@article{robbins_loudermilk_mozelewski_jones_scheller_2024, title={Fire regimes of the Southern Appalachians may radically shift under climate change}, volume={20}, ISSN={["1933-9747"]}, url={https://doi.org/10.1186/s42408-023-00231-1}, DOI={10.1186/s42408-023-00231-1}, abstractNote={Abstract
                Background
                Increased drought due to climate change will alter fire regimes in mesic forested landscapes where fuel moisture typically limits fire spread and where fuel loads are consistently high. These landscapes are often extensively modified by human land use change and management. We forecast the influence of varying climate scenarios on potential shifts in the wildfire regime across the mesic forests of the Southern Appalachians. This area has a long history of fire exclusion, land use change, and an expanding wildland urban interface. We considered interactions among climate, vegetation, and anthropogenic influences to forecast future fire regimes and changes to the forest structure. We used climate scenarios representing divergent drought patterns (overall drought trend and interannual variability) within a process-based fire model that captures the influence of climate, fuels, and fire ignition on wildfire patterns and suppression.
              
                Results
                Compared to simulations using historical climate (1972–2018), future total burned area (2020–2100: 782,302.7 (716,655.0–847,950.3) ha) increased by 42.3% under high drought variability (1,134,888.4 (1,067,437.2–1,202,339.6) ha), 104.8% under a substantial increase in drought trend (1,602,085.7 (1,511,837.5–1,692,334.0) ha), and 484.7% when combined (4,573,925.0 (4,434,910.5–4,712,939.5) ha). Landscape patterns of fire exclusion and suppression drove the spatial variability of fire return intervals (FRI). Our projections indicate wide spatial variability in future fire regimes with some areas experiencing multiple fires per decade while others experience no fire. More frequent fires corresponded with increased oak prevalence and a reduction in the biomass of mesic hardwoods and maple; however, mesic hardwoods remained prevalent under all fire intervals because of their contemporary dominance.
              
                Conclusions
                Our study illustrates how future drought–fire–management interactions and a history of fire exclusion could alter future fire regimes and tree species composition. We find that increasing trends in drought magnitude and variability may increase wildfire activity, particularly in areas with minimal fire suppression. In ecosystems where fuel moisture (and not load) is the standard limitation to fire spread, increased pulses of drought may provide the conditions for more fire activity, regardless of effects on fuel loading. We conclude the effects of climate and human management will determine the novel conditions for both fire regime and ecosystem structure.
              }, number={1}, journal={FIRE ECOLOGY}, author={Robbins, Zachary J. and Loudermilk, E. Louise and Mozelewski, Tina G. and Jones, Kate and Scheller, Robert M.}, year={2024}, month={Jan} }
 @article{robbins_xu_jonko_chitra-tarak_fettig_costanza_mortenson_aukema_kueppers_scheller_2023, title={Carbon stored in live ponderosa pines in the Sierra Nevada will not return to pre-drought (2012) levels during the 21st century due to bark beetle outbreaks}, volume={11}, ISSN={["2296-665X"]}, DOI={10.3389/fenvs.2023.1112756}, abstractNote={Outbreaks of several bark beetle species can develop rapidly in response to drought and may result in large transfers of carbon (C) stored in live trees to C stored in dead trees (10s of Tg C yr-1 in the western U.S. alone), which over time will be released back to the atmosphere. The western pine beetle (WPB) outbreak incited by the 2012–2015 mega-drought in the Sierra Nevada, California, U.S., could portend more frequent and/or severe bark beetle outbreaks as the temperature warms and drought frequency and intensity increase in the future. However, changes in the frequency and/or severity (resultant levels of host tree mortality) of beetle outbreaks are difficult to predict as outbreaks are complex with non-linear and eruptive processes primarily driven by interactions among beetle populations, the demography of hosts and other tree species, and climate and weather. Using an insect phenology and tree defense model, we projected the future likelihood of WPB outbreaks in the Sierra Nevada with climate drivers from different Earth System Models. Our goal was to understand how host (ponderosa pine, PIPO) recovery and future warming and drought affect the frequency and severity of WPB outbreaks and their C consequences. Our projections suggested that by 2100 the C stored in live PIPO (mean: 1.98 kg C m-2, 95% CI: 1.74–2.21 kg C m-2) will not return to levels that occurred before the 2012–2015 drought (2012: ∼2.30 kg C m-2) due to future WPB outbreaks. However, differences in climate models indicate a wide range of possible WPB outbreak frequencies and severities. Our results suggest that total plot basal area is the most significant factor in the mortality rate of PIPO by WPB in any given year, followed by drought severity and temperature. High levels of host basal area, higher temperature, and extreme drought all contribute to the frequency and severity of future WPB outbreaks. While PIPO basal area may decline under increased drought and warming, limiting high-stand basal area (>60 m2 ha-1) may reduce the severity of future WPB outbreaks in the Sierra Nevada.}, journal={FRONTIERS IN ENVIRONMENTAL SCIENCE}, author={Robbins, Zachary J. and Xu, Chonggang and Jonko, Alex and Chitra-Tarak, Rutuja and Fettig, Christopher J. and Costanza, Jennifer and Mortenson, Leif A. and Aukema, Brian H. and Kueppers, Lara M. and Scheller, Robert M.}, year={2023}, month={Mar} }
