Theses and Dissertations at Montana State University (MSU)
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Item A comprehensive study on forest management and wildfire trends in dry western coniferous regions of the United States(Montana State University - Bozeman, College of Agriculture, 2024) Schonenberg, Richard Herman; Co-chairs, Graduate Committee: Anthony Hartshorn and William KleindlIn response to the growing concern over the escalating severity of wildfires in dry coniferous forests across the Western United States, this study aims to evaluate the effectiveness of Managed Fire Operations (MFO) in mitigating wildfire severity. By leveraging satellite-derived fire severity data, specifically the Difference Normalized Burn Ratio (dNBR), I conducted a comprehensive analysis comparing fire severity between forest lands with and without MFO, subsequently affected by wildfires. Employing a paired study design, I analyzed wildfire events from 1985 to 2021 within dry coniferous forests, limited to south-facing slopes with moderate terrain gradients, using fire perimeter data from the Monitoring Trends in Burn Severity (MTBS) program. Geospatial analysis identified areas where wildfire perimeters intersected with MFO-designated zones, enabling the delineation of new burn perimeters for each wildfire. The results revealed that regions subjected to MFO before wildfires experienced a statistically significant decrease in fire severity compared to areas without MFO (Wilcoxon signed-rank test, p-value < .01). Ecoregion stratification revealed notable variations, with the Northwestern Forested Mountains showing an estimated median fire severity difference nearly three times greater than the Temperate Sierras. Further stratification by time since MFO implementation displayed consistent, modest reductions in fire severity across two intervals (0-15 years and 15-40 years), with minimal variations between the temporal categories. These results highlight the effectiveness of MFO in mitigating wildfire severity and emphasize the importance of regional context and temporal factors in evaluating MFO efficacy in Western U.S. dry coniferous forests over the past four decades.Item Spatial and temporal dynamics of conifer expansion in southwest Montana(Montana State University - Bozeman, College of Agriculture, 2022) Haygood, Nathaniel Paul; Chairperson, Graduate Committee: Craig Carr; This is a manuscript style paper that includes co-authored chapters.Since the mid-19th century, pinyon-juniper woodlands in western North America have experienced an expansion in range and density and a corresponding degradation in the provision of ecological goods and services including forage production, watershed function, biological diversity, and habitat values. While this is well-documented in other systems, there is little information characterizing shifts in tree range and abundance within the northern extent of these juniper and pine woodlands. The purpose of this research project was twofold: 1) identify and improve understanding of Rocky Mountain juniper and limber pine age distribution and compare these data to other systems, and 2) evaluate understory dynamics along the gradient of woodland development to assess impacts to understory species composition and abundance, as tree densities and range increase. We aged 278 trees across 38 plots in southwest Montana. We recorded soil moisture throughout the growing season (May-July), aspect, elevation, soil texture, herbaceous production and diversity, and tree density and canopy cover from 2019-2021. Greater than 95% of all trees were under 100 years old and the oldest tree (juniper) was 247 years old. Across the study site, limber pine was younger than Rocky Mountain juniper and appeared to prefer different sites. Maximum and mean juniper age was higher on dry sites with high sand content and lower on moist sites with low sand content in the top 15cm of the soil profile. Understory shrub and cool-season perennial grass cover was negatively influenced by heavy tree canopy cover on southwest to southeast aspects. The results from this study indicate 1) limber pine and Rocky Mountain juniper generally occupy different sites, 2) juniper and pine stand age is lower on north facing aspects with coarse soils and higher soil moisture content in late spring and early summer. Currently, increasing conifer dominance on north facing aspects appears to minimally impact cool-season perennial grass cover and production. Comparatively, increasing conifer dominance on south facing slopes may reduce cool-season perennial grass and shrub cover and production. We recommend the inclusion of these findings, as land managers seek to sustain delivery of necessary ecological goods and services.Item Long-term environmental history of two low-elevation mixed-conifer forests, Mission Valley, Montana(Montana State University - Bozeman, College of Letters & Science, 2021) LaPierre, Kari Richard; Chairperson, Graduate Committee: David McWethyLow elevation mixed-conifer forests are widespread throughout the Northern Rocky Mountains, yet there are few long-term environmental histories from these structurally and compositionally heterogenous ecosystems. We reconstructed >10,000 years of vegetation change, fire activity, and human presence (e.g., pollen, charcoal, biomarkers) for two closed-basin lakes in mixed-conifer forests in the Mission Valley, western Montana. Environmental reconstructions highlight periods of pronounced changes in climate, vegetation, and fire activity. The late glacial period (>18,000-11,000 cal yr. BP) was characterized by post-glacial warming, generally wet conditions, establishment of mixed-conifer forests and infrequent fires. Following an abrupt, short-lived return to Juniper/Douglas fir parkland associated with the Younger Dryas (~12,900-11,500 cal yr. BP), warming temperatures during the early Holocene (11,000-6,000 cal yr. BP) promoted the expansion of open parkland/grasslands and frequent fire activity until cooler summers and warm, wet winters facilitated the development of modern-day closed mixed-conifer forests. Organic biomarker analyses indicate human presence within the Rainbow Lake watershed for millennia c. 7,000-3,000 cal yr. BP. Regional fire frequency increased during this period at Rainbow Lake, suggesting a possible increased role of human influence.Item Recent and historical water use strategies of western U.S. conifers(Montana State University - Bozeman, College of Letters & Science, 2017) Clute, Timothy; Chairperson, Graduate Committee: David Roberts; Jia Hu (co-chair); Jia Hu was a co-author of the article, 'Investigating the variation in inter- and intraspecific physiological plant hydraulic traits across an elevation gradient' which is contained within this thesis.; Jia Hu was a co-author of the article, 'Historic influence of climate on conifer water status in northwestern Montana' which is contained within this thesis.Understanding the physiological traits that trees utilize to manage water use can reveal important insights into how and why they occur in their realized habitat. Among the tools to investigate these traits include measuring trees' seasonal water status, deciphering trees' rooting depth, and measuring the trees' vulnerability to cavitation. However, it is equally important to understand how complex landscape heterogeneity will affect both the inter- and intraspecific variation of these physiological traits. This thesis seeks to quantify the variation of the physiological traits used to manage water status among three common Rocky Mountain conifers; Pseudotsuga menziesii (a plastic species occurring across xeric and mesic sites), Pinus ponderosa (a xeric species), and Picea engelmannii (a mesic species), occurring across an elevation gradient. Furthermore, it aims to link tree maintenance of water status to source water, and understand how the importance of source water is reflected in the tree ring record. In the first chapter, I sought to quantify inter- and intraspecific variation of these three species by measuring diurnal and seasonal water status, seasonal water use, and xylem vulnerability to cavitation at a low elevation xeric site, composed of P. ponderosa and P. menziesii, and a high elevation mesic site, composed of P. engelmannii and P. menziesii. We found good evidence for interspecific variation in the physiological traits to manage water status at both sites. However, we did not find strong evidence for intraspecific variation in these same traits within our plastic species (P. menziesii). In the second chapter, we investigated how stable isotopes in tree rings reflected seasonal source water use as well as the atmospheric conditions the trees were growing under. At the low elevation site, we found evidence that the tree ring isotopes were likely reflecting both seasonal precipitation inputs as well as the atmospheric growing conditions. At the high elevation site, trees likely only reflected the atmospheric growing conditions and did not reflect seasonal water use.Item Red squirrel (Tamiasciurus hudsonicus) midden site selection and the influence of conifer species compositions on midden occurrence in the Cooke City Basin of Montana(Montana State University - Bozeman, College of Agriculture, 2017) Elkins, Eric Kyle; Chairperson, Graduate Committee: Bok SowellThroughout the Greater Yellowstone Ecosystem (GYE), whitebark pine (WBP: Pinus albicaulis) seeds serve as an important fall food source for threatened Yellowstone grizzly bears (Ursus arctos). Grizzly bears depend on red squirrel (Tamiasciurus hudsonicus) midden sites to obtain WBP seeds. In light of recent WBP population declines, managers are concerned about the negative effects that loss of WBP may have on grizzly bears. Therefore, managing WBP for grizzly bears is facilitated by understanding red squirrel habitat requirements and identifying areas that are most likely to contain middens. Previous studies indicate that red squirrel middens are most prevalent in subalpine mixed conifer forests with interspersed WBP, but a critical gap remains in identifying a conifer species composition that is ideal for midden sites. We studied red squirrel habitat selection in the Cooke City Basin (CCB) of Montana to identify variables associated with midden sites and midden area. We also examined conifer species compositions to identify a composition where middens are most likely to occur. Habitat variables, midden counts, and midden area measurements were collected in 810, 30 meter diameter circular plots equally spaced along 27 transect lines in the CCB. General linear mixed models (GLMM) were used to assess variables associated with red squirrel midden site selection, and linear mixed models (LMM) were used to assess variables associated with midden area. Results of the GLMM indicated that red squirrel midden occurrence probability is positively associated with the amount of hillshade (light) and canopy cover in a conifer stand. Additionally, midden occurrence increased as the percent WBP in a stand increased up to 44 percent, but decreased thereafter. Results of