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• 1. [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

• 2. [Article] Mixed-Severity Fire Effects on Biological Legacies and Vegetation Response in Pseudotsuga Forests of Western Oregon's Central Cascades, USA

  Mixed-severity fire occurrence is increasingly recognized in Pseudotsuga forests of the Pacific Northwest, but questions remain about how tree mortality varies, and forest structure is altered, across ...

  Citation × Citation Citation Abstract Copy Title: Mixed-Severity Fire Effects on Biological Legacies and Vegetation Response in Pseudotsuga Forests of Western Oregon's Central Cascades, USA Author: Dunn, Christopher J. Mixed-severity fire occurrence is increasingly recognized in Pseudotsuga forests of the Pacific Northwest, but questions remain about how tree mortality varies, and forest structure is altered, across the disturbance gradient observed in these fires. Therefore, we sampled live and dead biological legacies at 45 one ha plots, with four 0.10 ha nested plots, stratified across an unburned, low, moderate and high-severity fire gradient. We used severity estimates based on differenced Normalized Burn Ratio (dNBR), and captured a disturbance gradient, but plots in our low-severity class underestimated fire effects because of misclassification or delayed mortality. We estimated probability of mortality for shade-intolerant (Douglas-fir, incense-cedar, sugar pine) and shade-tolerant (western hemlock, western redcedar, true fir) trees from 5,079 sampled trees and snags. The probability of mortality was higher for shade-tolerant species across all fire-severity classes, and decreased with increasing DBH except for western hemlock. Only large, shade-intolerant trees survived high-severity fire. Post-fire snag fall and fragmentation were estimated from 2,746 sampled snags and logs. The probability of snag fall decreased with increasing DBH for all species, and was positively correlated with fire severity, except for Douglas-fir that had a higher probability following low-severity fire. Snag fragmentation was positively correlated with DBH and fire severity for all species. We also estimated the coefficient of variation within- and among-plots by fire severity class, as well as across all sampled conditions. Structural attributes varied more within- than among-plots, likely a result of increasing sub-hectare patchy mortality as fire intensity increased. Although vertical and horizontal structural diversity increased at sub-hectare scales, the coefficient of variation was highest for all structural attributes when compared across all fire severity classes. Therefore, the range of fire effects observed in mixed-severity fires may be functionally important in creating structural complexity across landscapes, which is an important attribute of old-growth forests in the Pacific Northwest. Understory vegetation response to mixed-severity fires has not been characterized for these forests even though the majority of vegetation diversity is found in these vegetation layers. Therefore, we sampled forest structure (1000 m² circular plots) and understory vegetation (100 m² plots) at 168 collocated plots stratified across unburned, low, moderate and high-severity conditions 10 years (Tiller Complex) and 22 years (Warner Fire) post-fire. We focused on shrub species, but sampled forbs, graminoids, ferns and moss as functional groups. Offsite colonization and fire stimulated soil seedbanks increased the total species richness from 23 to 46. The life-history strategies of residual and colonizing species resulted in three dominant species response-curves to the magnitude of disturbance: 1) 'disturbance-sensitive', when relative abundance was highest in unburned plots and continued to decline with increasing fire severity, 2) 'disturbance-stimulated', when relative abundance was highest following low or moderate-severity fire and 3) 'disturbance-amplified', when relative abundance increased with increasing fire severity. Residual and colonizing species assemblages promoted five or six distinct understory communities, dominantly driven by legacy tree basal area rather than the proportion of basal area killed. Understory communities were rarely associated with one disturbance severity class as fire refugia, variation in overstory and understory fire severity, and compensatory conditions offset fire effects. Early-seral habitats were the most different from unburned forests, but were not the only post-fire conditions important across these burned landscapes. Interactions among live and dead forest structures following low or moderate-severity fire, and the vegetation response to these conditions, are also unique to the post-fire landscape and likely important for various wildlife species. Therefore, if ecological forestry paradigms focus dominantly on creating old-growth structure or early-seral habitats, they might exclude important conditions that contribute to the landscape structural complexity created by mixed-severity fires. Additionally, tree regeneration response to mixed-severity fires has not been characterized for these forests even though they offer insight into one aspect of the resilience of these ecosystems to disturbance. Therefore, we sampled forest structure (1000 m² circular plots) and regeneration dynamics (100 m² plots) at 168 collocated plots stratified across unburned, low, moderate and high-severity conditions 10 years (Tiller Complex) and 22 years (Warner Fire) post-fire. The largest marginal increase in tree mortality (stems ha⁻¹) occurred between unburned and low-severity fires, given preferential mortality of small trees and shade-tolerant species, but basal area mortality had the largest marginal increase moving from moderate to high-severity. Pairwise comparisons of legacy tree basal area between low and moderate-severity weren’t as significant as other comparisons, but did capture a gradient of increasing fire effects. Quadratic mean diameter and canopy base height were positively correlated with fire severity as incrementally larger trees were killed and canopy ascension followed. Regeneration density increased regardless of severity, relative to unburned forests (median density of 1,384 trees ha⁻¹), but the highest median density (16,220 trees ha⁻¹) followed low-severity fire at the Tiller Complex and moderate-severity fire (14,472 trees ha⁻¹) at Warner Fire. Plot-level average species richness was highest following these same fire severity classes, supporting the Intermediate Disturbance Hypothesis. Statistically distinct regeneration communities occurred across the fire severity gradient at both fire sites. The relative abundance of shade-tolerant tree species decreased as fire severity increased, except for a divergent response following stand-initiation at the Warner Fire. While divergent successional pathways were evident within a couple decades following stand-initiation, low or moderate-severity fires also modified successional trajectories and may be the most functionally important disturbance