Zaret K and Holz A (2024) Exploration of large-scale vegetation transition in wet ecosystems: a c... more Zaret K and Holz A (2024) Exploration of large-scale vegetation transition in wet ecosystems: a comparison of conifer seedling abundance across burned vs. unburned forest-peatland ecotones in Western Patagonia.
High-severity fires increasingly threaten the resilience of forests by disrupting regeneration me... more High-severity fires increasingly threaten the resilience of forests by disrupting regeneration mechanisms. Postfire resilience mechanisms of temperate Araucaria-Nothofagus forests in southern South America, recently affected by severe fire, are poorly understood. We evaluated the effect of burn severity on stand structure and regeneration density (seedlings and resprouts, < 4 cm DBH) after fires. We surveyed 117 plots, seven years postfire, in pure stands of A. araucana, Nothofagus obliqua, and Nothofagus pumilio. Araucaria araucana is fire resistant except during intense fires, whereas the two Nothofagus are easily killed by fire. Araucaria araucana and N. obliqua resprout following fire whereas N. pumilio does not resprout and instead depends on new seedling establishment to regenerate following fire. We quantified stem survival (stems with green foliage crown, > 4 cm DBH), shrub and grass cover, and topographic characteristics (e.g., slope, aspect). Using GLMs, we modeled seedlings and resprouts abundance as a function of these variables. We found that high-severity fires caused high stem mortality (98 %) and promoted shrubs and grass growth. Higher post-fire surviving live stems increases seedlings abundance for all species and A. araucana resprouts. High-severity fires do not threaten N. obliqua regeneration due to its resprouting ability. However, A. araucana and N. pumilio stands burned at high-severity may struggle to regenerate naturally, leading to replacement by shrublands or grasslands. Our results imply that successful recovery of Araucaria-Nothofagus forests in these areas require restoration protocols that integrate information on fire severity and topographic variability affecting sites suitable for post-fire regeneration.
Conifer forest resilience may be threatened by increasing wildfire activity and compound disturba... more Conifer forest resilience may be threatened by increasing wildfire activity and compound disturbances in western North America. Fire refugia enhance forest resilience, yet may decline over time due to delayed mortality-a process that remains poorly understood at landscape and regional scales. To address this uncertainty, we used high-resolution satellite imagery (5-m pixel) to map and quantify delayed mortality of conifer tree cover between 1 and 5 years postfire, across 30 large wildfires that burned within three montane ecoregions in the western United States. We used statistical models to explore the influence of burn severity, topography, soils, and climate moisture deficit on delayed mortality. We estimate that delayed mortality reduced live conifer tree cover by 5%-25% at the fire perimeter scale and 12%-15% at the ecoregion scale. Remotely sensed burn severity (1-year postfire) was the strongest predictor of delayed mortality, indicating patch-level fire effects are a strong proxy for fire injury severity among surviving trees that eventually perish. Delayed mortality rates were further influenced by long-term average and short-term postfire climate moisture deficits, illustrating the impact of drought on fire-injured tree survival. Our work demonstrates that delayed mortality in conifer forests of the western United States can be remotely quantified at a fine grain and landscape scale, is a spatially extensive phenomenon, is driven by fire-climate-environment interactions, and has important ecological implications.
Background Warmer climate conditions are altering fire regimes globally, eroding the capacity of ... more Background Warmer climate conditions are altering fire regimes globally, eroding the capacity of forest ecosystems to resist and recover from natural disturbances like wildfire. Severe and rapidly repeated wildfires are promoting tree regeneration failure of obligate-seeders and/or fire-sensitive species in temperate forests of the Southern Hemisphere. We collected post-fire field data to evaluate whether forest structure and tree regeneration responses varied between two Andean forest study areas dominated by the threatened Gondwanan conifer Araucaria araucana and Nothofagus species (southern beeches)-one area burned once, the other reburned after 13 years. Results Tree mortality was high across species after a single high severity and/or repeated wildfire, although some A. araucana trees were able to survive both events. Post-fire seedling regeneration of A. araucana and Nothofagus spp. was poor in areas affected by severe wildfires, and the latter was absent from reburned plots. A key driver of this regeneration failure was increasing distance to live seed source trees, which was negatively correlated with these species' post-fire seedling abundances. In contrast, species with the capacity to regenerate via resprouting (A. araucana, N. alpina, N. obliqua) did so after a single high severity fire; however, only a single Nothofagus species (N. alpina) resprouted abundantly after a reburn. Conclusions Our findings suggest that high severity and short-interval fires can drastically change the structure of and limit post-fire tree regeneration in Araucaria-Nothofagus forests, promoting alternative post-fire forest ecosystem trajectories. Resprouting species of the Nothofagus genus, especially N. alpina, exhibit the greatest resilience to these emerging fire patterns. These forests are currently facing an unprecedented climatic shift toward greater fire activity, where resprouting is the favored regeneration strategy. If the occurrence of severe and short-interval fires increases in the coming decades, as predicted, we expect Araucaria-Nothofagus forests to shift toward a drier, more flammable shrubland ecosystem state.
Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)-a ... more Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)-a culturally, ecologically and economically important species-has recently experienced anomalously high mortality rates and partial canopy dieback. We
Context Western Chilean Patagonia is an isolated temperate region with an important proportion of... more Context Western Chilean Patagonia is an isolated temperate region with an important proportion of intact forest landscapes (IFL) that was subjected to large-scale fires over 60 years ago. However, there is no empirical evaluation of the land cover dynamics to establish the forest loss and recovery, and the effect on the landscape structure and function, and remnant IFL following the fires. Objectives The present study addressed the following questions: (1) What have been the main trends of the land cover dynamics between 1984 and 2018 following earlier fires, and how have these trends shaped the spatial patterns and potential carbon stock of forests in western Patagonia? (2) What proportion of forest landscape remains intact following fires in this region? Methods We selected the Coyhaique Province (1,231,910 ha) in western Chilean Patagonia as the study area. Land cover maps for three dates (1984, 2000, 2018) were used to evaluate landscape dynamics after fires. A map of persistence and change occurrence was made to estimate the IFL area over the 1984-2018 period. Landscape metrics were used to assess landscape structure change, and potential carbon stock was estimated based on a literature review. Results Following fires, the main land cover changes between 1984 and 2018 were loss of ~ 32,600 ha of old-growth forest and a recovery of ~ 69,000 ha of second-growth forest. The increase in secondgrowth forest area mainly resulted from loss of agricultural cover (~ 41% of the area). Despite these changes, ~ 61% of the area could potentially remain as IFL after fires. Over the 1984-2018 period, a slight increase in fragmentation of old-growth forest, and a
The introduction of non-native tree species for large-scale afforestation may alter the fire regi... more The introduction of non-native tree species for large-scale afforestation may alter the fire regime of native ecosystems by modifying fuel proprieties. We quantified changes in fuel abundance and structure resulting from the establishment of commercial Pinus spp. plantations in Araucaria araucana ecosystems in northwestern Patagonia, Argentina. Specifically, we assessed the amount, distribution, and condition (live/dead) of surface and standing fine fuel in A. araucana stands with mature pine plantations (i.e. > 20 cm dbh) and in stands dominated only by A. araucana (control). Our study shows that both types of stands are prone to wildfires, but pine plantations have fuel characteristics that imply greater flammability due to higher fuel load and vertical continuity in the understory and in the overstory canopy. In the absence of fuel mitigation practices, A. araucana stands with plantations exhibit greater flammability than the control A. araucana stands, potentially promoting the occurrence and spread of fires of greater severity. This study contributes to understanding the effects of enrichment planting of pines, and possibly pine invasions, on A. araucaria ecosystem flammability and their potential consequences on fire behavior.