the LMM indicated that midden area is positively associated with total canopy cover. We identified that a conifer species composition of approximately 44 percent WBP and a 56 percent mixture of subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) is ideal for midden sites. We concluded that managing for areas within subalpine zone mixed conifer forests containing similar compositions should be a priority to ensure availability of prime habitat for midden sites and associated WBP seeds for grizzly bears.Item Conifer transpiration in a montane watershed : environmental controls and methodological uncertainties(Montana State University - Bozeman, College of Letters & Science, 2015) Looker, Nathaniel Thomas; Chairperson, Graduate Committee: Jia Hu; Justin Martin, Zachary Hoylman, Kelsey Jencso and Jia Hu were co-authors of the article, 'Topographic mediation of sap flow dynamics in a forested watershed' submitted to the journal 'Ecohydrology' which is contained within this thesis.; Justin Martin, Kelsey Jencso and Jia Hu were co-authors of the article, 'Uncertainty in conifer transpiration due to sapwood traits' submitted to the journal 'Agricultural and Forest Meteorology' which is contained within this thesis.Transpiration, the evaporative loss of water from vegetation through stomata, is intertwined with biological, meteorological, and hydrologic processes from the scale of leaves to continents. Among the methods of estimating transpiration, thermometric sap flow sensors have gained favor due to their high temporal resolution and capacity to integrate across sub-canopy variability. Heterogeneity in growing conditions through space and time may contribute substantial uncertainty to sap flow-based inferences; hence, quantification of spatiotemporal variability in sap flow dynamics and plant physiological traits is required to improve confidence in transpiration estimates. In this study, I employ sap flow sensors to address the environmental sensitivity of water use dynamics in conifers growing in contrasting topographic positions (elevations, aspects, and hillslope positions) within a watershed; additionally, I explore how variability in sapwood traits may contribute to uncertainty in sap flow-based estimates of whole-tree transpiration. To these ends, I measured sap flow in 32 trees of 5 species throughout the 2014 growing season and estimated sapwood traits in those individuals and in co-occurring trees. I related sap flow dynamics to environmental variables (vapor pressure deficit and soil moisture) through lag analyses and nonlinear regression. A Monte Carlo-based simulation and mixed effects variance decomposition served to quantify the sensitivity of transpiration to variability in sapwood traits and the magnitude of that variability across ecological scales. The response of sap flow to vapor pressure deficit varied with species and across topographic positions, and sap flow dynamics diverged spatially as soil moisture declined during the summer. The simulation suggested that a common assumption (constant wood thermal diffusivity) could impart up to 100% overestimation of transpiration in trees with sapwood properties similar to those I observed. Taken together, these findings underscore the importance of sampling sap flow and sapwood traits at the spatiotemporal scale for which inferences are to be drawn.Item Inhibition of conifers growing under a deciduous canopy : degrees, seasonality and causes of suppression(Montana State University - Bozeman, College of Letters & Science, 2009) Dickman, Garrett Joseph; Chairperson, Graduate Committee: Theodore W. WeaverThe physiological response of conifers to a deciduous overstory is unstudied in the cool temperate zone despite the widespread occurrence of the association. The object of this study is therefore to determine the degree that understory conifers are inhibited by a deciduous overstory, and to identify the factors responsible. Thus, three conifer species (Juniperus scopulorum, Pseudotsuga menziesii, Abies lasiocarpa) growing with and without a deciduous Populus canopy were contrasted in regards to environment, seasonal performance, and physiological response to environmental factors. Understory conifer stands were darker, and especially so when the overstory had leaves. Understory environments of P. menziesii and A. lasiocarpa were cooler and had lower vapor pressure deficits than open sites, but were similar in relative humidity and soil water. In contrast, understory sites of J. scopulorum had similar temperature, vapor pressure deficit, and soil water. Relative to conifers in the open, photosynthesis of understory conifers was reduced by roughly the same amount (~10%), regardless of photosynthetic rate. While at intermittent times photosynthesis was reduced by much greater amounts, the expected increase of summertime suppression (>10%) was not seen. Understory conifer suppression was not linked to any seasonally limited resource, such as light, water, or nutrients. The slight suppression over the year was due to an unidentified factor, perhaps the lower temperature of the understory environment (-2C), or seemingly unlikely, an allelopathic effect from Populus. In instantaneous measures, photosynthesis was better correlated with day of year and RH than light, VPD, or temperature. The correlated phenomena relate to season and precipitation rather than canopy condition under study. Slight photosynthetic inhibition of understory conifers is supported by similar observations of suppression in the diameter growth and one-year's twig growth.