Showing papers by "Thomas T. Veblen published in 2003"

Interactions between fire and spruce beetles in a subalpine rocky mountain forest landscape

Abstract: Interactions between natural disturbances are widely recognized as important determinants of vegetation patterns in forested landscapes but have only rarely been in- vestigated quantitatively. In a subalpine forest landscape in northwestern Colorado, we quantified spatial associations of fire and spruce beetle ( rufipennis ) outbreaks over more than a century and developed a multivariate logistic model of probability of occurrence of spruce beetle outbreaks. The study area, an ;2800 km 2 landscape in White River National Forest, was affected by severe widespread fires around 1879 and by a spruce beetle outbreak in the 1940s, which affected most of the forests that were dominated by Picea engelmannii(Engelmann spruce) and Abies lasiocarpa(subalpine fir). The boundaries of the fires ca. 1879 and of the 1940s spruce beetle outbreaks were digitized based on an 1898 vegetation map and modern aerial photograph interpretation, and overlaid in a Geo- graphic Information System. The areas disturbed by the ca. 1879 fires and the 1940s beetle outbreak were also overlaid with 303 fires recorded after 1950 as well as with topographic and forest structural variables. Forests that had burned in 1879 were less affected by the 1940s outbreak than older stands. On the other hand, areas affected by the 1940s spruce beetle outbreak showed no higher susceptibility to subsequent fires. A multivariate logistic model indicated that, al- though fire history had the greatest effect on stand susceptibility to spruce beetle outbreak, dominance of neighboring stands by spruce as well as elevation were also important pre- dictors of outbreaks. Predictive modeling of spatial interactions between fire and spruce beetle disturbances needs to consider the high degree of variability in the nature of these interactions related to contingencies such as time since last major disturbance, topographic position, and weather during windows of potential interaction.

Effects of Fire and Spruce Beetle Outbreak Legacies on the Disturbance Regime of a Subalpine Forest in Colorado

Abstract: Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low-severity fire. Stands that originated following stand-replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low-severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low-severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.

Regional and local effects of disturbance and climate on altitudinal treelines in northern Patagonia

Abstract: In this field study we analysed the regional and local scale effects of disturbance and climate on altitudinal treelines dominated by Nothofagus pumilio in northern Patagonia. We compared two regions west and east of the Andes at 40° S, slopes with warm vs cool aspects and undisturbed vs locally disturbed treelines. This spatial framework allowed us to test (1) for differences among treelines affected by different types of local disturbance and (2) the traditional hypothesis that low temperature limits treeline. Contingency tables and ANOVA showed that local disturbance occurred more frequently than expected on slopes with cool aspects, steep slope angles and concave slope configuration. Disturbed treelines were locally lowered with longer ecotones and lower krummholz growth rates and vegetation cover than undisturbed treelines. Three-way ANOVA showed the significant influences of study area (regional climate) and aspect (local climate) on treeline elevation, krummholz growth rates and density, t...

Subalpine forest development following a blowdown in the Mount Zirkel Wilderness, Colorado

Abstract: We documented the occurrence of a 1934 blowdown in a subalpine forest in northwestern Colorado, USA. Prior to the blowdown, the stand was dominated by old-growth Picea engelmannii - Abies lasiocarpa forests. Although blowdowns are believed to trigger outbreaks of Dendroctonus rufipennis (spruce beetle), we found no detectable increase in beetle caused mortality. Forest recovery was by both release of the previously suppressed regeneration and by new seedling establishment. Both recovery pathways were dominated by Abies. The blowdown thus caused a shift in species dominance from Picea to Abies; 65 yr after the blowdown, the fallen logs and tip up mounds continue to provide favourable habitat for seedling establishment of both species. The present study shows that the legacy of blowdowns can influence forest dynamics for decades following the disturbance event.

Historic range of variability of mountain forest ecosystems: concepts and applications

Abstract: Concepts of historical range of variability (HRV) have taken on an increasingly important role in resource planning and the management of mountain forest ecosystems. This essay draws on examples fr...

