Droughts, Wildfires, and Forest Carbon Cycling: A Pantropical Synthesis

2019 ◽  
Vol 47 (1) ◽  
pp. 555-581 ◽  
Author(s):  
Paulo M. Brando ◽  
Lucas Paolucci ◽  
Caroline C. Ummenhofer ◽  
Elsa M. Ordway ◽  
Henrik Hartmann ◽  
...  
Keyword(s):  
Tropical Forests ◽  
Tree Mortality ◽  
Scientific Evidence ◽  
Alternative Stable States ◽  
Forest Carbon ◽  
Autotrophic Respiration ◽  
Stable States ◽  
Threshold Conditions ◽  
The Tropics ◽  
Forest C

Tropical woody plants store ∼230 petagrams of carbon (PgC) in their aboveground living biomass. This review suggests that these stocks are currently growing in primary forests at rates that have decreased in recent decades. Droughts are an important mechanism in reducing forest C uptake and stocks by decreasing photosynthesis, elevating tree mortality, increasing autotrophic respiration, and promoting wildfires. Tropical forests were a C source to the atmosphere during the 2015–2016 El Niño–related drought, with some estimates suggesting that up to 2.3 PgC were released. With continued climate change, the intensity and frequency of droughts and fires will likely increase. It is unclear at what point the impacts of severe, repeated disturbances by drought and fires could exceed tropical forests’ capacity to recover. Although specific threshold conditions beyond which ecosystem properties could lead to alternative stable states are largely unknown, the growing body of scientific evidence points to such threshold conditions becoming more likely as climate and land use change across the tropics. ▪ Droughts have reduced forest carbon uptake and stocks by elevating tree mortality, increasing autotrophic respiration, and promoting wildfires. ▪ Threshold conditions beyond which tropical forests are pushed into alternative stable states are becoming more likely as effects of droughts intensify.

scholarly journals Estimating aboveground carbon density and its uncertainty in Borneo’s structurally complex tropical forests using airborne laser scanning

2018 ◽  
Author(s):  
Tommaso Jucker ◽  
Gregory P. Asner ◽  
Michele Dalponte ◽  
Philip Brodrick ◽  
Christopher D. Philipson ◽  
...  
Keyword(s):  
Tropical Forests ◽  
Laser Scanning ◽  
Canopy Cover ◽  
Carbon Stocks ◽  
Forest Carbon ◽  
Airborne Laser Scanning ◽  
Tropical Asia ◽  
Airborne Laser ◽  
Dipterocarp Forests ◽  
The Tropics

Abstract. Borneo contains some of the world’s most biodiverse and carbon dense tropical forest, but this 750 000-km2 island has lost 62 % of its old-growth forests within the last 40 years. Efforts to protect and restore the remaining forests of Borneo hinge on recognising the ecosystem services they provide, including their ability to store and sequester carbon. Airborne Laser Scanning (ALS) is a remote sensing technology that allows forest structural properties to be captured in great detail across vast geographic areas. In recent years ALS has been integrated into state-wide assessment of forest carbon in Neotropical and African regions, but not yet in Asia. For this to happen, new regional models, need to be developed for estimating carbon stocks from ALS in tropical Asia, as the forests of this region are structurally and compositionally distinct from those found elsewhere in the tropics. By combining ALS imagery with data from 173 permanent forest plots spanning the lowland rain forests of Sabah, on the island of Borneo, we develop a simple-yet-general model for estimating forest carbon stocks using ALS-derived canopy height and canopy cover as input metrics. An advanced feature of this new model is the propagation of uncertainty in both ALS- and ground-based data, allowing uncertainty in hectare-scale estimates of carbon stocks to be quantified robustly. We show that the model effectively captures variation in aboveground carbons stocks across extreme disturbance gradients spanning tall dipterocarp forests and heavily logged regions, and clearly outperforms existing ALS-based models calibrated for the tropics, as well as currently available satellite-derived products. Our model provides a simple, generalised and effective approach for mapping forest carbon stocks in Borneo, and underpins ongoing efforts to safeguard and facilitate the restoration of its unique tropical forests.

scholarly journals Palaeo-trajectories of forest savannization in the southern Congo

2019 ◽  
Vol 15 (8) ◽  
pp. 20190284
Author(s):  
Julie C. Aleman ◽  
Olivier Blarquez ◽  
Hilaire Elenga ◽  
Jordan Paillard ◽  
Victor Kimpuni ◽  
...  
Keyword(s):  
Tropical Forests ◽  
Vegetation Structure ◽  
Fire Regime ◽  
Alternative Stable States ◽  
Fire Regimes ◽  
Ecosystem Structure ◽  
Anthropogenic Influences ◽  
Stable States ◽  
Positive Feedbacks ◽  
Modern Landscape

