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• 21. [Article] Exploration of statistical methods for synthesizing the effects of variable-retention harvesting on multiple taxa

  Variable-retention harvesting was proposed to reduce loss of biodiversity and ecosystem processes associated with late-seral Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest. The Demonstration ...

  Citation × Citation Citation Abstract Copy Title: Exploration of statistical methods for synthesizing the effects of variable-retention harvesting on multiple taxa Author: Lam, Tzeng Yih Variable-retention harvesting was proposed to reduce loss of biodiversity and ecosystem processes associated with late-seral Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest. The Demonstration of Ecosystem Management Options experiment was established to test this hypothesis. Analysis presents various challenges to drawing statistical inferences about treatment effects. This dissertation explored novel statistical methods for understanding the response of multiple forest taxa to variable-retention harvesting. Excessive zero counts are common among terrestrial small mammal species that are captured infrequently. Zero-inflated and hurdle models are appealing tools for analyzing these data. A simulation was performed to understand the properties and robustness of these models. When true mean abundance was low, the estimated parameters from these models were highly unstable. Goodness of fit criteria could not discern among the processes generating the data. The Poisson and negative binomial Generalized Linear Models (GLMs) were fitted to four small mammal species with different rates of capture. Predictors included several variables representing vegetation structure. These models and overdispersed Poisson were then specified as Generalized Linear Mixed Models (GLMMs) to account for nesting and blocking in the experimental design. The fitted GLMs indicated that predictors were not consistent among models for the infrequently captured species. Differences in estimated coefficients between GLMs and GLMMs were noticeable. The overdispersed Poisson GLMM was suggested to be most suitable. Structural Equation Modeling (SEM) is suitable for modeling interactions of many cause-and-effect relationships in forest ecosystems. SEM was applied to understand overstory-understory relationships of late-seral herb species under mature forest conditions and immediately after variable-retention harvesting. In undisturbed forests, light attenuation, belowground competition and stand age were the primary drivers of late-seral herb cover. After variable-retention harvesting, microclimatic stresses were inferred to primarily affect late-seral species diversity and composition. Logging debris had little discernible effect on the change in the late-seral herb community. The explored statistical models complement conventional methods for studying the effects of variable-retention harvesting. These models address distributional issues of response data and provide further insight into the complex processes driving managed forest ecosystems. Future analyses should apply a suite of statistical models to gain different perspectives. Title: Exploration of statistical methods for synthesizing the effects of variable-retention harvesting on multiple taxa Author: Lam, Tzeng Yih Variable-retention harvesting was proposed to reduce loss of biodiversity and ecosystem processes associated with late-seral Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest. The Demonstration of Ecosystem Management Options experiment was established to test this hypothesis. Analysis presents various challenges to drawing statistical inferences about treatment effects. This dissertation explored novel statistical methods for understanding the response of multiple forest taxa to variable-retention harvesting. Excessive zero counts are common among terrestrial small mammal species that are captured infrequently. Zero-inflated and hurdle models are appealing tools for analyzing these data. A simulation was performed to understand the properties and robustness of these models. When true mean abundance was low, the estimated parameters from these models were highly unstable. Goodness of fit criteria could not discern among the processes generating the data. The Poisson and negative binomial Generalized Linear Models (GLMs) were fitted to four small mammal species with different rates of capture. Predictors included several variables representing vegetation structure. These models and overdispersed Poisson were then specified as Generalized Linear Mixed Models (GLMMs) to account for nesting and blocking in the experimental design. The fitted GLMs indicated that predictors were not consistent among models for the infrequently captured species. Differences in estimated coefficients between GLMs and GLMMs were noticeable. The overdispersed Poisson GLMM was suggested to be most suitable. Structural Equation Modeling (SEM) is suitable for modeling interactions of many cause-and-effect relationships in forest ecosystems. SEM was applied to understand overstory-understory relationships of late-seral herb species under mature forest conditions and immediately after variable-retention harvesting. In undisturbed forests, light attenuation, belowground competition and stand age were the primary drivers of late-seral herb cover. After variable-retention harvesting, microclimatic stresses were inferred to primarily affect late-seral species diversity and composition. Logging debris had little discernible effect on the change in the late-seral herb community. The explored statistical models complement conventional methods for studying the effects of variable-retention harvesting. These models address distributional issues of response data and provide further insight into the complex processes driving managed forest ecosystems. Future analyses should apply a suite of statistical models to gain different perspectives. Close

• 22. [Article] Height-related trends in structure and function of Douglas-fir foliage

  This dissertation investigated the impacts of tree height upon a range of physiological and structural characteristics of Douglas-fir foliage; relationships between structural and functional trends with ...

