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• 11. [Article] Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests

  Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For ...

  Citation × Citation Citation Abstract Copy Title: Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests Author: Priebe, Jim E. Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For much of the 20th century, forest management on public lands emphasized timber production through the use of even-aged management practices. There has been increasing recognition, however, that traditional even-aged management approaches are unable to support species that rely on the complex, heterogeneous structures provided by old-growth forests. In response, public forest managers have redirected their focus to developing more ecologically sustainable forests capable of meeting a broad array of objectives including an increasing emphasis on the development of late-seral and old-growth characteristics. Thinning has been identified as a promising method for promoting late-seral characteristics in managed stands. Recent long-term studies have shown that thinning stands does indeed accelerate the development of at least some late-seral structure characteristics, particularly when varying levels of thinning intensity and non-uniform retention patterns are incorporated into silvicultural prescriptions. Likewise, thinning has also shown some ability to increase the abundance of late-seral associated plant species in the understory. The impacts of thinning on vegetation dynamics are complicated by external factors such as natural disturbance events and the influence of pre-treatment vegetation on post-treatment communities. Within the context of managing for late-seral attributes, thinning is used to imitate natural disturbance processes. However, this does not preclude the occurrence of natural disturbances, which may either disrupt or compound treatment effects. Initial site conditions create another potential complication for the development of late-seral attributes by limiting the potential for change in understory communities. While some studies have shown that thinning improves late-seral plant abundance, others have found that the legacy of pre-treatment vegetation has a stronger impact on post-treatment communities. This study focused on the impacts of ice storm disturbance and pre-treatment vegetation on the understory of mature Douglas-fir forests using the ongoing Mature Forest Study (MFS), a long-term silvicultural experiment evaluating the effects of thinning and understory vegetation management treatments, as a framework. The first study examined the impact of an ice storm (glaze disturbance) on planted understory trees. Specifically, I looked at the effect of understory tree species, tree size, and overstory neighborhood environment on the type (bending, crown loss), source (ice loading, falling debris), and severity of damage experienced by planted understory trees at one of the MFS sites. Tree species, size, and overstory environment all affected the amount of understory glaze damage. Frequency and severity of damage both varied among underplanted tree species. In general, smaller trees were more prone to being bent, while larger trees were more susceptible to crown loss. The Douglas-fir component of the overstory provided enough additional sheltering that the increased risk to understory trees from falling debris was balanced by a corresponding decrease in the odds of damage by ice loading. This was not the case for the hardwood component; increasing risk of damage to understory trees from falling debris with increasing hardwood basal area drove an overall increase in the risk of understory damage as hardwood basal area increased. This study suggests that species, tree size, and overstory environment all need to be considered by managers hoping to reduce glaze damage risk to younger cohorts in multi-aged stands. The second study investigated the impacts of thinning intensity and herbicide application on the long-term (20-year post-treatment) development of understory vegetation communities on both of the MFS sites. Trends were examined with a focus on the ability of herbicide application, in concert with thinning treatments, to reduce the legacy of common pre-treatment species and promote the abundance of late-seral associates. Results indicated that both thinning intensity and herbicide application affected 20-year changes in understory plant community composition. Herbicide application was associated with a decrease in the abundance of common pre-treatment species, suggesting that it did reduce the legacy effect. However, this was not associated with any change in the abundance of late-seral species. While light thinning showed some ability to mitigate decline in late-seral species relative to higher intensity thinnings, there was no evidence of treatment interaction with herbicide application. These results suggest that while managers may be able to reduce the influence of initial site conditions on post-treatment vegetation communities, the use of herbicides offers little control over the successional trajectory of the understory. Light thinning appears to be the most effective means of increasing late-seral species abundance, although the use of herbicides to meet other management objectives is not contraindicated by the results of this study. Overall, these results suggest that the best options available to managers to both reduce glaze disturbance impacts to understory trees and hasten the development of late-seral plant communities are heavy thinning with unmanaged leave patches to provide late-seral refugia, or light thinning with gaps to provide growing space for better tree regeneration. Title: Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests Author: Priebe, Jim E. Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For much of the 20th century, forest management on public lands emphasized timber production through the use of even-aged management practices. There has been increasing recognition, however, that traditional even-aged management approaches are unable to support species that rely on the complex, heterogeneous structures provided by old-growth forests. In response, public forest managers have redirected their focus to developing more ecologically sustainable forests capable of meeting a broad array of objectives including an increasing emphasis on the development of late-seral and old-growth characteristics. Thinning has been identified as a promising method for promoting late-seral characteristics in managed stands. Recent long-term studies have shown that thinning stands does indeed accelerate the development of at least some late-seral structure characteristics, particularly when varying levels of thinning intensity and non-uniform retention patterns are incorporated into silvicultural prescriptions. Likewise, thinning has also shown some ability to increase the abundance of late-seral associated plant species in the understory. The impacts of thinning on vegetation dynamics are complicated by external factors such as natural disturbance events and the influence of pre-treatment vegetation on post-treatment communities. Within the context of managing for late-seral attributes, thinning is used to imitate natural disturbance processes. However, this does not preclude the occurrence of natural disturbances, which may either disrupt or compound treatment effects. Initial site conditions create another potential complication for the development of late-seral attributes by limiting the potential for change in understory communities. While some studies have shown that thinning improves late-seral plant abundance, others have found that the legacy of pre-treatment vegetation has a stronger impact on post-treatment communities. This study focused on the impacts of ice storm disturbance and pre-treatment vegetation on the understory of mature Douglas-fir forests using the ongoing Mature Forest Study (MFS), a long-term silvicultural experiment evaluating the effects of thinning and understory vegetation management treatments, as a framework. The first study examined the impact of an ice storm (glaze disturbance) on planted understory trees. Specifically, I looked at the effect of understory tree species, tree size, and overstory neighborhood environment on the type (bending, crown loss), source (ice loading, falling debris), and severity of damage experienced by planted understory trees at one of the MFS sites. Tree species, size, and overstory environment all affected the amount of understory glaze damage. Frequency and severity of damage both varied among underplanted tree species. In general, smaller trees were more prone to being bent, while larger trees were more susceptible to crown loss. The Douglas-fir component of the overstory provided enough additional sheltering that the increased risk to understory trees from falling debris was balanced by a corresponding decrease in the odds of damage by ice loading. This was not the case for the hardwood component; increasing risk of damage to understory trees from falling debris with increasing hardwood basal area drove an overall increase in the risk of understory damage as hardwood basal area increased. This study suggests that species, tree size, and overstory environment all need to be considered by managers hoping to reduce glaze damage risk to younger cohorts in multi-aged stands. The second study investigated the impacts of thinning intensity and herbicide application on the long-term (20-year post-treatment) development of understory vegetation communities on both of the MFS sites. Trends were examined with a focus on the ability of herbicide application, in concert with thinning treatments, to reduce the legacy of common pre-treatment species and promote the abundance of late-seral associates. Results indicated that both thinning intensity and herbicide application affected 20-year changes in understory plant community composition. Herbicide application was associated with a decrease in the abundance of common pre-treatment species, suggesting that it did reduce the legacy effect. However, this was not associated with any change in the abundance of late-seral species. While light thinning showed some ability to mitigate decline in late-seral species relative to higher intensity thinnings, there was no evidence of treatment interaction with herbicide application. These results suggest that while managers may be able to reduce the influence of initial site conditions on post-treatment vegetation communities, the use of herbicides offers little control over the successional trajectory of the understory. Light thinning appears to be the most effective means of increasing late-seral species abundance, although the use of herbicides to meet other management objectives is not contraindicated by the results of this study. Overall, these results suggest that the best options available to managers to both reduce glaze disturbance impacts to understory trees and hasten the development of late-seral plant communities are heavy thinning with unmanaged leave patches to provide late-seral refugia, or light thinning with gaps to provide growing space for better tree regeneration. Close

