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1. Prolonged tropical forest degradation due to compounding disturbances: Implications for CO 2 and H 2 O fluxes

   https://doi.org/10.1111/gcb.14659  

   Brando, Paulo M. ; Silvério, Divino ; Maracahipes‐Santos, Leonardo ; Oliveira‐Santos, Claudinei ; Levick, Shaun R. ; Coe, Michael T. ; Migliavacca, Mirco ; Balch, Jennifer K. ; Macedo, Marcia N. ; Nepstad, Daniel C. ; et al ( June 2019 , Global Change Biology)

   Drought, fire, and windstorms can interact to degrade tropical forests and the ecosystem services they provide, but how these forests recover after catastrophic disturbance events remains relatively unknown. Here, we analyze multi‐year measurements of vegetation dynamics and function (fluxes of CO₂and H₂O) in forests recovering from 7 years of controlled burns, followed by wind disturbance. Located in southeast Amazonia, the experimental forest consists of three 50‐ha plots burned annually, triennially, or not at all from 2004 to 2010. During the subsequent 6‐year recovery period, postfire tree survivorship and biomass sharply declined, with aboveground C stocks decreasing by 70%–94% along forest edges (0–200 m into the forest) and 36%–40% in the forest interior. Vegetation regrowth in the forest understory triggered partial canopy closure (70%–80%) from 2010 to 2015. The composition and spatial distribution of grasses invading degraded forest evolved rapidly, likely because of the delayed mortality. Four years after the experimental fires ended (2014), the burned plots assimilated 36% less carbon than the Control, but net CO₂exchange and evapotranspiration (ET) had fully recovered 7 years after the experimental fires ended (2017). Carbon uptake recovery occurred largely in response to increased light‐use efficiency and reduced postfire respiration, whereas increased water use associated with postfire growth of new recruits and remaining trees explained the recovery in ET. Although the effects of interacting disturbances (e.g., fires, forest fragmentation, and blowdown events) on mortality and biomass persist over many years, the rapid recovery of carbon and water fluxes can help stabilize local climate.

    

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2. Disturbance‐accelerated succession increases the production of a temperate forest

   https://doi.org/10.1002/eap.2417  

   Gough, Christopher M. ; Bohrer, Gil ; Hardiman, Brady S. ; Nave, Lucas E. ; Vogel, Christoph S. ; Atkins, Jeff W. ; Bond‐Lamberty, Ben ; Fahey, Robert T. ; Fotis, Alexander T. ; Grigri, Maxim S. ; et al ( August 2021 , Ecological Applications)

   Many secondary deciduous forests of eastern North America are approaching a transition in which mature early‐successional trees are declining, resulting in an uncertain future for this century‐long carbon (C) sink. We initiated the Forest Accelerated Succession Experiment (FASET) at the University of Michigan Biological Station to examine the patterns and mechanisms underlying forest C cycling following the stem girdling‐induced mortality of >6,700 early‐successionalPopulusspp. (aspen) andBetula papyrifera(paper birch). Meteorological flux tower‐based C cycling observations from the 33‐ha treatment forest have been paired with those from a nearby unmanipulated forest since 2008. Following over a decade of observations, we revisit our core hypothesis: that net ecosystem production (NEP) would increase following the transition to mid‐late‐successional species dominance due to increased canopy structural complexity. Supporting our hypothesis, NEP was stable, briefly declined, and then increased relative to the control in the decade following disturbance; however, increasing NEP was not associated with rising structural complexity but rather with a rapid 1‐yr recovery of total leaf area index as mid‐late‐successionalAcer,Quercus, andPinusassumed canopy dominance. The transition to mid‐late‐successional species dominance improved carbon‐use efficiency (CUE = NEP/gross primary production) as ecosystem respiration declined. Similar soil respiration rates in control and treatment forests, along with species differences in leaf physiology and the rising relative growth rates of mid‐late‐successional species in the treatment forest, suggest changes in aboveground plant respiration and growth were primarily responsible for increases in NEP. We conclude that deciduous forests transitioning from early to middle succession are capable of sustained or increased NEP, even when experiencing extensive tree mortality. This adds to mounting evidence that aging deciduous forests in the region will function as C sinks for decades to come.