Privately-owned forests in the Pacific Northwest (PNW) are important potential carbon sinks and p... more Privately-owned forests in the Pacific Northwest (PNW) are important potential carbon sinks and play a large role in carbon sequestration and storage. Non-industrial private forest (NIPF) owners constitute a substantial portion of overall forest landownership in productive regions of the PNW; however, little is known about their preferences for non-market incentive programs aimed at increased carbon storage and sequestration, specifically by limiting timber harvest, and how those preferences might impact the outcome of forest carbon programs. We simulated landscape-scale outcomes of hypothetical forest carbon incentive programs in western Oregon (USA) by combining empirical models of NIPF owners' participation with spatially explicit forest carbon storage and sequestration data. We surveyed landowners to determine their willingness to enroll in various hypothetical forest management incentive programs that varied in terms of harvest restrictions, contract length, annual payment and incentive payment amounts, and cost-share percentages, as well as the program framing (e.g., carbon versus forest health). We used multinomial logistic regression to model whether landowners might enroll based on program attributes, landowners' attitudes toward climate change and forest management, past and planned future forest harvest activities, and socio-demographics. We found that 36% of respondents stated that they would probably or definitely enroll in at least one of the hypothetical programs they were shown while 21% of respondents refused all programs that they were offered. Our final model of landowner willingness to enroll indicated that higher annual and higher cost-share payments were the strongest positive predictors of whether landowners would enroll vs. not enroll. Landowners' willingness to enroll was not influenced by program framing as either a "forest carbon" or a "forest health"; however, landowner attitudes toward climate change were the next strongest positive predictor of enrollment after annual and cost-share payments. By simulating landowner enrollment in six policy relevant program scenarios, we illustrate that carefully designed forest carbon incentive programs for NIPF owners could have tangible carbon protection benefits (16.25 to 50.31 MMT CO2e cumulative) at relatively low costs per MT CO2e ($3.60 to $7.70). We highlight tradeoffs between maximizing enrollment in forest carbon incentive programs and providing longer term protection of carbon. This research contributes to the literature on the design of potential forest carbon incentive programs and communication about forest carbon management, as well as aims to aid policy makers and program administrators that seek ways to engage private landowners in carbon-oriented forest management.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in w... more Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in western North America, where forest recovery within high-severity burn patch interiors can be slow, yet successful over long temporal periods (decades to centuries). Increasing fire frequency and high-severity burn patch size, under a warming climate, however, may challenge post-fire forest recovery, promoting landscape-level shifts in forest structure, composition, and distribution of non-forest patches. Crucial to a delay and/or impediment to this shift, fire refugia (i.e., remnant seed sources) may determine forest recovery trajectories and potential forest state-transitions. To examine how fire refugia attributes (i.e. extent, composition, and structure) interact with local climate and environmental conditions to determine post-fire forest recovery responses, we developed fine-grain maps of fire refugia via remote sensing and conducted field-based assessment of post-fire conifer tree establishment largely originating (i.e., dispersed) from fire refugium in the Central Cascade Range of the Pacific Northwest United States. We found that limitations on seed availability, represented by the distance 2-weighted density (D 2 WD) of fine-grain refugia extent, largely explained post-fire tree establishment responses within our relatively mesic and cool subalpine study sites. Interactions between seed availability, climate, and environmental conditions indicated that the structural attributes of refugia (e.g., tree height) and site abiotic/biotic environmental controls (e.g., climate water deficit, canopy cover, and coarse woody debris cover) interplayed to constrain or enhance species-specific tree establishment responses. Importantly, these interactions illustrate that when seed availability is critically low for a given area, climate-environment conditions may strongly determine whether forests recover following fire(s). Toward modelling and predicting tree establishment responses and potential forest state-transitions after large stand-replacing fires(s), our study demonstrates the importance of accurately quantifying seed availability via the fine-grain extent, configuration, and attributes of remnant seed source legacies.
species replace obligate seeders, reducing local diversity. We established 120 field plots across... more species replace obligate seeders, reducing local diversity. We established 120 field plots across a burn severity gradient in two study sites: one affected by a single (burned 2015), and the second by two fires (burned 2002 and 2015), where vascular plant species abundance, among other biotic, abiotic, and topographic variables were estimated. We found that burn (high) severity is the main driver of post-fire understory assemblages, resulting in communities less competitive and heterogeneous, with an increasing number of exotic species. Also, post-fire responses are resulting in communities in which the high abundance of flammable taxa and post-fire resprouter species at the early-seral stage may lead to large-scale transitions from mesic forest ecosystems to dry, open forest and fire-prone shrublands in reburned areas. Our results highlight the ecological importance of short-interval and severe wildfires as leading factors in the transition of post-fire understory communities of Araucaria-Nothofagus forests to a system dominated by post-fire obligate resprouters, where tree species are less represented compromising the recovery of these ecosystems. These findings improve the understanding of the current post-fire processes that affect flammability feedbacks and contribute to a baseline of the current patterns in a world of altered fire regimes.
The photographers and figure sources credited herein retain all rights to their images. All other... more The photographers and figure sources credited herein retain all rights to their images. All other elements of the document are published under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0).
