Outline placeholderAbstractIntroductionInteractions Across Spatial ScalesForest Dieback and Associated Insect OutbreaksWildfire PropagationLinked Effects of Vegetation Surface Cover on Fire and Erosion ProcessesDisturbance Interactions Within and Across Spatial ScalesACKNOWLEDGEMENTSREFERENCESInteractions Across Spatial Scalesamong Forest Dieback, Fire, andErosion in Northern New MexicoLandscapesCraig D. Allen*US Geological Survey, Jemez Mountains Field Station, Los Alamos, New Mexico 87544, USAABSTRACTEcosystem patterns and disturbance processes atone spatial scale often interact with processes atanother scale, and the result of such cross-scaleinteractions can be nonlinear dynamics withthresholds. Examples of cross-scale pattern-processrelationships and interactions among forest die-back, fire, and erosion are illustrated from northernNew Mexico (USA) landscapes, where long-termstudies have recently documented all of these dis-turbance processes. For example, environmentalstress, operating on individual trees, can cause treedeath that is amplified by insect mortality agents topropagate to patch and then landscape or evenregional-scale forest dieback. Severe drought andunusual warmth in the southwestern USA sincethe late 1990s apparently exceeded species-specificphysiological thresholds for multiple tree species,resulting in substantial vegetation mortality acrossmillions of hectares of woodlands and forests inrecent years. Predictions of forest dieback acrossspatial scales are constrained by uncertaintiesassociated with: limited knowledge of species-spe-cific physiological thresholds; individual and site-specific variation in these mortality thresholds; andpositive feedback loops between rapidly-respond-ing insect herbivore populations and their stressedplant hosts, sometimes resulting in nonlinear‘‘pest‘‘ outbreak dynamics. Fire behavior alsoexhibits nonlinearities across spatial scales, illus-trated by changes in historic fire regimes wherepatch-scale grazing disturbance led to regional-scale collapse of surface fire activity and subsequentrecent increases in the scale of extreme fire eventsin New Mexico. Vegetation dieback interacts withfire activity by modifying fuel amounts and con-figurations at multiple spatial scales. Runoff anderosion processes are also subject to scale-depen-dent threshold behaviors, exemplified by ecohy-drological work in semiarid New Mexicowatersheds showing how declines in ground sur-face cover lead to non-linear increases in barepatch connectivity and thereby accelerated runoffand erosion at hillslope and watershed scales.Vegetation dieback, grazing, and fire can changeland surface properties and cross-scale hydrologicconnectivities, directly altering ecohydrologicalpatterns of runoff and erosion. The interactionsamong disturbance processes across spatial scalescan be key drivers in ecosystem dynamics, asillustrated by these studies of recent landscapechanges in northern New Mexico. To better antic-ipate and mitigate accelerating human impacts tothe planetary ecosystem at all spatial scales,improvements are needed in our conceptual andquantitative understanding of cross-scale interac-tions among disturbance processes.Key words: disturbance interactions; forest die-back; fire; erosion; fire history; cross-scale rela-tionships; thresholds; New Mexico; southwesternUSA.Received 11 September 2006; accepted 10 April 2007; published online20 June 2007.*Corresponding author; e-mail: [email protected] (2007) 10: 797–808DOI: 10.1007/s10021-007-9057-4797INTRODUCTIONEcosystem patterns and disturbance processes atone spatial scale often interact with processes atanother scale, and the result of such cross-scaleinteractions can be nonlinear dynamics withthresholds (Gunderson and Holling 2002). Suchinteractions change the pattern-process relation-ships across scales such that fine-scale processes caninfluence a broad spatial extent, or broad-scaledrivers can interact with fine-scale processes todetermine system dynamics (Peters and others2007). Non-linear disturbance propagation andamplification across spatial scales commonly in-volve positive feedback loops or spatial pattern-process thresholds, differing from linear relation-ships observed with simple repetition of patternsfrom fine to broad scales. Cross-scale interactionsinvolving disturbance processes are increasinglyrecognized as important drivers of ecosystemdynamics (Nepstad and others 2001; Turner 2005;Burkett and others 2005; Bodin and others 2006;Peters and others 2007). Spatially explicit ap-proaches differ from non-spatial approaches intheir emphasis on the importance of the interactionof disturbance transfer processes with spatial het-erogeneity (Peters and others 2007). This paperillustrates examples of nonlinear interactions acrossspatial scales among forest dieback, fire, and ero-sion from landscapes in the southwestern USA(Figure 1), particularly northern New Mexico,where long-term studies centered on BandelierNational Monument in the Jemez Mountains haverecently documented all of these disturbance pro-cesses (Allen 1989).Methods used to develop the presented examplesinclude: dendrochronological dating of fire scars(Touchan and others 1996) and tree populationdemographies (Swetnam and Betancourt 1998),weekly dendrometer measurements of ponderosapine (Pinus ponderosa) and pin˜on (Pinus edulis) tree-growth since 1991 and 1993 (CD Allen, unpub-lished data), monitoring of basal cover of herba-ceous vegetation at centimeter resolution along3 km of permanently marked line-intercept tran-sects since 1991 (CD Allen, unpublished data),photographic and remote-sensing delineation offorest dieback (Allen and Breshears 1998; Bres-CLIMATE (drought, temperature);OVERGRAZING by livestockHigh tree density, death of individual treesPatchy stand-scale mortalityLandscape-scaleforest mortalityFORESTDIEBACKFIREFine Intermediate BroadBeetle outbreak overwhelms even healthy treesBeetle populationgrows rapidly,colonizing more treesPhysiological tree stress, beetles attack weakand dying treesWithin-patchfuel connectivityFire ignition and surface fire spreadBetween-patch surface and canopyfuel connectivityTorching and between-patch fire spreadLong-distance fire spread beyond L fuel connectionsExplosive crown fire. Self-generated weatherEROSIONGrass cover, within-patch bare connectivityRunoff and erosion,drier microsite,within IC patchHillsloperunoff/erosionPercolating network