John B. Dickie

The global spectrum of plant form and function

Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today’s terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.

Whole-plant trait spectra of North American woody plant species reflect fundamental ecological strategies

The adaptation of plant species to their biotic and abiotic environment is manifested in their traits. Suites of correlated functional traits may reflect fundamental tradeoffs and general plant strategies and hence represent trait spectra along which plant species can vary according to their respective strategies. However, the functional interpretation of these trait spectra requires the inspection of their relation to plant performance. We employed principle coordinate analysis (PCoA) to quantify fundamental whole-plant trait spectra based on 23 traits for 305 North American woody species that span boreal to subtropical climates. We related the major axes of PCoA to five measures of plant performance (i.e., growth rate, and tolerance to drought, shade, water-logging and fire) for all species and separately for gymnosperms and angiosperms. Across all species a unified gymnosperm-angiosperm trait spectrum (wood density, seed mass, rooting habit) is identified, which is correlated with drought tolerance. Apart from this, leaf type and specific leaf area (SLA) strongly separate gymnosperms from angiosperms. For gymnosperms, one trait spectrum emerges (seed mass, rooting habit), which is positively correlated with drought tolerance and inversely with shade tolerance, reflecting a tradeoff between these two strategies due to opposing trait characteristics. Angiosperms are functionally more diverse. The trait spectra related to drought tolerance and shade tolerance are decoupled and three distinct strategies emerge: high drought tolerance (low SLA, dense wood, heavy seeds, taproot), high shade tolerance (high SLA, shallow roots, high toxicity, opposite arranged leaves), and fast growth/stress intolerance (large maximum heights, soft wood, light seeds, high seed spread rate). In summary, our approach reveals that complex suits of traits and potential tradeoffs underlie fundamental performance strategies in forests. Studies relying on small sets of plant traits may not be able to reveal such underlying strategies.

Experimental investigations into the feasibility ofex situ preservation of palm seeds; an alternative strategy for biological conservation of this economically important plant family

Given the widespread belief that the conservation of palms, especially the large-trunked species, is only accomplished throughin situ preservation or in plantations, this paper explores the feasibility of a third approach, i.e. cryogenic preservation of their seedsex situ. Seeds of the following palm species were subjected to air-drying to assess their tolerance of dessication:Washingtonia filifera (L. Linden) H. Wendl.;Sabal mexicana Mart. (syn. S. texana; Zong, 1990);Schippia concolor Burret;Orbignya cohune (Mart.) Dahlgren ex Standley;Acoelorraphe wrightii (Griseb. & H. Wendl.) H. Wendl. ex Becc.;Desmoncus orthacanthos Mart.;Attalea crassispatha (Mart.) Burret;Zambia antillarum (Desc.) L.H. Bailey;Pinanga malaiana (Mart.) Scheff.;Pinanga aff. polymorpha Becc.;Daemonorops verticillaris (Griff.) Mart. Of these, only two (W. filifera andS. mexicana) survived drying to moisture contents around 5% (fresh weight basis). Seeds of the remaining spp. would be difficult or impossible to conserveex situ in seedbanks or cryostores. Data are presented to show that the response ofO. cohune embryos to drying is similar to other recalcitrant (dessication intolerant) seeds, while seeds ofA. wrightii may belong to an intermediate seed storage category with limited tolerance of drying. The results are discussed in relation to the inadequacy of current knowledge as a basis for decisions on the broad scaleex situ conservation of palm germplasm.

Maximizing the Phylogenetic Diversity of Seed Banks

Ex situ conservation efforts such as those of zoos, botanical gardens, and seed banks will form a vital complement to in situ conservation actions over the coming decades. It is therefore necessary to pay the same attention to the biological diversity represented in ex situ conservation facilities as is often paid to protected-area networks. Building the phylogenetic diversity of ex situ collections will strengthen our capacity to respond to biodiversity loss. Since 2000, the Millennium Seed Bank Partnership has banked seed from 14% of the worlds plant species. We assessed the taxonomic, geographic, and phylogenetic diversity of the Millennium Seed Bank collection of legumes (Leguminosae). We compared the collection with all known legume genera, their known geographic range (at country and regional levels), and a genus-level phylogeny of the legume family constructed for this study. Over half the phylogenetic diversity of legumes at the genus level was represented in the Millennium Seed Bank. However, pragmatic prioritization of species of economic importance and endangerment has led to the banking of a less-than-optimal phylogenetic diversity and prioritization of range-restricted species risks an underdispersed collection. The current state of the phylogenetic diversity of legumes in the Millennium Seed Bank could be substantially improved through the strategic banking of relatively few additional taxa. Our method draws on tools that are widely applied to in situ conservation planning, and it can be used to evaluate and improve the phylogenetic diversity of ex situ collections.