Characterizing wildfire regimes where wildfires are uncommon is challenged by a lack of empirical... more Characterizing wildfire regimes where wildfires are uncommon is challenged by a lack of empirical information. Moreover, climate change is projected to lead to increasingly frequent wildfires and additional annual area burned in forests historically characterized by long fire return intervals. Western Oregon and Washington, USA (westside) have experienced few large wildfires (fires greater than 100 hectares) the past century and are characterized to infrequent large fires with return intervals greater than 500 years. We evaluated impacts of climate change on wildfire hazard in a major urban watershed outside Portland, OR, USA. We simulated wildfire occurrence and fire regime characteristics under contemporary conditions (1992-2015) and four mid-century (2040-2069) scenarios using Representative Concentration Pathway (RCP) 8.5. Simulated mid-century fire seasons expanded in most scenarios, in some cases by nearly two months. In all scenarios, average fire size and frequency projections increased significantly. Fire regime characteristics under the hottest and driest mid-century scenarios illustrate novel disturbance regimes which could result in permanent changes to forest structure and composition and the provision of ecosystem services. Managers and planners can use the range of modeled outputs and simulation results to inform robust strategies for climate adaptation and risk mitigation.
Increasing concentrations of greenhouse gases (GHGs) are causing global climate change and decrea... more Increasing concentrations of greenhouse gases (GHGs) are causing global climate change and decreasing the stability of the climate system. Long-term solutions to climate change will require reduction in GHG emissions as well as the removal of large quantities of GHGs from the atmosphere. Natural climate solutions (NCS), i.e., changes in land management, ecosystem restoration, and avoided conversion of habitats, have substantial potential to meet global and national greenhouse gas (GHG) reduction targets and contribute to the global drawdown of GHGs. However, the relative role of NCS to contribute to GHG reduction at subnational scales is not well known. We examined the potential for 12 NCS activities on natural and working lands in Oregon, USA to reduce GHG emissions in the context of the state's climate mitigation goals. We evaluated three alternative scenarios wherein NCS implementation increased across the applicable private or public land base, depending on the activity, and estimated the annual GHG reduction in carbon dioxide equivalents (CO 2 e) attributable to NCS from 2020 to 2050. We found that NCS within Oregon could contribute annual GHG emission reductions of 2.7 to 8.3 MMT CO 2 e by 2035 and 2.9 to 9.8 MMT CO 2 e by 2050. Changes in forest-based activities including deferred timber harvest, riparian reforestation, and replanting after wildfires contributed most to potential GHG reductions (76 to 94% of the overall annual reductions), followed by changes to agricultural management through no-till, cover crops, and nitrogen management (3 to 15% of overall annual reductions). GHG reduction benefits are relatively high per unit area for avoided conversion of forests (125-400 MT CO 2 e ha-1). However, the existing land use policy in Oregon limits the current geographic extent of active conversion of natural lands and thus, avoided conversions results in modest overall potential GHG reduction benefits (i.e., less than 5% of the overall annual reductions). Tidal wetland restoration, which has high per unit area carbon sequestration benefits (8.8 MT CO 2 e ha-1 yr-1), also has limited possible geographic extent resulting in low potential (< 1%) of state-level GHG reduction contributions. However, cobenefits such as improved habitat and water quality delivered by restoration NCS pathways are substantial. Ultimately, reducing GHG emissions and increasing carbon sequestration to combat climate change will require actions across multiple sectors. We demonstrate that
Increasing forest fuel aridity with climate change may be expanding mid-to-high-elevation forests... more Increasing forest fuel aridity with climate change may be expanding mid-to-high-elevation forests' vulnerability to large, severe, and frequent wildfire. Long-lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper-montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post-fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (<12-yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post-fire conifer regeneration was generally very poor among plots that experienced either a single high-severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre-fire dominant, shade-tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed postfire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low-then-high severity, or primarily fire-adapted pines, if high-then-low severity. Our findings suggest that these forests, affected by expansive high-severity and/or short-interval wildfire, may transition into a patchy, low-density, pine-dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire-adapted tree species, lower tree densities, and more non-forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid-to-high-elevation forests may begin resembling their drier, lower-elevation mixed-conifer counterparts in structure and composition.
Untangling the nuanced relationships between landscape, fire disturbance, human agency, and clima... more Untangling the nuanced relationships between landscape, fire disturbance, human agency, and climate is key to understanding rapid population declines of firesensitive plant species. Using multiple lines of evidence across temporal and spatial scales (vegetation survey, stand structure analysis, dendrochronology, and fire history reconstruction), we document landscape-scale population collapse of the
Aim: We examined whether and how tree radial-growth responses to climate have changed for the wor... more Aim: We examined whether and how tree radial-growth responses to climate have changed for the world's southernmost conifer species throughout its latitudinal distribution following rapid climate change in the second half of the 20th century. Location: Temperate forests in southern South America. Methods: New and existing tree-ring radial growth chronologies representing the
Zaret K and Holz A (2024) Exploration of large-scale vegetation transition in wet ecosystems: a c... more Zaret K and Holz A (2024) Exploration of large-scale vegetation transition in wet ecosystems: a comparison of conifer seedling abundance across burned vs. unburned forest-peatland ecotones in Western Patagonia.
