Understanding how complex phenotypes evolve and adapt to variable environments is a central question in evolutionary ecology. Complex phenotypes consist of numerous traits that tend to be highly integrated.

Phenotypic integration has two major implications for evolution and adaptation. Firstly, patterns of phenotypic integration provides information on the nature of selection that has shaped the current phenotype of the organism. Secondly, an understanding of phenotypic integration enables predictions of the capacity for complex phenotypes to respond to selection and adapt to new environments.

Our research group is currently examining how patterns of phenotypic integration constrains and promotes adaptation and diversity across environments.

Interpreting size-dependent reproduction in plants

Reproductive allometry is common in plant populations (large plants tend to have proportionally higher allocation to reproduction than small plants). This relationship is often interpreted as an artefact of a minimum size requirement for reproduction (Fig. 1a – after reproduction is initiated, relative allocation to reproduction quickly increases). However, plants tend to initiate reproduction at a a smaller size in adverse environments (Fig 1b). Thus, reproductive allometry should also be due to the way in which plant life histories are expressed across environments.

We are currently investigating how integration between plant life histories (i.e. size at reproduction) and allocation to reproductive and vegetative tissues can be used to understand plant strategies in complex environments.

Fig 1. Developmental trajectories of vegetative and reproductive allocation in plants showing where reproductive allometry is due to a) an invariant minimum size at reproduction, and b) plasticity or genetic variability in the minimum size at reproduction.

Further Reading

Bonser, S.P. and Aarssen, L..W. in review. Interpreting size dependent reproduction in plants.

Bonser, S.P. and Aarssen, L.W. 2001. Allometry and plasticity of meristem allocation throughout development in Arabidopsis thaliana. Journal of Ecology 89: 72-79.

Leaf functional variability in integrated plant phenotypes

The importance of leaf structure and function to plant evolution and ecology has generated great interest in developing a general understanding of leaf trait strategies. We are interpreting leaf functional variability by including biomass allocation patterns in integrated plant body plans.

Plant growth rate is a function of leaf mass. Stem architectural traits (e.g. vascular supply areas) control the amount of leaf mass on a stem. Environmental factors affecting stem architecture can also affect leaf allocation and growth rate. Thus, variability in leaf form and function should be due to a product of both selection on leaf physiology and constrains on total leaf allocation. This raises a number of important predictions that should help explain variability in leaf trait strategies within and between environments. For example, a plant should compensate for a loss in the production of total leaf tissue by producing light but highly productive leaves (Fig 2).

Fig 2. Leaf allocation controls growth rate in plants. A given growth rate can be achieved with less leaf biomass if leaves are constructed to be highly productive.

Fig 3. Water availability controls stem architecture and leaf function in Melaleuca quinqenervia.

Further Reading

Bonser, S.P. 2006. Form defining function: interpreting leaf functional variability in integrated plant phenotypes. Oikos 114: 87-90.

Mallit, K.L, Bonser, S.P., and Hunt, J. In review. The plasticity of phenotypic integration in response to light and water availability in the pepper grass, Lepidium bonariense.

Olliek, S, and Bonser, S.P. in prep. Functional responses of plant form to water limitation.

The evolution of competitive abilities

Competition for limiting resources generally reduces plant growth and fitness. A number of highly integrated traits related to biomass allocation, resource uptake, and physiological function determine competitive ability in plants. We are currently investigating a number of questions on the evolution of competitive ability, including:

● Is competitive ability related to seed production (fitness) across genotypes of Arabidopsis thaliana (Fig 4)?
● Do patterns of allocation, resource use, and physiological traits defining competitive ability change across Eucalyptus species (and genotypes) (Fig 5)?
● Is competitive ability related to the evolution of life histories in fire disturbed communities (Fig 6)?

Fig 4. Testing for a relationship between competitive ability and fitness in A. thaliana.

Fig 5. An experiment investigating competition between Eucalyptus camaldulensis seedlings and grasses.

Fig 6. Sprouter and non sprouter tree seedlings growing with and without competitors in a study investigating the relationship between competitive ability and life history evolution.

Further Reading

Ladd, B., Pepper, D.A. and Bonser, S.P. in review. Adversity selection and competition between Eucalyptus seedlings and herbaceous plants on a geographical-climate gradient.

Chew, S. and Bonser, S.P., in review. The evolution of competitive ability in sprouter and nonsprouter tree seedlings.