Tropical savannah and forest are thought to represent alternative stable states in ecosystem structure in some climates. The implication is that biomes are maintained by positive feedbacks, e.g. with fire, and that historical distributions could play a role in determining modern ones. In this context, climate alone does not govern transitions between biomes, and understanding the causes and pathways of such transitions becomes crucial. Here, we use a multi-proxy analysis of a 2000-year core to evaluate modes of transition in vegetation structure and fire regimes. We demonstrate a first transition ca 1540 BP, when a cyclic fire regime entered a forested landscape, eventually resulting, by ca 1060 BP, in a transition to a more open savannah-like or mosaicked structure. This pattern may parallel currently accelerating fire regimes in tropical forests suggesting that fires can savannize forests, but perhaps more slowly than feared. Finally, ca 540 BP, a drought combined with anthropogenic influences resulted in a conclusive transition to savannah, probably resembling the modern landscape in the region. We show here that fire interacted with drought to transition forest to savannah, suggesting that disturbance by fire can be a major driver of biome change.

scholarly journals Carbon Dynamics in a Human-Modified Tropical Forest: A Case Study Using Multi-Temporal LiDAR Data

2020 ◽  
Vol 12 (3) ◽  
pp. 430 ◽  
Author(s):  
Yhasmin Mendes de Moura ◽  
Heiko Balzter ◽  
Lênio S. Galvão ◽  
Ricardo Dalagnol ◽  
Fernando Espírito-Santo ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Tropical Forests ◽  
Carbon Dynamics ◽  
Carbon Stocks ◽  
Forest Carbon ◽  
Selective Logging ◽  
Carbon Accounting ◽  
Lidar Data ◽  
Multi Temporal ◽  
The Tropics

Tropical forests hold significant amounts of carbon and play a critical role on Earth´s climate system. To date, carbon dynamics over tropical forests have been poorly assessed, especially over vast areas of the tropics that have been affected by some type of disturbance (e.g., selective logging, understory fires, and fragmentation). Understanding the multi-temporal dynamics of carbon stocks over human-modified tropical forests (HMTF) is crucial to close the carbon cycle balance in the tropics. Here, we used multi-temporal and high-spatial resolution airborne LiDAR data to quantify rates of carbon dynamics over a large patch of HMTF in eastern Amazon, Brazil. We described a robust approach to monitor changes in aboveground forest carbon stocks between 2012 and 2018. Our results showed that this particular HMTF lost 0.57 m·yr−1 in mean forest canopy height and 1.38 Mg·C·ha−1·yr−1 of forest carbon between 2012 and 2018. LiDAR-based estimates of Aboveground Carbon Density (ACD) showed progressive loss through the years, from 77.9 Mg·C·ha−1 in 2012 to 53.1 Mg·C·ha−1 in 2018, thus a decrease of 31.8%. Rates of carbon stock changes were negative for all time intervals analyzed, yielding average annual carbon loss rates of −1.34 Mg·C·ha−1·yr−1. This suggests that this HMTF is acting more as a source of carbon than a sink, having great negative implications for carbon emission scenarios in tropical forests. Although more studies of forest dynamics in HMTFs are necessary to reduce the current remaining uncertainties in the carbon cycle, our results highlight the persistent effects of carbon losses for the study area. HMTFs are likely to expand across the Amazon in the near future. The resultant carbon source conditions, directly associated with disturbances, may be essential when considering climate projections and carbon accounting methods.

scholarly journals A Matter of Life and Death: Alternative Stable States in Trees, From Xylem to Ecosystems

2020 ◽  
Vol 3 ◽  
Author(s):  
William M. Hammond
Keyword(s):  
Tree Mortality ◽  
Alternative Stable States ◽  
Physiological Mechanism ◽  
Stable State ◽  
Stable Condition ◽  
State Theory ◽  
Future Climate ◽  
Environmental Drivers ◽  
Stable States ◽  
Hydraulic Failure

Global forests are experiencing widespread climate-induced mortality. Predicting this phenomenon has proven difficult, despite recent advances in understanding physiological mechanisms of mortality in individual trees along with environmental drivers of mortality at broad scales. With heat and drought as primary climatic drivers, and convergence on hydraulic failure as a primary physiological mechanism, new models are needed to improve our predictions of Earth’s forests under future climate conditions. While much of ecology focuses on equilibrium states, transitions from one stable state to another are often described with alternative stable state theory (ASST), where systems can settle to more than one stable condition. Recent studies have identified threshold responses of hydraulic failure during tree mortality, indicating that alternative stable states may be present. Here, I demonstrate that the xylem of trees has characteristics indicative of alternative stable states. Through empirical evidence, I identify a catastrophic shift during hydraulic failure which prevents trees from returning to pre-droughted physiological states after environmental stressors (e.g., drought, heat) are relieved. Thus, the legacy of climate-induced hydraulic failure likely contributes to reduced resilience of forests under future climate. I discuss the implications and future directions for including ASST in models of tree mortality.

