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1. Wildfire catalyzes upward range expansion of trembling aspen in southern Rocky Mountain beetle‐killed forests

   https://doi.org/10.1111/jbi.14302  

   Nigro, Katherine M. ; Rocca, Monique E. ; Battaglia, Mike A. ; Coop, Jonathan D. ; Redmond, Miranda D. ( December 2021 , Journal of Biogeography)

   Climate warming is expected to drive upward and poleward shifts at the leading edge of tree species ranges. Disturbance has the potential to accelerate these shifts by altering biotic and abiotic conditions, though this potential is likely to vary by disturbance type. In this study, we assessed whether recent wildfires and spruce beetle outbreaks promoted upward range expansion of trembling aspen.

   The San Juan Mountains of southern Colorado, USA (37°34′–37°50′N, 106°49′–107°21′W).

   Populus tremuloides.

   We used aerial imagery to determine the upper elevational limit of adult aspen and conducted seedling surveys at and above this upper limit in burned and unburned areas, which had already incurred high canopy mortality due to spruce bark beetle (Dendroctonus rufipennis) outbreaks. We compared characteristics of burned versus unburned bark beetle‐killed sites and assessed microsite conditions related to aspen seedling establishment using generalized linear models and interaction indices.

   Aspen seedling establishment occurred upslope of its previous range within burns, but not in unburned areas, despite severe beetle‐driven canopy mortality across all sites before the fire. Aspen seedling establishment was associated more with the light and mineral soil created by fire than the presence of nearby seed sources. Aspen seedlings were associated with nurse objects such as logs and rocks at the highest elevations, where these objects may ameliorate a range of stressors associated with the high elevation range boundary.

   Not all disturbance types are equal in promoting tree species migrations at the leading edge. Range shifts can be highly localized, and microsites are important for driving local range expansions in transitional environments. The mosaic of future disturbances across the landscape will drive forest compositional shifts, depending on the disturbance types and the species they promote.

    

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2. Vulnerability of northern rocky mountain forests under future drought, fire, and harvest

   https://doi.org/10.3389/ffgc.2023.1146033  

   Stenzel, Jeffrey E. ; Kolden, Crystal A. ; Buotte, Polly C. ; Bartowitz, Kristina J. ; Walsh, Eric W. ; Hudiburg, Tara W. ( September 2023 , Frontiers in Forests and Global Change)

   Novel climate and disturbance regimes in the 21st century threaten to increase the vulnerability of some western U.S. forests to loss of biomass and function. However, the timing and magnitude of forest vulnerabilities are uncertain and will be highly variable across the complex biophysical landscape of the region. Assessing future forest trajectories and potential management impacts under novel conditions requires place-specific and mechanistic model projections. Stakeholders in the high-carbon density forests of the northern U.S. Rocky Mountains (NRM) currently seek to understand and mitigate climate risks to these diverse conifer forests, which experienced profound 20th century disturbance from the 1910 “Big Burn” and timber harvest. Present forest management plan revisions consider approaches including increases in timber harvest that are intended to shift species compositions and increase forest stress tolerance. We utilize CLM-FATES, a dynamic vegetation model (DVM) coupled to an Earth Systems Model (ESM), to model shifting NRM forest carbon stocks and cover, production, and disturbance through 2100 under unprecedented climate and management. Across all 21st century scenarios, domain forest C-stocks and canopy cover face decline after 2090 due to the interaction of intermittent drought and fire mortality with declining Net Primary Production (NPP) and post-disturbance recovery. However, mid-century increases in forest vulnerability to fire and drought impacts are not consistently projected across climate models due to increases in precipitation that buffer warming impacts. Under all climate scenarios, increased harvest regimes diminish forest carbon stocks and increase period mortality over business-as-usual, despite some late-century reductions in forest stress. Results indicate that existing forest carbon stocks and functions are moderately persistent and that increased near-term removals may be mistimed for effectively increasing resilience.

    

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3. Expanding the invasion footprint: Ventenata dubia and relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA

   https://doi.org/10.1111/avsc.12511  

   Tortorelli, Claire M. ; Krawchuk, Meg A. ; Kerns, Becky K. ; Fraser, ed., Lauchlan ( August 2020 , Applied Vegetation Science)

   A recently introduced non‐native annual grass,Ventenata dubia, is challenging previous conceptions of community resistance in forest mosaic communities in the Inland Northwest. However, little is known of the drivers and potential ecological impacts of this rapidly expanding species. Here we (1) identify abiotic and biotic habitat characteristics associated with theV. dubiainvasion and examine how these differ betweenV. dubiaand other problematic non‐native annual grasses,Bromus tectorumandTaeniatherum caput‐medusae; and (2) determine how burning influences relationships betweenV. dubiaand plant community composition and structure to address potential impacts on Inland Northwest forest mosaic communities.