• 12. [Article] Fertilization impacts on Swiss needle cast disease severity in western Oregon

  This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/forest-ecology-and-management/. To the best of our knowledge, one ...

  Citation × Citation Citation Abstract Copy Title: Fertilization impacts on Swiss needle cast disease severity in western Oregon Author: Shaw, David C., Maguire, Douglas A., Mulvey, Robin L. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/forest-ecology-and-management/. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. Title: Fertilization impacts on Swiss needle cast disease severity in western Oregon Author: Shaw, David C., Maguire, Douglas A., Mulvey, Robin L. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/forest-ecology-and-management/. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. Close

• 13. [Article] Sources of Variation in the Self-Thinning Boundary Line for Three Species with Varying Levels of Shade Tolerance

  The species self-thinning boundary line has been widely analyzed with a variety of statistical techniques. Most previous studies in the forestry literature have reported that the relationship does not ...

  Citation × Citation Citation Abstract Copy Title: Sources of Variation in the Self-Thinning Boundary Line for Three Species with Varying Levels of Shade Tolerance Author: Temesgen, Hailemariam, Weiskittel, Aaron, Gould, Peter The species self-thinning boundary line has been widely analyzed with a variety of statistical techniques. Most previous studies in the forestry literature have reported that the relationship does not differ across a range of stand and site factors, but these studies have primarily used statistical techniques that make model fitting subjective or interpretation of covariate significance difficult. There is growing evidence that the use of stochastic frontier analysis is an effective statistical means for objectively fitting the species self-thinning boundary line, offering the opportunity to test the influence of additional covariates. Using extensive even-aged coastal Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco), western hemlock (Tsuga heterophylla), and red alder (Alnus rubra Bong.) data sets in the Pacific Northwest, we examined the assumption that the intercept and slope of the species self-thinning boundary line are insensitive to stand and site factors. Likelihood ratio tests indicated that site index, stand origin (natural versus planted), and purity (proportion of basal area in the primary species) significantly influenced the species self-thinning boundary line intercept for each of the species examined in this study. In Douglas-fir and western hemlock, the slope of the self-thinning boundary was also dependent on stand origin as well as site index in the case of Douglas-fir. Fertilization did not significantly influence the intercept or slope of the species self-thinning line for Douglas-fir and western hemlock. In addition, the inclusion of site aspect and dryness index marginally improved the red alder model of self-thinning, but neither site soil nor climatic variables improved the models for western hemlock and Douglas-fir. Thus, the species self-thinning boundary line can vary significantly among stands, and its variation is driven by several distinct factors; stochastic frontier analysis is an effective tool for identifying these factors. Title: Sources of Variation in the Self-Thinning Boundary Line for Three Species with Varying Levels of Shade Tolerance Author: Temesgen, Hailemariam, Weiskittel, Aaron, Gould, Peter The species self-thinning boundary line has been widely analyzed with a variety of statistical techniques. Most previous studies in the forestry literature have reported that the relationship does not differ across a range of stand and site factors, but these studies have primarily used statistical techniques that make model fitting subjective or interpretation of covariate significance difficult. There is growing evidence that the use of stochastic frontier analysis is an effective statistical means for objectively fitting the species self-thinning boundary line, offering the opportunity to test the influence of additional covariates. Using extensive even-aged coastal Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco), western hemlock (Tsuga heterophylla), and red alder (Alnus rubra Bong.) data sets in the Pacific Northwest, we examined the assumption that the intercept and slope of the species self-thinning boundary line are insensitive to stand and site factors. Likelihood ratio tests indicated that site index, stand origin (natural versus planted), and purity (proportion of basal area in the primary species) significantly influenced the species self-thinning boundary line intercept for each of the species examined in this study. In Douglas-fir and western hemlock, the slope of the self-thinning boundary was also dependent on stand origin as well as site index in the case of Douglas-fir. Fertilization did not significantly influence the intercept or slope of the species self-thinning line for Douglas-fir and western hemlock. In addition, the inclusion of site aspect and dryness index marginally improved the red alder model of self-thinning, but neither site soil nor climatic variables improved the models for western hemlock and Douglas-fir. Thus, the species self-thinning boundary line can vary significantly among stands, and its variation is driven by several distinct factors; stochastic frontier analysis is an effective tool for identifying these factors. Close

• 14. [Article] Impacts of Swiss needle cast on overstory Douglas-fir forests of the western Oregon Coast Range

  Article appears in Forest Ecology and Management (http://www.elsevier.com/wps/find/journaldescription.cws_home/503310/description#description) and is copyrighted by Elsevier.

  Citation × Citation Citation Abstract Copy Title: Impacts of Swiss needle cast on overstory Douglas-fir forests of the western Oregon Coast Range Author: Stone, Jeffrey K., Black, Bryan A., Shaw, David C. Article appears in Forest Ecology and Management (http://www.elsevier.com/wps/find/journaldescription.cws_home/503310/description#description) and is copyrighted by Elsevier. Title: Impacts of Swiss needle cast on overstory Douglas-fir forests of the western Oregon Coast Range Author: Stone, Jeffrey K., Black, Bryan A., Shaw, David C. Article appears in Forest Ecology and Management (http://www.elsevier.com/wps/find/journaldescription.cws_home/503310/description#description) and is copyrighted by Elsevier. Close

• 15. [Article] Temporal effect of vegetation management on growth and wood quality of conifers in a western Oregon plantation

  Concern over the increasing proportion of juvenile wood grown in second growth plantations has led to a large amount of research on the effects of common silvicultural practices on wood quality. Lacking ...

