Chapter Seven - Critical role and collapse of tropical mega-trees: A key global resource
Introduction
Tropical forests play a prominent role at local, regional and global scales in relation to biodiversity persistence, provision of livelihood goods (e.g. timber, fibre, fuelwood, medicinal plants) and ecosystem services (e.g. climate regulation, carbon sequestration and storage) essential for human well-being (Beer et al., 2010; Bonan, 2008; Houghton et al., 2015; Malhi, 2012). Tropical forests thus represent a key component of global sustainability (Malhi et al., 2014), but old-growth tropical forests worldwide continue to be relentlessly converted into degraded human-modified landscapes (Laurance et al., 2014; Lewis et al., 2015).
In this context, it is speculated that a large portion of the direct or indirect benefits provided by tropical forests are disproportionately contributed by very large trees or ‘mega-trees’ (see Fig. 1 for definition of a ‘mega-tree’ and key features). This is particularly the case of lowland tropical forests, in which mega-tree species attain the highest levels of species richness and biomass contribution (Balzotti et al., 2017; Corlett and Primack, 2011; Gonmadje et al., 2017). To provide a single example, Southeast Asian forests and their flora contain over 500 mega-tree species (Ghazoul, 2016). Although mega-trees usually represent a small fraction of the overall tree abundance, they account for most of the forest biomass, thereby contributing the lion's share of effects on biogeochemical cycles and ecosystem processes such as carbon storage, water cycling and nutrient dynamics (Bastin et al., 2018; Lutz et al., 2018; Nepstad et al., 1994; Sist et al., 2014; Slik et al., 2013). Moreover, by over-towering the prevailing forest canopy and interacting with the hotter, drier and more open conditions of the emergent stratum, mega-trees add non-redundant complexity to the vertical vegetation structure and control forest microclimate, providing unique microhabitats for myriad of organisms, many of which are obligate mega-tree attendants and/or commensals (Cockle et al., 2011a, Cockle et al., 2011b; Woods et al., 2015). Mega-tree species can also be exceptionally long-lived (i.e. over hundreds of years), enhancing their long-term value and the ecological benefits they provide (Chambers et al., 1998; Laurance et al., 2004). This brief description supports the notion that mega-tree species represent a unique physical, functional and ecological life-history strategy that cannot be replaced by other plant functional groups.
However, exceptional stature also imposes costs (see Falster and Westoby, 2003), such as higher investment in supporting stem and root structures in relation to photosynthetic foliage, thereby reducing reproductive output (Kawecki, 1993); increasing risk of hydraulic failure (i.e. lower resistance to cavitation) under extreme drought conditions (Brum et al., 2019); and higher breakage rates due to strong winds (Laurance et al., 2000). Many of these features partly explain why mega-trees are highly sensitive to natural and human disturbances. Currently, mega-trees are exposed to an increasing myriad of threats operating from local (e.g. selective logging; Sist et al., 2014) to regional (e.g. forest fragmentation, Laurance et al., 2000; Santos et al., 2008; severe droughts and wildfires, Barlow et al., 2003; Nepstad et al., 2007) and global scales (e.g. climate change; Bennett et al., 2015), with unanticipated impacts on biodiversity persistence, provision of ecosystem services and human well-being.
Despite such a disproportionately important role, a robust synthesis of the importance of mega-trees is still lacking. Although some reviews on the ecology and conservation of large ancient trees are available (Lindenmayer and Laurance, 2016, Lindenmayer and Laurance, 2017), these provide little emphasis on the ecological role and conservation status of mega-trees in tropical forest environments, thus leaving several gaps to be properly described and/or filled. A more comprehensive examination on the extent to which mega-trees support tropical biodiversity through countless direct and indirect interactions with a myriad of taxa and ecological groups is still required. Moreover, tropical mega-trees support several ecosystem functions and services, forest products and cultural benefits, most of which are yet to be acknowledged and properly described. Threats to tropical mega-trees must also be interpreted in the context of global trends/changes, to provide sound guidelines for mega-tree conservation action embedded in global agendas. By underappreciating the multiple contribution of tropical mega-trees and their requirements, we risk not only the persistence of this global heritage but also the opportunity to develop tropical forest regions sustainably.