magnitude because it has the greatest potential to increase compositional and structural diversity. Incorporating mixed-severity fire effects into landscape management of Pseudotsuga forests could increase structural complexity at stand and landscape-scales. Title: Mixed-Severity Fire Effects on Biological Legacies and Vegetation Response in Pseudotsuga Forests of Western Oregon's Central Cascades, USA Author: Dunn, Christopher J. Mixed-severity fire occurrence is increasingly recognized in Pseudotsuga forests of the Pacific Northwest, but questions remain about how tree mortality varies, and forest structure is altered, across the disturbance gradient observed in these fires. Therefore, we sampled live and dead biological legacies at 45 one ha plots, with four 0.10 ha nested plots, stratified across an unburned, low, moderate and high-severity fire gradient. We used severity estimates based on differenced Normalized Burn Ratio (dNBR), and captured a disturbance gradient, but plots in our low-severity class underestimated fire effects because of misclassification or delayed mortality. We estimated probability of mortality for shade-intolerant (Douglas-fir, incense-cedar, sugar pine) and shade-tolerant (western hemlock, western redcedar, true fir) trees from 5,079 sampled trees and snags. The probability of mortality was higher for shade-tolerant species across all fire-severity classes, and decreased with increasing DBH except for western hemlock. Only large, shade-intolerant trees survived high-severity fire. Post-fire snag fall and fragmentation were estimated from 2,746 sampled snags and logs. The probability of snag fall decreased with increasing DBH for all species, and was positively correlated with fire severity, except for Douglas-fir that had a higher probability following low-severity fire. Snag fragmentation was positively correlated with DBH and fire severity for all species. We also estimated the coefficient of variation within- and among-plots by fire severity class, as well as across all sampled conditions. Structural attributes varied more within- than among-plots, likely a result of increasing sub-hectare patchy mortality as fire intensity increased. Although vertical and horizontal structural diversity increased at sub-hectare scales, the coefficient of variation was highest for all structural attributes when compared across all fire severity classes. Therefore, the range of fire effects observed in mixed-severity fires may be functionally important in creating structural complexity across landscapes, which is an important attribute of old-growth forests in the Pacific Northwest. Understory vegetation response to mixed-severity fires has not been characterized for these forests even though the majority of vegetation diversity is found in these vegetation layers. Therefore, we sampled forest structure (1000 m² circular plots) and understory vegetation (100 m² plots) at 168 collocated plots stratified across unburned, low, moderate and high-severity conditions 10 years (Tiller Complex) and 22 years (Warner Fire) post-fire. We focused on shrub species, but sampled forbs, graminoids, ferns and moss as functional groups. Offsite colonization and fire stimulated soil seedbanks increased the total species richness from 23 to 46. The life-history strategies of residual and colonizing species resulted in three dominant species response-curves to the magnitude of disturbance: 1) 'disturbance-sensitive', when relative abundance was highest in unburned plots and continued to decline with increasing fire severity, 2) 'disturbance-stimulated', when relative abundance was highest following low or moderate-severity fire and 3) 'disturbance-amplified', when relative abundance increased with increasing fire severity. Residual and colonizing species assemblages promoted five or six distinct understory communities, dominantly driven by legacy tree basal area rather than the proportion of basal area killed. Understory communities were rarely associated with one disturbance severity class as fire refugia, variation in overstory and understory fire severity, and compensatory conditions offset fire effects. Early-seral habitats were the most different from unburned forests, but were not the only post-fire conditions important across these burned landscapes. Interactions among live and dead forest structures following low or moderate-severity fire, and the vegetation response to these conditions, are also unique to the post-fire landscape and likely important for various wildlife species. Therefore, if ecological forestry paradigms focus dominantly on creating old-growth structure or early-seral habitats, they might exclude important conditions that contribute to the landscape structural complexity created by mixed-severity fires. Additionally, tree regeneration response to mixed-severity fires has not been characterized for these forests even though they offer insight into one aspect of the resilience of these ecosystems to disturbance. Therefore, we sampled forest structure (1000 m² circular plots) and regeneration dynamics (100 m² plots) at 168 collocated plots stratified across unburned, low, moderate and high-severity conditions 10 years (Tiller Complex) and 22 years (Warner Fire) post-fire. The largest marginal increase in tree mortality (stems ha⁻¹) occurred between unburned and low-severity fires, given preferential mortality of small trees and shade-tolerant species, but basal area mortality had the largest marginal increase moving from moderate to high-severity. Pairwise comparisons of legacy tree basal area between low and moderate-severity weren’t as significant as other comparisons, but did capture a gradient of increasing fire effects. Quadratic mean diameter and canopy base height were positively correlated with fire severity as incrementally larger trees were killed and canopy ascension followed. Regeneration density increased regardless of severity, relative to unburned forests (median density of 1,384 trees ha⁻¹), but the highest median density (16,220 trees ha⁻¹) followed low-severity fire at the Tiller Complex and moderate-severity fire (14,472 trees ha⁻¹) at Warner Fire. Plot-level average species richness was highest following these same fire severity classes, supporting the Intermediate Disturbance Hypothesis. Statistically distinct regeneration communities occurred across the fire severity gradient at both fire sites. The relative abundance of shade-tolerant tree species decreased as fire severity increased, except for a divergent response following stand-initiation at the Warner Fire. While divergent successional pathways were evident within a couple decades following stand-initiation, low or moderate-severity fires also modified successional trajectories and may be the most functionally important disturbance magnitude because it has the greatest potential to increase compositional and structural diversity. Incorporating mixed-severity fire effects into landscape management of Pseudotsuga forests could increase structural complexity at stand and landscape-scales. Close