  Citation × Citation Citation Abstract Copy Title: Height-related trends in structure and function of Douglas-fir foliage Author: Woodruff, David R. This dissertation investigated the impacts of tree height upon a range of physiological and structural characteristics of Douglas-fir foliage; relationships between structural and functional trends with height; and compensatory mechanisms that mitigate height-related growth constraints. Height-related trends in foliar physiological and anatomical characteristics were examined both within trees as well as between trees of different heights. Emphasis was placed on discovering the mechanistic nature of the observed growth constraints and adaptive strategies. This research was conducted in the interest of enhancing our knowledge of the causes of age- and size-related decline in forest productivity. The research conducted for this dissertation explored several previously uninvestigated mechanisms for size-related reductions in forest productivity that are associated with tree foliar structure and function. Height-related trends in foliar turgor associated with the gravitational hydrostatic pressure gradient were identified as well as subsequent impacts on leaf and shoot morphology. Osmotic potential declined with height providing evidence of osmotic adjustment to offset the height-related decline in leaf turgor. This osmotic adjustment occurred only seasonally, and not during the spring when turgor maintenance is most important for leaf expansion. Gradients in leaf turgor were reflected in height-related trends in leaf and shoot morphology. Previous work has documented height-related trends in foliar morphological characteristics and earlier studies have examined osmotic adjustment as an adaptation to drought and salinity stress. Height-related trends in foliar morphological characteristics have typically been attributed to light gradients along vertical profiles. The work in this dissertation is the first to evaluate the effects of the gravitational component of water potential on the osmotic, morphological and growth characteristics of foliage along a height gradient in tall trees. Trends in leaf hydraulic efficiency and safety along a height gradient were examined, and correlations between changes in leaf hydraulic conductance (Kleaf) and changes in stomatal conductance (gs) were identified that provided evidence of a control mechanism for gs. A consistent relationship was discovered between changes in Kleaf and gs providing evidence that leaf tracheid embolism is associated with stomatal closure during periods of declining water availability. Hydraulic vulnerability curves of foliage collected at different heights indicated that increased height is correlated with reduced hydraulic efficiency and increased hydraulic safety. These opposing trends suggest the existence of a compensatory mechanism in foliage that functions to reduce hydraulic vulnerability at the expense of transport capacity. The research in this dissertation is the first to examine the dynamic relationships between leaf hydraulics and stomatal behavior in temperate conifers, and the first to examine how these characteristics are affected by tree height. Height-related trends in leaf xylem anatomical characteristics were examined and strong correlations were identified between these anatomical characteristics and the patterns of leaf hydraulic efficiency and safety that were observed in previous work for this dissertation. Theoretical estimates of leaf hydraulic efficiency (Kleaf-theoretical) based upon the Hagen-Pouseille equation and measured leaf tracheid anatomical characteristics were strongly correlated with laboratory measurements of Kleaf, providing further evidence of a causal relationship between height-related trends in both leaf tracheid anatomical properties and leaf hydraulic function. Earlier studies have documented connections between leaf anatomical characteristics and leaf physiological characteristics such as correlations between leaf architecture and gas exchange and leaf architecture and transport efficiency. The research in this dissertation however, is the first to examine the impact of tree height on leaf anatomical characteristics and associated shoot physiological properties. Trends in gas-exchange along a height gradient were identified, independent of the immediate effects of path length and gravitational resistance. Photosynthesis under ambient [CO2] declined with height in a manner that was consistent with height-related trends in mesophyll anatomy and independent of leaf nitrogen content. Analyses of mesophyll resistance from assimilation versus internal CO2 concentration (A-Ci) curves indicated that height-related trends in mesophyll resistance were correlated with trends in both photosynthesis and mesophyll anatomy along a height gradient. Analyses of integrated and instantaneous water use efficiency indicated that height-related trends in integrated water use efficiency are attributable to the effects of path length and gravity on stomatal behavior as opposed to a height-related trend in intrinsic foliar properties. The research in this dissertation is the first to isolate the influence of height-related trends in foliar structure on different gas exchange properties such as assimilation and mesophyll conductance, independent of the direct effects of vertical gradients in xylem tension on stomatal and photosynthetic physiology. Title: Height-related trends in structure and function of Douglas-fir foliage Author: Woodruff, David R. This dissertation investigated the