Here we provide a comprehensive overview of the importance of tropical mega-trees in relation to biodiversity persistence, ecosystem services, and economic and cultural values. We particularly focus on their biodiversity benefits across eight categories of supporting mechanisms. We also describe and integrate all contemporary threats to mega-trees as an expected consequence of the conversion of old-growth forests into degraded human-modified landscapes that are increasingly exposed to climate change. To tackle this alarming trajectory, we argue in favour of ‘productive forest landscapes’ as a strategy to protect mega-tree species and highlight their positive connections with the sustainable development goals and other global targets. To achieve this ambitious goal, we highlight (1) key gaps to be filled, such as the relative contribution of mega-tree species to the diversity of tropical floras, NTFP opportunities and an acknowledgement of mega-trees as priceless ‘spiritual entities’ in contemporary and ancient human cultures, and (2) public forestry policies as a key strategy of socioeconomic development. Our narrative is based on a review of over a thousand papers systematically compiled through the Web of Science and Google Scholar covering many aspects of the ecology, evolution, distribution and conservation of mega-trees (for Methods, see Box 1). We acknowledge this contribution is far from exhaustive, but we hope to stimulate debate on sustainable use of tropical forests and the required research and policy agendas that consider mega-trees as highly sensitive keystone species.
Section snippets
Supporting biodiversity
Mega-trees per se contribute greatly to tropical biodiversity, but the extent to which this operates and the biotic diversity involved remains a knowledge gap. We refer to hundreds to thousands of tree species with direct and indirect effects on tropical forest biodiversity. Biodiversity-supporting services provided by mega-tree species are intrinsically associated with their complex and colossal three-dimensional structure (Fig. 2). This includes large crowns, low-angled bifurcations and
Economic value of mega-trees
Tropical forests provide many commercially valuable products, including both timber and non-timber forest products. They also provide globally important ecosystem services, the market value of which is still poorly quantified, such as carbon sequestration/storage and water cycling. Timber and non-timber forest products have been exploited and traded by humans for centuries, while the carbon market is a recent and promising economic avenue for remaining forest stands (Box 2). Although these
Contemporary threats to tropical mega-trees
The majority of mega-tree species are locally rare and may also exhibit narrow geographic distributions, potentially making them naturally vulnerable to extinction (Hubbell et al., 2008; Newbold et al., 2018; Rabinowitz, 1981). To give but one example, at least 90 Shorea emergent dipterocarp tree species are endemic to Borneo (Ghazoul, 2016). In addition to such natural rarity, mega-tree species vulnerability can be summarized into three major threats: landscape modification (i.e. habitat loss,
Synthesis and way forward
Research on the complex and manifold ecological roles exerted by mega-tree species—on biodiversity persistence, provision of ecosystem services and livelihoods of forest-dependent peoples—is at best embryonic. As a thought-provoking exercise we could assume that the relative biomass sustained by mega-tree species represent a proxy for the amount of foliage, fruits, flowers, litter and microhabitats mega-trees offer to plant attendants and their networks of interactions, which in many cases
Acknowledgements
This review was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior—Brazil (CAPES)—Finance code 001, the Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq (grant 403770/2012-2), and a Newton Fund Institutional Partnership award between the University of East Anglia and Universidade Federal de Pernambuco. B.X.P. was funded by a PhD scholarship from CNPq. I.R.L. and M.T. also thank CNPq for productivity grants. We thank Jannah Oliveira for hand-drawing all
References (209)
- et al.
Uncertainty in the biomass of Amazonian forests: an example from Rondônia, Brazil
For. Ecol. Manag.
(1995) - et al.
Abundance, growth and mortality of very large trees in Neotropical lowland rain forest
For. Ecol. Manag.
(1996) - et al.
Supply of tree-holes limits nest density of cavity-nesting birds in primary and logged subtropical Atlantic forest
Biol. Conserv.
(2010) - et al.
Linking fungi, trees, and hole-using birds in a Neotropical tree-cavity network: pathways of cavity production and implications for conservation
For. Ecol. Manag.
(2012) - et al.
Reduced availability of large seeds constrains Atlantic forest regeneration
Acta Oecol.
(2012) - et al.
Biased seed rain in forest edges: evidence from the Brazilian Atlantic forest
Biol. Conserv.
(2006) - et al.
Nesting and nest trees of stingless bees (Apidae: Meliponini) in lowland dipterocarp forests in Sabah, Malaysia, with implications for forest management
For. Ecol. Manag.
(2003) - et al.
Scaling from traits to ecosystems: developing a general trait driver theory via integrating trait-based and metabolic scaling theories
Adv. Ecol. Res.
(2015) - et al.
Plant height and evolutionary games
Trends Ecol. Evol.
(2003) - et al.
Is multiple-use forest management widely implementable in the tropics?
For. Ecol. Manag.
(2008)
Multiple use management of tropical production forests: how can we move from concept to reality?
For. Ecol. Manag.
The relationship between leaf area index and microclimate in tropical forest and oil palm plantation: forest disturbance drives changes in microclimate
Agric. For. Meteorol.
Interception storage capacities of tropical rainforest canopy trees
J. Hydrol.
Determination of deforestation rates of the world's humid tropical forests
Science
Big-sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests
Glob. Chang. Biol.