    

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3. 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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4. Dynamic subcanopy leaf traits drive resistance of net primary production across a disturbance severity gradient

   https://doi.org/10.3389/ffgc.2023.1150209  

   Haber, Lisa T. ; Atkins, Jeff W. ; Bond-Lamberty, Ben P. ; Gough, Christopher M. ( July 2023 , Frontiers in Forests and Global Change)

   Across the globe, the forest carbon sink is increasingly vulnerable to an expanding array of low- to moderate-severity disturbances. However, some forest ecosystems exhibit functional resistance (i.e., the capacity of ecosystems to continue functioning as usual) following disturbances such as extreme weather events and insect or fungal pathogen outbreaks. Unlike severe disturbances (e.g., stand-replacing wildfires), moderate severity disturbances do not always result in near-term declines in forest production because of the potential for compensatory growth, including enhanced subcanopy production. Community-wide shifts in subcanopy plant functional traits, prompted by disturbance-driven environmental change, may play a key mechanistic role in resisting declines in net primary production (NPP) up to thresholds of canopy loss. However, the temporal dynamics of these shifts, as well as the upper limits of disturbance for which subcanopy production can compensate, remain poorly characterized. In this study, we leverage a 4-year dataset from an experimental forest disturbance in northern Michigan to assess subcanopy community trait shifts as well as their utility in predicting ecosystem NPP resistance across a wide range of implemented disturbance severities. Through mechanical girdling of stems, we achieved a gradient of severity from 0% (i.e., control) to 45, 65, and 85% targeted gross canopy defoliation, replicated across four landscape ecosystems broadly representative of the Upper Great Lakes ecoregion. We found that three of four examined subcanopy community weighted mean (CWM) traits including leaf photosynthetic rate ( p = 0.04), stomatal conductance ( p = 0.07), and the red edge normalized difference vegetation index ( p < 0.0001) shifted rapidly following disturbance but before widespread changes in subcanopy light environment triggered by canopy tree mortality. Surprisingly, stimulated subcanopy production fully compensated for upper canopy losses across our gradient of experimental severities, achieving complete resistance (i.e., no significant interannual differences from control) of whole ecosystem NPP even in the 85% disturbance treatment. Additionally, we identified a probable mechanistic switch from nutrient-driven to light-driven trait shifts as disturbance progressed. Our findings suggest that remotely sensed traits such as the red edge normalized difference vegetation index (reNDVI) could be particularly sensitive and robust predictors of production response to disturbance, even across compositionally diverse forests. The potential of leaf spectral indices to predict post-disturbance functional resistance is promising given the capabilities of airborne to satellite remote sensing. We conclude that dynamic functional trait shifts following disturbance can be used to predict production response across a wide range of disturbance severities. 

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5. Testing conceptual models of early plant succession across a disturbance gradient

   https://doi.org/10.1111/1365-2745.13120  

   ( January 2019 , Journal of Ecology)

   Studies of succession have a long history in ecology, but rigorous tests of general, unifying principles are rare. One barrier to these tests of theory is the paucity of longitudinal studies that span the broad gradients of disturbance severity that characterize large, infrequent disturbances. The cataclysmic eruption of Mount St. Helens (Washington, USA) in 1980 produced a heterogeneous landscape of disturbance conditions, including primary to secondary successional habitats, affording a unique opportunity to explore how rates and patterns of community change relate to disturbance severity, post‐eruption site conditions and time.

   In this novel synthesis, we combined data from three long‐term (c.30‐year) studies to compare rates and patterns of community change across three ‘zones’ representing a gradient of disturbance severity: primary successional blast zone, secondary successional tree blowdown/standing snag zone and secondary successional intact forest canopy/tephra deposit zone.

   Consistent with theory, rates of change in most community metrics (species composition, species richness, species gain/loss and rank abundance) decreased with time across the disturbance gradient. Surprisingly, rates of change were often greatest at intermediate‐severity disturbance and similarly low at high‐ and low‐severity disturbance. There was little evidence of compositional convergence among or within zones, counter to theory. Within zones, rates of change did not differ among ‘site types’ defined by pre‐ or post‐eruption site characteristics (disturbance history, legacy effects or substrate characteristics).

   Synthesis.The hump‐shaped relationships with disturbance severity runs counter to the theory predicting that community change will be slower during primary than during secondary succession. The similarly low rates of change after high‐ and low‐severity disturbance reflect differing sets of controls: seed limitation and abiotic stress in the blast zone vs. vegetative re‐emergence and low light in the tephra zone. Sites subjected to intermediate‐severity disturbance were the most dynamic, supporting species with a greater diversity of regenerative traits and seral roles (ruderal, forest and non‐forest). Succession in this post‐eruption landscape reflects the complex, multifaceted nature of volcanic disturbance (including physical force, heating and burial) and the variety of ways in which biological systems can respond to these disturbance effects. Our results underscore the value of comparative studies of long‐term, ecological processes for testing the assumptions and predictions of successional theory.

    

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