 @article{robbins_loudermilk_reilly_o'brien_jones_gerstle_scheller_2022, title={Delayed fire mortality has long-term ecological effects across the Southern Appalachian landscape}, volume={13}, ISSN={["2150-8925"]}, url={https://doi.org/10.1002/ecs2.4153}, DOI={10.1002/ecs2.4153}, abstractNote={AbstractFire is a critical ecological process to the forests of the Southern Appalachians. Where fire was excluded from forest types that historically burned frequently, unanticipated changes can occur when fire is reintroduced. For example, the development of new fuel characteristics can change the patterns of fire mortality and associated ecological responses. To test the fire effects of delayed fire mortality (mortality initiated by fire that occurs subsequent to the fire year) in the Southern Appalachians, USA, we developed a fire‐effects model using both field studies and remote sensing. We then simulated these effects at a landscape scale to estimate broader ecological effects. Fire‐effects models that accounted for delayed mortality increased landscape biomass removed annually (~23%) and increased the number of sites with high light conditions (leaf area index < 4) when compared to simulations that only account for immediate mortality. While delayed mortality occurred across species and age classes, it was especially prevalent among older trees (>100 years old) and fire‐resistant species (Quercus spp.). Overall, regeneration (trees <20 years old) changed very little, even with the inclusion of delayed mortality. This evidence suggests that, even when accounting for delayed mortality, individual fires are unlikely to shift the landscape composition toward the conditions of forests prior to fire exclusion and may even increase mesophication long term due to the loss of overstory dominant xeric trees.}, number={6}, journal={ECOSPHERE}, publisher={Wiley}, author={Robbins, Zachary J. and Loudermilk, E. Louise and Reilly, Matthew J. and O'Brien, Joseph J. and Jones, Kate and Gerstle, Christopher T. and Scheller, Robert M.}, year={2022}, month={Jun} }
 @article{mozelewski_robbins_scheller_2022, title={Forecasting the influence of conservation strategies on landscape connectivity}, volume={6}, ISSN={["1523-1739"]}, DOI={10.1111/cobi.13904}, abstractNote={AbstractMaintaining and enhancing landscape connectivity reduces biodiversity declines due to habitat fragmentation. Uncertainty remains, however, about the effectiveness of conservation for enhancing connectivity for multiple species on dynamic landscapes, especially over long time horizons. We forecasted landscape connectivity from 2020 to 2100 under four common conservation land‐acquisition strategies: acquiring the lowest cost land, acquiring land clustered around already established conservation areas, acquiring land with high geodiversity characteristics, and acquiring land opportunistically. We used graph theoretic metrics to quantify landscape connectivity across these four strategies, evaluating connectivity for four ecologically relevant species guilds that represent endpoints along a spectrum of vagility and habitat specificity: long‐ versus short‐distance dispersal ability and habitat specialists versus generalists. We applied our method to central North Carolina and incorporated landscape dynamics, including forest growth, succession, disturbance, and management. Landscape connectivity improved for specialist species under all conservation strategies employed, although increases were highly variable across strategies. For generalist species, connectivity improvements were negligible. Overall, clustering the development of new protected areas around land already designated for conservation yielded the largest improvements in connectivity; increases were several orders of magnitude beyond current landscape connectivity for long‐ and short‐distance dispersing specialist species. Conserving the lowest cost land contributed the least to connectivity. Our approach provides insight into the connectivity contributions of a suite of conservation alternatives prior to on‐the‐ground implementation and, therefore, can inform connectivity planning to maximize conservation benefit.}, journal={CONSERVATION BIOLOGY}, author={Mozelewski, Tina G. and Robbins, Zachary J. and Scheller, Robert M.}, year={2022}, month={Jun} }
 @article{buotte_koven_xu_shuman_goulden_levis_katz_ding_ma_robbins_et al._2021, title={Capturing functional strategies and compositional dynamics in vegetation demographic models}, volume={18}, ISSN={["1726-4189"]}, DOI={10.5194/bg-18-4473-2021}, abstractNote={Abstract. Plant community composition influences carbon, water, and
energy fluxes at regional to global scales. Vegetation demographic models
(VDMs) allow investigation of the effects of changing climate and
disturbance regimes on vegetation composition and fluxes. Such investigation
requires that the models can accurately resolve these feedbacks to simulate
realistic composition. Vegetation in VDMs is composed of plant functional
types (PFTs), which are specified according to plant traits. Defining PFTs
is challenging due to large variability in trait observations within and
between plant types and a lack of understanding of model sensitivity to
these traits. Here we present an approach for developing PFT
parameterizations that are connected to the underlying ecological processes
determining forest composition in the mixed-conifer forest of the Sierra
Nevada of California, USA. We constrain multiple relative trait
values between PFTs, as opposed to randomly sampling within the range of
observations. An ensemble of PFT parameterizations are then filtered based
on emergent forest properties meeting observation-based ecological criteria
under alternate disturbance scenarios. A small ensemble of alternate PFT
parameterizations is identified that produces plausible forest composition
and demonstrates variability in response to disturbance frequency and
regional environmental variation. Retaining multiple PFT parameterizations
allows us to quantify the uncertainty in forest responses due to variability
in trait observations. Vegetation composition is a key emergent outcome from
VDMs and our methodology provides a foundation for robust PFT
parameterization across ecosystems.
