Browsing by Author "Hart, Sarah, advisor"
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Item Open Access Interactions and impacts of multiple bark beetle outbreaks in the southern Rocky Mountains(Colorado State University. Libraries, 2022) Tutland, Niko Joseph, author; Hart, Sarah, advisor; Redmond, Miranda, committee member; Rocca, Monique, committee memberIn conifer forests of western North America, outbreaks of native bark beetles are important biotic disturbances that influence forest structure and function. Subalpine forests in the southern Rocky Mountains (SRM) are home to multiple species of host-specific bark beetles that can affect the same stand concurrently or successively, potentially interacting to affect landscape patterns of tree mortality and subsequent disturbance dynamics. Despite their importance, the causes and consequences of interactions among bark beetle disturbances are poorly understood. To examine how outbreaks of different bark beetle species interact, we conducted two studies in the SRM. First, using broad-scale geospatial data, we explored the extent and severity of overlapping outbreaks of multiple bark beetle species across the SRM. We found that forest stand susceptibility to outbreaks of multiple bark beetle species is limited by host tree distributions, and that overlap of outbreaks was relatively uncommon. Furthermore, cumulative tree mortality was rarely higher in stands that experienced overlapping outbreaks compared to stands that experienced single-species outbreaks. These results suggest that forest trajectories in mixed-species stands will not be altered when multiple bark beetle outbreaks co-occur, compared to single-species outbreaks. Second, in a multi-scale study including both field and geospatial data, we explored how history of stand-replacing fires and a severe spruce beetle (Dendroctonus rufipennis) outbreak affected future susceptibility to subalpine fir (Abies lasiocarpa) decline (SFD) from outbreaks of the western balsam bark beetle (Dryocoetes confusus) and associated fungal pathogens. At the landscape scale, we found that disturbance history had limited effects on the susceptibility to future SFD, with a weak trend towards higher susceptibility in younger forests. At the stand scale, however, we found no effect of disturbance history on stand structure or composition traits that have been shown to influence susceptibility to SFD. Individual tree-scale analyses revealed that tree and local neighborhood traits were most important for determining likelihood of SFD. Weak connections between disturbance history, stand structure and composition, and SFD underscore the variability in forest recovery after disturbance, and highlight the need for future research. Together, these results from these studies indicate that, while climate change may be amplifying tree mortality events, subalpine forests in the SRM may be broadly resilient to co-occurring and successive disturbances.Item Open Access Occurrence, distribution, and driving environmental factors of quaking aspen regeneration by seed in the Cameron Peak Fire burn scar(Colorado State University. Libraries, 2024) Carter, Sarah, author; Hart, Sarah, advisor; Rhoades, Charles, committee member; Rocca, Monique, committee memberAs a result of the increasing frequency and severity of wildfires in the mountain west region of North America, greater mortality of montane and subalpine forests has led to changes in forest regeneration patterns and species composition. Increased drought conditions pre- and post-fire due to warming climate and destruction of existing seed have led to loss of historically conifer-dominant forests. This has subsequently opened a niche for post-fire aspen establishment, particularly through seed. The understanding of aspen regeneration by seed is understudied in comparison to the more broadly emphasized vegetative reproduction: a process which is limited spatially by the presence of surviving root networks and a lack of adaptive capacity of clone genetics. In this study, we aimed to (1) quantify the presence and density of post-fire aspen seedling establishment and (2) assess the environmental drivers of post-fire seedling establishment in a recent burn scar in northern Colorado. Two growing seasons following the fire, we conducted field surveys at 38 sites within the Cameron Peak Fire burn scar. We aimed to quantify regeneration of all tree species, including aspen as well as the dominant pre-fire conifers ponderosa pine (Pinus ponderosa), lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii). Across our study area we found widespread establishment of aspen seedlings, particularly at high elevations, where soil moisture is less limiting. Given the occurrence of aspen seedlings within a site, we found seedlings were most likely to occur in moss seedbeds, near large coarse woody debris, and within microsite concavities, where soil moisture availability is likely higher. Collectively, our findings highlight the importance of moisture availability for the germination and initial survival of aspen seedlings. Further we found occurrence of aspen seedlings far outweighed that of any conifer species. These findings support projected changes in forest composition, species dominance, and range shift following stand replacing fire to favor early successional species such as aspen. The successful dispersal and establishment of aspen seeds in large, high severity burned patches have potential to facilitate the range shift of aspen forests towards higher elevations. These implications become more prevalent as changes in climate increase the risk of high severity fires and loss of seed sources, while decreasing suitability for montane and subalpine forest species to persist and regenerate.Item Open Access The influence of provenance on radial growth and drought resilience of lodgepole pine in Fraser Experimental Forest, Colorado, U.S.A.