   Blue Mountains of the Inland Northwest, USA.

   We measured environmental and plant community characteristics in 110 recently burned and nearby unburned plots. Plots were stratified to capture a range ofV. dubiacover, elevations, biophysical classes, and fire severities. We investigated relationships betweenV. dubia, wildfire, environmental, and plant community characteristics using non‐metric multidimensional scaling and linear regressions.

   Ventenata dubiawas most abundant in sparsely vegetated, basalt‐derived rocky scablands interspersed throughout the forested landscape. Plant communities most heavily invaded byV. dubiawere largely uninvaded by other non‐native annual grasses.Ventenata dubiawas abundant in both unburned and burned areas, but negative relationships betweenV. dubiacover and community diversity were stronger in burned plots, where keystone sagebrush species were largely absent after fire.

   Ventenata dubiais expanding the overall invasion footprint into previously uninvaded communities. Burning may exacerbate negative relationships betweenV. dubiaand species richness, evenness, and functional diversity, including in communities that historically rarely burned. Understanding the drivers and impacts of theV. dubiainvasion and recognizing how these differ from other annual grass invasions may provide insight into mechanisms of community invasibility, grass‐fire feedbacks, and aid the development of species‐specific management plans.

    

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4. Less fuel for the next fire? Short‐interval fire delays forest recovery and interacting drivers amplify effects

   https://doi.org/10.1002/ecy.4042  

   Braziunas, Kristin H. ; Kiel, Nathan G. ; Turner, Monica G. ( April 2023 , Ecology)

   As 21st‐century climate and disturbance dynamics depart from historic baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100–300 year), severe fire. We sampled paired short‐interval (<30‐year) and long‐interval (>125‐year) post‐fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short‐interval fire, climate, topography, and distance to unburned live forest edge interact to affect post‐fire forest regeneration? (2) How do forest biomass and fuels vary following short‐interval versus long‐interval severe fires? Mean post‐fire live tree stem density was an order of magnitude lower following short‐interval versus long‐interval fires (3240 vs. 28,741 stems ha⁻¹, respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer–drier climate was associated with higher seedling densities even after short‐interval fire, likely relating to regional variation in serotiny of lodgepole pine (Pinus contortavar.latifolia). Unlike conifers, density of aspen (Populus tremuloides), a deciduous resprouter, increased with short‐interval versus long‐interval fires (mean 384 vs. 62 stems ha⁻¹, respectively). Live biomass and canopy fuels remained low nearly 30 years after short‐interval fire, in contrast with rapid recovery after long‐interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short‐interval plots also had half as much dead woody biomass compared with long‐interval plots (60 vs. 121 Mg ha⁻¹), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short‐interval versus long‐interval fires will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short‐interval fires to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of a future fire.

    

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5. Widespread regeneration failure in forests of Greater Yellowstone under scenarios of future climate and fire

   https://doi.org/10.1111/gcb.15726  

   Rammer, Werner ; Braziunas, Kristin H. ; Hansen, Winslow D. ; Ratajczak, Zak ; Westerling, Anthony L. ; Turner, Monica G. ; Seidl, Rupert ( July 2021 , Global Change Biology)

   Changing climate and disturbance regimes are increasingly challenging the resilience of forest ecosystems around the globe. A powerful indicator for the loss of resilience is regeneration failure, that is, the inability of the prevailing tree species to regenerate after disturbance. Regeneration failure can result from the interplay among disturbance changes (e.g., larger and more frequent fires), altered climate conditions (e.g., increased drought), and functional traits (e.g., method of seed dispersal). This complexity makes projections of regeneration failure challenging. Here we applied a novel simulation approach assimilating data‐driven fire projections with vegetation responses from process modeling by means of deep neural networks. We (i) quantified the future probability of regeneration failure; (ii) identified spatial hotspots of regeneration failure; and (iii) assessed how current forest types differ in their ability to regenerate under future climate and fire. We focused on the Greater Yellowstone Ecosystem (2.9 × 10⁶ ha of forest) in the Rocky Mountains of the USA, which has experienced large wildfires in the past and is expected to undergo drastic changes in climate and fire in the future. We simulated four climate scenarios until 2100 at a fine spatial grain (100 m). Both wildfire activity and unstocked forest area increased substantially throughout the 21st century in all simulated scenarios. By 2100, between 28% and 59% of the forested area failed to regenerate, indicating considerable loss of resilience. Areas disproportionally at risk occurred where fires are not constrained by topography and in valleys aligned with predominant winds. High‐elevation forest types not adapted to fire (i.e.,Picea engelmannii–Abies lasiocarpaas well as non‐serotinousPinus contortavar.latifoliaforests) were especially vulnerable to regeneration failure. We conclude that changing climate and fire could exceed the resilience of forests in a substantial portion of Greater Yellowstone, with profound implications for carbon, biodiversity, and recreation.

    

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