impacts of tree height upon a range of physiological and structural characteristics of Douglas-fir foliage; relationships between structural and functional trends with height; and compensatory mechanisms that mitigate height-related growth constraints. Height-related trends in foliar physiological and anatomical characteristics were examined both within trees as well as between trees of different heights. Emphasis was placed on discovering the mechanistic nature of the observed growth constraints and adaptive strategies. This research was conducted in the interest of enhancing our knowledge of the causes of age- and size-related decline in forest productivity. The research conducted for this dissertation explored several previously uninvestigated mechanisms for size-related reductions in forest productivity that are associated with tree foliar structure and function. Height-related trends in foliar turgor associated with the gravitational hydrostatic pressure gradient were identified as well as subsequent impacts on leaf and shoot morphology. Osmotic potential declined with height providing evidence of osmotic adjustment to offset the height-related decline in leaf turgor. This osmotic adjustment occurred only seasonally, and not during the spring when turgor maintenance is most important for leaf expansion. Gradients in leaf turgor were reflected in height-related trends in leaf and shoot morphology. Previous work has documented height-related trends in foliar morphological characteristics and earlier studies have examined osmotic adjustment as an adaptation to drought and salinity stress. Height-related trends in foliar morphological characteristics have typically been attributed to light gradients along vertical profiles. The work in this dissertation is the first to evaluate the effects of the gravitational component of water potential on the osmotic, morphological and growth characteristics of foliage along a height gradient in tall trees. Trends in leaf hydraulic efficiency and safety along a height gradient were examined, and correlations between changes in leaf hydraulic conductance (Kleaf) and changes in stomatal conductance (gs) were identified that provided evidence of a control mechanism for gs. A consistent relationship was discovered between changes in Kleaf and gs providing evidence that leaf tracheid embolism is associated with stomatal closure during periods of declining water availability. Hydraulic vulnerability curves of foliage collected at different heights indicated that increased height is correlated with reduced hydraulic efficiency and increased hydraulic safety. These opposing trends suggest the existence of a compensatory mechanism in foliage that functions to reduce hydraulic vulnerability at the expense of transport capacity. The research in this dissertation is the first to examine the dynamic relationships between leaf hydraulics and stomatal behavior in temperate conifers, and the first to examine how these characteristics are affected by tree height. Height-related trends in leaf xylem anatomical characteristics were examined and strong correlations were identified between these anatomical characteristics and the patterns of leaf hydraulic efficiency and safety that were observed in previous work for this dissertation. Theoretical estimates of leaf hydraulic efficiency (Kleaf-theoretical) based upon the Hagen-Pouseille equation and measured leaf tracheid anatomical characteristics were strongly correlated with laboratory measurements of Kleaf, providing further evidence of a causal relationship between height-related trends in both leaf tracheid anatomical properties and leaf hydraulic function. Earlier studies have documented connections between leaf anatomical characteristics and leaf physiological characteristics such as correlations between leaf architecture and gas exchange and leaf architecture and transport efficiency. The research in this dissertation however, is the first to examine the impact of tree height on leaf anatomical characteristics and associated shoot physiological properties. Trends in gas-exchange along a height gradient were identified, independent of the immediate effects of path length and gravitational resistance. Photosynthesis under ambient [CO2] declined with height in a manner that was consistent with height-related trends in mesophyll anatomy and independent of leaf nitrogen content. Analyses of mesophyll resistance from assimilation versus internal CO2 concentration (A-Ci) curves indicated that height-related trends in mesophyll resistance were correlated with trends in both photosynthesis and mesophyll anatomy along a height gradient. Analyses of integrated and instantaneous water use efficiency indicated that height-related trends in integrated water use efficiency are attributable to the effects of path length and gravity on stomatal behavior as opposed to a height-related trend in intrinsic foliar properties. The research in this dissertation is the first to isolate the influence of height-related trends in foliar structure on different gas exchange properties such as assimilation and mesophyll conductance, independent of the direct effects of vertical gradients in xylem tension on stomatal and photosynthetic physiology. Close

• 23. [Article] Thinning effects on stand and tree growth : different perspectives on same old questions

  The search for the stand density that optimizes growth and hence generates the maximum amount of wood is one of the philosopher’s stone in modern forestry. Since scientific methods were applied to forestry, ...