                    }, number={14}, journal={BIOGEOSCIENCES}, author={Buotte, Polly C. and Koven, Charles D. and Xu, Chonggang and Shuman, Jacquelyn K. and Goulden, Michael L. and Levis, Samuel and Katz, Jessica and Ding, Junyan and Ma, Wu and Robbins, Zachary and et al.}, year={2021}, month={Jul}, pages={4473–4490} }
 @article{vakili_shakeri_motahari_farahani_robbins_scheller_2021, title={Resistance and Resilience of Hyrcanian Mixed Forests Under Natural and Anthropogenic Disturbances}, volume={4}, ISSN={["2624-893X"]}, DOI={10.3389/ffgc.2021.640451}, abstractNote={Biological disturbances are integral to forest ecosystems and have pronounced effects on forest resistance, resilience, and diversity. The Hyrcanian mixed forest, in northern Iran, is at risk of declining resistance, resilience, and diversity due to ongoing pressure from land use change, harvesting, and biological disturbances. We analyzed the resistance and resilience of this area under two biological disturbances (i.e., oak charcoal fungus, Biscogniauxia mediterranea, and alder leaf beetle, Galerucella lineola) and in concert with proposed harvesting. We used a simulation modeling approach whereby we simulated 12 combinations of biological disturbances and harvesting scenarios using the LANDIS-II landscape change model. We estimated the correlation between forest resistance and resilience and tree species diversity to harvesting and biological disturbance. We analyzed the full species composition and age class for 30 and 100 years after disturbances in order to assess resistance as the change in species composition over time. We considered resilience as the ability to recover from a disturbance back to a similar initial state. Results indicate a positive effect of biological disturbances and harvesting on diversity. Our simulations resulted in a negative relationship between diversity-resistance and diversity-resilience within high diversity areas. Our simulation of the Hyrcanian forest reveals that harvesting and biological disturbances, as tested, fulfill the goal of maintaining forest diversity. However, increasing diversity does not always follow by increasing forest resistance and resilience. Scenarios with oak charcoal fungus, both with and without harvesting indicate the lowest decrease in resilient and resistant.}, journal={FRONTIERS IN FORESTS AND GLOBAL CHANGE}, author={Vakili, Mehdi and Shakeri, Zahed and Motahari, Saeed and Farahani, Maryam and Robbins, Zachary James and Scheller, Robert M.}, year={2021}, month={Jul} }
 @article{robbins_xu_aukema_buotte_chitra-tarak_fettig_goulden_goodsman_hall_koven_et al._2021, title={Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California}, volume={10}, ISSN={["1365-2486"]}, DOI={10.1111/gcb.15927}, abstractNote={AbstractQuantifying the responses of forest disturbances to climate warming is critical to our understanding of carbon cycles and energy balances of the Earth system. The impact of warming on bark beetle outbreaks is complex as multiple drivers of these events may respond differently to warming. Using a novel model of bark beetle biology and host tree interactions, we assessed how contemporary warming affected western pine beetle (Dendroctonus brevicomis) populations and mortality of its host, ponderosa pine (Pinus ponderosa), during an extreme drought in the Sierra Nevada, California, United States. When compared with the field data, our model captured the western pine beetle flight timing and rates of ponderosa pine mortality observed during the drought. In assessing the influence of temperature on western pine beetles, we found that contemporary warming increased the development rate of the western pine beetle and decreased the overwinter mortality rate of western pine beetle larvae leading to increased population growth during periods of lowered tree defense. We attribute a 29.9% (95% CI: 29.4%–30.2%) increase in ponderosa pine mortality during drought directly to increases in western pine beetle voltinism (i.e., associated with increased development rates of western pine beetle) and, to a much lesser extent, reductions in overwintering mortality. These findings, along with other studies, suggest each degree (°C) increase in temperature may have increased the number of ponderosa pine killed by upwards of 35%–40% °C−1 if the effects of compromised tree defenses (15%–20%) and increased western pine beetle populations (20%) are additive. Due to the warming ability to considerably increase mortality through the mechanism of bark beetle populations, models need to consider climate's influence on both host tree stress and the bark beetle population dynamics when determining future levels of tree mortality.}, journal={GLOBAL CHANGE BIOLOGY}, author={Robbins, Zachary J. and Xu, Chonggang and Aukema, Brian H. and Buotte, Polly C. and Chitra-Tarak, Rutuja and Fettig, Christopher J. and Goulden, Michael L. and Goodsman, Devin W. and Hall, Alexander D. and Koven, Charles D. and et al.}, year={2021}, month={Oct} }