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1. Pacific Northwest conifer forest stand carrying capacity under future climate scenarios

   https://doi.org/10.1111/nrm.12381  

   Heiderman, Ryan R.; Kimsey, Mark J. (November 2023, Natural Resource Modeling)

   Abstract Maximum stand density index (SDI_MAX) represents the carrying capacity of a forest stand based on the relationship between the number of trees and their size. Plot‐level inventory data provided through a collaborative network of federal, state, and private forest management groups were utilized to develop SDI_MAXmodels for important Pacific Northwest conifers of western Washington and Oregon, USA. The influence of site‐specific climatic and environmental variables was explored within an ensemble learning model. Future climate projections based on global circulation models under different representative CO₂concentration pathways (RCP 4.5 and RCP 8.5) and timeframes (2050s and 2080s) were utilized in a space‐for‐time substitution to understand potential shifts in modeled SDI_MAX. A majority of the region showed decreases in carrying capacity under future climate conditions. Modeled mean SDI_MAXdecreased 5.4% and 11.4% for Douglas‐fir (Pseudotsuga menziesii(Mirb.) Franco) dominated forests and decreased 6.6% and 8.9% for western hemlock (Tsuga heterophylla(Raf.) Sarg.) and Pacific silver fir (Abies amabilis), dominated forests under the RCP 4.5 in the 2050s and RCP 8.5 in the 2080s, respectively. Projected future conditions often fall outside the range of any contemporary climate profile, resulting in what may be referred to as extramural conditions. Within the study region, 45% and 46% of climate variables included in the final model were extramural for the Douglas‐fir and hemlock models, respectively, under RCP 8.5 in the 2080s. Although extrapolating beyond the range of input data is not appropriate and many unknowns remain regarding future climate projections, these results allow for general interpretations of the direction and magnitude of potential shifts in forest carrying capacity. 

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2. Restoration Thinning in a Drought‐Prone Idaho Forest Creates a Persistent Carbon Deficit

   https://doi.org/10.1029/2020JG005815  

   Stenzel, J. E.; Berardi, D. M.; Walsh, E. S.; Hudiburg, T. W. (March 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Western US forests represent a carbon sink that contributes to meeting regional and global greenhouse gas targets. Forest thinning is being implemented as a strategy for reducing forest vulnerability to disturbance, including mortality from fire, insects, and drought, as well as protecting human communities. However, the terrestrial carbon balance impacts of thinning remain uncertain across regions, spatiotemporal scales, and treatment types. Continuous and in situ long‐term measurements of partial harvest impacts to stand‐scale carbon and water cycle dynamics are nonetheless rare. Here, we examine post‐thinning carbon and water flux impacts in a young ponderosa pine forest in Northern Idaho. We examine in situ stock and flux impacts during the 3 years after treatment as well as simulate the forest sector carbon balance through 2050, including on and off‐site net emissions. During the observation period, increases in tree‐scale net primary production (NPP) and water use persistence through summer drought did not overcome the impacts of density reduction, leading to 45% annual reductions of NPP. Growth duration remained constrained by summer drought in control and thinned stands. Ecosystem model and life cycle assessment estimates demonstrated a net forest sector carbon deficit relative to control stands of 27.0 Mg C ha⁻¹in 2050 due to emissions from dead biomass pools despite increases to net ecosystem production. Our results demonstrate dynamics resulting in carbon losses from forest thinning, providing a baseline with which to inform landscape‐scale modeling and assess tradeoffs between harvest losses and potential gains from management practices. 

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3. Relative density of United States forests has shifted to higher levels over last two decades with important implications for future dynamics

   https://doi.org/10.1038/s41598-021-98244-w  

   Woodall, C. W.; Weiskittel, A. R. (December 2021, Scientific Reports)

   null (Ed.)