Karyosystematics of the Australasian stipoid grass Austrostipa and related genera: chromosome sizes, ploidy, chromosome base numbers and phylogeny

Abstract. Mitotic metaphase chromosomes were counted in 29 taxa, representing 11 subgenera of Austrostipa, and in 11 species from nine related genera of the grass subfamily Pooideae. Karyotype features were also measured. The cytogenetic data were mapped on molecular phylogenetic trees based on nuclear ITS and plastid 3′ trnK DNA sequence data. The trees showed four different main lineages within Austrostipa, but supported only two of the 13 acknowledged subgenera. The phylogenetic positions of the genera Anemanthele, Achnatherum, Nassella and Oloptum indicated paraphyly of the genus Austrostipa. In nuclear-sequence data, Anemanthele was nested within Austrostipa; however, in plastid-sequence data, both were sisters. The newly obtained chromosome counts in Austrostipa showed that most species have 2n = 44, the other 2n = 66. Presuming a chromosome base number of x = 11, the counts corresponded with ploidy levels of 4x and 6x respectively. Karyotype data of Austrostipa and Anemanthele were very similar. Chromosome counting in further genera suggested chromosome base numbers of x = 9, 10, 11, 12 and 13. Chromosome sizes of the phylogenetically derived tribe Stipeae were smaller than those of the earliest diverging Pooideae lineages Nardeae, Meliceae and Phaenospermateae. The mechanisms of chromosome evolution and the origin of the considerable variation in chromosome base numbers in the subfamily Pooideae are discussed in the context of chromosome evolution and biosystematics.

A research agenda for seed-trait functional ecology

Trait-based approaches have improved our understanding of plant evolution, community assembly and ecosystem functioning. A major challenge for the upcoming decades is to understand the functions and evolution of early life-history traits, across levels of organization and ecological strategies. Although a variety of seed traits are critical for dispersal, persistence, germination timing and seedling establishment, only seed mass has been considered systematically. Here we suggest broadening the range of morphological, physiological and biochemical seed traits to add new understanding on plant niches, population dynamics and community assembly. The diversity of seed traits and functions provides an important challenge that will require international collaboration in three areas of research. First, we present a conceptual framework for a seed ecological spectrum that builds upon current understanding of plant niches. We then lay the foundation for a seed-trait functional network, the establishment of which will underpin and facilitate trait-based inferences. Finally, we anticipate novel insights and challenges associated with incorporating diverse seed traits into predictive evolutionary ecology, community ecology and applied ecology. If the community invests in standardized seed-trait collection and the implementation of rigorous databases, major strides can be made at this exciting frontier of functional ecology.

Ecological correlates of seed dormancy differ among dormancy types: a case study in the legumes

Seed germination and seedling survival are crucial in determining species distributions, and plant population and community dynamics (Larson & Funk, 2016). By controlling the timing of germination, seed dormancy plays an important role in seedling survival, particularly in seasonal environments (Willis et al., 2014). The recent paper by Rubio de Casas et al. (2017) examined the seed size and global distributions of dormant compared with nondormant (ND) members of the Fabaceae, finding that dormant seeds are typically smaller than ND seeds, and are clearly associated with, and evidently have adaptive value in, more seasonal environments. This paper has made a valuable contribution to our understanding of the ecology and evolution of seed dormancy within the Fabaceae. However, the term ‘seed dormancy’ (defined at its simplest as a ‘block to the completion of germination of an intact viable seed under favourable conditions’ (p. 502); Finch-Savage & Leubner-Metzger, 2006) encompasses a diverse range of different physiological and structural mechanisms, which can be broadly split into discrete dormancy ‘classes’ (e.g. Baskin & Baskin, 2014) and can differ in prevalence among habitats, climatic zones, and taxonomic lineages (Finch-Savage & Leubner-Metzger, 2006). Within the Fabaceae, species with ‘dormant’ seeds can be assigned one of three dormancy classes: physiological dormancy (PD; where dormancy is broken in response to particular environmental cues such as warm or cold temperature), physical dormancy (PY; where the seed coat is impermeable to water thereby preventing imbibition; i.e. ‘hard seededness’), and combinational dormancy (PYPD; where a seed has both physical and physiological dormancy) (Baskin & Baskin, 2014). PY is the dominant dormancy class within the Fabaceae, with PD and PYPD making up a small minority of species. Although PY and PD both have the effect of delaying the onset of seed germination; these two types of seed dormancy are mechanistically very different, and may be the outcome of different selection pressures. For example, while PD is only considered an adaptation that controls the timingof germination tooptimize seedling survival, PY (‘hard seededness’) has also been hypothesized to reduce mammalian predation (Paulsen et al., 2013) and to ensure longevity in the soil (Murdoch, 2014). PY and PDmay also differ in terms of their relationships with other seed traits, for instance desiccation sensitivity was found in 12.3% of the PD species in a dataset of trees and shrubs used by Tweddle et al. (2003), but only in 1.4% of PY and none of the PYPD species. Furthermore, at least one author has considered PY to be ameans of ‘endogenous quiescence’ (Murdoch, 2014), rather than a type of dormancy per se. Rubio de Casas et al. (2017) compared all dormant species in the Fabaceae for which they had data with the ND species, and found some compelling results. Without wishing to enter a semantic discussion on the definition of ‘dormancy’,we simplywish to highlight that PY andPDare two very distinct means of achieving germination delay. Here, we are interested in extending the work of Rubio de Casas et al. (2017) by considering these different dormancy classes separately. We will demonstrate that while the PY and PYPD species conform to the patterns described in the previous study, the few dormant members of the Fabaceae that lackPY (i.e. thePDspecies) donot; highlighting the ecological, physiological, and potentially the evolutionary dissimilarities that exist among the different dormancy classes. Lumping of these dormancy classes may obscure detail of the underlying selection pressures and evolutionary processes involved in the control of germination timing.

Seed banking not an option for many threatened plants

The Global Strategy for Plant Conservation requires 75% of threatened plant species conserved ex situ by 2020. Currently, ex situ conservation focuses on conventional seed banking, yet this method is unsuitable for many threatened species. The 75% target is unattainable without urgent investment into alternative techniques.