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Browsing by Author "Rocca, Monique E., committee member"

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    Seeing the forest for the seeds: masting and mortality in dry forest and woodland ecosystems of the Rocky Mountains and Colorado Plateau
    (Colorado State University. Libraries, 2022) Wion, Andreas P., author; Redmond, Miranda D., advisor; Pearse, Ian S., committee member; Stevens-Rumann, Camille S., committee member; Rocca, Monique E., committee member
    For forests to persist on the landscape, tree recruitment must keep pace with tree mortality. Larger, more frequent, and more severe disturbances have raised concerns about the capacity for water-limited forests and woodlands to recover under increasingly hotter climates. For most conifer species, seed availability is the fundamental prerequisite to new tree recruitment. In addition, most conifer species are also masting species, meaning seed production is highly variable among years and synchronous within a population. Masting creates boom and bust patterns of seed availability that shape forest dynamics. In many species, it remains unknown what drives masting or how the mosaic of climate and competition shape patterns of seed production across species ranges. This limits our ability to forecast forest demography across large spatial areas and under uncertain climate futures. This dissertation is an exploration into two key processes driving forest persistence and loss, mast seeding and tree mortality, in two pine species native to dry forests and woodlands of the Rocky Mountains and Colorado Plateau: piñon pine and ponderosa pine. In chapter one, I examine the spatiotemporal drivers of masting in piñon pine across its latitudinal distribution in Colorado and New Mexico. I demonstrate that masting was driven by favorable weather conditions acting during key phenological periods of cone development: initiation and pollination. Cone production was sensitive to the spatial variability in long-term climate - cooler and wetter sites produced more cones, more frequently than hotter and drier sites. In chapter two, I examined cone production in ponderosa pine across a large portion of this species' distribution between Arizona and Wyoming. In contrast to piñon pine, I found that individual-level factors like tree size, age, and stand density, were better predictors of cone production than long-term climate or annual weather. In chapter three, I examined patterns of cone production between these two species jointly and related them to large scale modes of climate variability, like the North American monsoon and the El Niño Southern Oscillation. I found similar patterns, where piñon pine cone production was strongly associated with climate while ponderosa pine was not. Range wide synchrony in masting was associated with the North American monsoon, and regional north-south anti-synchrony was associated with the phase of the El Niño Southern Oscillation. Collectively, these chapters highlight contrasting masting patterns among these two, co-occurring pine species of dry forests and woodlands of the Rocky Mountains and Colorado Plateau. I explore the consequences of these contrasting life history strategies for forest management in this region, as well as the potential impacts of a changing climate and disturbance regime on seed production. In the final chapter of this dissertation, I evaluated four drought metrics on their ability to correctly predict piñon pine die-off following a severe drought in 2018. The results highlight how some of these simple climate metrics can be used as an indicator of piñon pine die-off in future, hotter drought events, and I provide a simple framework for evaluating mortality risk at regional scales.
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    The response of a Rocky Mountain forest system to a shifting disturbance regime
    (Colorado State University. Libraries, 2019) Carlson, Amanda R., author; Sibold, Jason S., advisor; Assal, Timothy J., committee member; Hobbs, N. Thompson, committee member; Rocca, Monique E., committee member
    Climate change is likely to drive widespread forest declines and transitions as temperatures shift beyond historic ranges of variability. Warming temperatures and shifting precipitation patterns may lead to increasing disturbances from wildfire, insect outbreaks, drought, and extreme weather events, which may greatly accelerate rates of ecosystem change. However, the role of disturbance in shaping forest response to climate change is not well understood. Better understanding the impacts of changing disturbance patterns on forest decline and recovery will allow us to better predict how forest ecosystems may adapt to a warming world. Severe wildfires and bark beetle outbreaks are currently affecting large areas of forest throughout western North America, and increasing disturbance size and severity will have uncertain impacts on forest persistence. The goal of my dissertation was to investigate the factors shaping disturbance response in a region of the San Juan Mountains, Colorado, which has undergone impacts from a high-severity spruce beetle outbreak and wildfire in the last 15 years. I conducted three separate studies in the burn area of the West Fork Complex wildfire, which burned in 2013, and in surrounding beetle-affected spruce-fir forests. The goals of each study were to 1) assess whether the severity of spruce beetle outbreaks occurring before wildfire resulted in compounded disturbance interactions affecting vegetation recovery, 2) determine how the severity of each disturbance type influenced fine-scale below-canopy temperature patterns across the landscape, and 3) assess how conifer seedling regeneration densities were influenced by effects of disturbance severity on seed dispersal, temperature, and vegetation structure. I found that disturbances influenced seedling regeneration and ecosystem resilience through several mechanisms. First, pre-fire beetle outbreak severity was negatively correlated with post-fire vegetation cover, indicating that the combined disturbances were inhibiting regeneration beyond what may have been expected with fire alone. Second, disturbances had significant effects on below-canopy temperatures, with burned areas ~0.5 °C warmer than unburned forest areas and differences in overnight minimum temperatures resulting from loss of live canopy in unburned, beetle-killed forests. Third, the large fire size and high severity resulted in very little spruce seed dispersal or conifer regeneration in most of the burned area, while spruce regeneration in unburned forest was negatively correlated with increasing overstory mortality from the spruce beetle. My results indicate that disturbance is playing an important role in determining the future trajectory of the forest in my study area. The West Fork Complex fire has caused a severe ecosystem transformation, has increased landscape exposure to warming temperatures, and is preventing forest re-establishment as a result of a lack of seed sources. The spruce beetle outbreak has not resulted in such a severe transformation, but is possibly leading to reduced forest resilience by reducing spruce seedling re-establishment and by altering fuel structures to make forests more prone to high soil burn severity if fire follows within ~10 years. Warming of below-canopy microclimates is not exacerbated by spruce beetle outbreak, and is rather partially offset by cooling of overnight temperatures. These findings provide insights into how forest responses to climate change may be shaped by disturbance processes, which are occurring with increasing severity and frequency worldwide.