  Citation × Citation Citation Abstract Copy Title: Temporal effect of vegetation management on growth and wood quality of conifers in a western Oregon plantation Author: Goracke, Heidi S. Roe Concern over the increasing proportion of juvenile wood grown in second growth plantations has led to a large amount of research on the effects of common silvicultural practices on wood quality. Lacking is research on the effect of timing and duration of vegetation control on wood quality near the pith of young trees. This study was designed to quantify differences in specific gravity, ring width, percent latewood, biomass increment and eight-year diameter growth of Douglas-fir seedlings among different vegetation control regimes applied over the first five years of stand establishment. Largest year-eight volumes were observed in plots having five consecutive years of vegetation control, resulting in 239% more volume than control plots with no vegetation control. Mean specific gravity values ranged from 0.41 to 0.45 in rings 3, 4, and 5 from the pith and from 0.37 to 0.41 in rings 6, 7, and 8, with no significant treatment effect. Average ring width increased with increasing years of vegetation control for both ring segments, reaching consistent lengths of approximately 30 mm after three years of initial treatment. Percent latewood decreased with increasing years of vegetation control, ranging from 3.73% to11.13% in rings 3, 4, and 5. No significant treatment effect was observed for percent latewood in rings 6, 7, and 8, indicating consistent ring width production over time. Biomass increment was significantly affected by treatment, increasing by a maximum of 376% with increasing intensity of vegetation control. The lack of significant differences in specific gravity among treatments and the significant gains in volume and biomass from a greater intensity of vegetation control suggests a lack of adverse impacts on wood quality. Future silvicultural treatment and age at the time of harvest will likely have a more significant influence on the end-use quality of the wood. Title: Temporal effect of vegetation management on growth and wood quality of conifers in a western Oregon plantation Author: Goracke, Heidi S. Roe Concern over the increasing proportion of juvenile wood grown in second growth plantations has led to a large amount of research on the effects of common silvicultural practices on wood quality. Lacking is research on the effect of timing and duration of vegetation control on wood quality near the pith of young trees. This study was designed to quantify differences in specific gravity, ring width, percent latewood, biomass increment and eight-year diameter growth of Douglas-fir seedlings among different vegetation control regimes applied over the first five years of stand establishment. Largest year-eight volumes were observed in plots having five consecutive years of vegetation control, resulting in 239% more volume than control plots with no vegetation control. Mean specific gravity values ranged from 0.41 to 0.45 in rings 3, 4, and 5 from the pith and from 0.37 to 0.41 in rings 6, 7, and 8, with no significant treatment effect. Average ring width increased with increasing years of vegetation control for both ring segments, reaching consistent lengths of approximately 30 mm after three years of initial treatment. Percent latewood decreased with increasing years of vegetation control, ranging from 3.73% to11.13% in rings 3, 4, and 5. No significant treatment effect was observed for percent latewood in rings 6, 7, and 8, indicating consistent ring width production over time. Biomass increment was significantly affected by treatment, increasing by a maximum of 376% with increasing intensity of vegetation control. The lack of significant differences in specific gravity among treatments and the significant gains in volume and biomass from a greater intensity of vegetation control suggests a lack of adverse impacts on wood quality. Future silvicultural treatment and age at the time of harvest will likely have a more significant influence on the end-use quality of the wood. Close

• 16. [Article] Using Remote Sensing and Process-based Growth Modeling to Predict Forest Productivity Across Western Oregon

  Accurate measurement of forest productivity is fundamental to understand the carbon balance of forested ecosystems. Recent changes in climate highlight the importance of developing methods to measure ...

  Citation × Citation Citation Abstract Copy Title: Using Remote Sensing and Process-based Growth Modeling to Predict Forest Productivity Across Western Oregon Author: McAdam, Erick Accurate measurement of forest productivity is fundamental to understand the carbon balance of forested ecosystems. Recent changes in climate highlight the importance of developing methods to measure forest productivity so that sound economic and environmental decision can be made. Efforts to measure forest productivity across the landscape using remote sensing suffer from limitations inherent in individual sensors. We combine approaches to estimate forest productivity on Douglas-fir plantations using optical passive remote sensing to estimate stand ages, single-pass LiDAR sampling to estimate structural properties, and a process-based growth model (3PG) to assess variation in soil properties across western Oregon. Our results indicate that it is possible to estimate both site index and difficult-to-obtain soil properties across the landscape using two types of remote sensing instruments in combination with modeling. While we believe that the approach is sound, we found that the quality of extrapolated climatic data very much affects the derived values of soil properties assessed by modeling. In contrast, estimates of maximum leaf area index with LiDAR proved a reliable independent means to assess site productivity because plantations reach maximum values in less than two decades after establishment. Title: Using Remote Sensing and Process-based Growth Modeling to Predict Forest Productivity Across Western Oregon Author: McAdam, Erick Accurate measurement of forest productivity is fundamental to understand the carbon balance of forested ecosystems. Recent changes in climate highlight the importance of developing methods to measure forest productivity so that sound economic and environmental decision can be made. Efforts to measure forest productivity across the landscape using remote sensing suffer from limitations inherent in individual sensors. We combine approaches to estimate forest productivity on Douglas-fir plantations using optical passive remote sensing to estimate stand ages, single-pass LiDAR sampling to estimate structural properties, and a process-based growth model (3PG) to assess variation in soil properties across western Oregon. Our results indicate that it is possible to estimate both site index and difficult-to-obtain soil properties across the landscape using two types of remote sensing instruments in combination with modeling. While we believe that the approach is sound, we found that the quality of extrapolated climatic data very much affects the derived values of soil properties assessed by modeling. In contrast, estimates of maximum leaf area index with LiDAR proved a reliable independent means to assess site productivity because plantations reach maximum values in less than two decades after establishment. Close

• 17. [Article] Approaches for Characterizing Plant Physiological Responses to Environmental Stress

  It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, ...

