Browsing by Author "Rocca, Monique E., advisor"

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Open Access
Forest regeneration trajectories in mountain pine beetle-disturbed forests of Rocky Mountain National Park
(Colorado State University. Libraries, 2010) Diskin, Matthew, author; Rocca, Monique E., advisor; Romme, W. H. (William H.), committee member; Sibold, Jason S., committee member
A severe mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic in western North America has caused widespread mortality of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and drastically altered subalpine forest structure and composition over large areas. My research describes possible future forest regeneration trajectories by documenting tree survivorship in Rocky Mountain National Park, CO, and then projecting future forest conditions using an established forest growth model, the Forest Vegetation Simulator. In 2008, I measured stand structure and tree species composition in lodgepole pine-dominated forests in the western portion of the Park. I defined five lodgepole pine forest types that varied with respect to the abundance of seedlings and non-lodgepole pine species. These forest types formed the foundation for further analyses to describe variability in post-epidemic forest regeneration trajectories. Chapter One documents surviving forest stand structure and composition in the Park, and shows that surviving trees, including larger canopy trees, saplings, and seedlings, were plentiful in most of the post-epidemic forests. Lodgepole pine remained the dominant species in most areas, but modest increases in the relative abundance of subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry ex Engelm.), and aspen (Populus tremuloides Michx.) had occurred. Forest structure and composition varied considerably among the five forest types after the epidemic, setting the stage for variable future forest regeneration trajectories that were explored in Chapter Two. Future forest conditions were projected over a 100 year time period and are presented in Chapter Two. The projections show that the beetle-disturbed forests remain forested in the future, and emphasize that the most important mechanism for forest renewal is the release of surviving trees, rather than post-epidemic tree seedling establishment. Projected future forest conditions varied depending on the forest type, but indicate that basal area and quadratic mean diameter recovery occurs within 40-100 years in most areas. Spruce, fir, and aspen become dominant in the projected future forests on approximately 60% of the landscape, while lodgepole pine remains dominant in the forests where it formed pure stands prior to the epidemic.
Open Access
Incorporating the disturbance process of fire into invasive species habitat suitability models
(Colorado State University. Libraries, 2008) Silverman, Joel, author; Rocca, Monique E., advisor; Laituri, Melinda J., committee member; Martin, Patrick H., committee member
This study is motivated by the difficulties land managers face while attempting to simultaneously maintain the natural role of fire in ecosystems and prevent the spread and proliferation of invasive plants. I developed habitat suitability models to predict the responses of three invasive species to fire and other environmental variables: one species in each of three National Parks. For each species, model comparisons tested whether the inclusion of nationally-available data on burn severity, time since fire, and fire occurrence could improve habitat suitability models relative to non-burn data alone. Each species demonstrated significant responses to fire, although incorporation of fire information into the models improved model performance for some species more than for others.
Open Access
Is thin and chip an ecologically viable fuels reduction option? Initial results in Black Hills ponderosa pine forests
(Colorado State University. Libraries, 2011) Cueno, Katherine L., author; Rocca, Monique E., advisor; Rhoades, Chuck, Affiliate, committee member; Brown, Cynthia, committee member
Across the dry western forests of the United States, accumulated hazard fuels increase the risk of catastrophic wildfires. Chipping or mastication of mechanically thinned fuels is becoming a common fuels reduction technique that aims to both reduce the risk of catastrophic fire and dispose of non-merchantable thinned material. We conducted an experiment to examine the ecological effects of thinning and chipping in ponderosa pine forests at two National Park Service locations in the Black Hills, South Dakota. By using an unthinned control and a thin-only treatment to compare to thin-chip treatment we were able to separate the effects of thinning from the effects of wood chip application. A greenhouse experiment was used to assess the effects of wood chip depth on seedling emergence and growth of several grass and forb species common to the study sites. Thin-only and thin-chip treatments greatly reduced hazard fuels by lowering pole tree density by 96%. Thinning did not elicit much of an understory response in the first year following treatment, while wood chip application caused slight decreases in understory plant richness and cover. Thin-chip plots had one-third lower graminoid cover than unthinned plots and half the number of annual species richness than thin-only plots. There was no difference in non-native species cover or richness, or in overall plant community composition as a result of treatments. Ponderosa pine germinated equally well in wood chips as in other areas. We observed a small decrease in NO3--N in thin-chip plots at one study site, but no effect at the other site. In the greenhouse, increasing wood chip depth created an increasing barrier to seedling emergence and growth in both grass and forbs. Complete suppression of plant emergence in the greenhouse occurred at wood chip depths ≥ 6 cm and plant biomass was undetectable at wood chip depths ≥ 3 cm. Our initial results suggest that thin-chip is a viable fuels treatment option. Although wood chip application slightly reduced some measures of understory cover and richness, the results we detected were subtle. Future examination will determine if delayed thinning effects reverse the slightly suppressive effects of wood chip application on understory vegetation.
Open Access
The interactive effects of climate and disturbance on tree species distributions
(Colorado State University. Libraries, 2015) Renwick, Katherine M., author; Rocca, Monique E., advisor; Hobbs, N. Thompson, committee member; Sibold, Jason, committee member; Stohlgren, Thomas J., committee member
Climate change is expected to alter species distributions as ranges shift to track favorable temperature and precipitation regimes. Range shifts are already being observed across a wide range of taxa, but many species are not keeping pace with the rate of recent climate warming. This is particularly true for tree species, which often experience significant migration lags due to a variety of non-climatic factors that can hinder range expansion or delay range retreats. Because many other species depend on trees for food or habitat, migration lags in tree species may have cascading impacts on a wide range of taxa that would otherwise face few barriers to migration. The importance of understanding how climate change will affect tree species distributions prompted several related research questions: 1) What factors contribute to the observed lags in tree species distributions? 2) Can biotic disturbances accelerate climate-driven shifts at the range margins of trees species? 3) How important is climate in determining landscape-scale vegetation patterns? My dissertation research addresses these questions using an integrated approach that draws on exiting literature, field sampling, and statistical models to inform our understanding of potential climate change impacts on tree species distributions. Observations of contemporary tree species migrations occurring throughout the world suggest that migration lags are pervasive and can be caused by a wide variety of abiotic factors and biotic processes. Tree migrations are likely to occur episodically when migration constrains are overcome, resulting in temporal variability in the migration rate. Physical disturbances such as fire can reduce competition and initiate periods of rapid change, but the effects of biological disturbances such as insect outbreaks are more nuanced. A case study examining the impacts of climate change and mountain pine beetle (Dendroctonus ponderosae) disturbance at lodgepole pine (Pinus contorta) range margins suggests that while biological disturbances may accelerate a range retreat by killing mature trees, they do not initiate range expansion for the target species. The impact of non-climatic constraints on current tree species distributions was also evident at the landscape scale, and climatic variables alone proved insufficient to explain patterns of co-occurrence among tree species. Together, these findings suggest that Rocky Mountain tree species will not uniformly shift upward in elevation as the climate continues to warm. Range shifts will likely be episodic and idiosyncratic, and forecasts based solely on climate data may over-estimate the rate and under-estimate the landscape-scale heterogeneity of potential distribution changes.