(Colorado State University. Libraries, 2024) Warnick, Katarina J., author; Hart, Sarah, advisor; Tinkham, Wade, advisor; Battaglia, Mike, committee member; Dahl, Jamie, committee memberClimate change poses unprecedented challenges to forested ecosystems, particularly in drought-affected areas in the western United States, where increased temperatures and shifting precipitation regimes are negatively impacting locally adapted tree species. In response, researchers and land managers are exploring innovative forest adaptation strategies to maintain forest resilience, such as assisted migration. This study examines the potential of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) to be utilized in forest assisted migration efforts in western North America, drawing from the understanding that this widespread species displays intraspecies variation in growth-climate associations due to local adaptations across its extensive geographic range. We make use of a provenance study established in the early 1980s in Fraser Experimental Forest, Colorado, U.S.A. to examine the effect of seed source, or provenance, on growth-climate responses and drought resilience of lodgepole pine in its southernmost range. We investigate two primary research questions. First, does provenance climate influence the radial growth response of lodgepole pine trees? Second, do lodgepole pine trees from climatically drier provenances exhibit greater resilience to drought, as measured by annual radial growth before and after drought events? To address our first question, we employed dendrochronological methods and a generalized linear mixed-effects modeling approach utilizing climate data to analyze the variation in overall radial growth of trees explained by the difference in provenance and study site climates during our study period, 1992-2021. In addressing our second question, we utilized the Standardized Precipitation Evapotranspiration Index (SPEI) and metrics of quantifying drought resilience to determine the impact of provenance site aridity on tolerance to drought during our selected drought period, 2001-2002. Our analyses revealed significant differences in radial growth between trees from different provenances. Specifically, we found that trees originating from provenance sites with greater growing season average vapor pressure deficit, yet higher precipitation, exhibited increased radial growth. In analyzing drought resilience, we found that trees from provenances with higher average temperatures and greater evapotranspiration showed greater resilience to drought, aligning with previous research linking seed source climate aridity to drought tolerance. Interestingly, we also found that trees from sites with greater climate moisture index compared to their growing site exhibited greater radial growth resilience to drought, suggesting that certain populations may be less sensitive to moisture deficits. Our research sheds light on the relationship between provenance climate with radial growth response and drought resilience in an assisted migration context. Our findings suggest that provenance climate aridity may confer resilience to trees during resource-limited events. More broadly, our study underscores the potential of utilizing local adaptations and intraspecies variability of tree species in climate adaptation efforts aimed at mitigating the impact of climate change in forested ecosystems.Item Open Access Western spruce budworm outbreak associated with wet periods in the Colorado Front Range: a multicentury reconstruction(Colorado State University. Libraries, 2022) Santiago, Olivia, author; Hart, Sarah, advisor; Redmond, Miranda, committee member; Bombaci, Sara, committee memberWestern spruce budworms (WSB) are one of the most important native defoliators of coniferous forests in North America, causing widespread reductions in tree growth and elevated levels of tree mortality during outbreak. Like other eruptive insects, outbreaks of WSBs are thought to be influenced by a broad regional driver such as climate. Documentation of WSB outbreak is limited to the mid-20th century, constraining our understanding of how climate may influence outbreak. Previous research has linked periods of outbreak with both anomalously wet and dry periods in the Southwest and Pacific Northwest, respectively. To assess this relationship in the Colorado Front Range, I used tree ring methods to reconstruct periods of WSB outbreak over 437 years (1564-2001) at 15 sites. I detected 102 outbreak periods lasting on average 9.7 years (±1.0) with 27.7 years (±7.2) between outbreak periods. These outbreaks were regionally synchronous and were most common during wet periods that followed periods of average or low moisture. Considering the potential impacts of WSB defoliation on the provisioning of key ecosystem services, including carbon sequestration, provisioning of timber, and wildlife habitat, the findings provide valuable information for forest scientists and managers responsible for promoting forest health and resilience.