scholarly journals Estimating aboveground carbon density and its uncertainty in Borneo's structurally complex tropical forests using airborne laser scanning

2018 ◽  
Vol 15 (12) ◽  
pp. 3811-3830 ◽  
Author(s):  
Tommaso Jucker ◽  
Gregory P. Asner ◽  
Michele Dalponte ◽  
Philip G. Brodrick ◽  
Christopher D. Philipson ◽  
...  
Keyword(s):  
Tropical Forests ◽  
Laser Scanning ◽  
Carbon Stocks ◽  
Forest Carbon ◽  
Airborne Laser Scanning ◽  
Tropical Asia ◽  
Aboveground Carbon ◽  
Airborne Laser ◽  
Dipterocarp Forests ◽  
The Tropics

Abstract. Borneo contains some of the world's most biodiverse and carbon-dense tropical forest, but this 750 000 km2 island has lost 62 % of its old-growth forests within the last 40 years. Efforts to protect and restore the remaining forests of Borneo hinge on recognizing the ecosystem services they provide, including their ability to store and sequester carbon. Airborne laser scanning (ALS) is a remote sensing technology that allows forest structural properties to be captured in great detail across vast geographic areas. In recent years ALS has been integrated into statewide assessments of forest carbon in Neotropical and African regions, but not yet in Asia. For this to happen new regional models need to be developed for estimating carbon stocks from ALS in tropical Asia, as the forests of this region are structurally and compositionally distinct from those found elsewhere in the tropics. By combining ALS imagery with data from 173 permanent forest plots spanning the lowland rainforests of Sabah on the island of Borneo, we develop a simple yet general model for estimating forest carbon stocks using ALS-derived canopy height and canopy cover as input metrics. An advanced feature of this new model is the propagation of uncertainty in both ALS- and ground-based data, allowing uncertainty in hectare-scale estimates of carbon stocks to be quantified robustly. We show that the model effectively captures variation in aboveground carbon stocks across extreme disturbance gradients spanning tall dipterocarp forests and heavily logged regions and clearly outperforms existing ALS-based models calibrated for the tropics, as well as currently available satellite-derived products. Our model provides a simple, generalized and effective approach for mapping forest carbon stocks in Borneo and underpins ongoing efforts to safeguard and facilitate the restoration of its unique tropical forests.

Fire Feedbacks with Vegetation and Alternative Stable States

2009 ◽  
Vol 18 (1) ◽  
pp. 159-173 ◽  
Author(s):  
Brian Beckage ◽  
Chris Ellingwood ◽  
Keyword(s):  
Alternative Stable States ◽  
Stable States ◽  
Fire Feedbacks

scholarly journals Forest carbon sink neutralized by pervasive growth-lifespan trade-offs

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
R. J. W. Brienen ◽  
L. Caldwell ◽  
L. Duchesne ◽  
S. Voelker ◽  
J. Barichivich ◽  
...  
Keyword(s):  
Tree Growth ◽  
Tree Mortality ◽  
Carbon Sink ◽  
Growth Stimulation ◽  
Forest Carbon ◽  
System Model ◽  
Earth System ◽  
Canopy Tree ◽  
Trade Offs ◽  
Almost All

Abstract Land vegetation is currently taking up large amounts of atmospheric CO2, possibly due to tree growth stimulation. Extant models predict that this growth stimulation will continue to cause a net carbon uptake this century. However, there are indications that increased growth rates may shorten trees′ lifespan and thus recent increases in forest carbon stocks may be transient due to lagged increases in mortality. Here we show that growth-lifespan trade-offs are indeed near universal, occurring across almost all species and climates. This trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment. Thus, current tree growth stimulation will, inevitably, result in a lagged increase in canopy tree mortality, as is indeed widely observed, and eventually neutralise carbon gains due to growth stimulation. Results from a strongly data-based forest simulator confirm these expectations. Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.

Are systems with strong underlying abiotic regimes more likely to exhibit alternative stable states?

Oikos ◽  
2005 ◽  
Vol 110 (2) ◽  
pp. 409-416 ◽  
Author(s):  
Raphael K. Didham ◽  
Corinne H. Watts ◽  
David A. Norton
Keyword(s):  
Alternative Stable States ◽  
Stable States
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