  Citation × Citation Citation Abstract Copy Title: Thinning effects on stand and tree growth : different perspectives on same old questions Author: Romero Castaño, Pablo The search for the stand density that optimizes growth and hence generates the maximum amount of wood is one of the philosopher’s stone in modern forestry. Since scientific methods were applied to forestry, numerous generations of foresters have studied the relations between stocking density and growth, obtaining diverse results and structuring acquired knowledge into different principles that continue to be debated by supporters and detractors. Three main principles are: 1) any reduction in stand density entails a decreasing in the growing capacity, 2) growth is constant for a range of stand densities and, 3) progressive reductions in stand density increase growth up to an optimum, after which growth starts decreasing. Thinning is the traditional tool that foresters use to manage stand density. Consequently, most of growth-growing stock relations are based on data from thinning experiments. In addition to the potential effects of thinning on growth of residual trees, the erratic behavior of periodic annual increment routinely found in field studies may be contributed by measurement errors, imputation inaccuracies, variation in initial conditions, imprecise treatment implementation, irregular mortality, or variation in definition of density and stocking. These sources of variation in response to thinning lead to different results and different interpretations of conformity to the principles listed above. The Black Rock Unit of the George T. Gerlinger Experimental Forest in Oregon holds one of the largest and oldest Douglas-fir thinning experiments with the native region of this species. The characteristics of the stand, particularly the age when treatments were carried out and the initial dimensions of the trees, broaden our perspective to the on results from other studies in other areas and in younger stands of both Douglas-fir and other species. Reductions in stand density generally lead to a change in the expected growth of the stand, most typically a reduction in growth more or less commensurate with the reduction in stand density, as further controlled by the initial diameter, height, and crown length of the residual trees. The direct effect of thinning is that portion of the growth response that is systematically related to the type, intensity, and timing of the thinning and that is not explained by the relationship of growth to conventional stand, tree, and site predictor variables in unthinned stands. Accurate forecasting of tree and stand growth after thinning therefore requires quantification of the direct effects of thinning and its change over time since thinning. In an effort to isolate the direct effect of thinning treatments at the stand and tree level, from effects of conventional stand, tree, and site variables, regression techniques were applied to data from a set of thinned and unthinned stands. After an initial period of no response, limited response, or even "shock", thinning increased stand growth inversely proportional to the intensity of treatment (if stands were compared at the same initial conditions). The overall direct effect of thinning was consequently an initial decrease relative to the growth rate of an unthinned stand with the same initial conditions, but then an increase growth after several years. Direct effects at the stand level were the aggregate result of direct effects of thinning on the constituent trees. These tree-level direct effects were more complicated than has previously been assumed in growth models. Trees of different diameter classes (correlated with crown classes) differed in their direct response to thinning. In general, thinning had little direct effect on growth patterns in the largest diameter class (largest 28%), increased the growth performance in mid-sized trees (next largest 22% by diameter), and reduced initially the average growth of smallest trees (smallest 19% by diameter). However, most diameter classes under most thinning intensities recovered quickly after a few years and direct thinning effects led to greater growth that expected for the same initial conditions in unthinned stands. The intensity of thinning increased this positive direct response among all size classes. These results help to understand the factors that control growth in Douglas-fir stands. Likewise, quantification of direct growth effects attributable to thinning add to our knowledge base on tree and stand-level responses to thinning. In regard to the stated growth-growing stock principles, the Black Rock thinning trials seemed to support the principle that stand growth declines in direct proportion to thinning intensity. The eventual growth increase stimulated by the direct thinning effect did not compensate for the reduced growing stock imposed by thinning. Furthermore, little evidence was found for uniform growth across a wide range in initial stand density. Finally, analyses of individual trees and percentiles of the diameter distribution corresponding roughly to crown classes demonstrated that the direct effect of thinning within a stand is not uniform across tree size, implying that stand-level multipliers for the direct thinning effect might gainfully be replaced by a tree-level function of relative tree size as well as thinning intensity and time since thinning. Title: Thinning effects on stand and tree growth : different perspectives on same old questions Author: Romero Castaño, Pablo The search for the stand density that optimizes growth and hence generates the maximum amount of wood is one of the philosopher’s stone in modern forestry. Since scientific methods were applied to forestry, numerous generations of foresters have studied the relations between stocking density and growth, obtaining diverse results and structuring acquired knowledge into different principles that continue to be debated by supporters and detractors. Three main principles are: 1) any reduction in stand density entails a decreasing in the growing capacity, 2) growth is constant for a range of stand densities and, 3) progressive reductions in stand density increase growth up to an optimum, after which growth starts decreasing. Thinning is the traditional tool that foresters use to manage stand density. Consequently, most of growth-growing stock