Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Monday, 20 May 2013

Below-ground ecology often gets ignored

One way a plant species will deal with the challenge of a changing climate is to track shifts in their climate envelope by dispersal and establishment in new locations that are climatically suitable for that species. Whilst this seems an obvious mechanism to deal with climate change, it fails to account for the fact that plants might be 'migrating' to very different soil types, and these soil types might be unsuitable for their establishment.
Differences in soil texture (which helps inform us about water holding capacity) and soil pH (which tells us something about nutrient availability) are two key ways in which soils might differ from Point A to Point B. So, plants that require free-drainage (like many Banksia in southern Australia that are found primarily on deep, acidic sands) probably won't do very well when they try to establish on poorly-draining clays.

But soils don't just provide nutrients and water. They are also the reservoir of soil biota that includes important groups such as mycorrhizal fungi.

Mycorrhizas are associations between fungi and plant roots that can be beneficial to both the plant and the fungi. The fungi link the plant with soil by acting as agents of nutrient exchange. The fungi receive carbohydrates as energy from the host plant root whilst nutrients such as phosphorus and zinc are passed back into the plant roots from the soil. Mycorrhizal associations may also reduce attack from root pathogens and increase the tolerance of the plant to adverse conditions such as heavy metals, drought, and salinity. In general, mycorrhizas play an important role in plant productivity.

It got me to thinking. How important are mycorrhizal associations when plant disperse to new soils? Could their absence (from the new soil types) inhibit colonisation potential of species trying to track their climate envelope shift?

While we know that mycorrhizal associations are important in plant communities (work in ecological restoration, for instance, has shown that some species are incredibly hard to re-establish if mycorrhizae are missing from the rehab site), it is hard to find much evidence for their existence in the plants I work with in southern Australia. In particular, what about the importance of mycorrhizal fungi for short-lived plants like annuals.

Hyalosperma praecox - one of the annual species
 I work with in grassy woodlands.
A feature of annual plants is their ability to grow rapidly after germination, establish one or more shoots that bear inflorescences, and then to flower and produce seed rapidly. The uptake of water, photosynthates and minerals into the developing flower heads and seeds occurs with some urgency because of the relatively short growing season. I got to wondering if mycorrhizae play a role here - after all, annual plants have very little time to grow roots to explore their soils for mineral nutrition and water.

The starting place to even begin to answer this question has to be: do annual plants form mycorrhizal associations in soils in the short-time that they are active? Using some old fashioned techniques (germinating annuals in their 'native' soil, then extracting them 4 weeks later, preparing roots by staining and using a microscope), we've recently found that many annual plants do indeed form mycorrhizal associations. This is the first study of its kind on grassland annuals in Australia, so rather exciting news.

Vesicles and hyphae, shown here by the staining, in a root

of the annual daisy Triptilodiscus pygmaeus

(Photo: Rohan Ball)
My Research Student Rohan Ball focused on observing what are called vesicular-arbuscular mycorrhizas (or VAM). The fungal hyphae here penetrate root cells and form intricately branched, shrub-like arbuscles within the cells and, at times, bladder-like vesicles as well. They are relatively easy to see, so we thought this was a good place to start.

Of the eight species we surveyed (all in the Asteraceae), we can confidently say that the roots of six species were colonised by VAM within 4 weeks of germination. You can see VAM in the photos as dark, stained areas - both vesicles and hyphae stain here quite clearly.  We are not yet sure what growth advantage this gives these developing plants, but we think it points to an interesting set of experiments. For instance, would VAM form in plant roots grown on 'non-native' soils (i.e. soils that are very different to those the species currently occur on)? If we remove VAM from the 'native' soil, does this affect growth and reproduction? Such questions are important because they have an applied outcome: are VAM necessary for annuals to recolonise agricultural lands? Will the lack of VAM affect colonisation potential of native species that disperse to new areas?