Showing papers by "Thomas T. Veblen published in 2004"

The Interaction of Fire, Fuels, and Climate across Rocky Mountain Forests

Abstract: Understanding the relative influence of fuels and climate on wildfires across the Rocky Mountains is necessary to predict how fires may respond to a changing climate and to define effective fuel management approaches to controlling wildfire in this increasingly populated region. The idea that decades of fire suppression have promoted unnatural fuel accumulation and subsequent unprecedentedly large, severe wildfires across western forests has been developed primarily from studies of dry ponderosa pine forests. However, this model is being applied uncritically across Rocky Mountain forests (e.g., in the Healthy Forests Restoration Act). We synthesize current research and summarize lessons learned from recent large wildfires (the Yellowstone, Rodeo-Chediski, and Hayman fires), which represent case studies of the potential effectiveness of fuel reduction across a range of major forest types. A “one size fits all” approach to reducing wildfire hazards in the Rocky Mountain region is unlikely to be effective and may produce collateral damage in some places.

Spatiotemporal influences of climate on altitudinal treeline in northern patagonia

Abstract: Tree radial growth and seedling establishment of Nothofagus pumilio at alpine treeline near 40° S latitude in Chile and Argentina show time- and site-dependent relationships to interannual-and decadal-scale climate variation. Six treelines were sampled at two spatial scales corresponding to regional and local climates. A shift in climate from cool–wet to warm–dry conditions facilitated comparison of climate–vegetation relationships during two distinct periods: 1957–1976 and 1977–1996. For each treeline, tree radial growth and seedling establishment were correlated against monthly and seasonal temperature, precipitation, moisture availability, and two indices of El Nino–Southern Oscillation (ENSO): southern oscillation index (SOI) and sea surface temperature (SST). Four key aspects of climatic influences on N. pumilio radial growth and seedling establishment were as follows. (1) The relationship between krummholz radial growth and temperature variation was nonlinear. (2) Moisture availability was the dominant climatic factor influencing seedling establishment, although temperature–precipitation interactions resulted in variability among study areas. (3) Climate conditions that facilitated Nothofagus pumilio seedling establishment were distinct, and often opposite, from those that enhanced radial growth. (4) The relationships of radial growth and seedling demography with climate and ENSO differed among study areas and have been unstable over the past 40 years. Observed spatial and temporal instabilities in vegetation–climate relationships demonstrate the complexity of treeline dynamics in northern Patagonia under a changing climate. We conclude that a directional increase in temperature, as predicted by current global climate scenarios, will not necessarily result in an upslope expansion of the N. pumilio forests growing at altitudinal treeline in northern Patagonia.

The persistence of quaking aspen (populus tremuloides) in the grand mesa area, colorado

Abstract: 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 been substantial concern that quaking aspen, the most widely distributed tree in North America and 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 reported decline has been attributed to conifer encroachment associated with fire exclusion, as well as 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 compare an 1898 map of vegetation and fires to a modern map of present forest cover types. Based on this comparison, a larger portion of the current landscape is dominated by quaking aspen relative to the late 19th century, a period of extensive burning in this area. During the 20th century, aspen was persistent over most of its extent, even in the absence of fire. Fires of the late 19th century also increased aspen cover in stands that were previously dominated by spruce and fir. The trend toward increased aspen was greater at lower elevations. The total area where spruce and fir have replaced aspen is small in comparison to the area where aspen has increased or has persisted. The successional replacement of aspen by conifers is more 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 19th century 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 ecological conditions, including dominance by particular species.

Nothofagus regeneration dynamics in south‐central chile: a test of a general model

Abstract: Studies in Southern Hemisphere temperate forests have yielded a general model of how the successional status of shade-intolerant Nothofagus is related to site conditions and disturbance. According to model predictions, regeneration of most Nothofagus species is dependent on coarse-scale disturbance at favorable sites where shade-tolerant rain forest species co-occur. In contrast, at suboptimal sites where tree species richness is low, regeneration is not dependent on coarse-scale disturbance. We used variation in forest composition and structure as well as disturbance patterns in the forests of Puyehue National Park, Chile, to test several predictions from this general model. The study area presents a complex mosaic of disturbance patterns, including the effects of the strong 1960 earthquake as well as fire and wind disturbance. We reconstructed stand development on sites affected by these disturbances, and compared these patterns with the structure of old-growth forests along an elevation gradient from s...

Carbon accumulation in Colorado ponderosa pine stands

Abstract: Woody encroachment and thickening have occurred throughout the western United States and have been proposed as important processes in the US carbon (C) budget despite large uncertainty in the magnitude of these effects. In this study we investigated ponderosa pine encroachment near Boulder, Colorado. We reconstructed a time series of forest structure to estimate changes in C storage by the trees. Advantages of this technique include the ability to estimate changes in C stocks over time with a single series of measurements (i.e., no historical measurements), and the ability to calculate accumulation rate changes through time. Substantial variation occurred in the C amounts and accumulation rates among the three plots resulting from differences in slope, aspect, and soil conditions. Accumulation rates increased exponentially as trees increased in size and additional trees established within the plots, and were highly variable among plots (0.09–0.7 Mg C·ha–1·year–1 during 1980–2001). These rates were less th...