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    The role of disturbance and local adaptation in altering tree regeneration responses to climate change
    (Colorado State University. Libraries, 2023) Nigro, Katherine M., author; Redmond, Miranda D., advisor; Battaglia, Michael A., committee member; Stevens-Rumann, Camille S., committee member; Rocca, Monique E., committee member
    Anthropogenic climate warming is predicted to reorganize the communities seen on the landscape today as species migrate to stay within their preferred climate niche. However, for long-lived trees, it is uncertain whether migrations will be able to keep pace with the rapidly changing climate. In addition, adaptations to local climate that have evolved within subpopulations of a species may become obsolete or maladaptive under climate change. Natural disturbances such as wildfire and insect outbreaks in forests may help to accelerate range shifts by reducing competition from other plant species and creating favorable microsites for establishment of novel species. For landscapes that are not recovering after disturbance events, human-assisted movement of populations within species (assisted gene flow) has been proposed to pre-adapt the forest to future climate conditions. In the first part of my dissertation, I analyze large-scale USDA Forest Service Forest Inventory and Analysis data from the interior western US and conduct a field study of trembling aspen in southern Colorado to understand how disturbances are interacting with climate change to alter species range shifts at broad and local scales. The first chapter of my dissertation shows that disturbances are catalyzing range shifts on a large-scale for the dominant tree species of the interior western US under both disturbed and undisturbed conditions, but mostly through reduced regeneration in hot and dry portions of species ranges rather than expansions into cooler and wetter areas. However, chapter 2 reveals that expansions are happening on more local scales for trembling aspen after wildfire, but not after beetle-kill mortality events. Therefore, this research shows that disturbance-facilitated species range expansions are limited to certain species and locations, which may not be enough to keep pace with climate warming. The second part of my dissertation investigates the potential for assisted gene flow in ponderosa pine by examining differences between seedlings from different populations and mother trees in both the greenhouse under drought and the field under a variety of microsite conditions. This research reveals that population climate and watering regime are important determinants of seedling growth in the greenhouse, but that ponderosa pine seedlings from hotter and drier populations do not possess adaptations typically associated with drought-tolerance and do not perform well when planted at the cooler range margin. In addition, mother tree sensitivity to climate was influential in seedling water use efficiency in the greenhouse and in the sensitivity of seedlings to microclimate conditions in the field, making the case that targeting seed collections from specific mother trees may be warranted. Overall, my dissertation research provides insight into how future disturbance events are likely to influence regeneration at species range margins and provides valuable information for land managers seeking to implement assisted gene flow at the upper elevational margin of ponderosa pine forests.
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    Tools for improved management of sagebrush avifauna and sagebrush rangelands
    (Colorado State University. Libraries, 2017) Timmer, Jennifer Marie, author; Aldridge, Cameron L., advisor; Augustine, David J., committee member; Rocca, Monique E., committee member; Fernandez-Gimenez, Maria E., committee member
    To view the abstract, please see the full text of the document.
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    Using native annual plants to suppress weedy invasive species in post-fire habitats
    (Colorado State University. Libraries, 2010) Herron, Christopher M., author; Paschke, Mark W., advisor; Rocca, Monique E., committee member; Brown, Cynthia Stokes, committee member; Meiman, Paul J., committee member
    Increasing fire frequencies and uncharacteristic severe fires have created a need for improved restoration methods across rangelands in western North America. Traditional restoration seed mixtures of perennial mid- to late-seral plant species may not be suitable for intensely burned sites that have been returned to an early-seral condition. Under such conditions native annual plant species are likely to be more successful at competing with exotic annual plant species such as Bromus tectorum L. We used a field study in Colorado and Idaho, USA to test the hypothesis that native annual plant species are better suited to post-fire restoration efforts compared to perennial plant species that are commonly used in traditional seed mixtures. Replicated test plots at four post-fire sites were assigned one of four treatments (1) native annual seed mixture, (2) standard perennial seed mixture, (3) combination of annual and perennial, and (4) an unseeded control. Results suggest that there is potential for native annual plant species to be effective competitors with weedy exotic species in post-fire restoration scenarios.