Chapter Seven - Critical role and collapse of tropical mega-trees: A key global resource

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Abstract

Very large tropical forest trees (‘mega-trees’) represent an irreplaceable habitat associated with large benefits in terms of biodiversity and ecosystem services. Here we provide a comprehensive overview of the importance of tropical mega-trees relative to biodiversity persistence, ecosystem services, and sociocultural value. We describe all contemporary threats to mega-trees as a result of the conversion of old-growth forests into human-degraded landscapes exposed to climate change. We reveal a myriad of taxa, functional groups and ecosystem services that are directly or indirectly supported by tropical mega-trees, such as highly illuminated crowns for epiphytes and large boles and branches sheltering many vertebrate and invertebrate taxa. Mega-trees also amplify the resource spectra associated with non-redundant niche space, including thermal buffering and extended vertical microclimates above the forest canopy. Mega-trees also make the largest contribution to forest productivity, aboveground biomass, and timber. However, protecting this resource is a major challenge due to the natural rarity of mega-trees, illegal logging, old-growth forest conversion and climate change. We argue for ‘productive forest landscapes’ as a strategy to protect mega-trees and highlight their connections with sustainable development goals and other global targets. This strategy poses economic, political, technological and social challenges, as sparing tropical forests still incurs high opportunity costs that few stakeholders can realistically afford. Despite these challenges, the future of tropical forests mega-trees and local livelihoods are inextricably linked, and productive forest landscapes would bring clear unanticipated benefits for future generations and tropical biodiversity.

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

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