  Citation × Citation Citation Abstract Copy Title: Approaches for Characterizing Plant Physiological Responses to Environmental Stress Author: Marias, Danielle E. It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, and responses to stress vary by functional type and developmental stage. Thus, it is crucial to investigate physiological responses to heat and drought stress on multiple species, populations, and growth stages with diverse approaches. In this dissertation, I employ a suite of physiological and modeling methods to inform our knowledge of plant physiological responses to environmental stress in Coffea arabica saplings, Pseudotsuga mensizeii (PSME) and Pinus ponderosa (PIPO) seedlings, and old-growth PIPO. In Chapter 2, I evaluate the effect of leaf age and methodology on the thermotolerance or heat tolerance of C. arabica leaf discs using chlorophyll fluorescence and electrolyte leakage methods. I found that mature leaves were more heat tolerant than expanding leaves, longer time between temperature exposure and measurement yielded more accurate thermotolerance assessments, and photochemistry was more heat-sensitive than cell membranes. To complement the second chapter investigating heat stress responses on detached leaf discs, Chapter 3 examines the effect of leaf age and heat stress duration (45 min or 90 min) on whole-plant physiological responses and capacity to recover in C. arabica by monitoring chlorophyll fluorescence (F[subscript V]/F[subscript M]), gas exchange, and foliar non-structural carbohydrate (NSC) dynamics in situ in response to a simulated heat wave (49°C) in a growth chamber. I found that the 90 min treatment resulted in greater photosynthetic damage and slower recovery than the 45 min treatment, expanding leaves recovered more slowly than in mature leaves, and both heat treatments inhibited flowering. A leaf energy balance model demonstrated that heat stress would be exacerbated by drought-induced stomatal closure. Heat treatment duration significantly impacted NSC dynamics that were closely related to reproduction and repair. Because seedling establishment governs species’ distributions, and because seedlings are particularly threatened by high temperatures at the soil surface, in Chapter 4 I examined the thermotolerance and heat stress responses of PIPO and PSME seedling populations from contrasting climates. Unexpectedly, I found that PSME was more heat tolerant the PIPO. I also monitored physiological recovery after exposure to a simulated heat wave (45°C) by measuring photosynthesis, F[subscript V]/F[subscript M], foliar NSC, and carbon stable isotope ratios (proxy for intrinsic water use efficiency, iWUE). Heat stress responses were consistent with phenotypic plasticity and reflected the conditions under which the plants were grown, while iWUE, a measure of potential drought resistance, was consistent with ecotypic differentiation and the climates from which the seedlings originated. To investigate responses to environmental stress on larger temporal and spatial scales without the challenges of making repeated physiological measurements on old-growth trees, in Chapter 5 I used long-term trajectories of tree-ring growth and carbon and oxygen isotopes of tree-ring cellulose (δ¹³Ccell, and δ¹⁸Ocell) to successfully predict the stand characteristics of two sets (upland, riparian) of old-growth PIPO using the Physiological Principles in Predicting Growth (3-PG) model, the δ¹³Ccell submodel, and a δ¹⁸Ocell submodel added by me. The expanded model helped to explain physiological drivers underlying the different tree-ring growth, δ¹³Ccell, and δ¹⁸Ocell trajectories measured at the upland and riparian sites. The combination of both δ¹⁸O and δ¹³Ccell submodels provided a useful and novel way to constrain 3-PG. This dissertation demonstrates an innovative strategy of applying diverse approaches to understand the physiological mechanisms behind vegetation responses to environmental stress. Title: Approaches for Characterizing Plant Physiological Responses to Environmental Stress Author: Marias, Danielle E. It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, and responses to stress vary by functional type and developmental stage. Thus, it is crucial to investigate physiological responses to heat and drought stress on multiple species, populations, and growth stages with diverse approaches. In this dissertation, I employ a suite of physiological and modeling methods to inform our knowledge of plant physiological responses to environmental stress in Coffea arabica saplings, Pseudotsuga mensizeii (PSME) and Pinus ponderosa (PIPO) seedlings, and old-growth PIPO. In Chapter 2, I evaluate the effect of leaf age and methodology on the thermotolerance or heat tolerance of C. arabica leaf discs using chlorophyll fluorescence and electrolyte leakage methods. I found that mature leaves were more heat tolerant than expanding leaves, longer time between temperature exposure and measurement yielded more accurate thermotolerance assessments, and photochemistry was more heat-sensitive than cell membranes. To complement the second chapter investigating heat stress responses on detached leaf discs, Chapter 3 examines the effect of leaf age and heat stress duration (45 min or 90 min) on whole-plant physiological responses and capacity to recover in C. arabica by monitoring chlorophyll fluorescence (F[subscript V]/F[subscript M]), gas exchange, and foliar non-structural carbohydrate (NSC) dynamics in situ in response to a simulated heat wave (49°C) in a growth chamber. I found that the 90 min treatment resulted in greater photosynthetic damage and slower recovery than the 45 min treatment, expanding leaves recovered more slowly than in mature leaves, and both heat treatments inhibited flowering. A leaf energy balance model demonstrated that heat stress would be exacerbated by drought-induced stomatal closure. Heat treatment duration significantly impacted NSC dynamics that were closely related to reproduction and repair. Because seedling establishment governs species’ distributions, and because seedlings are particularly threatened by high temperatures at the soil surface, in Chapter 4 I examined the thermotolerance and heat stress responses of PIPO and PSME seedling populations from contrasting climates. Unexpectedly, I found that PSME was more heat tolerant the PIPO. I also monitored physiological recovery after exposure to a simulated heat wave (45°C) by measuring photosynthesis, F[subscript V]/F[subscript M], foliar NSC, and carbon stable isotope ratios (proxy for intrinsic water use efficiency, iWUE). Heat stress responses were consistent with phenotypic plasticity and reflected the conditions under which the plants were grown, while iWUE, a measure of potential drought resistance, was consistent with ecotypic differentiation and the climates from which the seedlings originated. To investigate responses to environmental stress on larger temporal and spatial scales without the challenges of making repeated physiological measurements on old-growth trees, in Chapter 5 I used long-term trajectories of tree-ring growth and carbon and oxygen isotopes of tree-ring cellulose (δ¹³Ccell, and δ¹⁸Ocell) to successfully predict the stand characteristics of two sets (upland, riparian) of old-growth PIPO using the Physiological Principles in Predicting Growth (3-PG) model, the δ¹³Ccell submodel, and a δ¹⁸Ocell submodel added by me. The expanded model helped to explain physiological drivers underlying the different tree-ring growth, δ¹³Ccell, and δ¹⁸Ocell trajectories measured at the upland and riparian sites. The combination of both δ¹⁸O and δ¹³Ccell submodels provided a useful and novel way to constrain 3-PG. This dissertation demonstrates an innovative strategy of applying diverse approaches to understand the physiological mechanisms behind vegetation responses to environmental stress. Close