relations are based on data from thinning experiments. In addition to the potential effects of thinning on growth of residual trees, the erratic behavior of periodic annual increment routinely found in field studies may be contributed by measurement errors, imputation inaccuracies, variation in initial conditions, imprecise treatment implementation, irregular mortality, or variation in definition of density and stocking. These sources of variation in response to thinning lead to different results and different interpretations of conformity to the principles listed above. The Black Rock Unit of the George T. Gerlinger Experimental Forest in Oregon holds one of the largest and oldest Douglas-fir thinning experiments with the native region of this species. The characteristics of the stand, particularly the age when treatments were carried out and the initial dimensions of the trees, broaden our perspective to the on results from other studies in other areas and in younger stands of both Douglas-fir and other species. Reductions in stand density generally lead to a change in the expected growth of the stand, most typically a reduction in growth more or less commensurate with the reduction in stand density, as further controlled by the initial diameter, height, and crown length of the residual trees. The direct effect of thinning is that portion of the growth response that is systematically related to the type, intensity, and timing of the thinning and that is not explained by the relationship of growth to conventional stand, tree, and site predictor variables in unthinned stands. Accurate forecasting of tree and stand growth after thinning therefore requires quantification of the direct effects of thinning and its change over time since thinning. In an effort to isolate the direct effect of thinning treatments at the stand and tree level, from effects of conventional stand, tree, and site variables, regression techniques were applied to data from a set of thinned and unthinned stands. After an initial period of no response, limited response, or even "shock", thinning increased stand growth inversely proportional to the intensity of treatment (if stands were compared at the same initial conditions). The overall direct effect of thinning was consequently an initial decrease relative to the growth rate of an unthinned stand with the same initial conditions, but then an increase growth after several years. Direct effects at the stand level were the aggregate result of direct effects of thinning on the constituent trees. These tree-level direct effects were more complicated than has previously been assumed in growth models. Trees of different diameter classes (correlated with crown classes) differed in their direct response to thinning. In general, thinning had little direct effect on growth patterns in the largest diameter class (largest 28%), increased the growth performance in mid-sized trees (next largest 22% by diameter), and reduced initially the average growth of smallest trees (smallest 19% by diameter). However, most diameter classes under most thinning intensities recovered quickly after a few years and direct thinning effects led to greater growth that expected for the same initial conditions in unthinned stands. The intensity of thinning increased this positive direct response among all size classes. These results help to understand the factors that control growth in Douglas-fir stands. Likewise, quantification of direct growth effects attributable to thinning add to our knowledge base on tree and stand-level responses to thinning. In regard to the stated growth-growing stock principles, the Black Rock thinning trials seemed to support the principle that stand growth declines in direct proportion to thinning intensity. The eventual growth increase stimulated by the direct thinning effect did not compensate for the reduced growing stock imposed by thinning. Furthermore, little evidence was found for uniform growth across a wide range in initial stand density. Finally, analyses of individual trees and percentiles of the diameter distribution corresponding roughly to crown classes demonstrated that the direct effect of thinning within a stand is not uniform across tree size, implying that stand-level multipliers for the direct thinning effect might gainfully be replaced by a tree-level function of relative tree size as well as thinning intensity and time since thinning. Close

• 24. [Article] Interactions between ecosystem nitrogen and bedrock control long-term calcium sources in Oregon Coast Range forests

  Ecosystem nitrogen (N) supply strongly influences the availability and cycling of other essential nutrients in temperate forests, especially calcium (Ca). Short-term additions of N that exceed ecosystem ...

  Citation × Citation Citation Abstract Copy Title: Interactions between ecosystem nitrogen and bedrock control long-term calcium sources in Oregon Coast Range forests Author: Hynicka, Justin D. Ecosystem nitrogen (N) supply strongly influences the availability and cycling of other essential nutrients in temperate forests, especially calcium (Ca). Short-term additions of N that exceed ecosystem demands often increase dissolved nitrate fluxes and decrease soil pH, which can stimulate soil Ca loss. However, the long-term effects of high N supply on ecosystem Ca availability are more difficult to determine, and may depend on the Ca content of bedrock and mineral soils. To address this, we examined major and trace element concentrations and ⁸⁷Sr/⁸⁶Sr ratios that trace Ca sources in precipitation, foliage, soil pools, and bedrock at 24 forested sites in the Oregon Coast Range having a wide, natural range of soil N (0.16 - 0.97 % N, 0-10 cm) on contrasting basaltic and sedimentary bedrock. Using a suite of 17 site properties, we also evaluated whether soil N variation across sites was related to the five major state-factors of soil and ecosystem development: climate, organisms, topography, parent material, and time. We found that as soil N increased across sites, its ¹⁵N/¹⁴N ratio declined towards atmospheric values, suggesting that soil N variation reflects a biotic legacy of symbiotic N fixation inputs. In contrast, soil N variation was unrelated to 17 other metrics of soil forming factors that represented climate (mean annual precipitation, mean annual temperature, and distance from the coast), topography (slope, soil depth, and abundance of coarse rock fragments), parent material (within bedrock type bulk and 1 M HNO₃ leachable rock Ca chemistry), and proxies of soil age (Hurst's redness rating, effective cation exchange capacity, Ca in non-exchangeable soil residues, chemical index of alteration, weathering index of Parker, Ca in coarse soil fragments, and soil Ca loss relative to bedrock). These analyses highlight symbiotic N-fixing red alder as a keystone organismal state-factor that produces a wide range of soil N accumulation in coastal Oregon forests. