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1. Plant hydraulics, stomatal control, and the response of a tropical forest to water stress over multiple temporal scales

   https://doi.org/10.1111/gcb.16179  

   Detto, Matteo ; Pacala, Stephen W. ( April 2022 , Global Change Biology)

   Many tropical regions are experiencing an intensification of drought, with increasing severity and frequency. The ecosystem response to these changes is still highly uncertain. On short time scales (from diurnal to seasonal), tropical forests respond to water stress by physiological controls, such as stomatal regulation and phenological adjustment, to cope with increasing atmospheric water demand and reduced water supply. However, the interactions among biological processes and co‐varying environmental factors that determine the ecosystem‐level fluxes are still unclear. Furthermore, climate variability at longer time scales, such as that generated by ENSO, produces less predictable effects because it depends on a highly stochastic combination of factors that might vary among forests and even between events in the same forest. This study will present some emerging patterns of response to water stress from 5 years of water, carbon, and energy fluxes observed on a seasonal tropical forest in central Panama, including an increase in productivity during the 2015 El Niño. These responses depend on the combination of environmental factors experienced by the forest throughout the seasonal cycle, in particular, increase in solar radiation, stimulating productivity, and increasing vapor pressure deficit (VPD) and decreasing soil moisture, limiting stomata opening. These results suggest a critical role of plant hydraulics in mediating the response to water stress over a broad range of temporal scales (diurnal, intraseasonal, seasonal, and interannual), by acclimating canopy conductance to light and VPD during different soil moisture regimes. A multilayer photosynthesis model coupled with a plant hydraulics scheme can reproduce these complex responses. However, results depend critically on parameters regulating water transport efficiency and the cost of water stress. As these costs have not been properly identified and quantified yet, more empirical research is needed to elucidate physiological mechanisms of hydraulic failure and recover, for example embolism repair and xylem regrowth.

    

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2. Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought‐induced tipping point

   https://doi.org/10.1111/gcb.16933  

   Restrepo‐Coupe, Natalia ; O’Donnell Christoffersen, Bradley ; Longo, Marcos ; Alves, Luciana F. ; Campos, Kleber Silva ; da Araujo, Alessandro C. ; de Oliveira, Jr, Raimundo C. ; Prohaska, Neill ; da Silva, Rodrigo ; Tapajos, Raphael ; et al ( September 2023 , Global Change Biology)

   Understanding the effects of intensification of Amazon basin hydrological cycling—manifest as increasingly frequent floods and droughts—on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest “tipping points”. Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001–2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015–2016 El Niño drought and La Niña 2008–2009 wet events. We found that the forest responded strongly to El Niño‐Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). PartitioningETby an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress‐induced reductions in canopy conductance (G_s) droveTdeclines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higherTand lowerE, with little change in seasonalET. Both El Niño‐Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet‐season leaf area index. However, only during El Niño 2015–2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown ofG_sand significant leaf shedding). Drought‐reducedTandG_s, higherHandE, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post‐drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin‐scale threshold‐crossing changes in forest energy and water cycling, leading to slow‐down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.

    

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3. Sixteen hundred years of increasing tree cover prior to modern deforestation in Southern Amazon and Central Brazilian savannas

   https://doi.org/10.1111/gcb.15382  

   Wright, Jamie L. ; Bomfim, Barbara ; Wong, Corrine I. ; Marimon‐Júnior, Ben H. ; Marimon, Beatriz S. ; Silva, Lucas C. R. ( October 2020 , Global Change Biology)

   Tropical ecosystems are under increasing pressure from land‐use change and deforestation. Changes in tropical forest cover are expected to affect carbon and water cycling with important implications for climatic stability at global scales. A major roadblock for predicting how tropical deforestation affects climate is the lack of baseline conditions (i.e., prior to human disturbance) of forest–savanna dynamics. To address this limitation, we developed a long‐term analysis of forest and savanna distribution across the Amazon–Cerrado transition of central Brazil. We used soil organic carbon isotope ratios as a proxy for changes in woody vegetation cover over time in response to fluctuations in precipitation inferred from speleothem oxygen and strontium stable isotope records. Based on stable isotope signatures and radiocarbon activity of organic matter in soil profiles, we quantified the magnitude and direction of changes in forest and savanna ecosystem cover. Using changes in tree cover measured in 83 different locations for forests and savannas, we developed interpolation maps to assess the coherence of regional changes in vegetation. Our analysis reveals a broad pattern of woody vegetation expansion into savannas and densification within forests and savannas for at least the past ~1,600 years. The rates of vegetation change varied significantly among sampling locations possibly due to variation in local environmental factors that constrain primary productivity. The few instances in which tree cover declined (7.7% of all sampled profiles) were associated with savannas under dry conditions. Our results suggest a regional increase in moisture and expansion of woody vegetation prior to modern deforestation, which could help inform conservation and management efforts for climate change mitigation. We discuss the possible mechanisms driving forest expansion and densification of savannas directly (i.e., increasing precipitation) and indirectly (e.g., decreasing disturbance) and suggest future research directions that have the potential to improve climate and ecosystem models.

