1. Tropical forests account for one‐third of the total metabolic activity of the Earth’s land surface. Hence, understanding the controls on tropical forest photosynthesis and respiration, and the ...allocation of the products of photosynthesis to canopy, woody tissue and rhizosphere, is important to understand global ecosystem functioning. 2. I review how studies in tropical ecosystem ecology have progressed since their inception in the 1960s towards developing a quantitative, mechanistic and global description of the carbon cycle of tropical vegetation. 3. I present a synthesis of studies in tropical forest sites in the Americas and Asia for which gross primary productivity (GPP) has been reported, and a subset of these sites for which net primary productivity (NPP) and ecosystem carbon use efficiency (CUE) have been estimated. GPP ranges between 30 and 40 Mg C ha−1 year−1 in lowland moist tropical forests and declines with elevation. CUE in tropical forests is at the low end of the global range reported for forests. 4. A pathway and framework are presented to explain the link between photosynthesis and tropical forest biomass, and to explain differences in carbon cycling and biomass between forests. Variation in CUE and allocation of NPP can be as important as variation in GPP in explaining differences in tropical forest growth rates between sites. 5. Finally, I explore some of the key questions surrounding the functioning and future of tropical forests in the rapidly changing conditions of the early Anthropocene. 6. Synthesis.There have been significant recent advances in quantifying the carbon cycle of tropical forests, but our understanding of causes of variation amongst forests is still poor. Moreover, we should expect all tropical forests in the 21st century, whether intact or disturbed, to be undergoing rapid change in function and composition; the key challenge for tropical ecosystem ecologists is to determine and understand the major and most fundamental aspects of this change.
▶ This paper synthesises the latest evidence of both the carbon source from tropical forest conversion, and the carbon sink in tropical vegetation. ▶ Remote sensing analyses suggest that tropical ...biome conversion is a source of 1.3
±
0.2
Pg
C
year
−1 to the atmosphere in both periods 1990–1999 and 2000–2005. ▶ Extrapolation of forest inventories suggests that intact tropical biomes were a carbon sink of 1.1
±
0.3
Pg
C
year
−1. ▶ Tropical Asia probably a strong net carbon source and tropical Africa probably a strong net sink. ▶ The net balance of the tropical forest biomes is estimated to be +0.2
±
0.4
Pg
C
year
−1 (not significantly different from zero); this result is consistent with that from atmospheric inversion models that better represent vertical CO
2 profiles (+0.1
±
0.3
Pg
C
year
−1).
Global awareness of the significance of the role that tropical forests play in the global carbon cycle has never been greater, but much uncertainty still exists as to the exact magnitude of this role. Here I review and attempt to synthesise the latest evidence of both the carbon source from tropical forest conversion, and the carbon sink in tropical vegetation. Tropical biome conversion is estimated to be a source of 1.3
±
0.2
Pg
C
year
−1 to the atmosphere in both periods 1990–1999 and 2000–2005, whereas intact tropical biomes were estimated to be a net carbon sink of 1.1
±
0.3
Pg
C
year
−1. The ratios of carbon source to carbon sink are very different for the different tropical continents, reflecting different rates of conversion and different area of forest cover, with tropical Asia probably a strong net carbon source and tropical Africa probably a strong net sink. The net balance of the tropical forest biomes is estimated to be +0.2
±
0.4
Pg
C
year
−1 (not significantly different from zero) over both periods 1990–1999 and 2000–2005; this result is consistent with that from atmospheric inversion models that better represent vertical CO
2 profiles (+0.1
±
0.3
Pg
C
year
−1).
A prominent signal of the Anthropocene is the extinction and population reduction of the megabiota-the largest animals and plants on the planet. However, we lack a predictive framework for the ...sensitivity of megabiota during times of rapid global change and how they impact the functioning of ecosystems and the biosphere. Here, we extend metabolic scaling theory and use global simulation models to demonstrate that (i) megabiota are more prone to extinction due to human land use, hunting, and climate change; (ii) loss of megabiota has a negative impact on ecosystem metabolism and functioning; and (iii) their reduction has and will continue to significantly decrease biosphere functioning. Global simulations show that continued loss of large animals alone could lead to a 44%, 18% and 92% reduction in terrestrial heterotrophic biomass, metabolism, and fertility respectively. Our findings suggest that policies that emphasize the promotion of large trees and animals will have disproportionate impact on biodiversity, ecosystem processes, and climate mitigation.
The rapid anthropogenic climate change that is being experienced in the early twenty-first century is intimately entwined with the health and functioning of the biosphere. Climate change is impacting ...ecosystems through changes in mean conditions and in climate variability, coupled with other associated changes such as increased ocean acidification and atmospheric carbon dioxide concentrations. It also interacts with other pressures on ecosystems, including degradation, defaunation and fragmentation. There is a need to understand the ecological dynamics of these climate impacts, to identify hotspots of vulnerability and resilience and to identify management interventions that may assist biosphere resilience to climate change. At the same time, ecosystems can also assist in the mitigation of, and adaptation to, climate change. The mechanisms, potential and limits of such nature-based solutions to climate change need to be explored and quantified. This paper introduces a thematic issue dedicated to the interaction between climate change and the biosphere. It explores novel perspectives on how ecosystems respond to climate change, how ecosystem resilience can be enhanced and how ecosystems can assist in addressing the challenge of a changing climate. It draws on a Royal Society-National Academy of Sciences Forum held in Washington DC in November 2018, where these themes and issues were discussed. We conclude by identifying some priorities for academic research and practical implementation, in order to maximize the potential for maintaining a diverse, resilient and well-functioning biosphere under the challenging conditions of the twenty-first century. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.