High-severity fires increasingly threaten the resilience of forests by disrupting regeneration me... more High-severity fires increasingly threaten the resilience of forests by disrupting regeneration mechanisms. Postfire resilience mechanisms of temperate Araucaria-Nothofagus forests in southern South America, recently affected by severe fire, are poorly understood. We evaluated the effect of burn severity on stand structure and regeneration density (seedlings and resprouts, < 4 cm DBH) after fires. We surveyed 117 plots, seven years postfire, in pure stands of A. araucana, Nothofagus obliqua, and Nothofagus pumilio. Araucaria araucana is fire resistant except during intense fires, whereas the two Nothofagus are easily killed by fire. Araucaria araucana and N. obliqua resprout following fire whereas N. pumilio does not resprout and instead depends on new seedling establishment to regenerate following fire. We quantified stem survival (stems with green foliage crown, > 4 cm DBH), shrub and grass cover, and topographic characteristics (e.g., slope, aspect). Using GLMs, we modeled seedlings and resprouts abundance as a function of these variables. We found that high-severity fires caused high stem mortality (98 %) and promoted shrubs and grass growth. Higher post-fire surviving live stems increases seedlings abundance for all species and A. araucana resprouts. High-severity fires do not threaten N. obliqua regeneration due to its resprouting ability. However, A. araucana and N. pumilio stands burned at high-severity may struggle to regenerate naturally, leading to replacement by shrublands or grasslands. Our results imply that successful recovery of Araucaria-Nothofagus forests in these areas require restoration protocols that integrate information on fire severity and topographic variability affecting sites suitable for post-fire regeneration.
Conifer forest resilience may be threatened by increasing wildfire activity and compound disturba... more Conifer forest resilience may be threatened by increasing wildfire activity and compound disturbances in western North America. Fire refugia enhance forest resilience, yet may decline over time due to delayed mortality-a process that remains poorly understood at landscape and regional scales. To address this uncertainty, we used high-resolution satellite imagery (5-m pixel) to map and quantify delayed mortality of conifer tree cover between 1 and 5 years postfire, across 30 large wildfires that burned within three montane ecoregions in the western United States. We used statistical models to explore the influence of burn severity, topography, soils, and climate moisture deficit on delayed mortality. We estimate that delayed mortality reduced live conifer tree cover by 5%-25% at the fire perimeter scale and 12%-15% at the ecoregion scale. Remotely sensed burn severity (1-year postfire) was the strongest predictor of delayed mortality, indicating patch-level fire effects are a strong proxy for fire injury severity among surviving trees that eventually perish. Delayed mortality rates were further influenced by long-term average and short-term postfire climate moisture deficits, illustrating the impact of drought on fire-injured tree survival. Our work demonstrates that delayed mortality in conifer forests of the western United States can be remotely quantified at a fine grain and landscape scale, is a spatially extensive phenomenon, is driven by fire-climate-environment interactions, and has important ecological implications.
Background Warmer climate conditions are altering fire regimes globally, eroding the capacity of ... more Background Warmer climate conditions are altering fire regimes globally, eroding the capacity of forest ecosystems to resist and recover from natural disturbances like wildfire. Severe and rapidly repeated wildfires are promoting tree regeneration failure of obligate-seeders and/or fire-sensitive species in temperate forests of the Southern Hemisphere. We collected post-fire field data to evaluate whether forest structure and tree regeneration responses varied between two Andean forest study areas dominated by the threatened Gondwanan conifer Araucaria araucana and Nothofagus species (southern beeches)-one area burned once, the other reburned after 13 years. Results Tree mortality was high across species after a single high severity and/or repeated wildfire, although some A. araucana trees were able to survive both events. Post-fire seedling regeneration of A. araucana and Nothofagus spp. was poor in areas affected by severe wildfires, and the latter was absent from reburned plots. A key driver of this regeneration failure was increasing distance to live seed source trees, which was negatively correlated with these species' post-fire seedling abundances. In contrast, species with the capacity to regenerate via resprouting (A. araucana, N. alpina, N. obliqua) did so after a single high severity fire; however, only a single Nothofagus species (N. alpina) resprouted abundantly after a reburn. Conclusions Our findings suggest that high severity and short-interval fires can drastically change the structure of and limit post-fire tree regeneration in Araucaria-Nothofagus forests, promoting alternative post-fire forest ecosystem trajectories. Resprouting species of the Nothofagus genus, especially N. alpina, exhibit the greatest resilience to these emerging fire patterns. These forests are currently facing an unprecedented climatic shift toward greater fire activity, where resprouting is the favored regeneration strategy. If the occurrence of severe and short-interval fires increases in the coming decades, as predicted, we expect Araucaria-Nothofagus forests to shift toward a drier, more flammable shrubland ecosystem state.
Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)-a ... more Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)-a culturally, ecologically and economically important species-has recently experienced anomalously high mortality rates and partial canopy dieback. We
Context Western Chilean Patagonia is an isolated temperate region with an important proportion of... more Context Western Chilean Patagonia is an isolated temperate region with an important proportion of intact forest landscapes (IFL) that was subjected to large-scale fires over 60 years ago. However, there is no empirical evaluation of the land cover dynamics to establish the forest loss and recovery, and the effect on the landscape structure and function, and remnant IFL following the fires. Objectives The present study addressed the following questions: (1) What have been the main trends of the land cover dynamics between 1984 and 2018 following earlier fires, and how have these trends shaped the spatial patterns and potential carbon stock of forests in western Patagonia? (2) What proportion of forest landscape remains intact following fires in this region? Methods We selected the Coyhaique Province (1,231,910 ha) in western Chilean Patagonia as the study area. Land cover maps for three dates (1984, 2000, 2018) were used to evaluate landscape dynamics after fires. A map of persistence and change occurrence was made to estimate the IFL area over the 1984-2018 period. Landscape metrics were used to assess landscape structure change, and potential carbon stock was estimated based on a literature review. Results Following fires, the main land cover changes between 1984 and 2018 were loss of ~ 32,600 ha of old-growth forest and a recovery of ~ 69,000 ha of second-growth forest. The increase in secondgrowth forest area mainly resulted from loss of agricultural cover (~ 41% of the area). Despite these changes, ~ 61% of the area could potentially remain as IFL after fires. Over the 1984-2018 period, a slight increase in fragmentation of old-growth forest, and a
The introduction of non-native tree species for large-scale afforestation may alter the fire regi... more The introduction of non-native tree species for large-scale afforestation may alter the fire regime of native ecosystems by modifying fuel proprieties. We quantified changes in fuel abundance and structure resulting from the establishment of commercial Pinus spp. plantations in Araucaria araucana ecosystems in northwestern Patagonia, Argentina. Specifically, we assessed the amount, distribution, and condition (live/dead) of surface and standing fine fuel in A. araucana stands with mature pine plantations (i.e. > 20 cm dbh) and in stands dominated only by A. araucana (control). Our study shows that both types of stands are prone to wildfires, but pine plantations have fuel characteristics that imply greater flammability due to higher fuel load and vertical continuity in the understory and in the overstory canopy. In the absence of fuel mitigation practices, A. araucana stands with plantations exhibit greater flammability than the control A. araucana stands, potentially promoting the occurrence and spread of fires of greater severity. This study contributes to understanding the effects of enrichment planting of pines, and possibly pine invasions, on A. araucaria ecosystem flammability and their potential consequences on fire behavior.
