Interactions Across Spatial Scales among Forest Dieback,Fire, and Erosion in Northern New Mexico Landscapes |
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Authors: | Craig D Allen |
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Institution: | (1) US Geological Survey, Jemez Mountains Field Station, Los Alamos, New Mexico 87544, USA |
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Abstract: | Abstract
Ecosystem patterns and disturbance processes at one spatial scale often interact with processes at another scale, and the
result of such cross-scale interactions can be nonlinear dynamics with thresholds. Examples of cross-scale pattern-process
relationships and interactions among forest dieback, fire, and erosion are illustrated from northern New Mexico (USA) landscapes,
where long-term studies have recently documented all of these disturbance processes. For example, environmental stress, operating
on individual trees, can cause tree death that is amplified by insect mortality agents to propagate to patch and then landscape
or even regional-scale forest dieback. Severe drought and unusual warmth in the southwestern USA since the late 1990s apparently
exceeded species-specific physiological thresholds for multiple tree species, resulting in substantial vegetation mortality
across millions of hectares of woodlands and forests in recent years. Predictions of forest dieback across spatial scales
are constrained by uncertainties associated with: limited knowledge of species-specific physiological thresholds; individual
and site-specific variation in these mortality thresholds; and positive feedback loops between rapidly-responding insect herbivore
populations and their stressed plant hosts, sometimes resulting in nonlinear “pest” outbreak dynamics. Fire behavior also
exhibits nonlinearities across spatial scales, illustrated by changes in historic fire regimes where patch-scale grazing disturbance
led to regional-scale collapse of surface fire activity and subsequent recent increases in the scale of extreme fire events
in New Mexico. Vegetation dieback interacts with fire activity by modifying fuel amounts and configurations at multiple spatial
scales. Runoff and erosion processes are also subject to scale-dependent threshold behaviors, exemplified by ecohydrological
work in semiarid New Mexico watersheds showing how declines in ground surface cover lead to non-linear increases in bare patch
connectivity and thereby accelerated runoff and erosion at hillslope and watershed scales. Vegetation dieback, grazing, and
fire can change land surface properties and cross-scale hydrologic connectivities, directly altering ecohydrological patterns
of runoff and erosion. The interactions among disturbance processes across spatial scales can be key drivers in ecosystem
dynamics, as illustrated by these studies of recent landscape changes in northern New Mexico. To better anticipate and mitigate
accelerating human impacts to the planetary ecosystem at all spatial scales, improvements are needed in our conceptual and
quantitative understanding of cross-scale interactions among disturbance processes. |
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Keywords: | disturbance interactions forest dieback fire erosion fire history cross-scale relationships thresholds New Mexico southwestern USA |
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