• 18. [Article] Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands

  Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose ...

  Citation × Citation Citation Abstract Copy Title: Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands Author: Nabel, Mark R. Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose of our research was to assess the establishment, survival, and growth of naturally-regenerated and underplanted conifers 10-13 years after overstory thinning and understory vegetation control in 50-year-old Douglas-fir stands. Two sites within the Oregon Coast Range were used in our study. One site was relatively dry (~130 cm of annual precipitation) and contained a mostly pure Douglas-fir overstory. The second site was moister (~175 cm of annual precipitation) and contained a Douglas-fir/western hemlock mixed-species overstory. At each site, stands were thinned to basal areas ranging from approximately 18 to 32 m2/ha using either a uniform or gappy thinning pattern. In the gappy treatment, 20% of the total area was comprised of 0.06- and 0.10-ha gaps. Understory vegetation was controlled across a portion of each thinning plot using a broadcast application of herbicides prior to thinning. After thinning, the following species were underplanted: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and grand fir (Abies grandis, dry site only). Establishment of natural regeneration and survival of underplanted conifers was greater for all species under lower overstory densities at the drier site, but on the moister site, where western hemlock regenerated prolifically in the understory, no differences were detected. At both sites, underplanted and dominant naturally-regenerated conifers growing under lower overstory retention levels were generally taller and had larger diameters after ten years. Thinning pattern had no effect on the establishment, survival, or growth of understory conifers at the overall stand level, but the tallest individuals were located within gaps. Controlling competing vegetation increased the rate of establishment for naturally-regenerated Douglas-fir and the rate of survival for planted Douglas-fir and western hemlock. Vegetation control generally resulted in larger underplanted seedlings of all species after ten years, but the size of dominant naturally-regenerated conifers was not affected. Shade-tolerant species generally outperformed Douglas-fir in understories except where western redcedar suffered heavy browsing damage. Natural regeneration of western hemlock may contribute to the development of an understory conifer layer when a seed source is present in the overstory, but in mostly pure Douglas-fir stands, underplanting would likely be required to supplement sporadic and slow-growing natural regeneration. In either case, future thinnings of rapidly closing overstory canopies would be required to maintain the long-term development of an understory conifer layer. Title: Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands Author: Nabel, Mark R. Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose of our research was to assess the establishment, survival, and growth of naturally-regenerated and underplanted conifers 10-13 years after overstory thinning and understory vegetation control in 50-year-old Douglas-fir stands. Two sites within the Oregon Coast Range were used in our study. One site was relatively dry (~130 cm of annual precipitation) and contained a mostly pure Douglas-fir overstory. The second site was moister (~175 cm of annual precipitation) and contained a Douglas-fir/western hemlock mixed-species overstory. At each site, stands were thinned to basal areas ranging from approximately 18 to 32 m2/ha using either a uniform or gappy thinning pattern. In the gappy treatment, 20% of the total area was comprised of 0.06- and 0.10-ha gaps. Understory vegetation was controlled across a portion of each thinning plot using a broadcast application of herbicides prior to thinning. After thinning, the following species were underplanted: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and grand fir (Abies grandis, dry site only). Establishment of natural regeneration and survival of underplanted conifers was greater for all species under lower overstory densities at the drier site, but on the moister site, where western hemlock regenerated prolifically in the understory, no differences were detected. At both sites, underplanted and dominant naturally-regenerated conifers growing under lower overstory retention levels were generally taller and had larger diameters after ten years. Thinning pattern had no effect on the establishment, survival, or growth of understory conifers at the overall stand level, but the tallest individuals were located within gaps. Controlling competing vegetation increased the rate of establishment for naturally-regenerated Douglas-fir and the rate of survival for planted Douglas-fir and western hemlock. Vegetation control generally resulted in larger underplanted seedlings of all species after ten years, but the size of dominant naturally-regenerated conifers was not affected. Shade-tolerant species generally outperformed Douglas-fir in understories except where western redcedar suffered heavy browsing damage. Natural regeneration of western hemlock may contribute to the development of an understory conifer layer when a seed source is present in the overstory, but in mostly pure Douglas-fir stands, underplanting would likely be required to supplement sporadic and slow-growing natural regeneration. In either case, future thinnings of rapidly closing overstory canopies would be required to maintain the long-term development of an understory conifer layer. Close

