Fire and rain #2: water for ironbarks

Box-Ironbark Forest dominated by Red Ironbark above Gold-dust Wattle.

Here’s a big question to think about. Picture your favorite ecosystem, perhaps it’s a grassland, a heathland or a wet forest. Which ecological process has the biggest impact on how your ecosystem changes over time?

Presumably, the process you selected is the one that you think is most critical to retain (or avoid) in order to conserve a diverse, functioning ecosystem. If the process is altered or removed, ecosystem health will decline.

Now, think about this question: what if you’re wrong?

What if some other process is actually far more important, and the process you selected is actually rather trivial in the big picture?

I’ve been thinking about this question while wandering around box-ironbark forests in central Victoria, an ecosystem I’m pretty new to. The question I’ve asked myself is: what are the dominant processes that control how box ironbark forests function? I don’t have the answer, but the question has triggered lots of interesting discussions. But before we discuss box ironbark forests, let’s widen the canvas and think about the processes that affect most Australian ecosystems.

Fire and Rain

I imagine that the two most popular answers to the question above would be fire and water. Grazing is important in agricultural landscapes, but fire and water are arguably the most important drivers of ecosystem dynamics in most Australian ecosystems.

Fire controls ecosystem processes in lots of Australian ecosystems, especially forests, heathlands and savannahs (see recent blog). Fire regimes affect plant regeneration and mortality, and can alter ecosystem structure and habitat suitability for fauna.

In semi-arid and arid regions, the key factor that affects ecosystem processes isn’t fire, it’s water; how much water arrives in the first place (in floods and droughts), and how water is retained in, and moves through, the landscape.

Intact ecosystems are very good at retaining water. Falling rain hits ground plants and litter, and these slow the flow of water through the ecosystem. The more water that is held in the landscape, and the longer it is available, the greater the biological productivity.

By contrast, degraded dryland systems are very leaky. Degrading processes such as heavy grazing remove the plants and litter that intercept surface water. Water runs off faster and soils dry out faster, and this in turn, leads to further reductions in biological productivity. This creates a vicious cycle as ‘leaky ecosystems’ become more and more degraded.

To restore degraded dryland systems, arid zone ecologists focus on measures that slow the flow of water across the landscape. Understory and ground plants, fallen timber and leaf litter all create small barriers that prevent water from leaking, or running out of the system. Drifts of leaf litter on the ground improve water infiltration and soil condition (including organic carbon levels), so soils can store more water for longer.

In degraded dryland systems, a primary management goal is to improve the system’s ability to retain soil moisture.

It’s easy to view these two world views as applying to completely different ecosystems – fire is important in forests and water in deserts. But sharp boundaries are rare in nature. Some forests grow on poor, degraded, dry soils, and don’t burn that much. Retaining or restoring scarce resources is likely to be very important in these systems. Which brings us back (finally!) to box-ironbark forests.

Box Ironbark forest in central Victoria dominated by Red Ironbark (Eucalyptus tricarpa)
Box Ironbark forest in central Victoria dominated by Red Ironbark (Eucalyptus tricarpa)

Water for Elephants Ironbarks?

To return to our original question: which ecological processes have the biggest impact on how box-ironbark ecosystems change over time? How important are fire and water? I emphasise that I don’t know (and I don’t know anyone else who does), so this blog sketches out some early thoughts, and some research questions for future projects.

Box-ironbark forests are really interesting. They occur on soils that are naturally infertile, but which were made less productive by widespread gold mining in the mid-1800s. Mining degraded soils over vast areas, and many soils now have no A horizon at all. The loss of the original topsoil must have reduced soil water holding capacity and, by inference, the biological productivity of the forests as a whole.

ST Gill Bendigo gold mining
Gold mining in the 1800s stripped the topsoil from many Box Ironbark forests. Early illustration by S.T. Gill.

Box-ironbark forests in Victoria don’t burn very often. They don’t produce a lot of ground fuels, so they don’t carry frequent fires. Many of the dominant understory shrubs seem to regenerate well without fires (although fire undoubtedly promotes regeneration of many species). Unlike most forests in south-east Australia, there is little evidence that fire is an important driver of ecosystem dynamics. (Who knows what role fire may have played before European settlement, but that’s a matter for conjecture).

By contrast, box ironbark forests are undoubtedly subject to seasonal water limitation. Annual rainfall is moderate (450-600 mm on average) but summers are hot and dry. Annual rates of biomass accumulation are low compared to ecosystems on more fertile, well watered soils. Prolonged droughts (as seen during the past decade) killed many understory shrubs which, in turn, is thought to have contributed to declining numbers of birds during the drought.

Shallow soils with little leaf litter are easily eroded.

