The Editors of the Journal of Ecology are pleased to honour Professor Mark Westoby in our continuing Eminent Ecologist virtual issue series. The virtual issue is available on the Journal of Ecology website.
Assistant Editor, Journal of Ecology
It’s a great honour to have this opportunity to comment on some of our papers in Journal of Ecology over the years. Yet at the same time it feels a bit like grandad being encouraged to reminisce. It’s a sort of test, isn’t it? — of whether I’m still clear-headed enough not to drivel on too long? I wonder whether I’ll pass?
I’ll group the papers that have been chosen, and say a bit about each group. But also, I’ve been amazingly lucky in the many bright and interesting people who have been in our lab and given rise to these publications. So I’d like also to introduce those people via pictures and say something about where they are now.
Power law of self-thinning
Westoby, M. and J. Howell. 1982. Self-thinning in Trifolium subterraneum populations transferred between full daylight and shade. J. Ecol. 70:615-621.
Westoby, M. and J. Howell. 1986. The influence of population structure on self-thinning. J. Ecol. 74:343-359.
Harper’s (1967) presidential address “A Darwinian approach to plant ecology” kicked off a revolution in plant ecology. Up to the 1960s most plant ecology had been occupied with where species grew, and in what combinations. In contrast Harperian plant ecology was demographic, often experimental, often using simple model systems. And Journal of Ecology during the 1970s and 1980s was the pravda of the Harper style.
One of several inspirational strands in Harper (1967) was to bring interesting Japanese literature to the attention of english-speakers. This included the power law proposed by Tadaki and Shidei (1959) and especially by Yoda et al (1963). The power law describes a boundary in log biomass – log density space (Fig 1). [“Westoby blog fig 1.jpg” around here] Crowded stands start at lower right with many small individuals, then as plants grow bigger, competitive mortality (“self-thinning”) sets in. The law encapsulated important messages for plant ecologists trying to find their way forward from animal-centric population dynamics. One message was that it’s inadequate to describe a plant population by the single number N as you might for locusts or lemmings. For plants, you absolutely need information about size as well as about number. Another message was that the more favourable the growth conditions, the faster plants die.
After I moved to Australia in 1975 and began to switch research direction into plant ecology, this law was attractive because it seemed a potential hinge-point between the dynamics of numbers and the dynamics of biomass and energy (Westoby 1981, 1984). Also the law was mainly an empirical observation. There was a dimensional interpretation along the lines that individual mass increased to the power 3 as plants grew bigger, while numbers per area decreased to the power 2 – hence the -1/2 power scaling — but this sketchy theory had not been tested. The opportunity was there to experimentally manipulate plant shapes and other factors.
One question was about trajectories followed by stands in low light. At the time a number of possibilities were in play (reviewed in Westoby 1984). Westoby and Howell (1982) experimented by running stands (of sub clover in pots) in low and high light and then swapping them over. Low-light stands swapped to high light did indeed return to the high-light boundary line, rather than merely changing their direction of travel.
Self-thinning mortality is associated with a wide spread of individual plant sizes. Asymmetric competition for light spreads out the range of sizes (Westoby 1982), while mortality trims off the smallest and most suppressed individuals. Westoby and Howell (1986) reported four experiments where population size structures were manipulated, by sowing smaller and larger seeds and by sowing some individuals later than others. To cut a long story short, there was little evidence that size structure affected the self-thinning trajectory. Rather the main effect was in the other direction. Whatever size structures were created experimentally, the processes of competition and mortality tended to bring them back to being roughly symmetrical on log dry mass scales, and between one and two orders of magnitude broad among the surviving individuals.
Joc Howell was a part-time research assistant during these projects. She was an older undergrad – after children and a marriage breakdown she had decided botany was what she really loved and had come back to university. She subsequently went on to a career at the Royal Botanic Gardens Sydney. She wrote the book (literally) on Sydney-region vegetation (Benson & Howell 1995; Howell & Benson 2000). She is now retired and an energetic traveller and grandmother.
Other J Ecol papers on self-thinning from our lab were (Westoby 1980; Westoby & Howell 1981).
Prof. Mark Westoby