The allocation of the net primary productivity (NPP) of an ecosystem between canopy, woody tissue and fine roots is an important descriptor of the functioning of that ecosystem, and an important ...feature to correctly represent in terrestrial ecosystem models. Here, we collate and analyse a global dataset of NPP allocation in tropical forests, and compare this with the representation of NPP allocation in 13 terrestrial ecosystem models. On average, the data suggest an equal partitioning of allocation between all three main components (mean 34 ± 6% canopy, 39 ± 10% wood, 27 ± 11% fine roots), but there is substantial site-to-site variation in allocation to woody tissue versus allocation to fine roots. Allocation to canopy (leaves, flowers and fruit) shows much less variance. The mean allocation of the ecosystem models is close to the mean of the data, but the spread is much greater, with several models reporting allocation partitioning outside of the spread of the data. Where all main components of NPP cannot be measured, litterfall is a good predictor of overall NPP (r 2 = 0.83 for linear fit forced through origin), stem growth is a moderate predictor and fine root production a poor predictor. Across sites the major component of variation of allocation is a shifting allocation between wood and fine roots, with allocation to the canopy being a relatively invariant component of total NPP. This suggests the dominant allocation trade-off is a 'fine root versus wood' trade-off, as opposed to the expected 'root—shoot' trade-off; such a trade-off has recently been posited on theoretical grounds for old-growth forest stands. We conclude by discussing the systematic biases in estimates of allocation introduced by missing NPP components, including herbivory, large leaf litter and root exudates production. These biases have a moderate effect on overall carbon allocation estimates, but are smaller than the observed range in allocation values across sites.
Abstract
Tropical ecosystems adapted to high water availability may be highly impacted by climatic changes that increase soil and atmospheric moisture deficits. Many tropical regions are experiencing ...significant changes in climatic conditions, which may induce strong shifts in taxonomic, functional and phylogenetic diversity of forest communities. However, it remains unclear if and to what extent tropical forests are shifting in these facets of diversity along climatic gradients in response to climate change. Here, we show that changes in climate affected all three facets of diversity in West Africa in recent decades. Taxonomic and functional diversity increased in wetter forests but tended to decrease in forests with drier climate. Phylogenetic diversity showed a large decrease along a wet-dry climatic gradient. Notably, we find that all three facets of diversity tended to be higher in wetter forests. Drier forests showed functional, taxonomic and phylogenetic homogenization. Understanding how different facets of diversity respond to a changing environment across climatic gradients is essential for effective long-term conservation of tropical forest ecosystems.
Large herbivores and carnivores (the megafauna) have been in a state of decline and extinction since the Late Pleistocene, both on land and more recently in the oceans. Much has been written on the ...timing and causes of these declines, but only recently has scientific attention focused on the consequences of these declines for ecosystem function. Here, we review progress in our understanding of how megafauna affect ecosystem physical and trophic structure, species composition, biogeochemistry, and climate, drawing on special features of PNAS and Ecography that have been published as a result of an international workshop on this topic held in Oxford in 2014. Insights emerging from this work have consequences for our understanding of changes in biosphere function since the Late Pleistocene and of the functioning of contemporary ecosystems, as well as offering a rationale and framework for scientifically informed restoration of megafaunal function where possible and appropriate.
The Concept of the Anthropocene Malhi, Yadvinder
Annual review of environment and resources,
10/2017, Volume:
42, Issue:
1
Journal Article
Peer reviewed
Open access
The Anthropocene, the concept that the Earth has moved into a novel geological epoch characterized by human domination of the planetary system, is an increasingly prevalent framework for debate both ...in academia and as a wider cultural and policy zeitgeist. This article reviews the proliferation of literature surrounding this concept. It explores the origins and history of the concept, as well as the arguments surrounding its geological formalization and starting date ranging from the Pleistocene to the twentieth century. It examines perspectives and critiques of the concept from the Earth system sciences, ecological and geological sciences, and social sciences and humanities, exploring its role as a cultural zeitgeist and ideological provocation. I conclude by offering a personal perspective on the concept of the Anthropocene and its utility.
Global nutrient transport in a world of giants Doughty, Christopher E.; Roman, Joe; Faurby, Søren ...
Proceedings of the National Academy of Sciences - PNAS,
01/2016, Volume:
113, Issue:
4
Journal Article
Peer reviewed
Open access
The past was a world of giants, with abundant whales in the sea and large animals roaming the land. However, that world came to an end following massive late-Quaternarymegafauna extinctions on land ...and widespread population reductions in great whale populations over the past few centuries. These losses are likely to have had important consequences for broad-scale nutrient cycling, because recent literature suggests that large animals disproportionately drive nutrient movement. We estimate that the capacity of animals to move nutrients away from concentration patches has decreased to about 8% of the preextinction value on land and about 5%of historic values in oceans. For phosphorus (P), a key nutrient, upward movement in the ocean by marine mammals is about 23% of its former capacity (previously about 340 million kg of P per year). Movements by seabirds and anadromous fish provide important transfer of nutrients from the sea to land, totalling ∼150 million kg of P per year globally in the past, a transfer that has declined to less than 4% of this value as a result of the decimation of seabird colonies and anadromous fish populations. We propose that in the past, marine mammals, seabirds, anadromous fish, and terrestrial animals likely formed an interlinked system recycling nutrients from the ocean depths to the continental interiors, with marine mammals moving nutrients from the deep sea to surface waters, seabirds and anadromous fish moving nutrients from the ocean to land, and large animals moving nutrients away from hotspots into the continental interior.
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