Browsing by Author "Redmond, Miranda D., advisor"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Open Access Juvenile tree dynamics in changing landscapes: effects of overstory-mediated microclimates on dryland tree recruitment vary across climatic gradients(Colorado State University. Libraries, 2024) Hill, Edward M., author; Redmond, Miranda D., advisor; Ocheltree, Troy W., advisor; Bradford, John B., committee member; Smith, Melinda D., committee memberClimate change impacts the future viability of plant species and communities directly through effects on demographic processes and indirectly through structural dynamics. Regeneration, establishment, growth, and survival of juvenile trees can be especially vulnerable processes in forest and woodland community development because juvenile trees are typically not able to tolerate abiotic stress as effectively as more mature trees. Because of this elevated sensitivity to climate-related stressors, juvenile establishment patterns are fundamental to understanding long-term species persistence. Overstory tree structure is an important mediating influence of the impacts of climate change in forest and woodland communities, particularly through influences on resource availability. Fine-scale variation in overstory tree size, density, and species influence primary plant resource requirements, including light availability, atmospheric heat and moisture, precipitation throughfall and soil water availability, and soil nutrient availability. Juvenile trees of different species can benefit from buffering of microclimate conditions by overstory trees, like direct radiation and extreme temperature variation, and experience competitive interactions for light and soil resources, especially in resource-limited communities. Yet, juvenile trees can span a range of sizes and physical maturity and vary in their capacity to acquire resources or tolerate resource limitations, and therefore can differ in facilitative versus competitive relationships with overstory conditions. Amplifying the complexity of these relationships, interannual variation in weather conditions, such as drier or wetter years than normal, influences the degree to which juvenile trees experience facilitative or competitive relationships with overstory trees. Indeed, the extent of microclimate effects on regeneration processes depend in part on the complex covariance of air temperature and humidity (thus, vapor pressure deficit), moisture availability (precipitation and soil moisture), and photosynthetically active radiation (i.e., light). In the absence of temperature and moisture limitations, trees may benefit from additional light availability for photosynthesis; alternatively, if temperature or moisture conditions are limiting, juveniles may benefit more from buffering influences of overstory, at the expense of decreased light availability. For dry forests and woodlands of the western U.S. which are at the forefront of climate change-driven tree recruitment vulnerabilities, greater resolution into juvenile relationships with overstory structure, and microclimate buffering, will substantially enhance the ability to evaluate and predict the effects of increasingly marginal climate space on their persistence. In this dissertation, I evaluated juvenile tree regeneration, growth, and survival in dryland forest and woodland systems relative to the mediating influences of overstory trees, across ranges of juvenile sizes, interannual weather variation, and broad climatic and elevational environmental gradients. In Chapter 1, I investigated survival and growth of ponderosa pine and Douglas-fir newly germinated seedlings, and older, larger seedlings to variation in overstory structure and associated microclimate conditions at fine-spatial scales. This study showed that while newly germinated seedlings were more sensitive to interannual variation in microclimates, overall survival and growth of younger and older seedlings were highest in microclimates with above-average warm and dry air during early-growing season months, and above-average light conditions. Importantly, the structural and microclimate influences on survival and growth over three years of study were primarily associated with the first year of study during which spring weather was abnormally cool and more humid. These results illustrated the environmental context for the initiation of survival and growth trajectories observed in this study, and demonstrate both spatially and temporally narrow conditions in which survival and growth was collectively greatest for both species. In Chapters 2 and 3, I investigated physiological and growth differences of juvenile piñon pine in dead and live overstory microenvironments over two years following experimentally-induced mortality of overstory trees. In Chapter 2, I measured photosynthetic and stomatal conductance rates of juveniles from among the smallest to largest individuals present in a middle-elevation piñon-juniper site in the core of the geographic distribution of two-needle piñon pine. Larger juveniles in dead overstory environments showed the highest photosynthetic and stomatal conductance rates. However, juveniles of all sizes were overall similarly limited by lower soil moisture and hot and dry microclimate conditions in both live and dead overstory environments. Given these limitations, the results of this study demonstrate the susceptibility of all juvenile piñon trees to hot and dry microclimates, which can be exacerbated both by mortality of overstory trees and by projections of future hotter and drier climate in these woodlands. In Chapter 3, I measured branch growth of juvenile piñon trees at six different sites spanning a gradient of latitudinal climate differences from hotter and drier southern locations to cooler and dry northern locations, and local elevation gradients from low to mid to high elevation piñon-juniper woodlands. Growth in post-overstory mortality years relative to mean growth prior to overstory mortality ("growth ratios") of juveniles across sizes was consistently higher in dead compared to live overstory environments only for middle- and high-elevation sites in our mid-latitude study region of southwestern Colorado, which had cooler and wetter post-overstory mortality weather conditions compared to other regions. Moreover, differences among sites were likely also related to typical climate differences associated both with latitude, where drier sites at southern and northern latitudes showed little growth responses to overstory mortality, and associated with elevation, where growth ratios were highest at the highest elevation site which has more moderate temperature and precipitation conditions on average. The results of this dissertation provide evidence for microclimate and juvenile tree survival outcomes in a dry forest restoration treatment and show the impact of acute structural change following overstory tree die-off on physiological and growth activity of juvenile piñon pine. The findings presented here provide ecologists and land managers with new information on the nuances of spatially and temporally narrow regeneration niches of species in dry mixed-conifer forests, and potential patterns and mechanisms of juvenile piñon pine resilience – but also potential future sensitivity – following overstory mortality. Importantly, results of these studies emphasize the role of interannual variation in weather conditions in driving specific forest and woodland development trajectories.Item Open Access 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 memberFor 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.Item Open Access 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 memberAnthropogenic 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.