Annals of Botany Graduate Prize Winner

Matt Daws: Large-seeded species take more risks

We are delighted to award the first Annals of Botany Graduate Prize for a paper reporting work from a thesis to Dr Matthew Daws. In the paper, ‘Germination responses to water potential in neotropical pioneers suggest large-seeded species take more risks’, Daws et al. (2008) focus on mechanisms of niche partitioning relating to seeds and seedling establishment, addressing the questions of how trees avoid competition and extinction by using resources in different ways or occupying different micro-sites. Matt worked with a well-defined group of tree species in Panamanian tropical forests, termed ‘pioneers’, and showed that seed size has a major impact on the niches that tree seedlings colonize.

For the Editors, it was an enjoyable challenge to choose the prize winner given the number of eligible papers that we have published in volume 102 of Annals, and their wide range of fields: from ecology and biodiversity studies through to molecular biology, from fundamental to applied to environmental. All had already passed through rigorous refereeing and met the strictly applied criteria of being both novel and substantial, while also being of interest for a large proportion of botanists from throughout the world. Ultimately, key criteria for short-listing for the prize was work that makes a difference to our understanding of biology, where the results are such that one would want to incorporate them immediately into first-year undergraduate lectures.

Our winner stands out in meeting these criteria. His PhD work, reported in this paper, was carried out in Aberdeen, in a research group interested in how the high diversity of tree species is maintained in tropical forests. This is an interesting question because tropical forests can contain more than 300 tree species per hectare compared with just 171 tree species in the whole of North America. It is also important because biodiversity can affect ecosystem functioning and the provision of services to humans.

Pioneer species require gaps in the forest canopy for establishment, which is different from the majority of tropical forest trees that can germinate and persist in the shaded forest understorey. During his PhD, Matt spent 6 months in Panama characterizing the environment within canopy gaps, monitoring natural seedling emergence and collecting seeds for germination studies back in Scotland. This paper was part of an unfolding story about the ecological traits of species that underlie why species establish where they do and the consequences for maintaining species' richness. When gaps form, the environment in the soil surface layers changes in a number of ways: the quantity of light increases and changes in its wavelength distribution, the magnitude of diurnal temperature fluctuations increases and soil water availability goes down. These changes all affect seed germination and seedling emergence, and thus determine which tree species successfully regenerate. The magnitude of these changes increases with gap size, so bigger gaps are generally harsher environments for a germinating seed than smaller gaps. Matt showed, and wondered why, seed size is greater for species that typically specialize on the larger gaps. Previous papers from Matt and colleagues in Aberdeen highlighted that species with small seeds require light to germinate, while many large-seeded species do not respond to light but do require large temperature fluctuations in order to germinate; and secondly, that the growth rate of the radicle (the root produced by a germinating seed) increases with seed size; radicle growth rate is important for tolerating drought because it determines whether the seedling can outpace the drying front as the soil profile dries if there is no rainfall after germination. Thus seed size emerges as a key trait determining gap occupancy.

In this paper, he showed that large-seeded species also tend to germinate more quickly, and can germinate in drier soils, than smaller-seeded species. This fits the gap-size partitioning hypothesis because surface soils are drier and hotter in larger gaps. Therefore species that specialize on large gaps need to be able to tolerate these drier conditions. However, this strategy is risky, because it may cause germination to occur too early at the end of the dry season, before the wet season has become well-established, for example in response to an isolated rainfall event. Small-seeded species adopt a more conservative strategy that better suits their preferred (moister) small gap environments and their lesser ability to survive desiccation during germination, but as a result they do not get going so quickly at the start of the wet season. One implication of these results is that tree species' germination and emergence will be sensitive to changes in the pattern of rainfall during the transition from dry to wet season, or to changes in the frequency of dry spells during the wet season, which suggests changes in tree species' composition in response to climate change.

Matthew is currently working for an environmental consultancy, Earth-Water-Life Sciences, in Darwin, Australia, specializing in mine-site rehabilitation. He is applying seed and seedling knowledge to the challenges of re-establishing native vegetation in post-mining landscapes.