Box ironbark forests also seem to be pretty ‘leaky’. Soils are often bare and crusted, and rates of water infiltration are (presumably) very slow. The paucity of humus and organic matter means that their water holding capacity is also low. Because the ground surface is relatively open, there often aren’t a lot of barriers to impede the flow of water during heavy rains. Following heavy summer rains, water rushes down shallow drainage lines, washing leaf litter off the slopes and down to the gullies. This process must further reduce soil productivity. Indeed, this is the vicious cycle that arid zone ecologists try to avoid.

Piecing together these casual observations, one can make a good case that, to maintain biological productivity in box-ironbark forests, it’s useful to view these forests through the lens of arid zone ecology. This approach becomes even more compelling given the perils of climate change. As temperatures increase, box ironbark forests will come under increasing water stress.

As in arid ecosystems, a valuable management strategy to minimize the effects of climate change will be to increase the system’s ability to retain soil moisture.

I’m starting to wonder – is water availability a far more important issue than fire in box ironbark forests? Instead of this simple either/or question, a better question might be: how do fires (where they do occur) affect soil moisture retention in box ironbark forests? Do they make the system more or less leaky?

Litter for water, water for ecosystems

Small barriers trap and prevent leaf litter from being washed from the system.

How do we retain soil moisture in water-limited ecosystems? David Tongway, John Ludwig and other arid zone ecologists have studied this for decades now.

The goal isn’t to create ‘dams’ of deep water (this is a dryland system after all). Instead, the goal is to increase the capacity of topsoil and litter layers to hold and slow the flow of surface water. Anything that promotes ‘surface roughness’, slows the flow of water. Branches, leaves and twigs on the soil surface trap fine litter and create small barriers that hold water for longer periods. Soil accumulates, soil micro-organisms become more abundant, and system productivity increases.

In the longer term, as trees grow older and drop more larger limbs, fallen coarse woody debris (‘brown gold’) helps to capture even more litter and water. By contrast, processes that reduce leaf litter accumulation are likely to reduce soil water holding capacity, biological productivity and resilience to climate change.

Fallen timber and deep leaf litter beneath an old grey box. Old trees drop more branches than young trees.

A key goal for conservation management is to increase ecosystem resilience to changing climate. The most likely impact of climate change in south-eastern Australia is a decline in soil water due to higher evaporation as temperatures increase. So any process that retains soil moisture and reduces system ‘leakage’ seems like a great process to promote (or at least to investigate), yes?

Water for ironbarks may not be the goal, but water for soil development, soil productivity, and biological diversity seems like an important ecosystem process to promote. Ultimately, retaining leaf litter (fallen leaves, twigs and branches) and old trees (that produce more branches) may be the simplest way to hold soil water. Unfortunately, litter for water and resilient ecosystems doesn’t sound that sexy does it? Nevertheless, as climate change intensifies, it may be the issue that we have to focus on more and more.


Further reading

Environment Conservation Council (1997). Box – Ironbark Forests and Woodlands Investigation Resources and Issues Report. (ECC: Melbourne)

Ludwig J, Tongway D, Freudenberger D, Noble J, Hodgkinson K (1997). Landscape Ecology, Function and Management: Principles from Australia’s Rangelands. (CSIRO: Melbourne).

Mac Nally R, Bennett AF, Thomson JR, Radford JQ, Unmack G, Horrocks G, Vesk PA (2009). Collapse of an avifauna: climate change appears to exacerbate habitat loss and degradation. Diversity and Distributions 15(4), 720-730.

Taylor SG (2008) Leaf litter invertebrate assemblages in box-ironbark forest: composition, size and seasonal variation in biomass. Victorian Naturalist 125(1), 19-27.

Watson DM (2011) A productivity-based explanation for woodland bird declines: poorer soils yield less food. Emu 111(1), 10-18.

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15 thoughts

  1. I just want to say that I enjoy, and learn from, your blogs. I don’t know enough to contribute anything thoughtful, but just wanted to say that I’m glad that you write, and I read.

  2. Thank you Ian for another fantastic readable, stimulating post. In Panton Hill, North-east of Melbourne, in small remainders of Box-Ironbark forest, Eucalyptus sideroxylon sometimes occurs with E. obliqua, one of our more moisture dependant trees. There seem also to be indicators in the flora of Heathcote BIF (forests) of a higher moisture supply than that indicated by the large expanses of bare ground. These places are maybe only half arid?

    1. Hi Antblue, thanks for a great comment. I’ve been remiss in implying that all box-ironbark forests are ‘the same’, as they obviously vary a lot over their range. I hadn’t pictured forests containing messmate stringybark at all when I wrote this post. They would definitely be more mesic than the drier forests I was thinking of. How did the messmates (E. obliqua) fare during the recent drought? Did they die off more than the more drought hardy ironbarks? Thanks again, Ian.