Privately-owned forests in the Pacific Northwest (PNW) are important potential carbon sinks and p... more Privately-owned forests in the Pacific Northwest (PNW) are important potential carbon sinks and play a large role in carbon sequestration and storage. Non-industrial private forest (NIPF) owners constitute a substantial portion of overall forest landownership in productive regions of the PNW; however, little is known about their preferences for non-market incentive programs aimed at increased carbon storage and sequestration, specifically by limiting timber harvest, and how those preferences might impact the outcome of forest carbon programs. We simulated landscape-scale outcomes of hypothetical forest carbon incentive programs in western Oregon (USA) by combining empirical models of NIPF owners' participation with spatially explicit forest carbon storage and sequestration data. We surveyed landowners to determine their willingness to enroll in various hypothetical forest management incentive programs that varied in terms of harvest restrictions, contract length, annual payment and incentive payment amounts, and cost-share percentages, as well as the program framing (e.g., carbon versus forest health). We used multinomial logistic regression to model whether landowners might enroll based on program attributes, landowners' attitudes toward climate change and forest management, past and planned future forest harvest activities, and socio-demographics. We found that 36% of respondents stated that they would probably or definitely enroll in at least one of the hypothetical programs they were shown while 21% of respondents refused all programs that they were offered. Our final model of landowner willingness to enroll indicated that higher annual and higher cost-share payments were the strongest positive predictors of whether landowners would enroll vs. not enroll. Landowners' willingness to enroll was not influenced by program framing as either a "forest carbon" or a "forest health"; however, landowner attitudes toward climate change were the next strongest positive predictor of enrollment after annual and cost-share payments. By simulating landowner enrollment in six policy relevant program scenarios, we illustrate that carefully designed forest carbon incentive programs for NIPF owners could have tangible carbon protection benefits (16.25 to 50.31 MMT CO2e cumulative) at relatively low costs per MT CO2e ($3.60 to $7.70). We highlight tradeoffs between maximizing enrollment in forest carbon incentive programs and providing longer term protection of carbon. This research contributes to the literature on the design of potential forest carbon incentive programs and communication about forest carbon management, as well as aims to aid policy makers and program administrators that seek ways to engage private landowners in carbon-oriented forest management.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in w... more Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in western North America, where forest recovery within high-severity burn patch interiors can be slow, yet successful over long temporal periods (decades to centuries). Increasing fire frequency and high-severity burn patch size, under a warming climate, however, may challenge post-fire forest recovery, promoting landscape-level shifts in forest structure, composition, and distribution of non-forest patches. Crucial to a delay and/or impediment to this shift, fire refugia (i.e., remnant seed sources) may determine forest recovery trajectories and potential forest state-transitions. To examine how fire refugia attributes (i.e. extent, composition, and structure) interact with local climate and environmental conditions to determine post-fire forest recovery responses, we developed fine-grain maps of fire refugia via remote sensing and conducted field-based assessment of post-fire conifer tree establishment largely originating (i.e., dispersed) from fire refugium in the Central Cascade Range of the Pacific Northwest United States. We found that limitations on seed availability, represented by the distance 2-weighted density (D 2 WD) of fine-grain refugia extent, largely explained post-fire tree establishment responses within our relatively mesic and cool subalpine study sites. Interactions between seed availability, climate, and environmental conditions indicated that the structural attributes of refugia (e.g., tree height) and site abiotic/biotic environmental controls (e.g., climate water deficit, canopy cover, and coarse woody debris cover) interplayed to constrain or enhance species-specific tree establishment responses. Importantly, these interactions illustrate that when seed availability is critically low for a given area, climate-environment conditions may strongly determine whether forests recover following fire(s). Toward modelling and predicting tree establishment responses and potential forest state-transitions after large stand-replacing fires(s), our study demonstrates the importance of accurately quantifying seed availability via the fine-grain extent, configuration, and attributes of remnant seed source legacies.
species replace obligate seeders, reducing local diversity. We established 120 field plots across... more species replace obligate seeders, reducing local diversity. We established 120 field plots across a burn severity gradient in two study sites: one affected by a single (burned 2015), and the second by two fires (burned 2002 and 2015), where vascular plant species abundance, among other biotic, abiotic, and topographic variables were estimated. We found that burn (high) severity is the main driver of post-fire understory assemblages, resulting in communities less competitive and heterogeneous, with an increasing number of exotic species. Also, post-fire responses are resulting in communities in which the high abundance of flammable taxa and post-fire resprouter species at the early-seral stage may lead to large-scale transitions from mesic forest ecosystems to dry, open forest and fire-prone shrublands in reburned areas. Our results highlight the ecological importance of short-interval and severe wildfires as leading factors in the transition of post-fire understory communities of Araucaria-Nothofagus forests to a system dominated by post-fire obligate resprouters, where tree species are less represented compromising the recovery of these ecosystems. These findings improve the understanding of the current post-fire processes that affect flammability feedbacks and contribute to a baseline of the current patterns in a world of altered fire regimes.
The photographers and figure sources credited herein retain all rights to their images. All other... more The photographers and figure sources credited herein retain all rights to their images. All other elements of the document are published under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0).