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) and other geochemical analyses indicate that long-term Ca sources in foliage and exchangeable soil pools in Oregon Coast Range forests depend on an interactive effect between N availability and bedrock. Basaltic rocks contained nearly 20-times more Ca than sedimentary rocks across our sites, and this difference was reflected in Sr-isotope partitioning of base cation sources. Atmospheric sources dominated plant and soil pools in forests overlying Ca-poor sedimentary rock, regardless of variation in soil N, indicating extremely limited capacity of weathering to support forest Ca demands. In contrast, forests overlying basaltic rock obtained as much as 80% of Ca from rock weathering in low N sites, yet relied to a greater extent on atmospheric Ca as soil N increased, with less than 10% of Ca from rock weathering at sites with the highest soil N. Surprisingly, differences in fresh rock Ca content and base cation sources between sedimentary and basaltic sites was not reflected in ecosystem Ca availability, and instead increasing soil N caused similar declines in foliar and exchangeable Ca across both rock types. This illustrates that nutrient pool sizes do not necessarily reflect long-term nutrient supply, and highlights how coupled biogeochemical cycles within ecosystems can regulate nutrient loss and supply to biota. Broadly, our results highlight how interactions between biological and geologic factors can influence base cation sources in forest ecosystems. The sustainability of base cation supplies to forests may therefore depend greatly on variation in bedrock weathering at low N sites, yet converge to depend on atmospheric inputs in sites that receive high N loading from biological fixation or anthropogenic deposition. Title: Interactions between ecosystem nitrogen and bedrock control long-term calcium sources in Oregon Coast Range forests Author: Hynicka, Justin D. Ecosystem nitrogen (N) supply strongly influences the availability and cycling of other essential nutrients in temperate forests, especially calcium (Ca). Short-term additions of N that exceed ecosystem demands often increase dissolved nitrate fluxes and decrease soil pH, which can stimulate soil Ca loss. However, the long-term effects of high N supply on ecosystem Ca availability are more difficult to determine, and may depend on the Ca content of bedrock and mineral soils. To address this, we examined major and trace element concentrations and ⁸⁷Sr/⁸⁶Sr ratios that trace Ca sources in precipitation, foliage, soil pools, and bedrock at 24 forested sites in the Oregon Coast Range having a wide, natural range of soil N (0.16 - 0.97 % N, 0-10 cm) on contrasting basaltic and sedimentary bedrock. Using a suite of 17 site properties, we also evaluated whether soil N variation across sites was related to the five major state-factors of soil and ecosystem development: climate, organisms, topography, parent material, and time. We found that as soil N increased across sites, its ¹⁵N/¹⁴N ratio declined towards atmospheric values, suggesting that soil N variation reflects a biotic legacy of symbiotic N fixation inputs. In contrast, soil N variation was unrelated to 17 other metrics of soil forming factors that represented climate (mean annual precipitation, mean annual temperature, and distance from the coast), topography (slope, soil depth, and abundance of coarse rock fragments), parent material (within bedrock type bulk and 1 M HNO₃ leachable rock Ca chemistry), and proxies of soil age (Hurst's redness rating, effective cation exchange capacity, Ca in non-exchangeable soil residues, chemical index of alteration, weathering index of Parker, Ca in coarse soil fragments, and soil Ca loss relative to bedrock). These analyses highlight symbiotic N-fixing red alder as a keystone organismal state-factor that produces a wide range of soil N accumulation in coastal Oregon forests. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) and other geochemical analyses indicate that long-term Ca sources in foliage and exchangeable soil pools in Oregon Coast Range forests depend on an interactive effect between N availability and bedrock. Basaltic rocks contained nearly 20-times more Ca than sedimentary rocks across our sites, and this difference was reflected in Sr-isotope partitioning of base cation sources. Atmospheric sources dominated plant and soil pools in forests overlying Ca-poor sedimentary rock, regardless of variation in soil N, indicating extremely limited capacity of weathering to support forest Ca demands. In contrast, forests overlying basaltic rock obtained as much as 80% of Ca from rock weathering in low N sites, yet relied to a greater extent on atmospheric Ca as soil N increased, with less than 10% of Ca from rock weathering at sites with the highest soil N. Surprisingly, differences in fresh rock Ca content and base cation sources between sedimentary and basaltic sites was not reflected in ecosystem Ca availability, and instead increasing soil N caused similar declines in foliar and exchangeable Ca across both rock types. This illustrates that nutrient pool sizes do not necessarily reflect long-term nutrient supply, and highlights how coupled biogeochemical cycles within ecosystems can regulate nutrient loss and supply to biota. Broadly, our results highlight how interactions between biological and geologic factors can influence base cation sources in forest ecosystems. The sustainability of base cation supplies to forests may therefore depend greatly on variation in bedrock weathering at low N sites, yet converge to depend on atmospheric inputs in sites that receive high N loading from biological fixation or anthropogenic deposition. Close

• 25. [Article] Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress

  Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species ...

