Towards a general model of plant trait diversity

This is the title of my PhD, completed in 2010. The broad goal of my current work is to develop theory explaining what mixtures of plants grow where and why.

Plant traits capture fundamental trade-offs that determine ecological roles of species. Thus traits are key to understanding vegetation dynamics. Currently trait research in plant ecology has a strong empirical focus. By incorporating natural selection into models of vegetation, I will develop novel theory describing how trait mixtures vary with environmental conditions. These adaptive models will provide a platform for next- generation global vegetation models, able to capture the likely impacts of human activities impacts on ecosystem function and species diversity.

A Trait-, Size-, and Patch-structured Model (TSPM) of vegetation

Most large scale vegetation models lack interesting ecology: species are represented by a single, even average-sized individual, grown in a mono culture. To better understand vegetation dynamics, we need models that incorporate the realities of size-structured competition for light and other resources.

I recently published a model that incorporates the influences of traits, climate, competition, and disturbance on demography and size structure. The work was completed in collaboration with Åke Brännström and Ulf Dieckmann from IIASA in Austria , and Mark Westoby at Macquarie University, as a result of my participation in IIASA's Young Science Summer Program.

TSPMs offer a viable compromise between the detailed but noisy output of spatially explicit simulation models and the convenience of modelling idealized stands lacking internal population structure. We used our TSPM to investigate the influences of four functional traits, for which trade-offs are relatively well understood, on several key properties of vegetation. We are also using it to investigate the evolution of plant trait diversity.

New theory of offspring size, driven by competitive interactions

Why does offspring size vary across species? This is an old question in evolutionary ecology and one that has received lots of attention, both theoretical and empirical. Despite this, general theory explaining some major quantitative patterns - such as the link between adult and offspring size - was lacking. In 2008 I published "A general model for the scaling of offspring size and adult size", the first paper from my PhD thesis. There we argue that size-asymmetric competition for resources drives evolution of offspring size. We also show how our general model can be applied to taxa as different as mammals and plants, and used to understand the differences between them.

See my article in Australasian Science for a lay-persons explanation of the model and theory.

SMATR software

SMATR is a freely-available program, used for fitting standardised major axis lines to data and for making inferences about such lines. So far, SMATR has been used in over 100 publications across diverse fields of evolutionary biology and ecology. SMATR exists as an R package, and as stand-alone software. The latest edition of the R package also includes plotting capabilities.

For information about sma lines, see our 2006 review.

DNA repair genes

During 2004-2005 I was working with Jarle Breivik from the Medical Faculty at the University of Oslo (Norway) investigating the effect of DNA mismatch repair genes on the occurrence of simple repeat sequences (e.g. AAAAAA, TTTT, GGGGG, CCCCC) in the human genome. These repeat sequences have a high mutation rate and play an important role in cancer development. Curiously, we found that the mismatch repair genes, which repair mistakes in the repeat sequences, have an over abundance of the repeats they repair. It's no wonder the repair machinery breaks down now and them. We outlined a simple evolutionary hypothesis to explain these results, whereby a repair protein influences its own sequence through the dynamics of recombination [link].

Plant height strategies

This work was completed during 2002-2003 as part of my MSc, supervised by Mark Westoby at Macquarie University. The focus of my thesis was the comparative ecology of plant height. Amongst coexisting species maximum height can range over 4 orders of magnitude. My research quantified the relationship between species maximum height and properties of the stem and leaves at three study sites. Thesis abstract.

Field site picture gallery

Plant architecture

The way in which plants arrange their leaves has important consequences for light interception, self-shading and carbon gain. During 2000-2001 I investigated inter-specific differences in light interception due to leaf angle and leaf size[link]. To do this I combined 3D-digitising technology with the simulation software Yplant, developed by Bob Pearcy (UC Davis). This novel combination of technologies has since been used by a number of groups and is described in full on the Prometheus wiki.

I have also been involved in a number of other architectural studies:
- quantifying age-related declines in the carbon budgets of individual leaves among 10 woody species differing in leaf life span (with Peter Reich, David Ellsworth, Ian Wright, Mark Westoby, Tali Lee, and Jacek Oleksyn). [link] [Commentary by Anten & Poorter]
- comparing size-related changes in light interception efficiency between gymnosperms and angiosperms (with Chris Lusk). [link]
- studying effects of crown density and leaf clumping on light interception (with Remko Duursma).

Some digitised plants: Persoonia lanceolata, Banksia oblongifolia, Isopogon anemonifolius.