   Abstract Tree size-density dynamics can inform key trends in forest productivity along with opportunities to increase ecosystem resiliency. Here, we employ a novel approach to estimate the relative density (RD, range 0–1) of any given forest based on its current size-density relationship compared to a hypothetical maximum using the coterminous US national forest inventory between 1999 and 2020. The analysis suggests a static forest land area in the US with less tree abundance but greatly increased timber volume and tree biomass. Coupled with these resource trends, an increase in RD was identified with 90% of US forest land now reaching a biologically-relevant threshold of canopy closure and/or self-thinning induced mortality (RD > 0.3), particularly in areas prone to future drought conditions (e.g., West Coast). Notably, the area of high RD stands (RD > 0.6) has quintupled over the past 20 years while the least stocked stands (RD < 0.3) have decreased 3%. The evidence from the coterminous US forest RD distribution suggest opportunities to increase live tree stocking in understocked stands, while using density management to address tree mortality and resilience to disturbances in increasingly dense forests. 

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4. An experimental comparison of stand management approaches to sudden oak death: prevention vs restoration

   https://doi.org/10.1139/cjfr-2022-0328  

   Quiroga, Gissella Betzavel; Simler-Williams, Allison B; Frangioso, Kerri M; Frankel, Susan; Rizzo, David M.; Cobb, Richard (January 2023, Canadian Journal of Forest Research)

   Many coastal forests stretching from central California to southwest Oregon are threatened or have been impacted by the invasive forest pathogen Phytophthora ramorum, cause of sudden oak death. We analyzed a set of stand-level forest treatments aimed at preventing or mitigating disease impacts on stand composition, biomass, and fuels, using a before-after-control-intervention experiment with a revaluation after five years. We compared the effects of restorative management in invaded stands to preventative treatments in uninvaded forests. The restorative treatments contrasted two approaches to mastication, hand-crew thinning, and thinning with pile burning with untreated controls (N=30) while the preventative treatments were limited to hand-crew thinning (N=10). Half of the restoration treatments had basal sprouts removed two- and four-years after treatment. All treatments significantly reduced stand density and increased average tree size without significantly decreasing total basal area both immediately and five years after treatments. Preventative treatments also significantly increased dominance of timber species not susceptible to P. ramorum. Follow-up basal sprout removal in the restoration experiment appears to maintain treatment benefits to average tree size and may be associated with small decreases in stand density five years after initial treatment. Our study demonstrates that for at least five years, a range of common stand management practices can improve forests threatened or impacted by sudden oak death. 

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5. Soil microbial community response to ectomycorrhizal dominance in diverse neotropical montane forests

   https://doi.org/10.1007/s00572-023-01134-4  

   Edwards, Joseph D.; Krichels, Alexander H.; Seyfried, Georgia S.; Dalling, James; Kent, Angela D.; Yang, Wendy H. (January 2024, Mycorrhiza)

   Abstract Ectomycorrhizal (EM) associations can promote the dominance of tree species in otherwise diverse tropical forests. These EM associations between trees and their fungal mutualists have important consequences for soil organic matter cycling, yet the influence of these EM-associated effects on surrounding microbial communities is not well known, particularly in neotropical forests. We examined fungal and prokaryotic community composition in surface soil samples from mixed arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) stands as well as stands dominated by EM-associatedOreomunnea mexicana(Juglandaceae) in four watersheds differing in soil fertility in the Fortuna Forest Reserve, Panama. We hypothesized that EM-dominated stands would support distinct microbial community assemblages relative to the mixed AM-EM stands due to differences in carbon and nitrogen cycling associated with the dominance of EM trees. We expected that this microbiome selection in EM-dominated stands would lead to lower overall microbial community diversity and turnover, with tighter correspondence between general fungal and prokaryotic communities. We measured fungal and prokaryotic community composition via high-throughput Illumina sequencing of theITS2(fungi) and16SrRNA (prokaryotic) gene regions. We analyzed differences in alpha and beta diversity between forest stands associated with different mycorrhizal types, as well as the relative abundance of fungal functional groups and various microbial taxa. We found that fungal and prokaryotic community composition differed based on stand mycorrhizal type. There was lower prokaryotic diversity and lower relative abundance of fungal saprotrophs and pathogens in EM-dominated than AM-EM mixed stands. However, contrary to our prediction, there was lower homogeneity for fungal communities in EM-dominated stands compared to mixed AM-EM stands. Overall, we demonstrate that EM-dominated tropical forest stands have distinct soil microbiomes relative to surrounding diverse forests, suggesting that EM fungi may filter microbial functional groups in ways that could potentially influence plant performance or ecosystem function. 

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