• 19. [Article] Disturbance and dynamics of riparian forests in southwestern Oregon

  Fire exclusion has been associated with structural and compositional changes in many upland forests of the western United States, but little is known about the impacts on riparian forests, portions of ...

  Citation × Citation Citation Abstract Copy Title: Disturbance and dynamics of riparian forests in southwestern Oregon Author: Messier, Michael S. Fire exclusion has been associated with structural and compositional changes in many upland forests of the western United States, but little is known about the impacts on riparian forests, portions of the landscape protected for habitat and water quality. For this study, I characterized the historic disturbance and tree recruitment processes of riparian forests in southwestern Oregon. I identified the ways in which riparian forest structure and composition have changed over the past 100 to 150 years, with a particular focus on the effects of fire exclusion. I used dendro-ecological methods to analyze tree ring data collected in riparian forest stands of two vegetation types in the Rogue River basin of southwestern Oregon: the mesic 'Mixed Conifer' zone (Pinus-Pseudotsuga-Calocedrus-Abies) and the more xeric 'Interior Valley' zone (Pinus-Quercus-Pseudotsuga). My results suggest riparian forests in both vegetation zones developed with frequent disturbance by fire and that, since fire has been excluded, these forests have entered a new successional trajectory. In the Mixed Conifer zone, tree recruitment in the pre-settlement period appears to have been driven by the interaction between climatic fluctuations and mixed-severity fires, which historically maintained multi-aged stands of shade-intolerant species. Patches of highseverity fire within the mixed-severity matrix created large canopy gaps (>30 m in diameter) in which new cohorts of Douglas-fir (Pseudotsuga menziesii) could establish within existing stands and perpetuate Douglas-fir overstory dominance. Tree recruitment appears to have been closely linked Since fire exclusion became effective, there has been an increase in tree density and a shift toward the establishment of white fir (Abies concolor), a fire-sensitive, shade-tolerant species. In the Interior Valley zone, riparian forests were most likely shaped by a low-severity fire regime. Frequent fires likely killed most tree seedlings and maintained open savannas or woodlands with shade-intolerant hardwoods and scattered, open-grown conifers. Fire exclusion in the Interior Valley zone has resulted in exceptionally high survival rates in Douglas-fir recruited during the late 19th and early 20th centuries. This led to the establishment of a closed-canopy conifer forest with fewer shade-intolerant hardwoods, which are an important source of food and habitat for wildlife. My findings suggest future management guidelines for riparian forests in fire-prone forests of the Pacific Northwest should consider the substantial and potentially undesirable ecological effects of the current hands-off management policy in riparian forests. Future proposals to attempt restoration of pre-settlement forest structure and composition in upland forests should also consider including restoration treatments within adjoining riparian forests. Title: Disturbance and dynamics of riparian forests in southwestern Oregon Author: Messier, Michael S. Fire exclusion has been associated with structural and compositional changes in many upland forests of the western United States, but little is known about the impacts on riparian forests, portions of the landscape protected for habitat and water quality. For this study, I characterized the historic disturbance and tree recruitment processes of riparian forests in southwestern Oregon. I identified the ways in which riparian forest structure and composition have changed over the past 100 to 150 years, with a particular focus on the effects of fire exclusion. I used dendro-ecological methods to analyze tree ring data collected in riparian forest stands of two vegetation types in the Rogue River basin of southwestern Oregon: the mesic 'Mixed Conifer' zone (Pinus-Pseudotsuga-Calocedrus-Abies) and the more xeric 'Interior Valley' zone (Pinus-Quercus-Pseudotsuga). My results suggest riparian forests in both vegetation zones developed with frequent disturbance by fire and that, since fire has been excluded, these forests have entered a new successional trajectory. In the Mixed Conifer zone, tree recruitment in the pre-settlement period appears to have been driven by the interaction between climatic fluctuations and mixed-severity fires, which historically maintained multi-aged stands of shade-intolerant species. Patches of highseverity fire within the mixed-severity matrix created large canopy gaps (>30 m in diameter) in which new cohorts of Douglas-fir (Pseudotsuga menziesii) could establish within existing stands and perpetuate Douglas-fir overstory dominance. Tree recruitment appears to have been closely linked Since fire exclusion became effective, there has been an increase in tree density and a shift toward the establishment of white fir (Abies concolor), a fire-sensitive, shade-tolerant species. In the Interior Valley zone, riparian forests were most likely shaped by a low-severity fire regime. Frequent fires likely killed most tree seedlings and maintained open savannas or woodlands with shade-intolerant hardwoods and scattered, open-grown conifers. Fire exclusion in the Interior Valley zone has resulted in exceptionally high survival rates in Douglas-fir recruited during the late 19th and early 20th centuries. This led to the establishment of a closed-canopy conifer forest with fewer shade-intolerant hardwoods, which are an important source of food and habitat for wildlife. My findings suggest future management guidelines for riparian forests in fire-prone forests of the Pacific Northwest should consider the substantial and potentially undesirable ecological effects of the current hands-off management policy in riparian forests. Future proposals to attempt restoration of pre-settlement forest structure and composition in upland forests should also consider including restoration treatments within adjoining riparian forests. Close