Characterizing wildfire regimes where wildfires are uncommon is challenged by a lack of empirical... more Characterizing wildfire regimes where wildfires are uncommon is challenged by a lack of empirical information. Moreover, climate change is projected to lead to increasingly frequent wildfires and additional annual area burned in forests historically characterized by long fire return intervals. Western Oregon and Washington, USA (westside) have experienced few large wildfires (fires greater than 100 hectares) the past century and are characterized to infrequent large fires with return intervals greater than 500 years. We evaluated impacts of climate change on wildfire hazard in a major urban watershed outside Portland, OR, USA. We simulated wildfire occurrence and fire regime characteristics under contemporary conditions (1992-2015) and four mid-century (2040-2069) scenarios using Representative Concentration Pathway (RCP) 8.5. Simulated mid-century fire seasons expanded in most scenarios, in some cases by nearly two months. In all scenarios, average fire size and frequency projections increased significantly. Fire regime characteristics under the hottest and driest mid-century scenarios illustrate novel disturbance regimes which could result in permanent changes to forest structure and composition and the provision of ecosystem services. Managers and planners can use the range of modeled outputs and simulation results to inform robust strategies for climate adaptation and risk mitigation.
Increasing concentrations of greenhouse gases (GHGs) are causing global climate change and decrea... more Increasing concentrations of greenhouse gases (GHGs) are causing global climate change and decreasing the stability of the climate system. Long-term solutions to climate change will require reduction in GHG emissions as well as the removal of large quantities of GHGs from the atmosphere. Natural climate solutions (NCS), i.e., changes in land management, ecosystem restoration, and avoided conversion of habitats, have substantial potential to meet global and national greenhouse gas (GHG) reduction targets and contribute to the global drawdown of GHGs. However, the relative role of NCS to contribute to GHG reduction at subnational scales is not well known. We examined the potential for 12 NCS activities on natural and working lands in Oregon, USA to reduce GHG emissions in the context of the state's climate mitigation goals. We evaluated three alternative scenarios wherein NCS implementation increased across the applicable private or public land base, depending on the activity, and estimated the annual GHG reduction in carbon dioxide equivalents (CO 2 e) attributable to NCS from 2020 to 2050. We found that NCS within Oregon could contribute annual GHG emission reductions of 2.7 to 8.3 MMT CO 2 e by 2035 and 2.9 to 9.8 MMT CO 2 e by 2050. Changes in forest-based activities including deferred timber harvest, riparian reforestation, and replanting after wildfires contributed most to potential GHG reductions (76 to 94% of the overall annual reductions), followed by changes to agricultural management through no-till, cover crops, and nitrogen management (3 to 15% of overall annual reductions). GHG reduction benefits are relatively high per unit area for avoided conversion of forests (125-400 MT CO 2 e ha-1). However, the existing land use policy in Oregon limits the current geographic extent of active conversion of natural lands and thus, avoided conversions results in modest overall potential GHG reduction benefits (i.e., less than 5% of the overall annual reductions). Tidal wetland restoration, which has high per unit area carbon sequestration benefits (8.8 MT CO 2 e ha-1 yr-1), also has limited possible geographic extent resulting in low potential (< 1%) of state-level GHG reduction contributions. However, cobenefits such as improved habitat and water quality delivered by restoration NCS pathways are substantial. Ultimately, reducing GHG emissions and increasing carbon sequestration to combat climate change will require actions across multiple sectors. We demonstrate that
Increasing forest fuel aridity with climate change may be expanding mid-to-high-elevation forests... more Increasing forest fuel aridity with climate change may be expanding mid-to-high-elevation forests' vulnerability to large, severe, and frequent wildfire. Long-lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper-montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post-fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (<12-yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post-fire conifer regeneration was generally very poor among plots that experienced either a single high-severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre-fire dominant, shade-tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed postfire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low-then-high severity, or primarily fire-adapted pines, if high-then-low severity. Our findings suggest that these forests, affected by expansive high-severity and/or short-interval wildfire, may transition into a patchy, low-density, pine-dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire-adapted tree species, lower tree densities, and more non-forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid-to-high-elevation forests may begin resembling their drier, lower-elevation mixed-conifer counterparts in structure and composition.
Untangling the nuanced relationships between landscape, fire disturbance, human agency, and clima... more Untangling the nuanced relationships between landscape, fire disturbance, human agency, and climate is key to understanding rapid population declines of firesensitive plant species. Using multiple lines of evidence across temporal and spatial scales (vegetation survey, stand structure analysis, dendrochronology, and fire history reconstruction), we document landscape-scale population collapse of the
Aim: We examined whether and how tree radial-growth responses to climate have changed for the wor... more Aim: We examined whether and how tree radial-growth responses to climate have changed for the world's southernmost conifer species throughout its latitudinal distribution following rapid climate change in the second half of the 20th century. Location: Temperate forests in southern South America. Methods: New and existing tree-ring radial growth chronologies representing the
Uploads
Papers by Andrés Holz