Biology and Conservation of the Harpy Eagle in Venezuela and Panama
Selective logging in the Brazilian Amazon
Science
Variation in wood density determines spatial patterns in Amazonian forest biomass
Glob. Chang. Biol.
Assessing the economic value of traditional medicines from tropical rain forests
Conserv. Biol.
Topographic distributions of emergent trees in tropical forests of the Osa Peninsula, Costa Rica
Ecography
Anthropogenic disturbance in tropical forests can double biodiversity loss from deforestation
Nature
Fire-mediated dieback and a compositional cascade in Amazonian forests
Philos. Trans. R. Soc. B
Large tree mortality and the decline of forest biomass following Amazonian wildfires
Ecol. Lett.
Predicting shifts in the functional composition of tropical forests under increased drought and CO2 from trade-offs among plant hydraulic traits
Ecol. Lett.
Pan-tropical prediction of forest structure from the largest trees
Glob. Ecol. Biogeogr.
Do Neotropical peccary species (Tayassuidae) function as ecosystem engineers for anurans?
J. Trop. Ecol.
Terrestrial gross carbon dioxide uptake: global distribution and covariation with climate
Science
Impact of forest fragmentation on understory plant species richness in Amazonia
Conserv. Biol.
Larger trees suffer most during drought in forests worldwide
Nat. Plants
Vascular Epiphytes: General Biology and Related Biota
Wood identification of 18th century furniture: interpreting wood naming inventories
Int. J. Conserv. Sci.
Sacred groves: potential for biodiversity management
Front. Ecol. Environ.
Conservation status of tropical raptors
J. Raptor Res.
Projected strengthening of Amazonian dry season by constrained climate model simulations
Nat. Clim. Chang.
Forests and climate change: forcings, feedbacks, and the climate benefits of forests
Science
Density, distribution, and attributes of tree cavities in an old-growth tropical rain forest
Biotropica
Drought effects on litterfall, wood production and belowground carbon cycling in an Amazon forest: results of a throughfall reduction experiment
Philos. Trans. R. Soc. B
Competition, predation and nest niche shifts among tropical cavity nesters: ecological evidence
J. Avian Biol.
Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest
J. Ecol.
Tree species diversity in commercially logged Bornean rainforest
Science
Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages
Proc. R. Soc. B
Ancient trees in Amazonia
Nature
Vertical stratification of leaf-beetle assemblages (Coleoptera: Chrysomelidae) in two forest types in Panama
J. Trop. Ecol.
Sleeping site selection by agile gibbons: the influence of tree stability, fruit availability and predation risk
Folia Primatol.
Vertical stratification of an avian community in New Guinean tropical rainforest
Popul. Ecol.
Selected Species and Strategies to Enhance Income Generation From Amazonian Forests
Woodpeckers, decay, and the future of cavity-nesting vertebrate communities worldwide
Front. Ecol. Environ.
Selection of nest trees by cavity-nesting birds in the Neotropical Atlantic Forest
Biotropica
Low recruitment of trees dispersed by animals in African forest fragments
Conserv. Biol.
Tropical Rain Forests: An Ecological and Biogeographical Comparison
Cited by (34)
Sparing old-growth maximises conservation outcomes within selectively logged Amazonian rainforest
2023, Biological ConservationPotential losses of animal-dispersed trees due to selective logging in Amazonian forest concessions
2022, Trees, Forests and PeopleCitation Excerpt :At least ten percent of the large zoochorous trees across the landscape were logged, with site-level losses sometimes exceeding one third of all zoochorous trees. In addition, collateral damage from tree felling and infrastructure development, which was unaccounted for in our analysis, may further increase losses (Lima et al., 2020; Pinho et al., 2020). These losses may be substantial, as even RIL can result in the incidental mortality or severe damage to non-target large trees (Feldpausch et al., 2005).
Neglected diversity of crop pollinators: Lessons from the world's largest tropical country
2021, Perspectives in Ecology and ConservationIntensification of açaí palm management largely impoverishes tree assemblages in the Amazon estuarine forest
2021, Biological ConservationCitation Excerpt :Here, tree stem density explained 50% of the variation in tree species richness, and when tree density was below 60 stems/ha, at least one third of tree species were lost. A minimum number of tree stems should be targeted not only for the sake of the tree flora, but also because tree species, particularly large trees, respond to substantial proportion of the ecosystem services provided by tropical forests, including biodiversity persistence through several taxa (Campbell et al., 2018; Freitas et al., 2015; Pinho et al., 2020). Açaí production has already achieved an economic boom (>US$290 million/year), with global markets for açaí still expanding (açaí-related products are available in many wealth countries).