What makes a plant species fire-resistant?

I've just returned from the annual meeting of the Ecological Society of Australia in Hobart. This is the peak ecological conference in Australia for ecologists and it's always a chance to meet up with old friends, hear about some great new ecology, and to have your thinking challenged by leaders in the field.

Malcolm Gill

This year, Dr Malcolm Gill was awarded the ESA Gold Medal for his long and outstanding contribution to the field of fire ecology. You may be aware of Malcom's work (see here) - I first became aware of him in my Honours year when I read his important review on fire in Australia - Fire and the Australian flora: a review. Australian Forestry 38, 4-25. It's still a seminal review for its time that should be read by all new post-grads working in this field.

Malcolm highlighted, in his own dry way, just how much the field of fire ecology has come since then. In particular, he made the compelling case that plant species in fire-prone ecosystems are adapted to fire regimes and we should place our work in this context. While it is relatively easy to characterize individual fire events and plant community responses to single fires (often limited to the timescales studied by PhD students), these need to be couched in terms of fire return intervals, as well as components of the fires themselves (severity, extent, patchiness). Importantly, he stressed that adaptive characteristics such as thick bark, serotiny, soil seed banks do not guarantee species persistence.

This is something that I have been thinking about lately - that the response to fire of a species can be a population attribute, not a species attribute per se. Let me give you an example.

In 2003, the Victoria alps - from the foothills to the alpine peaks - was burnt in a very large bushfire during a particularly severe drought. The subalpine woodlands and forests, dominated by Eucalyptus pauciflora (Snow Gum), were extensively burned. Snow Gum is a thin-barked species and the above-ground stems seem very sensitive to fire of any intensity. Hence, stems die in just about any fire but individuals recover by vigorous resprouting from a lignotuber. Hence, fire would apparently have very little impact on Snow Gum populations.

By 2009, I started to wonder whether this was true. It dawned on me that in the subalpine woodlands I work in, individual Snow Gums trees had responded quite differently to the same fire event. And this difference was mediated by tree size (or more correctly, girth).


Snow Gum with small girths - fire-resistant?
(Photo: John Morgan)

Snow Gum with big girths - fire-sensitive?
(Photo: John Morgan)

A quick walk through the forest hinted that resprouting from lignotubers (both the number of stems produced, and their vigour) was most pronounced in plants with small girth. By contrast, very large plants had often succumbed to very low intensity fires. We've started to collect data on this response and I hope to soon have some numbers to support these observations. There clearly is a relationship between girth and the number of dormant buds waiting to be released after fire and I'm sure there are both anatomical and physiological reasons for the response. I'm looking forward to unravelling the mechanisms that underpin these observations.

Interestingly, Malcolm talked about similar findings for tropical savanna trees in Darwin. Hence, it's clear (to me) that population responses must be incorporated into our thinking about vegetation response to fire events. Population dynamics are largely ignored when we assign fire response traits to plant species (i.e. resistant versus sensitive), perhaps to our peril.

What limits recovery of semi-arid oldfields? Seed addition experiments reveal the importance of seed versus microsite limitation

Many native grasslands in southern Australia have been utilised for agriculture. Grazing and cropping are two widespread landuses that have pushed these ecosystems to the brink of extinction. But what happens if you remove agricultural disturbances? Can native ecosystems recover? I've been leading a research team for some time that has been asking: does the function, structure and composition of oldfields return after the cessation of cultivation?

The recovery of native communities after cultivation may be constrained by two key ecological factors: (a) the failure of species to reach a site due to poor dispersal or (b) their failure to survive once there. Seed addition is a common method to test for seed versus microsite limitation. Most studies, however, do not follow populations beyond seedling establishment (to see if long-term persistence occurs), nor do they measure seed dispersal (to see if seed movements really are limiting recovery).

Recently, my grad-student Andrew Scott and I set out to determine the constraints on the recovery of semi-arid grasslands in northern Victoria after cultivation. We examined dispersal across native grassland / oldfield boundaries (using Astroturf to catch seeds at increasing distances from boundaries) and also investigated the relative importance of seed and microsite limitation across multiple life-history stages and generations by adding seed of two grassland forbs that are absent from the early successional recovery after cultivation.

Perhaps unsurprisingly, seed trapping over two seasons showed little movement of native seeds into old fields; most species had extremely localized dispersal of only a few metres. Consequently, similarity between the seed rain and standing vegetation was moderate to high. Hence, seed dispersal is a key constraint to the recovery of oldfields. This might be overcome by the deliberate introduction of propagules.

Goodenia - one of the species we used in seed addition
experiments in northern plains grasslands to test for
seed versus microsite limitation.
(Photo: Andrew Scott)

Seed addition into oldfields showed that two annual species (Goodenia, Rhodanthe) were able to establish in all, and flower in most, oldfields in the first year. Seedling establishment increased with sowing density, consistent with seed limitation. However, the relative importance of microsite limitation increased over the life-spans of the species. Density-dependence reduced the number of flowering plants, resulting in a large decline in seedling density in the following generation. This decline continued so that the initial positive effects of sowing density on seedling numbers disappeared by the fourth generation and hence, the long-term persistence of populations is uncertain. This highlights that seed additions might overcome dispersal barriers, but it does not guarantee that sustainable populations will then develop.

Thus, by monitoring seed dispersal and following experimental populations beyond seedling establishment, a rare achievement in the seed addition research field, we showed that dispersal limits species distributions, but microsite plays an important role in limiting population growth and persistence. Perhaps what this points to is that multiple seed re-introductions (i.e. seeds added across years) will be necessary to recover oldfields.

This work will appear in Oecologia soon.