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
Nutrients, rainfall, stress
Wright, I.J. and M. Westoby. 1999. Differences in seedling growth behaviour among species: trait correlations, and trait shifts along nutrient compared to rainfall gradients. Journal of Ecology 87: 85-97.
Fonseca, C.R., J. McC. Overton, B. Collins and M. Westoby. 2000. Shifts in trait-combinations along rainfall and phosphorus gradients. Journal of Ecology 88: 964-977.
Wright, I.J., Westoby, M. & Reich, P.B. 2002. Convergence towards higher leaf mass per area in dry and nutrient-poor habitats has different consequences for leaf lifespan. Journal of Ecology 90:534-543
From the 1970s onward a key idea in thinking about plant strategies has been stress or adversity and how species adapt to it. This is most apparent in Grime’s CSR triangle (Grime 1979), where one of the two axes is about coping with stress, meaning conditions that permit only slow growth. The question is whether there is indeed a common trait syndrome that confers advantage under low soil nutrient, cold climate, shading by competitors, low rainfall, heavy metal contamination and any number of other stresses. Or alternatively, are distinctive trait syndromes expected to arise for each of these stressors? In Australia drought versus low nutrients is an important manifestation of this general question. Traditionally in the northern hemisphere, sclerophyll foliage has been seen mainly as drought adaptation. But along the east coast of Australia many sclerophyll shrublands occur under quite high rainfall without a distinct dry season. Beadle’s papers (Beadle 1954, 1962) began a shift to the now widespread view that sclerophylly can also be an adaptation to low nutrients. The historical evolution of sclerophylly is also connected to this debate. Australia was climatically rainforest through most of the Tertiary up to about 15 Mya, but some sclerophyll features appear in Australian clades much earlier than that (Hill 1998).
During the second half of the 1990s several studies in our lab took advantage of the circumstance that working from Sydney, vegetation on high vs low nutrient sites can be contrasted at different rainfall levels, and also high vs low rainfall sites can be contrasted at different nutrient levels. Our nutrient indicator was total P, since our interest was in the mixture of species traits in vegetation constructed at a site over tens to hundreds of years not in physiology over weeks to months. Phosphorus in natural soil arises from the rock type plus local redistribution of fine particles, unlike nitrogen which arises largely from N-fixation by the vegetation itself. Three of these studies were published in J Ecol. It’s also worth noting Cunningham et al (1999) which selected species to form phylogenetically independent divergences along both nutrient and rainfall dimensions.
Ian Wright’s first work in our lab addressed seedling relative growth rate. Slow potential RGR of seedlings, measured during exponential growth and under favourable conditions, had been established as an attribute of species occupying slow-growth situations since Grime and Hunt (1975). Wright and Westoby (1999) confirmed that specific leaf area SLA was the strongest single predictor of potRGR (as had been demonstrated during the 1980s especially by a fine body of Netherlands work (Lambers & Poorter 1992)). In particular, Wright & Westoby 1999 showed that the SLA-potRGR correlation applied across species within sites and also across both rainfall and nutrient contrasts. (This is not always the case – trait correlations across sites can arise from different selective forces and can behave differently compared to correlations within sites.)
Fonseca et al (2000) looked at many more species across more sites, but with only field-measured traits. Starting from high-nutrient volcanic soil sites in high rainfall near the coast, it was possible to group sites to form trajectories towards lower nutrient and towards lower rainfall. The question was whether these trajectories behaved similarly in site-mean trait-space. As might perhaps have been expected, the answer was partly yes and partly no. Combinations of SLA with leaf width behaved rather similarly along the two trajectories. But comparing at a common SLA, low nutrient sites were running at taller plant heights than low rainfall sites.
Wright et al (2002) showed another sort of exception to the proposition that traits would adjust similarly to different stresses. The downshift in SLA has different leaf morphology, and different consequences, at low rainfall compared to on low-nutrient soils. At low nutrient the SLA downshift confers mechanical strengthening and longer leaf lifespan, but at low rainfall it is associated with more nitrogen and photosynthetic capacity per leaf area, with stronger drawdown of internal CO2 concentration during photosynthesis, and consequently with higher water use efficiency.
Ian Wright joined the Macquarie lab initially as a research assistant after completing honours at U Melbourne, then shifted to a PhD. As a postdoc he coordinated the “glopnet” collaboration, known for the leaf economic spectrum (Wright et al. 2004). The citation impact from that work persuaded Macquarie’s administration to offer him a continuing position. He is currently an Assoc Prof at Macquarie.
Carlos Fonseca and Jake Overton were postdocs from Brasil (via Imperial College London) and USA respectively.
Carlos, with his wife Gislene Ganade who was also at Macquarie, are now professors at Universidade Federal do Rio Grande do Norte in Natal Brasil. Jake Overton is with Landcare Research in New Zealand, and is currently seconded to Panthera, a carnivore survey program in Zambia.
Bronwyn Collins was a research assistant while at Macquarie. She is now at the Australian National Herbarium in Canberra, coordinating the curation process and also student internships.
Prof. Mark Westoby