    

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4. Seasonality and Budgets of Soil Greenhouse Gas Emissions From a Tropical Dry Forest Successional Gradient in Costa Rica

   https://doi.org/10.1029/2020JG005647  

   Calvo‐Rodriguez, Sofia ; Kiese, Ralf ; Sánchez‐Azofeifa, G. Arturo ( September 2020 , Journal of Geophysical Research: Biogeosciences)

   Limited information on greenhouse gas emissions from tropical dry forest soils still hinders the assessment of the sources/sinks from this ecosystem and their contribution at global scales. Particularly, rewetting events after the dry season can have a significant effect on soil biogeochemical processes and associated exchange of greenhouse gases. This study evaluated the temporal variation and annual fluxes of CO₂, N₂O, and CH₄from soils in a tropical dry forest successional gradient. After a prolonged drought of 5 months, large emissions pulses of CO₂and N₂O were observed at all sites following first rain events, caused by the “Birch effect,” with a significant effect on the net ecosystem exchange and the annual emissions budget. Annual CO₂emissions were greatest for the young forest (8,556 kg C ha⁻¹yr⁻¹) followed by the older forest (7,420 kg C ha⁻¹yr⁻¹) and the abandoned pasture (7,224 kg C ha⁻¹yr⁻¹). Annual emissions of N₂O were greatest for the forest sites (0.39 and 0.43 kg N ha⁻¹yr⁻¹) and least in the abandoned pasture (0.09 kg N ha⁻¹yr⁻¹). CH₄uptake was greatest in the older forest (−2.61 kg C ha⁻¹yr⁻¹) followed by the abandoned pasture (−0.69 kg C ha⁻¹yr⁻¹) and the young forest (−0.58 kg C ha⁻¹yr⁻¹). Fluxes were mainly influenced by soil moisture, microbial biomass, and soil nitrate and ammonium concentrations. Annual CO₂and N₂O soil fluxes of tropical dry forests in this study and others from the literature were much lower than the annual fluxes in wetter tropical forests. Conversely, tropical dry forests and abandoned pastures are on average stronger sinks for CH₄than wetter tropical forests.

    

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5. A changing climate is snuffing out post‐fire recovery in montane forests

   https://doi.org/10.1111/geb.13174  

   Rodman, Kyle C. ; Veblen, Thomas T. ; Battaglia, Mike A. ; Chambers, Marin E. ; Fornwalt, Paula J. ; Holden, Zachary A. ; Kolb, Thomas E. ; Ouzts, Jessica R. ; Rother, Monica T. ; McGill, ed., Brian ( August 2020 , Global Ecology and Biogeography)

   Climate warming is increasing fire activity in many of Earth’s forested ecosystems. Because fire is a catalyst for change, investigation of post‐fire vegetation response is critical to understanding the potential for future conversions from forest to non‐forest vegetation types. We characterized the influences of climate and terrain on post‐fire tree regeneration and assessed how these biophysical factors might shape future vulnerability to wildfire‐driven forest conversion.

   Montane forests, Rocky Mountains, USA.

   1981–2099.

   Pinus ponderosa;Pseudotsuga menziesii.

   We developed a database of dendrochronological samples (n = 717) and plots (n = 1,301) in post‐fire environments spanning a range of topoclimatic settings. We then used statistical models to predict annual post‐fire seedling establishment suitability and total post‐fire seedling abundance from a suite of biophysical correlates. Finally, we reconstructed recent trends in post‐fire recovery and projected future dynamics using three general circulation models (GCMs) under moderate and extreme CO₂emission scenarios.

   Though growing season (April–September) precipitation during the recent period (1981–2015) was positively associated with suitability for post‐fire tree seedling establishment, future (2021–2099) trends in precipitation were widely variable among GCMs, leading to mixed projections of future establishment suitability. In contrast, climatic water deficit (CWD), which is indicative of warm, dry conditions, was negatively associated with post‐fire seedling abundance during the recent period and was projected to increase throughout the southern Rocky Mountains in the future. Our findings suggest that future increases in CWD and an increased frequency of extreme drought years will substantially reduce post‐fire seedling densities.

   This study highlights the key roles of warming and drying in declining forest resilience to wildfire. Moisture stress, driven by macroclimate and topographic setting, will interact with wildfire activity to shape future vegetation patterns throughout the southern Rocky Mountains, USA.

    

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