Ecological effects of the Hayman Fire - Part 2: Historical (pre-1860) and current (1860-2002) forest and landscape structure

Abstract: The term “landscape structure” refers to the configuration of vegetation and other land features over a large land area (usually an extent of many square kilometers). A landscape can be regarded as a mosaic composed of patches of different kinds— for example, different forest types, landforms, or human-built structures such as roads. The scientific discipline of landscape ecology is concerned with quantitatively describing the features of landscape mosaics, including, for example, the variety of patch types, the sizes and shapes of patches, and how different patch types are juxtaposed (Forman 1995). Landscape ecology also is concerned with understanding how the structure of a landscape influences its function – for example, what kind of habitat it provides for various plant and animal species, or how water and nutrients or pollutants move from place to place (Turner and others 2001). Another critical aspect of landscape structure is how it influences the spread of disturbances – including fire. We address this question about historical and recent landscape structure in the area burned by the Hayman Fire in two parts. First, to put the Hayman Fire of 2002 into context, we characterize the historical range of variability (HRV – see the part 1 in this chapter) in landscape structure of the major forest zones of the Colorado Front Range, and compare these reference conditions with current landscape structure. This comparison of current to reference conditions permits an assessment of the magnitude, causes, and significance of changes that have occurred in the last century, and identifies generally where in the Front Range the changes have been great or small. We then focus on the Hayman area itself, to identify the nature, magnitude, and significance of 20 century landscape changes within the area where the 2002 fire occurred. We begin with a general assessment of the Front Range as a whole before treating the Hayman area in detail for two reasons: first, the general overview provides a context for understanding the unique features of the Hayman landscape, and second, future Front Range fires are likely to occur in other forest zones that are not well represented in the Hayman area per se. Historical Range of Variability and 20th Century Changes in Landscape Structure of the Colorado Front Range

Altitudinal treelines of the southern Andes near 40ºS

Abstract: In the southern Andes near 40oS, altitudinal treelines are dominated by Nothofagus pumilio, a broadleaf deciduous angiosperm in the beech family (Fagaceae). Treeline elevations, ranging from 1100 to 1500 m a.s.l., are influenced by regional climate and volcanism. At the local scale, disturbance influences treeline elevation, ecotone length, and vegetation productivity. Decadal and interannual variation in climate related to El Nino-Southern Oscillation (ENSO) significantly affected radial growth of krummholz trees and seedling demography; however, climate-treeline relations were complex. Radial growth of krummholz trees and seedling demography responded differently to climate variation. These relations differed between climate regions and were unstable through time. We conclude that inter-annual variations in climate, such as those associated with ENSO, will be critical for successful reproduction and growth of Nothofagus pumilio at treeline in the Andes under the influence of global warming. Key words:Ar...

Climatic and Human Influences on Fire Regimes in Temperate Forest Ecosystems in North and South America

Abstract: Fire plays an integral role in regulating ecosystem structure and processes including biodiversity, nutrient cycling, ecosystem structure, resiliency, stability, and carbon flow (Boerner 1982; Agee 1993). These effects of fire on ecosystems are extremely sensitive to all components of the disturbance regime (frequency, intensity, scale, predictability; Pickett and White 1985). To properly evaluate the impact of human activities on fire — both now and in the future — it is critical that we have an accurate baseline of past fire occurrences. In particular, we need to understand the natural role of fire in ecosystems across a range of scales from stands or sites to broad regions (Turner et al. 1994). In addition, we need to understand the controls that govern fire regimes across landscapes, so that we can better understand both how natural systems worked in the past and how fire regimes might be changed in the future (Franklin et al. 1991; Veblen and Lorenz 1991; Alaback and McClellan 1993).

Ecological effects of the Hayman Fire - Part 4: Forest succession

Abstract: The ecosystems within the area that burned in the Hayman Fire have a long history of fire (see part 1 of this chapter). It follows, therefore, that all of the native species and populations in this area probably have one or more mechanisms for enduring fire or becoming reestablished after fire and that no native species is likely to become extinct as a result of the direct effects of the Hayman Fire. It also follows that active rehabilitation is not required for all of the burned area. In fact, much or even most of the area is likely to recover normally without intervention, and in some areas our well-intentioned rehabilitation efforts actually could interfere with natural recovery processes.

Natural Ecosystems I. The Rocky Mountains

Abstract: This assessment of climate-change effects on Rocky Mountain terrestrial ecosystems is prepare from information generated by a workshop focused on terrestrial systems of the Rocky Mountains, and held in Boulder, CO, on 29-30 September 2000 at the National Center for Atmospheric Research. It is a compilation of this workshop's discussion along with material from earlier workshops.