  3. Ian – very good post. Question: why is productivity necessarily correlated with ecological function? I’m not saying it is or isn’t, but am wondering why that assumption is commonly made? Even within a particular ecosystem, less productive soils can still support diverse life, and can often be more resistant to invasion. Is it a bad thing if an ecosystem is producing less aboveground vegetative production than could be produced if managed differently? Seems like an agricultural way to think about an ecological system – potentially. Anyway, the question just struck me while reading your post. A good sign that the post was a success!

    1. Hi Chris, great question, and more than I can do justice to in a short reply. Productivity (or available resources) must always influence ecological function, or the degree to which ecological functions are expressed. However, more resources (water or nutrients) don’t necessarily improve the functions that we desire to occur.

      Plant species richness is often low in undisturbed, productive sites (as dominant grasses outcompete smaller plants). Many of our best grassland remnants are on poor, dry sites. Poor conditions deter many exotic plants, which need more water or nutrients to grow big. Indeed, much of our grassland restoration is based on reducing resource levels, such as Paul Gibson-Roy’s fantastic work and our sugar experiments (see my sugar blogs for more info). So, in our grasslands we’re often trying to promote native biodiversity by reducing productivity levels if we can. 

      You raise a great question about, ‘what impact would it have if we increased productivity in these dry forests?’ Consistent with my speculative post, my simple answer is ‘I don’t know’. As always, complex interactions will make outcomes difficult to predict. However, we do know that deeper litter supports more invertebrates, and can speculate that this should support more skinks, lizards and insectivorous birds. Greater soil water should also promote more plant growth, but again, this will include weeds as well as native plants presumably. 

      I must emphasize that I’m not suggesting that we increase productivity levels beyond their ‘natural’ baseline level. That’s what we’ve done historically through agriculture, which has been disastrous for biodiversity. Instead my thoughts were focused more on restoring productivity in degraded sites, and trying to maintain moisture holding capacity against reductions imposed by climate change.

      In a nut shell …. Productivity is extremely important, as it underpins all ecological functions. However if we are concerned with biodiversity (rather than growing crops) we need to focus on how changes in productivity levels influence the species, ecosystems and processes that we desire. Chasing productivity, on it’s own, is unlikely to help biodiversity conservation. This is a huge topic, but I think we’re coming from the same place.

      Thanks again for the best question I’ve ever got on a blog! Best wishes, Ian

  4. I’m actually thinking about it – especially after tripping over this passage from Aldo Leopold this evening. (Please tell me Aldo Leopold is widely read in Australia?) I think it relates to at least some of our discussion:

    It’s the opening passage of his essay entitled, “Country”
    “There is much confusion between land and country. Land is the place where corn, gullies, and mortgages grow. Country is the personality of the land, the collective harmony of its soil, life, and weather. Country knows no mortgages, no alphabetical agencies, no tobacco road; it is calmly aloof to these petty exigencies of its alleged owners…

    …Poor land may be rich country, and vice versa. Only economists mistake physical opulence for riches. Country may be rich despite a conspicuous poverty of physical endowment, and its quality may not be apparent at first glance, nor at all times.”

    1. I look forward to your blog Chris. I think you’ve raised two important, but different, issues. (1) The fact that increasing ‘productivity’ (i.e. nutrient and water supplies) doesn’t necessarily improve the biological and functional attributes that we value as conservationists and restorationists, and (2) the larger philosophical difference between the economic exploitation of land, and its sustainable use, including broader social and ecological values and economic sustainability over the longer term.

      Rest assured that Aldo Leopold does get read here, but perhaps only by those who value old books and deep thoughts. I gave him a plug in an earlier blog of mine: https://ianluntresearch.wordpress.com/2011/04/25/reading-the-bush-juxtapositions-in-history/ Cheers Ian

  5. I’ve come to this blog quite late, but the idea of retaining available resources within the ecosystem rather than have them leak out is a crucial concept. And clearly water is an issue that we should give more attention to. We tend to forget the most striking manifestation of water being wasted from a landscape – dryland salinity. As a result of reduced rainfall since the 80s in SE Australia and better agricultural technology, this issue has fallen off the policy radar somewhat. However, water being lost from semi-arid woodlands is perhaps now becoming an issue of basic survival for trees, given that rainfall amounts appear to be falling and evaporation is increasing.
    Having not long returned from a trip to Israel, it is instructive to consider the work being done by researchers and foresters with JNF – in a process they call ‘savannisation’. This involves planting trees in areas that receive only 280 – 400 mm MAP. The effective rainfall available to each planted tree (which includes eucalypts in the northern Negev), is enhanced by mounding and plowing adjacent to the widely spaced rows. It definitely works, and useful to consider in already degraded areas (including farms) where we can’t rely on the accumulation of surface organic matter from existing trees to slow infiltration.

    1. Thanks very much for writing in. Great points. Water availability is going to get increasingly important in many regions as climate change intensifies further. I imagine that the issue will get a lot more attention in the future, especially if tree health declines dramatically. Thanks again, best wishes Ian

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