  Citation × Citation Citation Abstract Copy Title: Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress Author: Mosel, Jamie E. Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species dependent, and even within species different populations may demonstrate a spectrum of responses to drought, from susceptibility to resistance. As both loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii) have broad geographic ranges, they provide an opportunity to assess drought resistances of provenances associated with a variety of site climate conditions across their ranges. Furthermore, while studies of singular drought events have revealed important information about seedling stress responses, it has been shown that drought may impart effects even following the release from drought. In some cases, effects of drought exposure can be detrimental or lethal; however, exposure to non-lethal drought may also lead to drought acclimation, which could potentially enhance seedling functioning under subsequent droughts. This potential phenotypic plasticity, i.e. the extent to which a seedling can acclimate to drought conditions, may vary across species and across provenances within a species. This study used physiological (electron transport rate, fluorescence, and water potentials) and growth (biomass accumulation and height increment) responses for two goals: 1) to assess the drought resistances of three provenances of loblolly pine and three provenances of Douglas-fir associated with varying site climate conditions, and 2) to investigate whether previous exposure to a drought results in acclimation to a second drought. We hypothesized that: 1) drought responses would differ among the provenances in accordance with associated site climate conditions (i.e., provenances associated with mesic site climates as characterized by low climatic moisture deficit and high mean annual precipitation would be drought susceptible, and more xeric site climates as characterized by high climatic moisture deficit and low mean annual precipitation would be more drought resistant) as evidenced by physiological measures of electron transport rate, fluorescence, water potentials and growth; and 2) previous exposure to drought would result in acclimation to drought as evidenced by maintenance of physiological function (i.e., higher levels of electron transport and fluorescence) in previously drought exposed seedlings compared to previously unexposed seedlings. We also hypothesized that levels of acclimation would vary among provenances. The study yielded some evidence to support the first hypothesis regarding provenance differences in drought resistances in both species. Provenance drought resistances conformed largely to expectations, though differences were less than expected. In loblolly pine, although not statistically significant, during the second drought there was a pattern of lower maximum electron transport rates, which appeared sooner in the more mesic provenance than in the other two provenances. There were also provenance differences in seedling heights, with the most xeric of the three provenances being shorter at the beginning and the end of the study. In Douglas-fir, there were significant differences in provenance and in the interaction of treatment and provenance for maximum electron rates and fluorescence. Dark-adapted fluorescence was lower in the Coos Bay (mesic site climate) provenance during drought than in the Cascades (mesic site climate) and New Mexico (xeric site climate) provenances. The New Mexico provenance showed the least differences in fluorescence between droughted and watered treatments. The study also yielded some evidence to support the second hypothesis regarding drought acclimation in both species and among provenances. During the second drought, there was a pattern of higher maximum electron transport rates and fluorescence in previously drought exposed seedlings as compared to seedlings previously unexposed to drought. This difference was significant in the mesic provenance (North Carolina) of loblolly pine, with maximum electron transport rates significantly higher in the previously drought exposed treatment compared to the newly exposed treatment during and following the second drought. Patterns of lower electron transport rates in seedlings previously unexposed to drought compared to seedlings previously exposed to drought also appeared in Douglas-fir during the second drought, though not with statistical significance. However, the most xeric provenance showed the reverse pattern during and following the second drought, with lower maximum electron transport rates in the previously drought exposed treatment compared to the treatment previously unexposed to drought. Chlorophyll fluorescence values were significantly higher during the second drought in the previously droughted treatments compared to newly drought-exposed treatments in some provenances of loblolly pine and Douglas-fir. Lastly, seedlings exposed to an early drought had significantly lower final heights than seedlings unexposed to an early drought in both species, although the differences were greater in loblolly pine than Douglas-fir. It may be that growth acclimation, especially in the form of height reductions, influenced physiological responses during a second drought. Further studies are necessary to provide more conclusive evidence in support or against the two hypotheses. Nonetheless, this study provides valuable information on the drought responses of young, greenhouse-grown seedlings of two species that are widespread in North America and that are economically important throughout the world. Further studies in a wider range of age classes, incorporating field studies or more natural settings, may help better predict plant responses in the face of changing climate. Title: Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress Author: Mosel, Jamie E. Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species dependent, and even within species different populations may demonstrate a spectrum of responses to drought, from susceptibility to resistance. As both loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii) have broad geographic ranges, they provide an opportunity to assess drought resistances of provenances associated with a variety of site climate conditions across their ranges. Furthermore, while studies of singular drought events have revealed important information about seedling stress responses, it has been shown that drought may impart effects even following the release from drought. In some cases, effects of drought exposure can be detrimental or lethal; however, exposure to non-lethal drought may also lead to drought acclimation, which could potentially enhance seedling functioning under subsequent droughts. This potential phenotypic plasticity, i.e. the extent to which a seedling can acclimate to drought conditions, may vary across species and across provenances within a species. This study used physiological (electron transport rate, fluorescence, and water potentials) and growth (biomass accumulation and height increment) responses for two goals: 1) to assess the drought resistances of three provenances of loblolly pine and three provenances of Douglas-fir associated with varying site climate conditions, and 2) to investigate whether previous exposure to a drought results in acclimation to a second drought. We hypothesized that: 1) drought responses would differ among the provenances in accordance with associated site climate conditions (i.e., provenances associated with mesic site climates as characterized by low climatic moisture deficit and high mean annual precipitation would be drought susceptible, and more xeric site climates as characterized by high climatic moisture deficit and low mean annual precipitation would be more drought resistant) as evidenced by physiological measures of electron transport rate, fluorescence, water potentials and growth; and 2) previous exposure to drought would result in acclimation to drought as evidenced by maintenance of physiological function (i.e., higher levels of electron transport and fluorescence) in previously drought exposed seedlings compared to previously unexposed seedlings. We also hypothesized that levels of acclimation would vary among provenances. The study yielded some evidence to support the first hypothesis regarding provenance differences in drought resistances in both species. Provenance drought resistances conformed largely to expectations, though differences were less than expected. In loblolly pine, although not statistically significant, during the second drought there was a pattern of lower maximum electron transport rates, which appeared sooner in the more mesic provenance than in the other two provenances. There were also provenance differences in seedling heights, with the most xeric of the three provenances being shorter at the beginning and the end of the study. In Douglas-fir, there were significant differences in provenance and in the interaction of treatment and provenance for maximum electron rates and fluorescence. Dark-adapted fluorescence was lower in the Coos Bay (mesic site climate) provenance during drought than in the Cascades (mesic site climate) and New Mexico (xeric site climate) provenances. The New Mexico provenance showed the least differences in fluorescence between droughted and watered treatments. The study also yielded some evidence to support the second hypothesis regarding drought acclimation in both species and among provenances. During the second drought, there was a pattern of higher maximum electron transport rates and fluorescence in previously drought exposed seedlings as compared to seedlings previously unexposed to drought. This difference was significant in the mesic provenance (North Carolina) of loblolly pine, with maximum electron transport rates significantly higher in the previously drought exposed treatment compared to the newly exposed treatment during and following the second drought. Patterns of lower electron transport rates in seedlings previously unexposed to drought compared to seedlings previously exposed to drought also appeared in Douglas-fir during the second drought, though not with statistical significance. However, the most xeric provenance showed the reverse pattern during and following the second drought, with lower maximum electron transport rates in the previously drought exposed treatment compared to the treatment previously unexposed to drought. Chlorophyll fluorescence values were significantly higher during the second drought in the previously droughted treatments compared to newly drought-exposed treatments in some provenances of loblolly pine and Douglas-fir. Lastly, seedlings exposed to an early drought had significantly lower final heights than seedlings unexposed to an early drought in both species, although the differences were greater in loblolly pine than Douglas-fir. It may be that growth acclimation, especially in the form of height reductions, influenced physiological responses during a second drought. Further studies are necessary to provide more conclusive evidence in support or against the two hypotheses. Nonetheless, this study provides valuable information on the drought responses of young, greenhouse-grown seedlings of two species that are widespread in North America and that are economically important throughout the world. Further studies in a wider range of age classes, incorporating field studies or more natural settings, may help better predict plant responses in the face of changing climate. Close