• 20. [Article] The effects of mechanical damage on residual coastal Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) following commercial thinning

  Commercial thinning operations can result in damage to residual stems. A literature review revealed that little was known about the effects of residual logging wounds with regard to rotation-age commercial ...

  Citation × Citation Citation Abstract Copy Title: The effects of mechanical damage on residual coastal Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) following commercial thinning Author: Kiser, James D. (James Donald) Commercial thinning operations can result in damage to residual stems. A literature review revealed that little was known about the effects of residual logging wounds with regard to rotation-age commercial conifers, particularly Douglas-fir. An experiment to examine fungal colonization of Douglas-fir following logging damage showed that while damage was significant in total numbers of fungi, there were no significant differences in the numbers of total fungal taxa, known decay taxa, or frequency of fungi related to radial distance inward. The study fails to support existing models of active tree response to invasion and instead corroborates the model of passive response to fungal invasion following damage to wood tissue. An examination of wood tissue following wounding supports the compartmentalization of damaged wood tissue as described by the CODIT model. However the lack of evidence of compartmentalized tissue in the presence of fungi in undamaged trees supports the idea of passive response mechanisms rather than an active defense response. Reductions in wood quality volume were quantified for damaged trees. Consideration of a selective milling option is shown to increase value in the lower log by as much as 8.5 percent. Results also show significant increases in sapwood distance along the undamaged side of the bole. However, there were no significant differences in radial increment or in sapwood area before or after damage suggesting that damaged trees may delay conversion of sapwood to heartwood to maintain necessary sapwood area for crown requirements. An experiment was conducted to determine if tree and stand-level differences in growth and yield could be detected between damaged and undamaged trees following damage. No significant differences were found for total height or DBH to total height curves. Examination of basal area increment before and after thinning showed no significant differences. No significant difference was found for live crown length except between the damaged and adjacent undamaged trees; however, this may be explained by differences in diameter between the two groups. No significant differences were found for height to crown base or the height to crown base / tree diameter relationship. Title: The effects of mechanical damage on residual coastal Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) following commercial thinning Author: Kiser, James D. (James Donald) Commercial thinning operations can result in damage to residual stems. A literature review revealed that little was known about the effects of residual logging wounds with regard to rotation-age commercial conifers, particularly Douglas-fir. An experiment to examine fungal colonization of Douglas-fir following logging damage showed that while damage was significant in total numbers of fungi, there were no significant differences in the numbers of total fungal taxa, known decay taxa, or frequency of fungi related to radial distance inward. The study fails to support existing models of active tree response to invasion and instead corroborates the model of passive response to fungal invasion following damage to wood tissue. An examination of wood tissue following wounding supports the compartmentalization of damaged wood tissue as described by the CODIT model. However the lack of evidence of compartmentalized tissue in the presence of fungi in undamaged trees supports the idea of passive response mechanisms rather than an active defense response. Reductions in wood quality volume were quantified for damaged trees. Consideration of a selective milling option is shown to increase value in the lower log by as much as 8.5 percent. Results also show significant increases in sapwood distance along the undamaged side of the bole. However, there were no significant differences in radial increment or in sapwood area before or after damage suggesting that damaged trees may delay conversion of sapwood to heartwood to maintain necessary sapwood area for crown requirements. An experiment was conducted to determine if tree and stand-level differences in growth and yield could be detected between damaged and undamaged trees following damage. No significant differences were found for total height or DBH to total height curves. Examination of basal area increment before and after thinning showed no significant differences. No significant difference was found for live crown length except between the damaged and adjacent undamaged trees; however, this may be explained by differences in diameter between the two groups. No significant differences were found for height to crown base or the height to crown base / tree diameter relationship. Close