1603Ecological Applications,14(5), 2004, pp. 1603–1614q2004 by the Ecological Society of AmericaTHE PERSISTENCE OF QUAKING ASPEN (POPULUS TREMULOIDES)IN THE GRAND MESA AREA, COLORADODOMINIKKULAKOWSKI,1THOMAST. VEBLEN,ANDSARAHDRINKWATERDepartment of Geography, University of Colorado, Campus Box 260, Boulder, Colorado 80309 USAAbstract.Human activities have caused the decline of numerous species and ecosys-tems. To promote ecosystem resilience, recent management efforts aim to maintain eco-system patterns and processes within their historical range of variability. There has beensubstantial concern that quaking aspen, the most widely distributed tree in North Americaand the most important deciduous tree in the subalpine forests of the Rocky Mountains,has declined significantly in the western landscape during the 20th century. This reporteddecline has been attributed to conifer encroachment associated with fire exclusion, as wellas other causes. To assess long-term changes in the extent of quaking aspen in a 175 000-ha study area in western Colorado, we used a Geographic Information System to comparean 1898 map of vegetation and fires to a modern map of present forest cover types. Basedon this comparison, a larger portion of the current landscape is dominated by quaking aspenrelative to the late 19th century, a period of extensive burning in this area. During the 20thcentury, aspen was persistent over most of its extent, even in the absence of fire. Fires ofthe late 19th century also increased aspen cover in stands that were previously dominatedby spruce and fir. The trend toward increased aspen was greater at lower elevations. Thetotal area where spruce and fir have replaced aspen is small in comparison to the area whereaspen has increased or has persisted. The successional replacement of aspen by conifers ismore pronounced at higher elevations and where the predisturbance vegetation was dom-inated by conifers. The net effect of severe disturbances during and after the late 19thcentury has also increased aspen cover relative to the period that preceded these distur-bances. Thus, where the successional replacement of aspen by conifers is occurring today,such a trend may be within the range of historical variation. Long intervals between large,severe natural disturbances in ecosystems can result in a broad range of natural ecologicalconditions, including dominance by particular species.Key words: aspen; Colorado, USA; Geographic Information System (GIS); historical range ofvariability; fire;Populus;reference period; vegetation change.INTRODUCTIONRecent management activities often aim to manageecosystems within the range of their historical varia-tion. Because a range of conditions is usually difficultto describe, focus is often placed on a snapshot of struc-ture and composition that existed immediately prior tomajor modification of ecosystems by Euro-Americans(e.g., Mast et al. 1999). However, ecosystems are notstatic and, while a reference point is helpful in under-standing how ecosystems have evolved and perpetu-ated, it is still only a snapshot of a naturally fluctuatingsystem. Ecosystems that are shaped by infrequent eco-logical processes are likely to be characterized by broadfluctuations in structure and composition. Within suchsystems, the selection of only a single reference pointcan lead to an underestimation of the breadth of naturalconditions within that ecosystem and to inappropriatemanagement decisions. For example, the concern thatquaking aspen (Populus tremuloidesMichx.) is declin-Manuscript received 27 May 2003; revised 12 December2003; accepted 26 January 2004. Corresponding Editor: G. H.Aplet.1E-mail: [email protected] in the landscape of western North America is, insome areas, primarily based on trends during the 20thcentury. However, the early 20th century may not rep-resent the full range of variation of aspen dynamics.Instead, present aspen dynamics may be within thebounds of longer term ecosystem behavior.Quaking aspen is the most widely distributed tree inNorth America and the most widespread deciduous treein the subalpine zone of the U.S. Rocky Mountains(Little 1971). In the Rocky Mountains and in otherregions, aspen forests serve numerous important eco-logical functions. There has been substantial popularconcern that aspen has been declining during the 20thcentury in the western landscape and that, in fact, itspersistence may be ‘‘doomed’’ (Kay 1997). The suc-cessional replacement of aspen by conifers has beenargued to be outside of the range of historical variationand has been deemed an ‘‘environmental catastrophe’’(Club 20 1998). Aspen decline has been proposed tobe associated with changes in fire regimes, herbivorepopulations, and/or climate (Loope and Gruell 1973,Kay 1995, 2001, Romme et al. 1995, Baker et al. 1997,Club 20 1998, Kay and Bartos 2000, Bartos 2001).1604DOMINIK KULAKOWSKI ET AL.Ecological ApplicationsVol. 14, No. 5Aspen regeneration strategy has been described asprolific clonal resprouting following stand-replacingdisturbance such as fire (Bartos and Mueggler 1981,Brown and DeByle 1987). Severe fires kill both coniferand aspen canopy trees, which creates favorable con-ditions for the resprouting and dominance of the shade-intolerant aspen (Schier et al. 1985). New aspen seed-ling establishment can also occur following stand-re-placing fires, although such recruitment is rare (Turneret al. 2003). The shade-tolerant Engelmann spruce (Pi-cea engelmannii) and subalpine fir (Abies lasiocarpa)can establish in the understory of aspen-dominatedstands and numerous studies have reported aspen re-placement by conifers (e.g., Rogers 2002). The tran-sition from aspen to conifer dominance generally oc-curs during one aspen generation, but in some instancesmay take much longer (Crawford et al. 1998).While the perceived decline of aspen has been get-ting considerable attention among researchers, man-agers, and policy makers, not all studies have con-cluded that aspen is in decline. Even though aspen hasa limited ability to compete in low-light environments(Jones and Schier 1985), several studies have reportedjuvenile aspen beneath an aspen canopy, implying self-replacement and persistence of aspen within a stand(e.g., Langenheim 1962, Betters and Woods 1981,Crawford et al. 1998, Barnett and Stohlgren 2001).Aspen has been suggested to be persistent especiallyat lower elevation (Mueggler 1985, Romme et al. 2001)and where