remains of dead trees as a function of elevation and topographic position. Higher resolution (scale = 1:5,000), partial-coverage photographs from 1958 allowed us to sharpen our estimate of the timing of ecotone changes written documents on file at Bandelier National Monument further confirmed this estimate. Full-coverage photographs of the area, which existed for 1935, 1954, 1963, and 1975, and partial coverage photographs from 1951 were used to map vegetation patches in terms of ponderosa pine cover: areas with more than 10% cover were classified as forest, and the remainder of the area as piñon–juniper woodland. ![]() We quantified changes in the ecotone over a 40-yr period on the basis of Geographic Information System (GIS) analyses of a sequence of aerial photographs taken between 19-a period encompassing a severe regional drought in the mid-1950s ( 21, 22). Furthermore, as woody vegetation contains 80% of the world’s terrestrial carbon ( 16), an improved understanding of mortality-induced responses of woody vegetation to climate is essential for addressing the environmental and policy implications of climate variability and global change ( 17, 18). In coming decades, climate changes are expected to produce major shifts in vegetation distributions at unprecedented rates, in large part due to mortality ( 14) however, largely because of the lack of field data on vegetation mortality, current models do not represent adequately such rapid effects ( 14, 15). Moreover, most field studies and model-based assessments of vegetation responses to climate have focused on changes associated with natality and growth, which are inherently slow processes for woody plants-even though the most rapid changes in vegetation are caused by mortality rather than natality ( 13). Previous studies of woody ecotones document landscape-scale shifts in vegetation as occurring only over relatively long (decades to millennia) periods ( 8– 12). Persistent vegetation shifts are most clearly detected in the distributions and abundances of long-lived woody plants, namely, trees and shrubs ( 7). The responses of vegetation to variations in climate are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems ( 2– 5), with semiarid ecotones considered to be among the most sensitive ( 6). ![]() The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.ĭistributions of vegetation across landscapes depend on climate, perhaps best illustrated by the wholesale movement of plant species across geographic and topographic gradients during the last deglaciation ( 1). Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and piñon–juniper woodland shifted extensively (2 km or more) and rapidly (<5 years) through mortality of ponderosa pines in response to a severe drought. ![]() ![]() However, current models do not adequately provide for such rapid effects-particularly those caused by mortality-largely because of the lack of data from field studies. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates.
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