Functional trait representation differs between restoration plantings and mature tropical rainforest

https://doi.org/10.1016/j.foreco.2020.118304Get rights and content

Highlights

  • We present a landscape scale study of species composition of restoration plantings.

  • Species abundances were highly uneven.

  • Small-seeded, fast growing species were most abundant in restoration plantings.

  • Low abundance of large-seeded species did not result in lack of large tree species.

Abstract

The planting and attempted restoration of tropical forest landscapes is increasing rapidly across the globe. Two limiting aspects of large-scale forest restoration are the demand for appropriate quantities of seeds and seedlings of native species, and the ability to facilitate succession in planted sites. Species functional traits such as seed type, tree size, germination time, and wood density may influence the quantity of seedlings that can be produced for restoration, and the potential of these seedlings to persist and facilitate site succession. Therefore, it is important to understand the species composition and functional trait representation of restoration plantings. We explored the species composition and functional trait representation of 846 restoration plantings in the Australian Wet Tropics containing > 465,000 seedlings from 599 species, using seedling supply records from six nurseries over a six year period (2012–2017). Despite restoration plantings in the Australian Wet Tropics containing an impressive number of species, just 52 species contributed over half of all individual seedlings. We found that species with small animal-dispersed seeds and low wood density were more abundant, on average, and had greater representation in restoration plantings than in mature rainforest. Despite this, we did not find evidence that restoration plantings had a diminished capacity to grow tall or sequester carbon as there was no significant difference in the relative abundance of tall tree species or species with high wood density. Small seeded and fast growing species may be cheaper to produce in nurseries and may accelerate site succession as these characteristics are associated with pioneer and early successional species, however these traits are also associated with higher mortality rates. Understanding how functional trait representation influences the success of restoration plantings will require further insight into temporal aspects of site succession.

Introduction

Deforestation has resulted in substantial loss and fragmentation of the world’s tropical forests (Hansen et al., 2013; Taubert et al., 2018), reducing their ability to support biodiversity (Allnutt et al., 2008) and provide ecosystem services (Portela and Rademacher, 2001, Chazdon, 2008). Loss and fragmentation of tropical forests reduce population sizes (Brook et al., 2003, Ewers and Didham, 2006), gene flow (Hamilton, 1999, Ewers and Didham, 2006), and dispersal potential of both flora and fauna (Laurance et al., 2004, Hadley and Betts, 2009, Cole et al., 2011), exacerbating their vulnerability under changing climatic conditions and limiting migration to more suitable regions. Additionally, deforestation substantially impacts many ecosystem services provided by tropical forests including carbon cycling (Baccini et al., 2017), water cycling (Webb et al., 2005, Mahmood et al., 2013, Schlesinger and Jasechko, 2014), temperature regulation (Mahmood et al., 2013), and soil maintenance (Hartanto et al., 2003). In this context, ecological restoration is a useful tool for expanding and connecting habitat patches in fragmented landscapes to retain biodiversity value and provision of ecosystem services (DeFries et al., 2010; Hansen et al., 2013; Taubert et al., 2018).

Numerous global initiatives, such as the Bonn Challenge, recognize the importance of tropical forest restoration and are establishing pledges to restore 350 million hectares by 2030 (Chazdon et al., 2017, Dave et al., 2019). The sheer scale of plant material required to meet these initiatives is daunting. For example restoration projects in India and Ethiopia in 2019, planted 220 million trees and 350 million trees, respectively. However, it is not only the quantity of plant production that is important, but what species are selected and the implications this may have on plant survival, site succession, and the future ecological function of these restored forests. For example, Brancalion et al. (2018) compared the functional traits of planted and mature Atlantic forests in Brazil, and found that species with small animal-dispersed or wind-dispersed seeds were far more abundant in planted forests than in mature rainforest. Although species with these characteristics are common in naturally recovering forests (Goosem et al., 2016), planting millions of trees with similar traits could have important implications for successional trajectories because it may “inadvertently homogenize” plant communities across the landscape (Palma & Laurance, 2015).

Despite the potential for functional trait representation to influence recovery trajectories and biodiversity in tropical forest restoration, studies examining this are rare. After almost three decades of restoration activity in the Australian Wet Tropics, no studies have yet explored the ecological implications of planting composition. The aim of this study was to assess the relative abundance of species used in restoration plantings for both individual plantings, and the pool of all restoration plantings. Additionally, we aimed to compare species abundance relative to functional traits with direct influences on ecological function of restoration plantings, and the relative representation of these functional traits between restoration plantings and mature rainforest. The functional traits of seed type (size and dispersal mode), maximum height, wood density, and germination time were selected for comparison and binned into discreet functional groups based on previous ecological literature and available data. Two key questions were formulated: (1) How does seedling abundance at the level of individual plantings, and the pool of restoration plantings, differ between species and functional groups? (2) Does the relative representation of functional groups differ between restoration plantings and mature forest? We expected that species from functional groups associated with increased seed and seedling production (small seeds, shorter germination times, low wood density) would have higher abundance in restoration plantings, and that this reduction in the relative abundance of large-seeded, slow-growing species would result in a lower abundance of large tree species.

Section snippets

Study area

The Wet Tropics biogeographic region of north-eastern Australia, henceforth referred to as the ‘Australian Wet Tropics’, encompasses an area of ~ 900,000 ha of tropical forest (Kanowski et al., 2003), that spans 450 km of north-eastern Australia (Fig. 1). The region represents an area of significant floral and faunal biodiversity with a high level of endemism (Williams et al., 2009), and is considered an area of Outstanding Universal Value (UNESCO, n.d.). To date, the Australian Wet Tropics has

Species diversity and abundance

Over a six year period, rainforest restoration in the Australian Wet Tropics incorporated a highly diverse selection of 515 tree species, making up 435,435 individual seedlings. The abundance of species produced in nurseries was extremely uneven (Fig. 2) with the 52 most abundant species comprising > 50% of all individual plants produced (the 20 most abundant species are reported in Table A1). Hence, the overwhelming majority of species had low abundances across the pool of seedling records,

Discussion

We found that seedling supply records for plantings in the Australian Wet Tropics to be some of the most speciose when compared to previous studies, in Australia and globally, at the level of both individual plantings (Palma & Laurance, 2015) and across the entire region (Brancalion et al., 2018). Relative abundance of tree species was highly variable, however, species possessing functional traits associated with greater seed supply and faster seedling production (such as smaller seeds, shorter

Conclusion

Species with functional traits promoting greater seedling production and faster growth (small seeds, shorter germination times, lower wood density) were substantially more abundant when analysing seedling supply records for restoration across the Australian Wet Tropics. This resulted in restoration plantings having a greater abundance of seedlings from small-seeded animal-dispersed species than was present in adult trees of mature rainforest within the same bioregion. However, this did not

CRediT authorship contribution statement

Jayden E. Engert: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Nara O. Vogado: Formal analysis, Writing - original draft. Kylie Freebody: Resources, Writing - review & editing. Basil Byrne: Resources, Writing - review & editing. Judy Murphy: Resources, Writing - review & editing. Gaylene Sheather: Resources, Writing - review & editing. Peter Snodgrass: Resources, Writing - review & editing.

Acknowledgements

We thank the wonderful staff of the six nurseries that supplied records and provided feedback for this research, from; Cairns Regional Council Stratford Nursery, Cassowary Coast Regional Council Tully Nursery, Douglas Shire Regional Council Mossman Nursery, Queensland Parks and Wildlife Service Lake Eacham Nursery, Rainforest Rescue Daintree Nursery, and Tablelands Regional Council Community Revegetation Nursery.

Declaration of Competing Interest

None.

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